2016

By : Amira Abdullah Youssef

Ibrahim

To : Dr. Hussein Mokhtar

Material 2 – Civil Engineering

department – Level 3

Durability & Permeability of Concrete

1. General Definitions Durability is the ability to last a long time without significant

deterioration. A durable material helps the environment by

conserving resources and reducing wastes and the environmental

impacts of repair and replacement.

Permeability is defined as the property that governs the rate of flow

of a fluid into a porous solid.

2. Durability of concrete

Durability of concrete: ability to resist weathering action,

chemical attack, abrasion, or any process of deterioration.

A durable concrete is one that

performs satisfactiorily under

anticipated exposure conditions

during its life span. The material

and mix proportions used should

be such as to maintain its intigrity

and, if applicable, to protect

embeded metal from corrosion.

Even though concrete is a durable

material requiring a little or no

maintenance in normal

environment but when subjected

to highly aggressive or hostile

environments it has been found to

deteriorate resulting in premature

failure of structures or reach a state requiring costly repairs.

One of the main characteristics influcing durability of concrete is

its permeability to the ingree of water, oxygen, carbon dioxide,

chloride, sulphate and other potential deleterious substances. •

Most of the durability problems in the concrete can be

attributed to the volume change in the concrete. Volume

change in concrete is caused by many factors. The entire

hydration process is nothing but an internal volume change, the

effect of hydration, the pozolanic action, the sulphate attack,

the carbonation, the moisture movement, all type of shrinkages,

the effect of chlorides, corrosion of steel, comes under the

aspects of volume change in concrete. • The internal and

external restraints to volume change in concrete results in

cracks. It is the crack that promotes permeability and thus it

becomes a part of cyclic action, till such time that concrete

deteriorates, degrades, disrupts, and eventually fails.

ROLE OF WATER-CEMENT RATIO The volume change in concrete results in cracks and the cracks

are responsible for disintegration of concrete.

Permeability is the contributory factor to the volume change

with higher water-cement ratio being the fundamental cause of

higher permeability. Therefore, use of higher water-cement ratio-

permeability- volume change-cracks- disintegration- failure of

concrete is a cyclic process in concrete. Therefore, for a durable

concrete, use of lower possible water-cement ratio is the

fundamental requirement to produce dense and impermeable

concrete.

It is generally recognized that quality of hydration product and

the micro-structure of the concrete in case of low water- cement

ratio is superior to the quality of micro-structure that exists in the

case of higher water-cement ratio.

The lower water-cement ratio concretes are less sensitive to

carbonation, external

chemical attack and other

detrimental effects that

cause lack of durability of

concrete.

However, in lower water-

cement ratio concretes,

there is not enough water

available to fully hydrate all

cement particles, only

surface hydration of

cement particles takes

place leaving considerable

amount of unhydrated core

of cement grains. This

unhydrated core of cement

grains constitute strength in

reserve.

3. Role of Permeability The capillary pores in concrete serve as a conduit or provide

transport system for deteriorating agents. However, it may be

mentioned that the micro-cracks in initial stage are so fine that they

may not increase the permeability. But propogation of micro-cracks

with time due to drying shrinkage, thermal shrinkage, and externally

appied loads will increase the permeability of the system.

PERMEABILITY OF CEMENT Cement paste consists of C-S-H (gel), Ca(OH)2 , and both water

filled and empty capillary cavities. The gel has porosity to the extent

of 28 % with permeability of the order of 7.5 x 10-16 m/s which is

about one-thousandth of that of cement paste. Therefore,

contribution of gel pores to the permeability of cement paste is

minimal. The extent and size of capillary cavities or pore depend

upon water- cement ratio. At low water-cement ratio the extent of

capillary cavities is less and cavities are very fine which are easily

filled up within few days by hydration

product of cement. Only unduly large

cavities resulting from high water-

cement ratio (of the order of 0.7) will not

get filled up by product of hydration

and will remain unsegmented and are

responsible for the permeability of the

paste.

INTRODUCTION OF AGGREGATE,

large size of aggregate, increase the

permeability considerably. As explained

above, the increase in the permeability

is due to the development of micro-

cracks in the weak transition zone at

early age. The size of cracks in the

transition zone is reported to be much

bigger than that of capillary cavities

present in the cement paste.

4. Effect of Mineral Additives Concrete containing cement with 35 % fly ash has been found to

be 2 to 5 times less permeable than concrete manufactured with

OPC or blast-furnace slag cements. Moreover, concretes made

using pozzolanic cements have a better flexural/ compressive

strength ratio and tendency to cracking than cement made using

OPC.

5. Effect of Air-Entrainment An air-entrainment upto 6 %

can make the concrete more

impervious. The steam curing

of concrete using pozzolana

has been reported to

decrease the permeability due

to formation of coarser C-S-H

gel, lower drying shrinkage

and accelerated conversion

of Ca(OH)2 into cementing

product.

6. Factors Affecting Durability • The factors affecting durability are broadly divided into two groups

namely external factor and internal factor.

Durability External factors Physical, chemical, or mechanical

Environmental, such as extreme temperatures, abrasion, and

electrostatic action. Attack by natural or Industrial liquid and

gases Internal factors Permeability of Concrete Alkali aggregate

reaction Volume changes due to difference in thermal

properties of the aggregate and cement paste.

Physical causes of deterioration of concrete Cracking Surface

wear Structural loading Overloading and impact cyclic loading

Exposure to temperature Fire Freezing Thawing action Volume

changes due to Temperature Humidity De-icing salts Abrasion

Cavitation Erosion

7. Requirement for Durability • Exposure conditions

• General environment

• Freezing and thawing

• Exposure to sulphate attack

• Acid attack

• Sea water attack

• Abrasion, erosion and cavitation

• Carbonation

• Fire resistance

REQUIREMENT OF CONCRETE COVER • Shape and size of member

• Type and quality of constituent materials

• Concrete mix proportions

• Maximum cement content

• Chloride in concrete

• Sulphate in concrete

• Alkali aggregate reaction

• Compaction, finishing and curing of concrete.

8. General Environment • The general environment to which the concrete will be exposed

during its working life is classified to five levels of severity, namely, mild,

moderate, severe, very severe, extreme.

EFFECT OF WEATHERING-FREEZING AND THAWING • Lack of durability of

concrete due to freezing

and thawing of frost is not

so important in Indian

conditions. But is of great

importance in cold

countries.

• As the temperature of

saturated hardened

concrete is lowered, the

water held in the capillary

pores in the concrete

freezes in a similar manner

to the freezing in the

capillary in rock and expansion of concrete takes place. Hence, if

concrete mass is subjected to alternate cycles of freezing and

thawing, it has detrimental effect on the strength of concrete.

• Hence, while concreting in cold weather, the temperature of the

fresh concrete should be maintained above O0 C.

SEA WATER ATTACK • Sea water contains sulphates and hence attacks concrete in a

manner similar to the sulphate attack.

• The deterioration of concrete in sea water is often is not

characterized by the expansion, as found in concrete exposed to

sulphate attack. Attack of sea water causes errosion or loss of

constituents of concrete without undue expansion. Calcium hydroxide

and calcium sulphate (gypsum) are considerable soluble in sea water,

and this will increase the leaching action.

• Incase of reinforced concrete the absorption of salt results in

corrosion of reinforcement. The accumulation of the corrosion product

on the steel, causes rupture of the surrounding concrete. So that effect

of sea water is more sevee on reinforced concrete than on plain

concrete