QT1- Geoffrey Griffiths - Concrete Roads and Pavements

Concrete roads and pavements

Geoffrey Griffiths

Concrete is a particularly useful form of construction, which can be used in situations where pavements are subjected to considerable point loads, and aggressive environments; concrete pavements have a number of uses that make them beneficial when compared with alternative bituminous designs. They are specifically useful when:

• high point loads are expected • diesel spillage or other chemical spills may attack alternative materials • low subgrade strengths are expected • high pavement temperatures will be experienced, e.g. in tropical regions • heavy axle loads can be anticipated • poorly skilled labour is the only possible labour force

The specific advantage of a concrete pavement is that it is a rigid stiff plate, applying a load to a wide area. Concrete pavements are also relatively easy to construct. Readily available concrete and steel can be quickly and easily assembled and laid in association with the main construction works of a large building project. Many examples exist of large projects, constructed in various forms of concrete; motorways, industrial hard standing areas, airport taxiways, runways and aprons as well as simple bus lay-bys, all successfully constructed in various forms of concrete.

In its simplest form a concrete pavement can consist of a domestic drive laid as a humble 1 O0 mm mass concrete slab. The most sophisticated form is probably the continuous

22/2 Concrete roads and pavements

reinforced, exposed aggregate finish Whisper concrete (Charonnat et al., 1989) used in some motorway projects. The successful application of concrete is beyond question; the form of construction is evident throughout civil and building engineering projects. This chapter is intended to provide a brief introduction to the different methods of construction and common applications, which may be found in our current environment.

Three alternative styles of construction can be employed. Each system is appropriate to different applications; the three main types of construction are:

• jointed unreinforced concrete pavement (URC) • jointed reinforced concrete pavement (JRC) • continuously reinforced concrete pavement (CRC)

The systems are described and discussed separately in this section of the chapter.

22.2.1 Jointed unreinforced concrete pavement (URC) description . ~ ~ ..~.~ ~ ~ : ~ ~ . ~ ~.~. ~ ~.:~,~,~,~:.~ .~.~.~ ~:~.~ ~:,~,.:.~:~ .~.~.~.~.:.~ ~,.~,~, ~.~ , ~ . ~ . ~ :~.~ ~.~,~ ~ . ~ ...................... ~..~.~..~.~.~:~,.~,~:~ ~ ~ . ~ . ~ : ~ : ~:.~:~:~.~:~.,. ~.~,~ ~.~.:::::. :.::::.:.: ,::. :.::::.~:.:::.:: .:.:.:::::.~. :.::. ..................... :::::::::::::::::::::::::::::::::::::::::::::::::: ::::::.:.::.:::..: ::::::::::::::::::::::::::::::::::::::::::: ................... .:~.:.:.::::: :::~:.: .............

Jointed unreinforced concrete construction has been extensively used on major highway projects. The system is currently out of favour in UK highway schemes but is extensively used on general infrastructure projects and in other parts of the world. Figure 22.1 illustrates a typical industrial application. The pavement consists of a patchwork of concrete slabs joined together with dowel and tie bars or crack induced joints. Each slab

Figure 22.1 A typical industrial lorry yard.

Concrete roads and pavements 22/3

will consist of approximately square units. The detailing of the joint layout is crucial to the successful design, execution and operation of the pavement.

The system relies on the tensile capacity and flexural strength of the concrete to resist cracking and successfully carry a load. When the pavement is built, the size of the concrete panels is controlled by the shrinkage strain generated by the hardening process. As the concrete sets, gains strength and cools, shrinkage strains generate a tensile force in the pavement. The size of the concrete slab controls the magnitude of the force. If the tensile capacity of concrete is exceeded the slab cracks.

A number of rules govern pavement detailing. The most important feature is to ensure that the joints are detailed, designed and most important, spaced correctly. A pavement joint must be arranged to produce a patchwork of rough square panels, the longitudinal joints running in one direction and transfers joints arranged at 90 degrees. Joint spacing is controlled by standard practice and is a function of pavement thickness: thicker pavement slabs can have greater joint spacing. Table 22.1 (ACPA, 1992) details the accepted practice for joint spacing. It is noted that the recommended maximum ratio of longitudinal to transverse joint spacing is 1.25. Pavement joints may be constructed as dowelled or undowelled: current practice is to construct most pavements with dowelled tie bars. Removing the steel dowels reduces the efficiency of the joints and gives an increase in pavement thickness.

Table 22.1 Joint spacing for mass concrete pavements (ACPA, 1992).

Pavement thickness (mm)

Maximum recommended joint spacing (m)

Gravels and crushed rock Limestone aggregate

150 4.5 5.4 200 4.9 5.9 250 5.3 6.4 300 6.0 7.2

22.2.2 Typical applications for URC pavement

Several different types of mass concrete pavement are currently found in major construction projects.

22.2.2.1 Airfield pavements One of the most significant Western European applications of mass concrete pavements is currently the construction of airfield taxiways and aprons. Large, thick slipformed areas of concrete are constructed with sawn, crack induced joints. The pavements are built across a slipformed platform of typically 150 mm thick leanmix concrete. The leanmix acts as a support to the crack induced joints.

