Effects of weather and vibrations on concrete roads.pdf

8/10/2019 Effects of weather and vibrations on concrete roads.pdf

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Abstract

Past research works have proven that concrete is more economical, reliable and durable than

asphalt when it comes to road construction. Part of actualization of the vision 2030 requires

research projects on concrete to be undertaken in order to come up with long-term solutions to

present persistent road maintenance problems.

The objective of this report is to investigate the impact of weather, vibration and abuse/misuse of

concrete roads. The theoretical response of concrete to the above three problems was inspected

with a real time case study and inspection of Mbagathi Way. Concrete pavements are susceptible

to physical, chemical and biological weathering processes. On the other hand, durability of such

pavements are also affected by the climatic conditions of area. Moreover, concrete pavements

are rigid hence dynamic loading principles should be incorporated in design. Finally, road

misuse of any form is likely to reduce the pavement effectiveness.

Conclusively, durable and effective pavements can be achieved by integrating the solutions to

the above mentioned problems.


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Acknowledgements

My first gratitude goes to my dear family especially my mother, Mrs. Susan Koikai, whose

support has been instrumental for the completion of this research project.

Secondly, I would like to thank my supervisor Eng. Evans C. Goro for his advice, patience and

guidance throughout the process of preparing this report.

Above all I thank the almighty God for his grace and making this happen.


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Table of Contents

Abstract....................................................................................................................................... i

Dedication.................................................................................................................................. ii

Acknowledgements................................................................................................................... iii

Chapter One.................................................................................................................................. 1

Introduction........................................................................................................................ 1

1.1 History of Concrete Roads ........................................................................................... 1

1.2 Summary of Past Researches ........................................................................................ 3

1.3 Emergence of concrete roads in Kenya ........................................................................ 5

Chapter Two.................................................................................................................................. 6

Literature Review and Theoretical Analysis................................................. 6

2.1 Concrete as a Construction Material. ........................................................................... 6

2.1.1 Concrete basics ...................................................................................................... 6

2.1.2 Hydration Process ................................................................................................. 7

2.1.3 Curing Process..................................................................................................... 12

2.2 Weathering Processes ................................................................................................. 13

2.2.1 Properties of Concrete Surfaces .......................................................................... 14

2.2.2 Physical Causes of Concrete Deterioration ......................................................... 172.2.3 Deterioration Caused By Physical Weather/Environmental Conditions............. 18

2.2.4 Deterioration Due To Chemical Attack .............................................................. 21

2.3 Anomalies in Concrete Pavements (PASER Manual). ............................................... 28

2.3.1 Surface Defects ................................................................................................... 28

2.3.2 Pavement Cracks ................................................................................................. 32

2.3.3 Pavement Deformations ...................................................................................... 35

2.4 Direct Effects of Climatic Conditions on Concrete Pavements ................................. 38

2.4.1 Local Climate ...................................................................................................... 38

2.4.2 Oceanic Climate .................................................................................................. 41

2.4.3 Climate Change ................................................................................................... 41

2.4.4 Choice of Aggregate............................................................................................ 44

2.4.5 Concrete placing in Low Temperature Environments ........................................ 45


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2.4.6 Concrete Placing in High Temperature Environments ....................................... 46

2.4.7 Temperature Evolution ........................................................................................ 48

2.4.8 Impact of Extreme Temperature ......................................................................... 49

2.4.9 Unpredictable Precipitation ................................................................................. 49

Chapter Three............................................................................................................................. 50

Effects of Traffic Induced Vibration on Concrete................................... 50

3.1 Introduction ................................................................................................................ 50

3.2 Nature of Vibration ..................................................................................................... 51

3.2.1 Mechanism of Wave Generation ......................................................................... 51

3.2.2 Factors Affecting Magnitude of Vibration and Frequency ................................. 52

3.3 Effects on Concrete Pavements .................................................................................. 54

3.3.1 Concrete Slab Deflection .................................................................................... 54

3.3.2 Induced Tensile Stresses ..................................................................................... 55

3.3.3 Yielding of Concrete under Excessive Vibration ................................................ 56

Chapter Four............................................................................................................................... 57

Road Abuse/Misuse...................................................................................................... 57

4.1 Definition .................................................................................................................... 57

4.2 Overloading ................................................................................................................ 57

4.2.1 Truck Damages on Pavements from the AASHO Test Perspective ................... 58

4.3 Pre-Mature Use of Newly Constructed Roads ........................................................... 60

4.4 Oil Spillage ................................................................................................................. 60

4.4.1 Transmission Fluid .............................................................................................. 61

4.4.2 Lubricating Oil .................................................................................................... 61

4.4.3 Diesel Spillage..................................................................................................... 61

4.5 Use of Inappropriate Vehicles .................................................................................... 62

Chapter Five................................................................................................................................ 63

Inspection of Mbagathi Way.................................................................................. 63

5.1 Inspection Methodology ............................................................................................. 63

5.2 Results and Analysis ................................................................................................... 65

5.2.1 Defects due to weathering ................................................................................... 65


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5.2.2 Damages due to misuse/abuse ............................................................................. 68

5.3 Discussion ................................................................................................................... 70

5.4 Pavement rating .......................................................................................................... 71

Chapter Six.................................................................................................................................. 72

Mitigation Measures and Recommendations.............................................. 72

6.1 Control of Weathering ................................................................................................ 72

6.1.1 Reducing Freeze and Thaw ................................................................................. 72

6.1.2 Minimizing Thermal Cracking ............................................................................ 72

6.1.3 Abrasion Resistance ............................................................................................ 73

6.1.4 Reducing Sulphate Attack ................................................................................... 75

6.1.5 Minimizing Chloride Attack ............................................................................... 76

6.1.6 Minimizing Acid Attack...................................................................................... 77

6.2 Mitigation Measures of Effects Directly Pertaining To Weather Conditions ............ 77

6.2.1 Cold Weather Paving .......................................................................................... 77

6.2.2 Hot Weather Paving ............................................................................................ 77

6.2.3 Precautions Due To Rain..................................................................................... 79

6.3 Minimizing the Magnitude and Effect of Traffic Induced Vibrations on Concrete

Pavements. ............................................................................................................................. 80

6.3.1 Joint Performance ................................................................................................ 806.3.2 Other Measures ................................................................................................... 80

6.4 Measures to Curb Road Misuse .................................................................................. 81

6.5 Conclusion .................................................................................................................. 82

6.6 References .................................................................................................................. 83

6.7 Appendices ................................................................................................................. 87


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List of Tables

Table 2.1 Permeability of Cement Paste ....15

Table 2.2 Permeability of Aggregates...15

Table 2.3 Nairobis Climate..39

Table 2.4 Mombasas Climate...40

Table 5.3.1 Rating pavement surface condition (PASER Manual)...87


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List of Illustrations

Figures

Fig.2.1 Concrete composition (cement.org) ..7

Fig. 2.2 Rate of hydration vs. time.8

Fig. 2.3 Digital model of the hydration phases..10

Fig. 2.4 Heat evolution of Type I/II Portland cement paste as measured by conduction

calorimetry.10

Fig. 2.5 Typical development of the degree of hydration and compressive strength of a Type I

Portland cement over time.1 2

Fig. 2.6 Ice expansion in concrete.19

Fig. 2.7 Illustration of frost heave..20

Fig.2.8 Sulphate attack process..23

Fig. 2.9 Corrosion of Reinforcement in Concrete Pavement.24

Fig.2.10 Alkali-Silica reaction ...27

Fig. 2.11 Temperature projection...42

Fig. 2.12 Bar graph of coefficients of thermal expansion for various aggregates.44

