Concrete Durability

• Serious carbonation , chemical attack on the concrete ,cracking & spalling due to poor quality materials or workmanship and /or corrosion of the reinforcement are all signs of distress that indicate a concrete of low durability .
• Low durability may caused either by the environment to which the concrete is exposed or by some internal causes within the concrete itself .
• The external causes can be physical , chemical or mechanical . For example , thjey may be due to weathering effects , occurrences of extreme temperatures , abrasion , el;ectrolytic action & attack by natural or industrial liquids & gases .
• The extent of damage to the  concrete produced by these agenets depends largely on its quality .
• The internal cause include alkali-aggregates reaction & volume changes , as well as the permeability of the concrete .
• The last cause determines the vulnerability of concrete to external agencies , so that in order to be durable , concrete must be relatively  impermeable .
• Permeability determines the ease with which concrete can become saturated with water .
• Concrete with low permeability is generally more durable than one which is hihgly permeable .
• Permeability of concrete is a function of its porosity as well as the size , distribution & continuity of the pores .
• Therefore , the cement gel has a low permeability because of the extemely fone texture of the hardened cement paste which contains numerous but very fine pores .
• In the case of rocks , the pores , though fewer in number are much larger & lead higher permeability

Factors contributing to the degree of permeability of concrete include :
1. The quality of the cement : for the same water/cement ratio , coarse cement tends to produce a paste with a higher porosity than a fine cement .
2. The permeability of the aggregates :if the aggregates has a low permeability , its presence reduces the effective area over which flow can take place .

1. The quality & quantity of the cement paste :

the quality of the cment paste depends on the amount of cement in the mix . the water/cement ratio & the degree of hydration of the cement . Permeability decreases rapidly with the progress of hydration & it is also lower if the cement content of the paste is increased .

2. The degree of compaction of the concrete : A well - compacted concrete reduces the porosity & hyence the permeability of the concrete .

3. The standard of curing : permeability of steam -cured concrete is generally higher than that of wet-cured concrete .

4. The characteristics of any admixtures used in the mix : air-entraining is expected to increase the permeability of concrete although it reduces segreagation & bleeding & improbes workability .

5. The presence of cracks allow direct entry of moisture .