22.2.2.2 Industrial yards and hard standings Many large industrial sites are constructed in mass concrete. The method of construction is particularly useful for large lorry manoeuvring and turning areas. Typical industrial projects consist of 200 mm thick pavements in 4.5 m square grids with dowel and tie bar joints laid on to a 300 mm thick crushed rock sub-base.


22.2.2.3 Example: Mumbai (Bombay) dty streets A surprising application for mass concrete pavements can be found in Mumbai, India. The city authority currently uses mass concrete as a standard pavement option for city streets. The city engineers have found that system to be more durable than alternative asphaltic concrete systems in the hot humid climate. Concrete slabs are constructed using labour-intensive techniques. Joints are sawn, cut, and sealed in the normal manner. A standard brush finish is used. Service crossings are accommodated by omitting concrete panels at regular intervals. Figure 22.2 illustrates a typical street layout.

I - . . . . . . . . .

Figure 22.2 A Bombay street scene using mass concrete construction.

22.2.3 Jointed reinforced concrete pavement (JRC) description ...... ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ....................................... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . : : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Reinforced jointed concrete pavements are frequently constructed as a variation on the mass concrete theme. The reinforcing steel provides two functions; it controls cracking but also instils additional stiffness in the concrete slab. Reinforced concrete slabs are frequently used in place of mass concrete when

• workmanship and materials are suspect • the pavement will be subjected to large unplanned differential settlement forces

The recognized design methods allow a significant reduction in the pavement thickness to allow for the reinforcement structural contribution when compared to URC. Reinforced systems are generally constructed with longer slabs than the equivalent mass concrete pavement. Slabs can be designed and constructed up to 20 m long. A frequently accepted slab length is 10 m.

Reinforced systems can be designed as either cracked or uncracked slabs. Reinforcement is typically placed in the centre of the slab but some designers use reinforcement on each face. The minimum thickness of a reinforced concrete slab is approximately 150 mm; which is fixed by the practical problems involved in providing adequate cover to the


reinforcement. A distinct advantage of fixing the reinforcement in the centre of the slab is that the positive and negative moments are equally balanced thus allowing the slab to flex equally before cracking and failing. A disadvantage of reinforced slabs is that the wider joint spacing precludes the use of undowelled joints. The wider transverse joint spacing produces larger thermal strains resulting in increased joint movement when compared to mass concrete pavements. The movements can only be accommodated using dowel bars or tied joints. The movement is too large for untied joints.

22.2.4 Typical applications for JRC pavement

Jointed reinforced concrete is typically used when designers are not confident that their work force will construct a pavement correctly or a pavement may be subjected to high differential consolidation force.

22.2.4.1 Example: Bangkok city streets One of the best examples of the efficient use of reinforced concrete can be found in the city streets of Bangkok. The streets are subjected to a very aggressive trafficking regime within a very hot tropical climate. The city is also subjected to a number of subgrade problems that make the effective operation of bituminous pavements very problematic. The city is built across a very low-lying fiver delta; frequently subjecting the streets to severe flooding. Subgrade strengths are exceptionally low: a desiccated surface crust provides most of the strength at formation. Groundwater abstraction also produces massive differential consolidation problems. Poor workmanship is also a major additional problem. Each of these contributory factors produces an exceptionally difficult environment but the reinforced concrete pavements operate effectively despite the extreme environment. Pavements can be seen containing massive cracks, surface polishing and various forms of acid attack but the system generally functions to an acceptable level of service despite the fact that very little planned maintenance is carried out. Figure 22.3 illustrates a typical city street.

22.2.4.2 Western European industrial sites Many small industrial sites are built with reinforced concrete slabs if designers are not confident that a mass concrete alternative would be constructed correctly. If small areas of concrete are required it is frequently preferable to use a reinforced slab and ensure that an unskilled labour force will successfully complete the project without any problems.

22.2.5 Description of continuously reinforced concrete pavements CRCP

~:.: ::: :.:,~::, .: ::::::::::.:.~..:~: :::::::::::::::::::::::::::::::::::::: :~ :~: . :~ :::~::::~:~:.:~:::::: ~, ~ : ~ : : ~ .,: :~ ~ : : : : : : : :::~:~ ::~:~ .................... :~:..: :.::: : : : : .................... :.~ ~.~:::.:~::,:.: : ~ : . . ........................... ~ : : : : : , : :.: : . : : : .: : .: ,: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : ::::::::::::::::::::::::::::::::::::::::::::::::::: : : : : : : : : : : : : : : : : : : : : : : : : : : : : ..................... : : : . : , : : : : : , : . , : . : ........................... ....................... .................... : : : : : : : : : : : : : : : : : : : : : : : : . : , , . : . : : : : : : : : : : : : : : : : : : : : : :

This form of construction has been developed from reinforced concrete pavements. Continuous reinforced concrete pavements are constructed as long slabs with longitudinal reinforcement fixed at the centre of the slab. Figure 22.4 illustrates a typical CRC pavement. The longitudinal reinforcement is intended to control shrinkage cracking. A nominal amount of transverse reinforcement is also provided; to hold the longitudinal reinforcement in place. Essentially a continuously reinforced concrete slab consists of a regular section


Figure 22.3 Bangkok reinforced concrete pavement.