Fig.2.13 Temperature evolution.48

Fig 3.1 Time histories of slab deflection...55

Fig. 3.2 Relationship between loading and damage..59

Fig. 5.1 Methodology of data collection63

Fig.6.1 Relationship between resistance of aggregate to abrasion and concrete abrasion

loss. 75


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Plates

Plate 2.1 A section of Mbagathi way5

Plate 2.2 Wear and Polishing28

Plate 2.3 Map cracking..29

Plate 2.4 Pop-outs..29

Plate 2.5 Surface scaling30

Plate 2.6 Shallow reinforcement31

Plate 2.7 Spalling...31

Plate 2.8 Transverse slab cracks32

Plate 2.9 D- cracking.33

Plate 2.10 Corner cracks34

Plate 2.11 Meander cracks.34

Plate 2.12 Blow ups...35

Plate 2.13 Faulting.36

Plate 2.14 Heave36

Plate 2.15 Manhole cracks.37

Plate 2.16 Shoulder deformation37

Plate 2.17 Hailstones in Nyahururu...43

Plate 3.1 Irregularities on the road surface51

Plate 3.2 Model pavement.54


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Plates from Case Study

Plate 5.2 D-crack65

Plate 5.3 Corner cracks..65

Plate 5.4 Spalling along joints. ..65

Plate 5.5 Scaling.66

Plate 5.6 Polished Surfaces66

Plate 5.7 Transverse cracks66

Plate 5.8 Surface mortar worn away..67

Plate 5.9 Pop outs...67

Plate 5.10 Meander crack and encroachment of soil on pavement67

Plate 5.11 Engraved footprints.. .68

Plate 5.12 Dents on pavement due to excessive abrasive pressure68

Plate 5.13 Rutting...69

Plate 5.14 Exposed slab section.69


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Chapter One

Introduction

1.1

History of Concrete Roads

Roads constitute a major part of the nations infrastructure. It is important tohave a well-

functioning road network in order to ensure the economic wellbeing of a society or nation.

In year1909 the Wayne County Road Commission (Detroit, Michigan) introduced the world to a

new kind of road: Concrete road. During that year the only place where concrete roads were

available was Woodward Avenue, which is now northwest Detroit. Before then paved roads

were built using bricks, cobblestone or macadam. John McAdam, a Scottish engineer in the year

1820, pioneered the latter. This type of road consisted of single-sized aggregate layers of small

stones, with a coating of binder as a cementing agent.

Campaigned by a group of cyclists (League of American Wheelmen) who were catalyzed by the

need to make cycling more pleasurable than it had been on the areas rough and rutted roads, the

need for a better type of road construction was evident. Ever since construction of concrete roads

has been engaged in highway engineering. Subsequent improvements have been made over the

decades.

In Kenya, a road-network must be designed to fully sustain impact from large traffic volumes.

The traffic intensity as well as traffic loading increases over time. This calls for road pavements

with high wear resistance and little maintenance. A concrete road network is the immediate and

long-term solution to this problem.


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Concrete roads are 40-50% more expensive than ordinary bituminous roads. The former has a

life span of up to 5 decades with minimal maintenance but the latter lasts for about 1 decade with

an initial cost of about 9% of the cost set aside for maintenance.

In countries like Germany, Belgium and Switzerland, about 20 % of their major road networks

consist of concrete pavements. The difference in use of concrete roads between Kenya and the

above mentioned European countries can be explained by the conservatism in the local

construction industry and the fear of trying new trends. Another reason contributing to minimal

construction of such roads is the lack of solid national experience and continual practice among

the road contractors.

The underlying fact is that there is a need for more efficient pavement design techniques and

approaches that could reduce environmental impacts as well as providing pavements that are

more economic in the long run and are of a higher quality than those used today. Giving

preference to quality over cost, implementation of concrete road is a worthwhile undertaking. In

regard to the economical state of Kenya few adjustments can be introduced to harmonize quality

with cost within allowable limits of compromise and construct concrete roads that still maintain a

substantial advantage over bitumen roads.

This project will analytically deal with the effects of weather on concrete roads (weathering of

concrete and the direct impact of climatic conditions on concrete road construction) with respect

to the local climate patterns. However, relevant researches based on other climates will be

utilized.

Secondly, the effects of traffic induced vibration on roads will be examined bearing the fact that

bitumen and concrete respond to seismic disturbances quite differently due to their differing

material properties. This research will zero in on the effects of such vibrations on concrete roads.

Finally road misuse/abuse and their effects on concretes of pavements will be investigated. Roadmisuse/abuse is a common phenomenon in East Africa. This results in increased road

maintenance costs and even premature end of the pavements serviceable life. The forms of road

misuse/abuse will be discussed.


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In addition, published scientific papers, principally from university researchers and experienced

concrete consultants, revealed that chloride containing de-icing materials such as calcium

chloride, potassium chloride, and sodium chloride, can exacerbate a scaling problem as concrete

structures (including roads) goes through freeze-thaw cycles.

Research on pervious concrete has been done over the years and with significant success in its

implementation. Pervious concrete pavement has been in use for over 30 years in Florida and an

experimental road was constructed in England in the 1960s.


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1.3Emergence of concrete roads in Kenya

The construction of Mbagathi way (prototype concrete road in Kenya) commenced on 19th

August 2005 with a programmed completion date of 18 August 2006. However due to its

unprecedented nature, the occurrence of technical and operational difficulties pushed the

expected date of completion to 30th

June 2007, a time overrun of 11 months. (Kenya National

Assembly Official Report, 2007)

Plate 2.1 A section of Mbagathi way


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Chapter Two

Literature Review and Theoretical Analysis

2.1Concrete as a Construction Material.

2.1.1 Concrete basics

In its simplest form, concrete is a mixture of paste and aggregates. The paste, composed of

Portland cement and water, coats the surface of the fine and coarse aggregates. Through a

chemical reaction called hydration, the paste hardens and gains strength to form the rock-like

mass known as concrete.

Within this process lies the key to a remarkable trait of concrete: it's plastic and malleable when

newly mixed, strong and durable when hardened. These qualities explain why one material,

concrete, can build skyscrapers, bridges, sidewalks and superhighways, houses and dams.

A concrete mixture that does not have enough paste to fill all the voids between the aggregates

will be difficult to place and will produce rough, honeycombed surfaces and porous concrete. A

mixture with an excess of cement paste will be easy to place and will produce a smooth surface;

however, the resulting concrete is likely to shrink more and be uneconomical.


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Fig.2.1 Concrete composition (cement.org)

A properly designed concrete mixture will possess the desired workability for the fresh concrete

and the required durability and strength for the hardened concrete. Typically, a mix is about 10 to

15 percent cement, 60 to 75 percent aggregate and 15 to 20 percent water. Entrained air in many

concrete mixes may also take up another 5 to 8 percent.

Cement and water form a paste that coats each particle of stone and sand. Through a chemical

reaction called hydration, the cement paste hardens and gains strength. The character of the

concrete is determined by quality of the paste. The strength of the paste, in turn, depends on the

ratio of water to cement. The water-cement ratio is the weight of the mixing water divided by the

weight of the cement. Lowering the water-cement ratio as much as possible without sacrificing

the workability of fresh concrete produces high-quality concrete. Generally, using less water

produces a higher quality concrete provided the concrete is properly placed, consolidated, and

cured.

2.1.2 Hydration Process

The hydration process can be categorized in to two primary mechanisms:

Through solution;this involves dissolution of anhydrous compounds to their ionic

constituents, formation of hydrates in solution, and eventual precipitation due to their low

solubility


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Topochemicalorsolid-statehydration; reactions take place directly at the surface of the

anhydrous cement compounds without going into solution (Kurtis K. 1998).

Stages of hydration:

Fig. 2.2 Rate of hydration vs time

Stage 1: This stage brief because of the rapid formation of an amorphous layer of hydration

product around the cement particles, which separates them from the pore solution and prevents

further rapid dissolution. This is then followed by the induction period, during which almost no

reaction occurs.

Stage 2: This stage is identified as the induction period. Induction is a 1-2 hour period of

inactivity that separates the initial short burst of reaction that occurs when cement and water first

come into contact from the main hydration period that leads to set. This behaviour is vital

because it prevents the cement from setting too quickly. The precise nature of the induction

period, and in particular the reason for its end, is not fully agreed upon, due to differing opinions

among cement chemists.

Stage 3: This is the rapid reaction period, the rate of reaction increases rapidly, reaching a

maximum at a time that is usually less than 24 hours after initial mixing, and then decreases

rapidly again to less than half of its maximum value. This behaviour is due to the hydration of


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the C3S (tri-calcium silicate), and the rate of hydration is controlled by the rate at which the

hydration products nucleate and grow. Both the maximum reaction rate and the time at which it

occurs depend strongly on the temperature and on the average particle size of the cement. At the

end of this about 30% of the initial cement has hydrated, and the paste has undergone both initial

and final set. This stage is also is characterized by a continuous and relatively rapid deposition of

hydration products (primarily calcium silicate hydrate (C-S-H) gel and calcium hydroxide

Ca(OH)2) into the capillary porosity, which is the space originally occupied by the mix water.

This causes a large decrease in the total pore volume and a concurrent increase in strength. The

rapid formation of calcium silicate hydrate and calcium hydroxide is accompanied by significant

evolution of heat. The C-S-H forms a thickening layer around the cement grains. As the shells

grow outward, they begin to coalesce after about 12 hours after mixing; this time coincides with

the maximum rate of heat evolution (Fig. 2.4) and corresponding approximately to the

completion of setting. The shells are apparently sufficiently porous to allow the passage of water

in and dissolved cement minerals out. A gap begins to appear between the hydration shell and

the surface of the cement grain. The microstructure of the paste at this point consists of unreacted

cores of the cement particles surrounded by a continuous layer of hydration product, which has a

very fine internal porosity filled with pore solution, and larger pores called capillary pores.

In order for further hydration to take place, the dissolved ions from the cement must diffuse

outward and precipitate into the capillary pores, or water must diffuse inward to reach the

unreacted cement cores. These diffusion processes become slower as the layer of hydration

product around the cement particles becomes thicker.

Stage 4: This is the diffusion-limited reaction period.

(http://iti.northwestern.edu/cement/monograph/Monograph1_1.html)


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Fig. 2.3 Digital model of the hydration phases. The microstructure of cement paste as it hydrates

as simulated by a realistic digital image base model. The yellow phase is the main hydration

product, C-S-H gel. At the end of stage 3, the yellow rims of hydration product have become

interconnected, causing final set and giving paste some minimal strength. By 28 days the image

is dominated by C-S-H gel and the porosity has noticeably decreased. The final amount of

porosity will depend strongly on the initial w/c of the paste. (Images courtesy of National

Institute of Standards and Technology (NIST))

Fig. 2.4Heat evolution of Type I/II Portland cement paste as measured by conduction

calorimetry.


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Hydration will continue at a slow rate during Stage 4 until one of the three following criteria is

met:

All of the cement reacts. This indicates that the paste has a moderate or high w/c and was

cured correctly. While it is the best possible outcome for the given mix design, it does not

guarantee high quality concrete as the w/c may have been too high. If the cement contains

some large particles, full hydration of these particles may not occur for years. However

this is generally not the case with modern cements.

There is no more liquid water available for hydration. If the cement has a w/c less than

about 0.4, there will not be enough original mix water to fully hydrate the cement. If

additional water is supplied by moist curing or from rainfall, hydration may be able to

continue. However, it is difficult to supply additional water to the interior of largeconcrete sections. If the cement is improperly cured so that it dries out, hydration will

terminate prematurely regardless of the w/c. This is the worst-case scenario, as the

strength will be lower (perhaps significantly) than the value anticipated from the mix

design.

There is no more space available for new reaction product to form. When the capillary

porosity is reduced to a certain minimal level, hydration will stop even if there is

unreacted cement and a source of water. This is the best possible outcome, and it is only

possible if the w/c is less than about 0.4. Not only will the cement paste or concrete have

a high strength, but it will also have a low permeability and thus be durable.


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Fig. 2.5 Typical development of the degree of hydration and compressive strength of a Type I

Portland cement over time.

Succinctly, cement hydration is a continuous process by which the cement minerals are replaced

by new hydration products, with the pore solution acting as a necessary transition zone between

the two solid states. A node forms on the surface of each cement particle. The node grows and

expands until it links up with nodes from other cement particles or adheres to adjacent

aggregates.

This building up process results in progressive stiffening, hardening, and strength development.

The mixture is placed in forms while it is still workable.

2.1.3 Curing Process

Curing begins after the exposed surfaces of the concrete have hardened sufficiently to resist

marring. Curing ensures the continued hydration of the cement and the strength gain of the

concrete. Concrete surfaces are cured by sprinkling with water fog, or by using moisture-

retaining fabrics such as burlap or cotton mats. Other curing methods prevent evaporation of the

water by sealing the surface with plastic or special sprays (curing compounds).


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Moreover, concrete is available in many types, created by adjusting the proportions of the main

ingredients. In this way or by substitution for the cementitious and aggregate phases, the finished

product can be tailored to its application with varying strength, density, or chemical and thermal

resistance properties.

Various additives are added to cement to improve its properties while admixtures may be added

during mixing to modify the properties of the fresh concrete.

The ability to modify concrete properties on such a wide scale makes it advantageous over

bitumen; concrete is now being embraced as the most suitable candidate to substitute asphalt.

2.2

Weathering Processes

For the purpose of this research weathering has been adopted to mean the effects of natural

forces such as rain and sunlight and unnatural forces such as pollution on well-made concrete.

Weathering is the natural effect of time on engineering works and it would be unwise to ignore it

or relegate it to a position of little importance in the design process.

The weather almost always has some role in the occurrence of uncontrolled cracking. Air

temperature, wind, relative humidity and sunlight, influence concrete hydration and shrinkage.

These factors may heat or cool concrete or draw moisture from exposed concrete surfaces. The

subbase can be a heat sink that draws energy from the concrete in cold weather, or a heat source

that adds heat to the bottom of the slab during hot, sunny weather.

Under warm sunny conditions, the maximum concrete temperature will vary depending on the

time of day when the concrete is paved. Concrete paved in early morning will often reach higher

maximum temperatures than concrete paved during the late morning or afternoon because it

receives more radiant heat. As a result, concrete paved during the morning will generally have a

shorter sawing window, and often will exhibit more instances of uncontrolled cracking. In this

situation, sawing as a method of crack control becomes challenging.

After the concrete sets, uncontrolled cracking might occur when ambient conditions induce

differential thermal contraction. Differential contraction is a result of temperature differences

throughout the pavement depth. Research indicates that a sudden drop in surface temperature

more than 9.5C (15F) can result in cracking from excessive surface contraction. This degree of

temperature change is common all year-round in arid climates (North Eastern Kenya), and


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possible in most other climates when air temperatures drop significantly from day to night.

Differential contraction also may occur when a rain shower cools the slab surface, or when the

surface cools after removing insulating blankets from fast-track concrete.

In order to construct a road that will weather well, the following aspects should be closely taken

in to consideration:

1. The drainage design of the pavement.

2. The blend of materials and surface finish of the road.

2.2.1 Properties of Concrete Surfaces

Porosity and Permeabili ty

Dry concrete consists of interconnected pores resulting from the following technical challenges:

Due to the need for workability of concrete, more water than what is required for the

hydration of cement is used. This excess water is eventually trapped in voids leading to

formation of pores.

Trapped air during compaction. This is due to the impossibility of removing all entrapped

air during compaction process (Newman J. and Choo S. B 2003)

As mentioned insection 2.1.1concretescomponents mainly include coarse aggregate, fine

aggregates and cement paste. Intricately porosity and permeability can be rooted to these

individual components.