, ~ \~ -

Figure 22.4 CRC pavement.

of cracked square concrete plates connected together by the steel. The system is very similar to a mass and reinforced concrete; except that the cracks are formed in a random fashion and remain unsealed. A second feature of a continuous reinforced concrete system is that ground anchors are required at terminations. A CRC slab will move extensively under the influence of changing environmental temperatures. The ends of the slab are therefore anchored to prevent massive movement. If a continuously reinforced concrete slab is not provided with terminations a large bump or ripple will occur at the start of the bituminous material. Figure 22.5 illustrates a typical anchorage arrangement.


400 I-

S u b - b a s e - - - - - ~ - - - ~ ~ ~ Formation ~ ~ ~ ~ ~ - ~

0 0 0 0 CJ O~ w--

i i

Mix ST1 b l i n d i n g ~ __ concrete

2 I 2

_ 1 I

I 2 I 2

I" - - - - - -

6O0

Ground beam

Transverse and longitudinal steel some as adjoining CRC

J "~ 100 Key ",,, -,, -,, \

25 PVC duct placed at underside of sub-base and at mid deptl at 2000 centres

(4 No. in anchorage)

Figure 22.5 CRC ground anchor, taken from the UK Highways Agency Standard Details.

Crack spacing is essential to the efficient operation of this type of pavement. Transverse cracks must be spaced between 0.9 m and 4 m centres; if the cracks are too closely spaced the blocks of concrete can fail in shear as punch-outs. Cracks can also be spaced too widely; if the cracks are spaced too far apart aggregate interlock is lost across the joint. Crack spacing is controlled by the longitudinal reinforcement content which is currently fixed at 0.6% of the section area.

The surface finish is a particular engineering problem associated with this type of construction. Several different types of finish are currently used. A patent form of construction known as Whisper concrete (Charon et al., 1989) uses an exposed aggregate surface to provide a running surface. Figure 22.6 illustrates a section of Whisper concrete. The top

Figure 22.6 Whisper concrete.


40 mm layer of concrete is constructed in a thin overlay of high quality, air entrained 10 mm aggregate concrete. The surface is then sprayed with a retarding agent immediately after the concrete is laid. The surface can then be removed by wire brush as the concrete sets, thus removing the cement paste from the coarse aggregate matrix. Whisper concrete is very popular in mainland Europe but has been used only on a number of small experimental projects in the USA and UK. The system requires expensive specialist aggregates, skills and equipment, thus making it uneconomic on most projects. Whisper concrete is currently banned in new construction on UK trunk roads.

A thin bituminous 35 mm wearing course is currently considered the most practical UK form of surface finish to this type of construction. The wearing course is held in place using a bituminous pad coat. The main concrete slab can then be constructed in non-air- entrained material.

22.2.6 CRCP applications

This form of construction may not be confined to highway projects. It is practical to construct any major pavement in a continuous reinforced option. The efficient application is restricted by the limited design methods. No real analytical technique is currently recognized. The highway design methods are basically empirical.

22.2.6.1 Highway projects The system is found only on major motorways and trunk road schemes. This form of construction is usually the most economic type of pavement when a large aggregate source is available within the site area.

22.2.6.2 Airfield runways Liverpool Airport runway was constructed using continuously reinforced techniques and is a notable example of a successfully completed project.

A number of different factors contribute to the failure; each issue needs to be considered separately in undertaking the pavement design.

22.3.1 Fatigue cracking of concrete

The first design issue to consider is the cracking of concrete. This form of failure occurs when the concrete strain exceeds the tensile capacity of the material. Fatigue cracking is easily recognized as either comer breaks or longitudinal cracks following the line of wheel loading. Figure 22.7 illustrates a typical failure.


Figure 22.7 Fatigue cracking.

22.3.2 Loss of support

At high numbers of load repetitions, over 3000000 load applications, pavements with unbound sub base may fail as a result of undermined foundations. This type of failure is known as loss of support. Failure can be prevented using bound foundation materials.

22.3.3 Frost damage

Much debate has occurred around this issue. Some frost resistant high strength concretes can be produced but the technique is not accepted in the USA or UK. Frost damage is a major problem in concrete pavements. If the pavement is constructed in normal un-air- entrained concrete the surface can be quickly removed by the weathering action of frost. Figure 22.8 illustrates frost damage. The concrete must be air entrained. A number of researchers (BSI, 1997) have suggested that if concrete achieves a 50 N strength the material will not be susceptible to frost.

A typical standard of frost protected concrete will be achieved with a 5% plus o r - 1.5% air content. The air-entraining agent acts has a cracking agent, reducing the size of any bubbles to a point when the formation of ice lenses within the pores will not cause damage to the concrete matrix. In a conventional concrete ice lenses are formed in the voids contained within the structure of the concrete. The ice is then able to crack the concrete thus resulting in the formation of surface scaling.

Adding air-entraining agents reduces the concrete strength by approximately 10%.


Figure 22.8 Frost damaged concrete compared to non frost damaged concrete.

22.3.4 Surface abrasion

Surface abrasion is an important design consideration. The surface of a heavily trafficked pavement will quickly scrub and abrade away under the action of traffic if the concrete is of an inadequate strength. Figure 22.9 illustrates a good example of surface failure.

~ ..~

~ N

iii~i

Figure 22.9 Surface abrasion.


22.3.5 Skidding characteristics

Most national standards fail to specify a skidding requirement. The British highway specification is an exception and rigorously controls pavements' skidding characteristics. The standard specifies the polishing characteristics and aggregate abrasion values for the coarse aggregate. Different polished stone (PSV) and abrasion values (Highway Agency, 1999) are dictated for different highway characteristics. The same aggregate characteristics are used for both bituminous and concrete coarse aggregate. Other national standards fix skidding characteristics using the strength of the aggregate.

The UK standard uses a different surface texture for concrete roads. The surface texture is fixed at 1 mm plus or minus 0.25 mm.

Two altemative approaches may be used to undertake the design of a pavement. The two styles of design are:

• the empirical approach • the semi-empirical approach

Both methods relying on statistical calibration relationships to produce accurate designs.

22.4.1 Theempirical approach

The simplest method of designing highway pavements is to use the statistical back analysis equations presented in Mayhew and Harding (1987). The method forms the basis of UK highway pavement design. Two alternative equations are presented in the document.

• the relationship for estimating mass concrete pavement design lives • a second similar formula for estimating the design life of a reinforced concrete pavement

The relationships are then used to present design graphs for mass concrete, reinforced concrete, and continuously reinforced in Highways Agency pavements.

22.4.1.1 Mass concrete pavement design af ter Mayhew and Harding (1987)

In (L) = 5.094 In (H) + 3.466 In (S) + 0.4836 In (M) + 0.08718 In ( F ) - 40.78

(22.1) where the following terms are used: L = traffic loading in msa H = the intended pavement thickness in mm S = concrete 28-day mean compressive cube strength

M = the estimated equivalent modulus of the pavement support platform in MPa, derived from core data extracted from the pavement foundation

F = the percentage of failed bays


22.4.1.2 Reinforced concrete pavement design

In (L) = 4.786 In (H) + 1.418 In (R) + 3.171 In (S) + 0.3255 In (M) - 45.15

where R = the amount of longitudinal reinforcement in mm2/m. The following key assumptions are used with the expressions and are used to generate

the standard UK pavement designs:

• A standard 40 N concrete mix is assumed to produce concrete with a strength of 50 N/mm 2 at 28 days.

• The support platform is conservatively assumed to be a 270 MPa stiffness, which is represented by a 150 mm concrete leanmix over 350 mm of capping onto a 2% CBR formation.

• The equations are set to a 50% confidence. • The design graphs (Highways Agency) also assume that a kerb or 1 m wide concrete

strip, to prevent edge loading conditions occurring supports the pavement edge. The standard design graphs contain a reduction in thickness of approximately 10% when compared to the equations to allow for the edge beam stiffness.

22.4.2 The semi-empirical approach

A number of scientifically based rational design methods may be found in various publications. All these methods use modifications and adjustments to the Westergaard (1926) pavement stress/strain calculation techniques linked with a fatigue model to produce a design method. Some design methods use Westergaard (1926) comer loading conditions, some edge conditions and others internal loads. Figure 22.10 illustrates the standard Westergaard load cases. A summary of the most popular and common design methods is presented here:

1. AASHTO (1992) design method, using a corner loading condition 2. Modified AASHTO (1998) design method, using an internal loading condition 3. TR 550 (Chandler, 1982) design for heavy loads, Westergaard (1926) comer loading

condition

These are the most important design methods, many other national highway design standards, airfield pavement design methods and a number of methods for designing port

Edge (

Figure 22.10 Westergaard Ioadings.

) ©--

© Corner

Internal


facilities may be found. All of the design methods use variations, developments or adjustments on the basic Westergaard (1926) design equations.

The design methods combine concrete tensile fatigue models with the estimated pavement stress and Miners Rule (1945) to produce practical design methods. The most commonly used fatigue model is the Portland Cement Association method which assumes that a cement bound material has an infinite life if the magnitude of the stress impulse falls below 50% of the ultimate tensile strain.

22.4.2.1 The AASHTO design method (AASHTO, 1992) The AASHTO design method is the most commonly used pavement design technique in the world. The standard uses a derivation of the Spangler (1942) comer loading equation to produce mass concrete pavement designs. The original standard uses a graphical nomograph presentation derived from a complex equation to produce pavement thickness. The following variables may be adjusted in the design method:

• flexural strength • subgrade strength • drainage conditions • design confidence level and probability of survival • joint conditions and types • subgrade support condition

The standard is written using Imperial units. Pavement designs produced following Mayhew and Harding (1987) and AASHTO (1992, 1998) give the same pavement thickness when comparable input data is used. The AASHTO (1992) system is far more comprehensive and flexible than the Mayhew and Harding (1987) relationship.

22.4.2.2 The AASHTO relationship An alternative, more complex design method was published by AASHTO in 1998. The method is a modified internal Westergaard loading condition assessment of pavement stress linked to climatic conditions and the results obtained from an extensive monitoring programme. The standard includes a set of catalogue designs that may be applied to most common pavement problems. The results obtained from all three design methods are comparable.