When porosity decreases from 40 to 30%, the permeability of cement paste drops from 110 to 20

x 10-12 cm/sec. However, a decrease in porosity from 30% to 20% results in a small drop in

permeability. This observation is due to a reduction in number and size of large pores and

creation of tortuosity.


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Table 2.1 Permeability of Cement Paste.

Age (days) Permeability (cm/s 10 -11 )Fresh 20,000,000

5 4000

6 1000

8 400

13 50

24 10

Ultimate 6

Compared to 30 to 40 percent capillary porosity of typical cement pastes in hardened concrete,

the volume of pores in most natural aggregates is usually under 3 percent, and it rarely exceeds

10 percent.

However, the coefficient of permeability of aggregates are as variable as those of hydrated

cement pastes of water/cement ratios in the range 0.38 to 0.71. This is because some aggregates

have much higher permeability than the cement paste because their capillary pores are much

larger. In addition, Most of the capillary porosity in a mature cement paste lies in the range 10 to

100 nm, while pore size in aggregates are, on average, larger than 10 microns.

Table 2.2 Permeability of Aggregates

Type of Rock Permeability (cm/sec )

Dense Trap 2.47 x 10 -12

Quartz Diorite 8.24 x 10 -12

Marble 2.39 x 10 -10

Granite 5.35 x 10 -9

Sandstone 1.23 x 10 -8

I ni tial colour changes

Concrete is intrinsically grey, but by the time concrete has dried ad formwork removed, a thin

layer of calcite crystals will have formed on the surface. Calcite crystals (Calcium Carbonate)

result from the reaction between atmospheric carbon dioxide with calcium hydroxide, a product


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of cement hydration.

These crystals give the surface a white appearance. However due to the refractive index of these

crystals, the concrete regains its original grey colour when water is applied to the surface

(Newman J. and Choo S. B 2003).

I norganic growths

Changes in appearance of concrete roads may also be due to inorganic growths, which are a

consequence of interaction of the products of cement hydration with the atmosphere.

The integration of chemical and biological weathering processes

Carbonation of concrete surfaces provides suitable conditions for biological colonization.

Colonizers include algae, fungi and associated bacteria. For concrete roads, this phenomenon

may take place at the edges of the pavements where there is minimal contact with tires.

During rainy seasons, eroded soil may collect at colonized surfaces and this, together with dead

lichen, provides a footing for mosses and if unchecked more developed plants.

Degree of colonization depends on the permeability of concrete. Poor quality concrete will in

general accumulate more dirt and attract biological colonization (Newman J. and Choo S. B

2003).

Concrete surfaces attacked to a significant degree by bio-colonization have a relatively lower

skid resistance hence compromising the safety of motorists of planes if runways are considered.

In more advanced levels of bio-attack, plant growth at the sides of the pavements eventually

cause cracking of concrete pavements due to root penetration.


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2.2.2 Physical Causes of Concrete Deterioration

Abrasion and Erosion

Dry attrition (wear on pavements by traffic). Abrasion is the wear due to hard particles or hard

protuberances forced against and moving along a solid surface. Abrasion resistance is the ability

of a surface to resist being worn away by rubbing and friction (American Society for Testing and

Materials(ASTM), American Concrete Institute (ACI)).

This type of wear is caused by a rubbing action, plus an impact-cutting type of wear. This is

brought about by the use of chains on automobile and truck tires or metal vehicle wheels. As the

wheel revolves, it brings the metal into contact with the concrete surface with considerable

impact, a process that tends to cut the surface of the concrete. Wear is greatly increased by the

introduction of foreign particles, such as sand, small metal scraps, gravel or similar materials.

Abrasion can also take place through erosion. This is type of deterioration mainly occurs in areas

of poor carriageway drainage where suspended solids in rapidly flowing floods facilitate the

abrasion process. The action of the abrasive particles carried by the flowing water, of course, is

controlled largely by the velocity of the water, the angle of contact, the type of abrasive material,

and the general surrounding conditions.

Factors that affect abrasion resistance include:

Quality of aggregates

Compressive strength

Mixture proportioning

Concrete types

Finishing procedures

Curing and,

Surface treatment


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Cavitation

Cavitation-erosion is a result of complex flow characteristics of water over concrete surfaces. For

damage to occur, the rate of water flow normally has to exceed 12.2 m/s (40 ft/sec.). Fast water

and irregular surface areas of concrete can result in cavitation. The surface irregularity and water

speed create bubbles. The bubbles are carried downstream and have a lowered vapor pressure.

Once the bubbles reach a stretch of water that has normal pressure, the bubbles collapse. The

collapse is an implosion that creates a shock wave. Once the shock wave reaches a concrete

surface, the wave causes a very high stress over a small area. When this process is repeated,

pitting can occur.

2.2.3

Deterioration Caused By Physical Weather/Environmental Conditions

The most profound, weather caused, deterioration in concrete surfaces is through frost action.

Frost action can be classified in to two separate but related processes, frost heave and thaw

weakening. However technically related, freeze and thawing also stands out from the above

classifications.

Freeze and Thawing

The transformation of ice from liquid water generates a volumetric dilation of 9%.

As the water in moist concrete freezes, it produces hydraulic pressure in the pores of the

concrete. If the pressure developed exceeds the tensile strength of the concrete, the cavity will

dilate and rupture. However, some of the water may migrate through the boundary, decreasing

the hydraulic pressure.

Hydraulic pressure depends on:

Rate at which ice is formed.

Permeability of the material.

Distance to an "escape boundary."

If the transformation occurs in small capillary pores the ice crystals can also damage the cement

paste by literally pushing the capillary walls.


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Fig. 2.6 Ice expansion in concrete (Monterio P., 2008)

The accumulative effect of successive freeze-thaw cycles and disruption of paste and aggregate

can eventually cause expansion and cracking, scaling, and crumbling of the concrete.

Frost Heave

Frost heaving of soil is also caused by crystallization of ice within the larger soil voids and

usually a subsequent extension to form continuous ice lenses, layers, veins, or other ice masses.

An ice lens grows through capillary rise and thickens in the direction of heat transfer until the

water supply is depleted or until freezing conditions at the freezing interface no longer support

further crystallization. As the ice lens grows, the overlying soil and pavement will heave up

potentially resulting in a cracked, rough pavement. This problem occurs primarily in soils

containing fine particles (often termed frost susceptible soils), while clean sands and gravels

(small amounts of fine particles) are non-frost susceptible (NFS). Thus, the degree of frost

susceptibility is mainly a function of the percentage of fine particles within the soil.


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Fig. 2.7 Illustration of frost heave.

The three elements necessary for ice lenses and thus frost heave are:

1. Frost susceptible soil (significant amount of fines).

2.

Subfreezing temperatures (freezing temperatures must penetrate the soil and, in general,the thickness of an ice lens will be thicker withslowerrates of freezing).

3. Water (must be available from the groundwater table, infiltration, an aquifer, or held

within the voids of fine-grained soil).

Cracking occurs during differential heaving of the pavement. Differential heave is more likely to

occur at locations such as:

Where subgrades change from clean non frost susceptible (NFS) sands to silty frostsusceptible materials.

Abrupt transitions from cut to fill with groundwater close to the surface.

Where excavation exposes water-bearing strata.


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Drains, culverts, etc., frequently result in abrupt differential heaving due to different

backfill material or compaction and the fact that open buried pipes change the thermal

conditions (i.e., remove heat resulting in more frozen soil).

Thaw Weakening

Thawing is essentially the melting of ice contained within the subgrade. As the ice melts and

turns to liquid it cannot drain out of the soil fast enough and thus the subgrade becomes

substantially weaker (less stiff) and tends to lose bearing capacity. Therefore, loading that would

not normally damage a given pavement may be quite detrimental during thaw periods.

2.2.4 Deterioration Due To Chemical Attack

Frequently sound concrete has been unintentionally subjected to conditions that lead to

disintegration.