22.4.3 Continuous reinforced concrete pavements .......... .~:.~ ~ . ~ . ~ . ~ ~:.~ .~.:.~ ..................... .~ ~:~,: ~.~..~:~.~.:~ ~.~ ~ ~ ~ . ~ . : ~ ~ . ~ : ~ . ~ ,.,~.~.~:~ .~.~:~..~.. .~.: .~.~.~.,~.. .~.~..~: . ,~ . : ~. :~.~.~:~.~,~, . : ,~ ~ ~: .~ ~,~ ~ ~ ~ . : . ~ . ~ . ~ : , . ~. ~ . ~ . : : . : : : , ~: ::.: : : . : : : : , .~.~ ~ . : : : : : : : . : .. : : . : : : : : : : : : : : : . . : : ~ : : : : : : : : : : : : : : : : : : : : : ~:::. : : : : : : , . : : , . : : : : : . . ::::::. ~.~ : : : : . : : : . : : : : : , :::..~ . : . . : : : :~ : : : : . ............... : : : : : :::.::.:.:::.~, : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : .................... : : : : : : : : : : : : : : : : : : : : : : : : : : : : : . . . . . . . . . . . . . . . . : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : . : : : : . . . . .

Great difficulty is experienced in effectively designing continuous reinforced concrete pavements. Most of the common current design methods are based on modifications of either AASHTO (1992) or Mayhew and Harding (1987), both of which were originally produced for jointed reinforced concrete pavement design. A pressing need exists to review the existing design methods and produce a rational or semi-empirical design method truly focused on the specific problems associated with continuously reinforced concrete pavements. The design of CRC pavements must therefore be considered as more empirical than rational.


It has already been noted that joint design is fundamental to the efficient operation of a concrete pavement. Joints may be designed as either dowelled or undowelled with crack induced or formed joints. A maximum traffic level of 3 msa is considered to be the limit for an undowelled construction. Various techniques are currently available to form joints. If joints are formed using crack-induced techniques the crack or saw cut must extend to a depth of between 1/4 and a 1/3 of the slab depth. Joints are classified in three different types, each of which forms a separate function in the pavement.

22.5.1 Transverse contraction joints ................................. ~ .............................. ~. ..................................................... ~:,:,:~:::.:::.::.:~:~ ................................................... ~ ..................................................... ~:::.:::~ ............................ ~ ................................................................................................................... ~::,: ........................................ ~ ..................................... ~ ............... ..................................... :. ......................................................................... ~ ..................... ~ ........................................................... ~:. : : .................................................................................. .: ..................................................................... :::.::

Transverse joints are formed at regular intervals in mass concrete pavements and jointed reinforced concrete pavements at points where hydration cracking might occur. This type of joint is used in jointed reinforced concrete and mass concrete pavements. Figure 22.11 illustrates a standard joint design. It is important to note that the dowel bars should be debonded to allow free movement.

Pavement construction

URC SLAB

Dimension in mm

AIBI c /

13 I 3 I 16 ~ Approved compressible

\ Seal caulked material where Formed bullnosed arris \ / required not exceeding 5 radius " ~ . ~ /

Separation membrane Dowel bar covered for minimum of 3/4 of to clause 100 its length by an approved de-bonding

Dowel to be 25 mm dia when D is between 240 mm and 299 mm Dowel to be 30mm dia when D is 300mm and Above

Figure 22.11 Transverse joints, taken from the UK Highways Agency Standard Details.

22.5.2 Longitudinal joints

Longitudinal joints are required in all pavement types. The joint is needed to produce a regular crack between construction bays or at a point in CRC construction where the transverse reinforcement would induce an irregular crack. Longitudinal joints are generally constructed with tiebars. The tiebars are bonded to produce a positive connection between adjoining slabs. Figure 22.12 illustrates a typical arrangement.


Pavement construction

URC SLAB

Dimensions in mm

A B C

13 3 16

D/3 M in to D2/3 max

/

Formed '~c bullnosed quired arris not exceeding

~ 1 Seal 5 radius

Formed joint

m m

\

Approved compressible caulked material where

12 mm diameter

1 I

Protective coating

. . . . . . . . . . . . . . . . . .

Separation membrane

Figure 22.12 Longitudinal joint taken from the UK Highways Agency Standard Details.

22.5.3 Expansion or isolation joints

Expansion joints and isolation joints provide similar functions. They are intended to allow movement between slabs and/or hard objects. Expansion joints are essential to reinforced concrete pavements but may be omitted in mass concrete crack induced design options. Manholes and piled foundations must be isolated from a concrete slab. Concrete moves extensively under the influence of changing environmental temperatures. Detailing manhole, gullies, and the other protrusions through the concrete slab is essential to the success of the pavement. Figure 22.13 describes a standard arrangement.

Compressible caulked material where required

at corner Seal ~.....,.~

Rigid pavement

,o,nt // filler

Separation membrane

Isolation joint detail

Protruding surface feature, building or structural

Figure 22.13 Isolation joint.


22.5.4 Typical jointing related problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ .................. ~ .............................................................. ~ ~ ................. ............................................ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ ~ ......................... ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

The following construction problems are commonly encountered and illustrate the importance attached to good sound detailing. Many contractors fall foul to poor construction detailing problems. The following figures illustrate typical problems:

Figure 22.14 shows an incorrect joint location Figure 22.15 shows a joint which is too shallow Figure 22.16 shows poor detailing Figure 22.17 shows good manhole detailing Figure 22.18 shows badly debonded tie bars

Figure 22.14 Joint cut too late and incorrect joint location.


Figure 22.15 Joint cut too shallow.