Acids

Acids combine with calcium compounds in hydrated cement to form soluble substances that are

easily eroded, thus producing concrete disintegration. Acids attack by dissolving both hydrated

and unhydrated cement compounds as well as calcareous aggregate. In most cases, the chemical

reaction forms water-soluble calcium compounds, which are then leached away

Natural waters usually have a pH of more than 7 and seldom less than 6. Waters with a pH

greater than 6.5 may be aggressive if they contain bicarbonates. Any water that contains

bicarbonate ion also contains free carbon dioxide, which can dissolve calcium carbonate unless

saturation already exists. Water with this aggressive carbon dioxide acts by acid reaction and can

attack concrete and other Portland cement products whether or not they are carbonated.

Arguably the popular cause of acid spillage on highways is the lead-acid car battery in the event

of an accident. Concrete can be destroyed by prolonged contact with strong solutions of sulfuric,

sulfurous, hydrochloric, nitric, hydrobrombic and hydrofluoric acids. Acids with low pH values

are destructive to the predominantly alkaline concrete. Abrasive traffic aggravates the condition

Acid attack increases with:

Increase in acid concentration


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Constant and fast renewal of acidic solution at the concrete/liquid interface

Increase in temperature

Increase in pressure

Bases

When Portland cement is made with non-alkali-reactive aggregates, it is highly resistant to strong

solutions of most bases. Calcium, ammonium, barium and strontium hydroxides are normally

harmless. However, sodium hydroxide may cause damage.

Attack via Carbonation

This is the process whereby atmospheric carbon dioxide (CO2) enters the pore structure ofhardened cement paste and reacts with Ca(OH)2to form calcium carbonate (CaCO3). This

involves the following chemical reaction:

In this process, the pH of the pore water is reduced from 12,5 to 8,5 upon complete

carbonation. When the carbonation front reaches the reinforcing steel in the slabs of the concrete

pavement, the low pH causes the gamma-ferric oxide layer to become unstable and the steel is

de-passivated. If sufficient oxygen and moisture is available, the steel will start corroding with

subsequent loss in the load-bearing capacity of the pavement structure.

Carbonation moves as a "front" into the concrete. This front does not advance beyond a

particular point until all the Ca(OH)2at that point has been converted to CaCO3. Hence, the

amount of Ca(OH)2in the pore structure of the concrete also has an influence on the rate of

carbonation.

The rate of advance of the carbonation front can be expressed as:


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Where,

x= depth of carbonation

t = time of exposure

D= carbonation coefficient

(Ballim Y. 2012)

Sulphate attack

Excessive amounts of sulphates in soil or water can, over a period of years, attack and destroy

concrete pavements and other structures

Sulphates damage concrete by reacting with hydrated tricalcium aluminate (C3A) compounds in

the hardened cement paste and by infiltrating and depositing salts. Due to crystal growthpressure, these expansive reactions can disrupt the cement paste, resulting in cracking and

disintegration of the concrete.

Fig.2.8 Sulphate attack process


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Chlor ide Attack

Chloride attack occurs in coastal regions. However, this attack may also occur in other inland

environments e.g. L. Magadi but the effects in inland areas is quite insignificant compared to the

marine environment. Chloride attack is caused by the miniscule and highly mobile free chloride

ions penetrating the pavements with water as the transport medium.

Steel reinforcement embedded in concrete pavements is inherently protected against corrosion by

passivation of the steel surface due to the high alkalinity of the concrete. When a sufficient

amount of chlorides reaches the steel reinforcement it permeates the passivating layer and

increases the risk of corrosion. The resistivity of concrete can also be reduced, affecting the

corrosion rate of the steel.

Fig. 2.9 Corrosion of Reinforcement in Concrete Pavement

The transportation of chloride ions into concrete is a complicated process which involvesdiffusion, capillary suction, permeation and convective flow through the pore system and micro

cracking network, accompanied by physical adsorption and chemical binding.


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Diffusion

Diffusion mode of transport operates in fully saturated media such as fully submerged concrete

structures.

Capillary Suction and Absorption

This mode of transport occurs when concrete in not in permanent contact with a liquid such as in

the tidal zone, a non-steady state transport of the liquid prevails. In this case, the amount of

liquid absorbed at the surface of the concrete as well as the amount of liquid transported at any

distance from the surface is a function of time.

Permeability

Transport through permeability is applicable for concrete structures in contact with liquid under

a pressure head, such as in liquid-retaining structures. Dissolved chlorides and gases are

therefore transported by convection with the permeating water into concrete. The permeability of

concrete depends on the pore structures and the viscosity of the liquids or gases.

Migration

Migration is the transport of ions in electrolytes due to the action of an electrical field as the

driving force. In an electrical field, positive ions will move preferentially to the negative

electrode and negative ions to the positive one. Migration may generate a difference in

concentration in a homogeneous solution or may provoke a species flux in the direction of

concentration gradients. This mode of transport may occur accidentally when there is a stray

current leakage, or intentionally in concrete rehabilitation techniques.

Adsorption and Desorption

Adsorption is a fixation of molecules on solid surfaces due to mass forces in mono- or multi-

molecular layers. Desorption is liberation of adsorbed molecules from solid surfaces.

Adsorption of chlorides is controlled by the micropore structure and the characteristics of the

hydrated products, and in particular the specific surface area and surface charge of the pore

walls.


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Mixed modes

The transport mechanism depends on the boundary conditions as well as on the moisture state

and its distribution in the concrete element. Pure permeation of a chloride solution as well as

pure diffusion of chloride ions will prevail only for a moisture-saturated concrete in which no

capillary forces can be active. If dry or non-saturated concrete is exposed to a chloride solution,

however, capillary absorption is the dominant mechanism. Nevertheless, small hydraulic

pressure heads can support the ingress by permeation, and the diffusion of ions simultaneously

carries the ions also into narrow pore spaces where no capillary flow any longer occurs. Except

for concrete elements that are continuously submerged in seawater, these mixed modes of

chloride transport obviously prevail in most cases for concrete structures in service.

The net result of chloride attack is the corrosion of dowels and steel reinforcement in the

pavement. Compromise in the soundness of the reinforcement due to corrosion reduces the

flexural rigidity of the pavement. This consequently leads to the spalling of concrete and in some

cases catastrophic structural failure in load bearing capacity especially in bridges.

Alkali -sil ica reaction

Alkali-silica reaction (ASR) is a potentially harmful condition in concrete resulting from a

chemical reaction between some aggregate minerals and the high alkaline (pH) pore solutions

(2.1.2) found in concrete. Over time, the product of these chemical reactions, a gelatinous alkali-

silicate referred to as ASR gel, can absorb water and expand, leading to concrete cracking and

reduced service life.

The amount of gel formed in the concrete depends on the amount and type of silica in the

aggregate and the alkali hydroxide concentration in the concrete pore solution. The presence of

gel does not always coincide with distress. The reactivity is potentially harmful only when it

produces significant expansion.


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Fig.2.10 Alkali-Silica reaction

Typical indicators of deleterious ASR include a network of cracks that are perpendicular to joints, closed

or spalled joints, or relative displacements of adjacent slabs. Because ASR is slow, deterioration

often takes several years to develop. Alkali-silica reactions can cause serviceability problems and

can exacerbate other deterioration mechanisms, such as those that occur in frost, deicer, or Sulphate

exposures.

Salt Scaling

Scaling is a physical deterioration mechanism aggravated by the use of de-icing salts and

freezing and thawing. Salts that are used to melt snow and ice go into solution and penetrate concretespore structure, aggravating hydraulic pressures when the solution freezes. In addition, as the

water freezes to ice, the salts are concentrated at the freezing site. Unfrozen water migrates

toward the site due to osmosis. These osmotic pressures also cause cracking, scaling, and

disintegration. In addition to hydraulic and osmotic pressures, which are the primary cause of de-

icer scaling, salts may also crystallize upon drying, creating expansive pressures. Research has


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shown that relatively low concentrations of sodium chloride (2-4%) cause greater damage than

greater concentrations of sodium chloride (Klieger 1957).

2.3

Anomalies in Concrete Pavements (PASER Manual).

2.3.1 Surface Defects

Wear and polishing

A worn or polished surface may appear from traffic wearing off the surface mortar and skid

resistant texture. Extensive wear may cause slight ruts where water can collect and causehydroplaning. Sometimes traffic may polish aggregates smooth, causing the surface to be

slippery.

Plate 2.2 Wear and Polishing

Map cracking

A pattern of fine cracks usually spaced within several inches is called map cracking. It usually

develops into square or other geometrical patterns. Can be caused by improper cure or

overworking the surface during finishing. If severe, cracks may spall or surface may scale.


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Plate 2.3 Map cracking

Pop-outs

Individual pieces of large aggregate may pop out of the surface. This is often caused by chert or

other absorbent aggregates that deteriorate under freeze-thaw conditions.

Plate 2.4 Pop-outs

Scaling

Scaling is surface deterioration that causes loss of fine aggregate and mortar. More extensive

scaling can result in loss of large aggregate. Often caused by using concrete which has not been


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air-entrained, the surface becomes susceptible to freeze-thaw damage. Scaling is also aggravated

by the use of deicing chemicals.

Scaling can occur as a general condition over a large area or be isolated to locations where poor

quality concrete or improper finishing techniques caused loss of air entrainment. In severe cases,

deterioration can extend deep into the concrete. Traffic action may accelerate scaling in the

wheel paths.

Plate 2.5 Surface scaling

Shallow reinforcing

If the steel reinforcing bar or mesh is placed too close to the concrete surface it will lead to

concrete spalling. Corrosion of the steel creates forces that break and dislodge the concrete.

Often rust stains can be seen in the surface cracks before spalling occurs.


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Plate 2.6 Shallow reinforcement

Spalling

Spalling is the loss of a piece of the concrete pavement from the surface or along the edges of

cracks and joints. Cracking or freeze-thaw action may break the concrete loose, or spalling may

be caused by poor quality materials. Spalling may be limited to small pieces in isolated areas or

be quite deep and extensive.

Plate 2.7 Spalling


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2.3.2 Pavement Cracks

Tr ansverse slab cracks

Transverse cracks may appear parallel to joints and can be caused by thermal stresses, poor

subgrade support, or heavy loadings. They are sometimes related to slabs having joints spaced

too widely. Joints spaced more than 15 apart commonly develop mid -slab transverse cracks

Plate 2.8 Transverse slab cracks

D-cracks

Occasionally, severe deterioration may develop from poor quality aggregate. D-cracking

develops when the aggregate is able to absorb moisture. This causes the aggregate to break apart

under freeze-thaw action that leads to deterioration. Usually, it starts at the bottom of the slab

and moves upward.

Fine cracking and a dark discoloration adjacent to the joint often indicate a D-cracking problem.

Once this is visible on the surface the pavement material is usually severely deteriorated and

complete replacement is required.


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Plate 2.9 D- cracking

Corner cracks

Diagonal cracks near the corner of a concrete slab may develop, forming a triangle with a

longitudinal and transverse joint. Usually these cracks are within one foot of the corner of the

slab. They are caused by insufficient soil support or concentrated stress due to temperature

related slab movement. The corner breaks under traffic loading. They may begin as hairline

cracks.

Some corner cracks extend the full depth of the slab while others start at the surface and angle

down toward the joint. With further deterioration, more cracking develops; eventually the entire

broken area may come loose. This may be a localized failure or may point to widespread

maintenance problems.


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Plate 2.10 Corner cracks

Meander cracks

Some pavement cracks appear to wander randomly. They may cross a slab diagonally or

meander like a serpent. Meander cracks may be caused by settlement due to unstable subsoil or

drainage problems, or by utility trench settlement. Frost heave and spring thaw can also cause

them. They are often local in nature and may not indicate general pavement problems.

Plate 2.11 Meander cracks


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2.3.3 Pavement Deformations

Blowups

Concrete slabs may push up or be crushed at a transverse joint. This is caused by expansion of

the concrete where incompressible materials (sand, etc.) have infiltrated into poorly sealed joints.

As a result, there is no space to accommodate expansion. It is more common in older pavements

with long joint spacing.

Plate 2.12 Blow ups

Faulting

Joints and cracks may fault or develop a step between adjacent slabs. Faulting is caused by

pumping of subgrade soils and creation of voids. Heavy truck or bus traffic can rapidly

accelerate faulting. Longitudinal joints may fault due to settlement of an adjacent slab. Faulting

creates a poor ride and may cause slab deterioration.


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Plate 2.13 Faulting

Heave

Unstable or poorly drained subgrade soils may cause pavements to settle after construction.

Poorly compacted utility trenches may also settle. This may be a gentle swale or a fairly severe

dip.

Frost-susceptible soils and high water tables can cause pavements to heave during the winter

months. Extensive pavement cracking and loss of strength during the spring can result in severe

deterioration.

Plate 2.14 Heave


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Manhole and inl et cracks

Normal pavement movement due to frost heaving and movements due to changes in temperature

often cannot be accommodated in the pavement adjacent to a manhole or a storm sewer inlet.

Cracks and faulting may develop and the concrete slab may deteriorate further. These are often

localized defects that may not indicate a general pavement problem.

Plate 2.15 Manhole cracks

Curb or shoulder deformation

Concrete curb and gutter, or paved concrete shoulders, may separate from or settle along the

main pavement. The longitudinal joints between the pavement and curb or shoulder may open,

fault, or deteriorate like other longitudinal joints.

Plate 2.16 Shoulder deformation


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2.4Direct Effects of Climatic Conditions on Concrete Pavements

2.4.1 Local Climate

Climate has a considerable influence on road performance hence it should be taken to

consideration. Kenya is characterized by a very wide variety of climates, comprising:

Afro-alpine climate

Equatorial climate

Wet-tropical climate

Semi-arid climate

Arid climate

Very arid climate

Moreover, the pattern of the climatic zones is rather complex, since the Kenyan climates are

largely governed by altitude (Matheri, 2013).

The mean temperatures of all regions are arguably above 18C. The coastal region is humid with

an average temperature of 29C. Nairobi region has records of moderate rainfall with mean

temperatures of 18.1C. The Western region and Victoria basin have the highest rainfall records

accompanied by average temperatures of about 30C. Eldoret, Kitale and surrounding areas have

a cool climate and annual average temperature of about 17C.

Nakuru and central Kenya have substantially high rainfall records with maximum mean

temperature of 26C and minimums of 12C. Finally Eastern Kenya has very low rainfall records

and record high temperature fluctuations varying from highs of 40C during the day to lows of

20C at night (Kenya Meteorological Department).


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Table 2.4 Mombasas Climate

Mombasa's Climate

Month Precipitation Maximum Minimum Average

Sunlight

in cm F C F C Hours

January 1.0 2.5 88 31 75 24 8

February 0.7 1.8 88 31 75 24 9

March 2.5 6.4 88 31 77 25 9

April 7.7 19.6 86 30 75 24 8

May 12.6 32 82 28 73 23 6

June 4.7 11.9 82 28 73 23 8

July 3.5 8.9 80 27 72 22 7

August 2.5 6.4 81 27 71 22 8

September 2.5 6.4 82 28 72 22 9

October 3.4 8.6 84 29 73 23 9

November 3.8 9.7 84 29 75 24 9

December 2.4 6.1 86 30 75 24 9


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2.4.2 Oceanic Climate

The oceanic climate is characterized by very hot and humid environments notwithstanding the

salty conditions (high chloride levels).