High-quality concrete is important to the success of all concrete pavements. It is already noted from the design method that one of the most important issues influencing the bearing capacity of a concrete pavement is the strength of the concrete mix. Strength may be measured by either of the standard structural control techniques using compressive cubes or cylinders. Tests are normally conducted using standard 28-day cured cubes or cylinders. UK practice is to use cubes, European practice is to use cylinders, Americans use beams and cylinders. Standard structural relationships exist linking the two parameters.

22.6.1 Compressive cube strength and tensile strength relationships

~ ~ ~ ~ ~ : ~ : ~ ,~ , , ~ . ~ ~ ~ ~ ........................ ~ ~ ~ ,~,~: ~ ~ ~ ~,~ ~,~, ~ ~ . : ~ , ~ ~ , ,~ ~ : ~ ~ : ~ ~ ~ ~ ~ , ~ ~ : : : : : :~:~ ~ : ~ : :::::~: :::::: ~ : : : ::::::::::::::::::::::: :~ :~ :~::::::: ::, ~,~:~ :~::~: :::.~,::,:: ~: :::~:~ :::~ : : : : , : : : : : ~ : ~ ~:::~: :~ :~ : : ~ :~ : : ~:~:~: ~: ::~: :~::: ~::~::~ ~: : ~ : ~ :.~:~ ~::: ................ :~ ~ : ............... ============================================================== ~:,::~ :~ ~:~:~:::: ~ ~::~ ~ : : ~ : : : :~::~ ~ ~::::::~:~:: ~ ~:~::~:~:~::: : ~ ~

Concrete slabs are basically brittle unreinforced members which rely on the tensile capacity of the main structural element, concrete, to provide bearing capacity and durability.


Figure 22.16 Poor detailing.

Figure 22.17 Good quality detailing.

Concrete tensile strength is therefore fundamental to the efficient operation of a pavement. Tensile strength may be measured using two alternative techniques. The standard method is to use the Modulus of Rupture (R) to define the bending tensile capacity of concrete. The test is conducted on prisms using a bending test. The British Standard is BS 1881 Parts 109, 118 and 111 (BSI, 1983a, b, c). The test is a four-point bending test producing a tensile strain, which reflects the mass concrete beam strain capacity at failure.

An alternative technique known as the Brazilian Test or Indirect Tensile Strength test may also be used to control tensile strength. BS 1881 Part 117 defines the test (BSI,


Figure 22.18 Badly bonded tie bars.

1983d). Again standard relationships exist linking indirect tensile strength to the Modulus of Rupture. Figure 22.19 illustrates the different techniques for testing tensile strength.

A number of researchers have identified relationships linking tensile capacity to compressive cube strengths. Different relationships may be identified for gravel aggregates and crushed rock aggregates. The most commonly used relationships are those identified by Croney and Croney (1991):

For gravel aggregate Modulus of Rupture = 0.49 (fcu) °'55 For crushed rock aggregate Modulus of Rupture = 0.36 (fcu) °7°

Concrete .J Modulus of rupture prisms "-I )k

(~ Area o!tensile (~ cracking and failure

Concrete cylinder

/~.4~--------------- Hardboard strip

~ Brazilian test or - ~ J ~ , , indirect tensile test

~ - Tensile cracking occurs here

Figure 22.19 Tensile bending test, Modulus of rupture and the Indirect/Tensile test or Brazilian Test.


Similar relationships are also available for Indirect Tensile Strength and compressive Cylinder Strength.

22.6.2 Slip membranes . . . . . . . . . . . . . . . . . . . . , : : : . : : : : ................................ : : : : . ~ : : : : : : ~ . : ~ : : : ~: : . : : : : : : : : :~: : : : :.:~:.~::: : ::..::. :~:~:::: ~: ~::: . . . . . . . . . . . . . . . :: ................. : : : : . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ......................... .................. ........................................... ................. : ~ : : : . : : ......................... . ................. . : : : : : : ~ : : : ::: : ~ : : .................... ~ : ::.~ :: ~:: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .....

Slip membranes are important to the efficient performance of all types of pavement. A de- bonding layer is needed underneath a concrete pavement to prevent interaction between the base foundation and the slab but also to allow differential movement to occur. Plastic sheeting is the commonest method of providing a slip membrane. The sheeting is intended to:

• prevent loss of fines into the sub-base • form a separation layer between the concrete and the sub-base • reduce the risk of cracking between the joints during early shrinkage

If sawn crack induced joints are used plastic sheeting is not the most appropriate form of membrane. The plastic sheeting can allow too much free movement. If joints are sawn crack induced they may not break correctly at each joint. A bituminous sprayed layer is a more effective method of de-bonding providing partial separation thus producing cracks at every regular joint.

A number of different techniques are used throughout the world for constructing concrete pavements. Each is appropriate to different environments, economies and labour forces. It is important to recognize that slip forming is a useful technique in Western Europe, on large construction projects, but will not work on small building projects or third world schemes.