As discussed insection 2.2.4corrosion due to chloride penetration mainly occurs around coastal

regions. In response to climate change, corrosion initiation and damage is generally more likely

along the coast than other areas.

The risk of chloride-induced corrosion initiation of concrete structures along coasts increases

only slightly, depending on the region. At the same time, as temperatures increase in inland areas

where chloride and moisture are suitable for corrosion initiation, the risk of corrosion is likely to

increase.

In contrast to carbonation-induced corrosion, which always shows a greater change in warmer

regions, a greater change in chloride-induced corrosion may not necessarily happen in warm

areas per se, but in the areas where there is a greater increase in temperature.

2.4.3 Climate Change

Climate change is the variation in the earths global climate or in regional climates over time and

it involves changes in the variability or average state of the atmosphere.

Over the past decades it has been observed and verified that tropospheric and the temperature

over land is gradually increasing. This annual increment in temperature melts polar ice and

increases the sea level. This causes arguably inevitable change of atmospheric conditions (hence

climate) calls for adaptive design in concrete technology.

Some of the major challenges this poses to the construction industry include:

Abnormally high temperatures.

Longer periods of heavy precipitation hence frequent flooding.


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Prolonged dry periods.

Occasional rise of ground water table during long rainy seasons thus compromising the

bearing capacity of soil especially in class S1 soils where the California Bearing

Ratio(CBR) is just above 2.0 (Matheri 2013).

Fig. 2.11 Temperature projection

The above graph shows the projected average global temperature increase over the forthcoming

century if we remain on our current trajectory of economic growth and population increase

(peaking at 9 billion in 2050), but also incorporate new efficient technologies.

According to the Kenya Meteorological Department formally reported and incident in Nyahururu

where hail could not melt due cold weather. This is a direct indication of climate change since

this type of precipitation was unprecedented in the tropical climate region.


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Plate 2.17 Hailstones in Nyahururu


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2.4.4 Choice of Aggregate

In adaptive design to curb effects of climate change a good selection of aggregate should beconsidered. Different types of aggregates have varying magnitudes of thermal expansivity.

Increase in surface temperature due to global warming causes an increase in thermal expansion

of various materials. The differential thermal movement between the cement paste and the

aggregate is what can cause damage. Quartzite aggregate is the most prone to extreme heat

damage by cracking through the quartzite aggregate and bond failure between the cement paste

and the aggregate. This is because quartzite has the highest coefficient of thermal expansion

among the common aggregates. Limestone aggregate exhibits better heat resistance when

exposed to high atmospheric temperature since it has the least coefficient of expansion.

Lightweight aggregate also performs well due to low density (hence lower thermal expansion)

and adequate compressive strength.


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Fig. 2.12 Bar graph of coefficients of thermal expansion for various aggregates

Thermal expansion coefficient increases considerably with a drop of internal relative humidity

(e.g. Due to self-desiccation)

2.4.5 Concrete placing in Low Temperature Environments

During cold weather, hydration slows, slowing strength development. Concrete cools faster at

the surface than inside the slab, causing stress in the slab. If the stress is severe enough, the slab

will crack randomly.

From the local weather information it is safe to presume that cold weather concreting is not aparamount concreting method in Kenya. Cold weather concreting techniques are deployed where

the average daily air temperature is less than 5C (40F) and, the air temperature is not greater

than 10C (50F) for more than one-half of any 24 hour period. Kenyas weather conditions lie

arguably safely out of this bracket. However due to climate change and rather extreme weather

behaviour this matter should also be considered during mix design where concreting is to be

carried out in a region generally categorized as cold e.g. Limuru and its environs.

Some of the effects of cold weather on concreting include:

Freezing of pore water in concrete when the temperature gets as low as -1C.

Further depression of the freezing point when the ion concentration of the

unfrozen water goes up due to freezing of pore water.

Paralysis of the hydration process when temperatures fall below -4C. Forces

generated by the expansion of ice may be detrimental to the long-term integrity of

the concrete.

Succinctly, in cold weather concreting the hydration process should be accelerated using selected

set-accelerating admixtures or using Type III Portland cement.


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2.4.6 Concrete Placing in High Temperature Environments

Hot weather may be defined as any period of high temperature in which special precautions need

to be taken to ensure proper handling, placing, finishing and curing of concrete. This occurs

between February and March in the Kenyan climate. Hot temperatures are often accompanied by

high winds and dry air and can occur at any time, especially in arid and semi-arid regions of

Kenya. Hot weather conditions results in a rapid rate of evaporation of moisture from the surface

of the concrete, accelerated setting time, among other problems. However in the coastal regions

hot temperatures do not have as grave adverse effects as other regions in the country since high

relative humidity tends to reduce the effects of high temperature.

As discussed in preceding subtopics of this chapter, concrete sets as the cement hydrates and

hydration is an exothermic reaction. The rate of hydration accelerates with increasing concrete

temperature. On a significant level, the temperature of concrete is directly proportional to air

temperature. Concrete placed in a hot arid environment hydrates faster than concrete placed in

cooler highland areas. As concrete hydrates it sucks up water and grows crystals around the

aggregate particles. In hot environments the reaction is rapid; the crystals grow quickly but don't

have adequate time to acquire required strength. The immediate strength will be higher but the28-day strength will be compromised. If the temperature of concrete is 18 above normal (As

23C is considered the ideal temperature. Ref http://www.holcim.com.au) the ultimate

compressive strength will be 10% lower.

Summary of challenges of hot weather paving:

Concrete loses moisture more rapidly during hauling and placing.

Aggregate stockpiles dry out, affecting moisture consistency between batches.

Drying pavement subbase dries out before the mixture is placed, which then absorbs water

from the mixture.

Rapid water evaporation at the pavement surface can result in shrinkage cracks.

Difficulty in entraining air when temperatures are high. Entrained air is important for

pavement durability.


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Rapid setting of concrete, perhaps twice as fast, making finishing more difficult.

Sawing operations must proceed more rapidly. Additional saws may be required.

Nevertheless, once heat-related problems develop, it may be too late to fix them therefore

preventive measures should be adopted.

In hot weather, as the cement sets up, slump decreases rapidly and more mixing water is needed.

This can also contribute to lower strengths (as much as another 10% lower), and in integrally

colored concrete, can lead to variations in water content which can result in significant

differences in concrete color between adjacent pours.

In arid and semi-arid areas heat loss (cooling) during cold nights (due to minimal cloud cover)

leads to lower temperatures inside the structure, but at the same time to dangerous temperature

gradients during hot mid-day temperatures.


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2.4.7 Temperature Evolution

Fig.2.13 Temperature evolution

Thermal shr inkage

Subsequent temperature drop after placing of concrete in pavement construction may initiate

thermal shrinkage, consequently leading to thermal cracking. Thermal cracking occurs due to

excessive temperature differences within a concrete pavement or its surroundings. The

temperature drop is given by:

The temperature difference causes the cooler portion to contract more than the warmer portion,

which restrains the contraction. Thermal cracks appear when the restraint results in tensile

stresses that exceed the in-place concrete tensile strength. These temperature induced tensile

stresses are differential.


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Chapter Three

Effects of Traffic Induced Vibration on Concrete

3.1Introduction

Concrete pavement deterioration is a function of several parameters including thickness ofconcrete slab and subbase, material properties, boundary condition between concrete slab and

subbase, subgrade characteristics, environmental effects and configuration, magnitude and

position of the vehicular loads.

Vehicular loads have been considered as static loads in concrete pavement design guidelines as

dynamic analyses and experimental tests on concrete pavements in the past showed that dynamic

effects were not significant. The American Association of State Highway Officials research

(AASHO, 1962) showed that an increase in vehicle speed from 3.2 to 95.6 km/h decreases the

value of pavement response by about 29 per cent.

Analytical studies of concrete pavements under dynamic loads carried out by Stoner et al.