22.7.1 Traditional techniques

The labour-intensive method of construction is to erect wooden shutters along the line of each longitudinal joint and pour concrete into one continuous strip. Intermediate contraction joints are formed by either joint sawing, into the hardened concrete or dropping patent plastic crack inducers into the hardening pavement surface can be used to form intermediate contraction joints. The disadvantages of this method are:

• the technique is slow and labour-intensive • erecting shutters and laying concrete by hand is a skilled activity • joint sawing must be undertaken after the concrete has hardened but before any shrinkage

has taken place. The timing of the joint sawing is critical

The advantages are:

• the technique is flexible and requires no major capital expenditure • difficult pavement shapes can be worked around without problems


22.7.2 Slipforming ......................... ~ ~ , .............. ~ : ~ . ~ ,~ ~ ~ , ~ : ~ ~ : ~ , ~:~ ~,~ ~ : ~ : ~ , , . : : ~ : ~ : ~ ~ ~ ~ : : ~ : ~ ~: ~ ~ : ~ . .................. ================================================== : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : ======================================= ........................... ~ ~ : ~ : ~ ~ ~ ~ .......... ~ :::::::::::::::::::: ~ :::::::::::::::::::::: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :

Modem slipforming plant can produce an economical fast, well constructed alternative to hand-laying techniques. Slipforming machines have been employed on the construction of recent UK airfield projects. A typical slipformed pavement system will include the following construction:

• a stabilized formation layer, to allow good support to the slipformer • a sprayed bituminous slip membrane, in place of a plastic membrane

Mechanical dowel bar placing techniques can be employed in association with slip formed paving machines for mass concrete pavements. Slipforming is the most effective method of construction for continuously reinforced construction. Figure 22.20 illustrates a typical slipform system.

Figure 22.20 Slipforming a CRCP pavement, picture courtesy of Wirtgen slipform manufacturers.

The maintenance and repair of concrete pavement is an important area of specialist engineering application. A summary of the subject is presented in the current Highways Agency concrete pavement maintenance manual published in 2001. The first essential step in undertaking a repair is to identify the cause of failure. When the precise nature of failure is understood, it is then possible to move on and identify suitable repair techniques. The standard contains a set of flow charts to classify and evaluate contributory factors producing a defect.


The standard divides maintenance techniques into three different categories:

• emergency repairs • medium-term repairs • long-term repairs

Defects are classified into the various different types:

• jointing problems • surface abrasion, spalling or cobweb cracking problems • general forms of cracking including comer breaks, longitudinal cracks or shrinkage

cracking • joints stepping, slab rocking and loss of support through pumping • surface irregularities, pop outs or general durability problems

The severity of defect must be understood and evaluated in the design process but the following techniques may be used to remedy problems.

22.8.1 Emergency repairs • ~ .~ .~ ~ ~ . . ~ ................... ~ ~ . ~ . : ~ ~.~.~.~.~.~.~, ~.~:~..~.~.~.,.~, ~ ~.~.~ ~ :~ .~ ,~ .~ ~.~.~:~.~:~.~ ~ :~ .~ .~ , . ~ , . , . ~ .~ .~ . , ~ ,~ ,~ .~ . : . ~ .~ , . ~ .~ ~ ~ . ~.~ ~ ~.~.~ ~.~.~.~.:..~.~ :~.~.: ,::~.. :~: .~:~ .................. ..~ ~:~:.:~:~: ,.. ~::~..~.~ :.:::::: ~.::: ~ :.::.:.: ::: ~:~.:~.~:.~.~: : : .~:~ ~.:~:.~.::: ~ :~ :.~:.::.: . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . : : : : ~: :.~:. ............................... ~.~ ~ ~ ~.:..~:.~:~ : ~ .. :~. . . . . . . . . . . . . . . . . :::: : ~ . : : ~ : ~ :~:~ ~.~:~:~ .~ ~:. ~ .~ .~ : ~ : ..........................

22.8.1.1 Bay replacement Immediately after a concrete bay has failed the first line of defence is to knockout the bay between the existing joint and replace it with a bituminous inlay. This technique is considered as a very short-term emergency measure. The continuity of the concrete slab is lost as the weak bituminous material is placed in the gap left by the removal of the slab. Progressive chipping away and failure of the remaining adjoining slabs will quickly follow if the bituminous material is not replaced with concrete at the earliest opportunity.

22.8.2 Medium-term repairs

22.8.2.1 Cracks Any cracks identified within a pavement with a width greater than 0.5 mm should be repaired using some form of bituminous seal. If the cracks are not sealed detritus, dust and dirt will quickly fall down into the gap forcing open the joint.

Wide cracks, which have width greater than 1 mm, are considered to have a structural implication and must be repaired. Crack stitching techniques can be used to repair wide cracks but these techniques are undesirable and technically difficult. A stitch repair technique must also involve some form of pressure grouting. If a joint contains detritus or moisture, grouting techniques are unlikely to be successful.

22.8.2.2 Surface dressing Concrete pavement surfaces may be restored on a temporary basis by a simple application of a conventional bituminous surface dressing. This system is used to repair skidding problems caused by erosion of the pavement surface or aggregate polishing problems.


22.8.2.3 Pressure grouting to restore support A number of patent repair techniques are available which use pressure grouting to fill voids under the pavement. The systems are of mixed success and require highly skilled operators. These types of repair should only be attempted in exceptional circumstances.

22.8.3 Long-term repairs

22.8.3.1 Full depth bay replacement The most effective long-term repair is to replace completely badly cracked concrete bays. Bay replacement should only be attempted when it is clear that the majority of the concrete pavement is sound and likely to give many years further service.