(1990), Gillespie et al. (1993), Zaghloul and White (1993), Chatti et al. (1994), Bhatti and Stoner

(1998), Kim et al. (2002) and Shoukry and Fahmy (2002) showed that speed has significant

effects on slab deflection. However, a greater stress can be captured in concrete pavements if a

static analysis of concrete pavement is performed. On the other hand, Liu and Gazis (1999)

found that concrete pavements in the presence of pavement roughness experience a greater

tensile stress under dynamic loads than static loads.


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In an experimental study of plain concrete pavement resting on a subbase with low stiffness

under very heavy truck loads, it was found that velocity can noticeably change the value of slab

deflections or stresses (Izquierdo et al 1997).

Recent analytical studies on concrete pavements under moving axle group loads carried out by

Darestani et al. (2006) showed that vehicle speed has significant effect on responses of concrete

pavement even if the pavement has a smooth top surface

3.2Nature of Vibration

Moving traffic along a road creates a seismic environment. The induced vibrations are irregular

in nature. During peak hours of traffic flow the vibration frequency is at its highest and the

opposite is true during periods of minimal flow of vehicular traffic.

3.2.1 Mechanism of Wave Generation

Like most vibration problems, traffic vibrations can be characterized by a source-path- receiver

scenario Vehicle contact with irregularities in the road surface (e.g., potholes, cracks and uneven

manhole covers) induces dynamic loads on the pavement. These loads generate stress waves,

which propagate in the soil, eventually reaching other sections of the road and even the

foundations of adjacent buildings and causing them to vibrate.

Plate 3.1 Irregularities on the road surface


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Traffic vibrations are mainly caused by heavy vehicles such as buses and trucks. Passenger cars

and light trucks rarely induce vibrations that are perceptible in other sections of the road.

Therefore small vibration levels induced by road traffic could trigger damage by topping up

residual strains in the adjacent road sections. When a bus or a truck strikes an irregularity in the

road surface, it generates an impact load and an oscillating load due to the subsequent axle hop

of the vehicle. The impact load generates ground vibrations that are predominant at the natural

vibration frequencies of the soil whereas the axle hop generates vibrations at the hop frequency

(a characteristic of the vehicles suspension system). If the natural frequencies of the soil

coincide with any of the natural frequencies of the adjacent sections of the concrete road

resonance occurs and vibrations will be amplified.

Unlike irregularities such as manhole covers or potholes, normal road surface roughness induces

continuous dynamic loads on the road. If the road surface roughness includes a harmonic

component that, at the posted speed, leads to a forcing frequency that coincides with any of the

natural frequencies of the vehicle and/or those of the soil, substantial vibration may be induced.

This effect is familiar to car drivers travelling over dirt or gravel roads with ripples (termed the

washboard effect). At a certain speed, the vehicle shudders excessively but the vibration sub-

sides at higher or lower speeds.

Road traffic tends to produce vibrations with frequencies predominantly in the range from 5 to

25 Hz (oscillations per second) with the amplitude of the vibrations ranges between 0.005 and 2

m/s2 (0.0005 and 0.2 g) measured as acceleration, or 0.05 and 25 mm/s measured as velocity.

3.2.2 Factors Affecting Magnitude of Vibration and Frequency

1. Vehicle weight.

2. Condition of the road.

3. Speed and suspension system of vehicle.

4. Type of soil and stratification


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5. Section properties of the road.

6. Distance of section from irregularities causing vibration.

The above factors are interdependent. The greater the frequency of potholes and irregularities the

higher the frequency of the vibration. Similarly, the greater the speed the greater the magnitude

of vibration. Interdependence occurs for instance: the effect of the suspension system type

depends on vehicle speed and road roughness. For low speeds and smooth road conditions, the

effect of the type of suspension system is quite insignificant. But for high speeds and rough

roads, the type of suspension system has a significant contribution. The effect of vehicle speed

depends on the roughness of the road. Generally, the rougher the road, the more speed affects the

vibration amplitude.

Vibration amplitudes and the predominant frequencies are influenced significantly by the soil

type and stratification. The lower the stiffness and damping of the soil, the higher the vibration.

For impact loads, ground vibrations are highest at the natural frequencies of the site. At these

frequencies, the soil, like any structural system, offers the least resistance and hence the greatest

response to loads. For soils, the natural frequencies depend on stiffness and stratification.

Typically, traffic vibrations are worst in areas underlain by a soft clay soil layer that is between 7

and 15 m deep. In these areas, the natural frequencies of the soil can coincide with those of

houses and their floors, leading to resonance or amplified vibration.


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3.3Effects on Concrete Pavements

3.3.1 Concrete Slab Deflection

Research carried out on a model pavement (plate 3.2) using a semi-trailer truck with a gross

weight of 477.3 kN shows that slab deflection decreases from the corner of a free edge towards

mid-span and confined edge. Slab deflection at the corner is about 60 per cent greater than those

at the middle of free edge. Concrete slab deflection is strongly affected by truck speed so that

dynamic amplification varies between 55 per cent and 313 per cent depending on the pavement

type, boundary condition between concrete slab and subbase and location of measurement.

Greater dynamic amplifications occur along the confined longitudinal edge of the test section

though the slab deflection values of these points are relatively lower than those along the freelongitudinal edge.

Fig. 3.2 shows time history of slab deflections for different speeds at the middle of a free

longitudinal edge. The critical truck speed (which creates maximum slab deflection) depends on

several factors such as the location of instrumentation and type of pavements. Hence, medium

speed in some cases results in greater slab deflection.

Plate 3.2 Model pavement


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Fig 3.1 Time histories of slab deflection

The position of the dowels in the depth of a pavement slab has significant effects on the

magnitude of deflection. The slab deflection significantly decreases when dowels are positioned

at the mid-depth of the concrete slab. On the other hand, lower slab deflections occur where

dowels are placed close to the top surface layer of the concrete slab.

3.3.2 Induced Tensile Stresses

Tensile stresses in concrete slabs are affected by vehicle speed. Using the same 477.3 kN truck

dynamic amplification of tensile stresses varies between -10.8 and +108.9 %. The magnitude of

tensile stresses decreases when truck speed increases.

A comparison between maximum induced tensile stresses individual at speeds indicates that

tensile stresses at transverse joints increase when dowels are located at the mid-depth of the

concrete slab.


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Incorporating the effect of dowel location on tensile stresses and slab deformation adds up to the

fact that an optimum depth for dowel location is between 0.25d and 0.5d, wheredis the depth

of the concrete slab. This may aid in minimizing joint faulting.

Tensile stresses in jointed reinforced concrete pavements are greater than those in jointed plain

(unreinforced) concrete pavements. The recommended position of the longitudinal steel is

between 1/3 and 1/2 of the depth of the slab as measured from the surface. However, effects of

reinforcement location on pavement dynamic tensile stresses in the current study are still unclear

at this stage, as analyses of time history responses have not lead to a specific conclusion.

3.3.3 Yielding of Concrete under Excessive Vibration

Concrete immediately begins to yield to the vibration when the pavement is subjected to an

oscillating force having a frequency close to its own natural frequency. Cracking reduces

pavement stiffness and, consequently, lowers its natural frequency. Experimental results show

that the higher is the level of prestress the higher are the eigen frequencies of the prestressed

concrete beam. This increase in the natural frequency can be ascribed to the closure of the micro-

cracks produced in the concrete by the shrinkage realized by the prestressing force.

Conclusively, dynamic analysis is required to accurately predict pavement failure, especially for

jointed plain concrete pavement. Fatigue cracking is affected by axle group types and speed.

Damage location may be close to transverse joints, at midpoint or in some cases at quarter point

of slab.


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Legally loaded heavy vehicles cause a relatively small amount of damage to road pavement

structures, as opposed to overloaded heavy vehicles which are responsible for approximately

60% of the damage to the road. These trucks subject the pavement (concrete and asphalt) to high

stres

Effects of weather and vibrations on concrete roads.pdf

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