22.8.3.2 Joint repairs Many techniques exist which can give exceptionally good long-term service. Joint repairs require very careful design and detailing. If long-term joint repairs are considered appropriate it is recommended that section 8 of the repair manual (Burkes Green and Partners, 2001) is carefully examined.

A number of research techniques are currently being investigated which will further develop the technology of concrete pavements.

22.9.1 Fibre-reinforced concrete ..... : ~ : ~ ..................... ~ ~ , ~ ~ ~:~:~ ~ ~ : ~ : ~ : ~ : : :~ , :~ ~:~ ~ : ~ : ~ : ~ ~ :,~ , : ~ ~ :~:~ ~,,~ :~,~ :~ ~ : : ~ ::~ ~::,~:~:~ : ~ , ~ , : ~ ~ : ~ ~ : ~ : ~ ======================== ,:::::: :::: : : : : : : : ,:,::~:: ::::: :~::: : : : , : ,~ , : : : : :::::: :::::~: , : : : : : : : : : ::::: :::::,~: ::: :::::: ::::::: ::::,: : : : : ::::::: : : : : : : : : : : : : : : : :::::: : : : : : : : : : : :::::: :::::: : :~: : ::::: :::: : : : : : : :~: : :~: : : : : : : : ~:::,: ~ : : : : : : : : : : ::::::::::::::::: :::::: : : : : :,~:~ :::::::: :::::: ::,::: ::::::: : : : : : ::::,,:::: : : : : : : : : : : :~ : : : : ::::::: ::::::: : : : ~ : :~::: ::::::: : : : : : : :~: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :

A number of researchers are examining the possibilities of using steel fibres as enhancing material to mass concrete pavements. High tensile steel fibres are extensively used in internal industrial floor slabs. The techniques are applied to heavily loaded industrial slabs. Steel fibres are added at the rate of approximately 0.4% to a cement bound roadbase material. The fibres enhance the durability of the mix, increasing the failure ductility. The point when initial cracking occurs remains unaltered but the ultimate strength of the mix is increased substantially before failure occurs.

22.9.2 Thin overlays and white topping

A number of interesting experiments are currently being conducted involving the use of continuously reinforced overlays to existing bituminous pavements. Heavily rutted bituminous material can be removed and a thin 150 mm CRCP system installed as a replacement. The system is particularly effective for heavily trafficked motorway slow lanes and may become a standard future repair technique.


This chapter is intended as a brief introduction to the basic concepts of cement bound pavements. The views expressed here are the personal opinions of the author and should not be considered as design standards or definitive guidance. If serious design work is considered appropriate, based on advice given in this chapter, it is recommended that the primary sources of information quoted as references are used for detailed design.

Charonnat Y., Lefebvre J. and Sainton A. (1989) Optimization of non-skid characteristics in construction of concrete pavements, 4th International Conference on Concrete Pavement Design and Rehabilitation, April, Purdue University.

American Concrete Pavement Association (1992) Proper Use of Isolation and Expansion Joints in Concrete Pavements, ISA400.0IE

BSI (1997) BS 5328 Part 1 1997, Guide to specifying concrete. The Highways Agency (1999) Design Manual for Roads and Bridges, Vol. 7, HD 36/99 Surfacing

Materials for New and Maintenance Construction. Mayhew, H.C. and Harding, H.M. (1987) Research Report 87, Thickness design of concrete roads,

Transport and Road Research Laboratory, ISSN 0266-5247. The Highways Agency Design Manual for Roads and Bridges, Vol. 7, HD 26/01 Pavement Design. Westergaard, H.M. (1926) Stresses in concrete pavements computed by theoretical analysis. Public

Roads, Vol. 7, No 2. Also Proceedings of the 5th Annual Meeting of the Highway Research Council, Washington DC 1926 as Computation of Stresses in Concrete Pavements.

American Association of State Highway and Transportation Officials (1992), AASHTO Guide for Design of Pavement Structures, ISBN: 1-56051-055-2.

American Association of State Highway and Transportation Officials (1998) AASHTO Guide for Design of Pavement Structures Part H - Rigid Pavement Design and Rigid Pavement Joint Design. ISBN 1-56051-078-1

Chandler, J.W.E. (1982) Technical Report 550, Design of floors on ground. Cement and Concrete Association.

Miner, M.A. Cumulative damage in fatigue, ASME Transactions, 67, A159. Spangler, M.G. (1942), Stresses in the Comer Region of Concrete Pavements, Bulletin 157. Engineering

Experiment Station, Iowa State College, Ames. Ioannides, A.M., Thompson, M.R. and Barenberg, E.J. (1985), Westergaard solutions reconsidered.

Transportation Research Record, 1043. Croney, D. and Croney, E (199 l) The Design and Performance of Road Pavements, second edition.

McGraw Hill International, ISBN: 0-07-707408-4. BSI (1983a) BS 1881 Parts 109, Method for making test beams from fresh concrete. BSI (1983b) BS 1881, Part 111, Method of normal curing of test specimens (20°C). BSI (1983c) BS 1881 Part 118, Method for determination of flexural strength. BSI (1983d) BS 1881 Part 117, Method for determination of tensile splitting strength. Burkes Green and Partners (2001) Concrete Pavement Maintenance Manual.

QT1- Geoffrey Griffiths - Concrete Roads and Pavements

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