What is durability of concrete?

It is the ability to resist weathering action, chemical attack, abrasion, or any process of deterioration.

Durability and sustainability

Durability of concrete structure of paramount importance for the following reason

• Financial investment especially in infrastructure projects is very high. Due to this, building durable structures should be sustained and practiced by the construction industry.
• It is the responsibility of the designer, consultant, engineer to design and build the structure keeping the environmental condition, usage etc in mind. This ensures the long life of the building and adds to durability of concrete.
• Also the selection of right material and construction practices by the contractor are vital practices to be followed.

How do we ensure ‘durable concrete’?

The following is an important list of control factors that need attention in the production of durable concrete.

• Structural design of concrete including the cover to reinforcement.
• The concrete mix design and specification (Water to Cement ratio, maximum cement content, type of cement).
• Workability and cohesiveness.
• Batching/ mixing/ transportation/ placing and compacting.
• The curing process.
• Maintenance and usage during service life.

Reasons for deterioration

Deterioration of concrete can take place basically due to porosity.

Concrete has porosity of several types:

• Capillary pores: Depending on the water cement ratio, the number of capillary pores can increase. The table shows the age at which capillary pores get blocked with different water cement ratios. It clearly shows that if W/C ratio is more than 0.7, the capillary pores will never be blocked and remain for the entire lifetime of the structure. Meaning, the durability of the structure/ concrete is in equation.

Water Cement Ratio	Age at which capillary pores become blocked
0.40	3 days
0.45	7 days
0.50	14 days
0.55	14 days
0.60	3 months
0.70	Over 1 year
Over 0.70	Infinity

Coefficient of Permeability represents the level of permeability of concrete. The table below gives the coefficient of permeability of concrete made using same materials but with different W/C ratios:

Coefficient of Permeability for different W/C   

Sr. No.	W/C	Coefficient of Permeability (Valenta)
1	0.35	1.05 × 10-3
2	0.50	10.30 × 10-3
3	0.65	1000 × 10-3

When the w/c ratio is increased two-fold, the coefficient of permeability increases thousand-fold. This clearly explains how important w/c ratio is to durability and strength of concrete.

• Entrapped air: Proper compaction methods should be followed to ensure removal of all entrapped air within the concrete.
• Honey combs: Proper concrete mix and good concrete practice are the key tools to eliminate honeycombs in concrete.
• Cracks: Good concrete practice, right choice of materials, right design and maintenance – all contribute to minimize cracks in concrete.

Concrete in Aggressive Environments.

Deterioration of concrete takes place in stages. The speed of deterioration and damage to concrete will greatly depend on the following:

Physical Factors

• Continuous or discountinuous cracks.4weathering effects.
• Cyclic loading of impact loading.

Chemical Factors

• Environmental actions or aggressive chemicals, iron and their penetration.

Entry of Chemicals into Concrete and their impact

Trigger	Impact
Chloride	Enters through capillary pores, corrodes reinforcement, causes swelling of concrete
Water	Enters through capillary pores, corrodes reinforcement
Carbon Dioxide	Reacts with concrete and deteriorates it
Sulphates	Reacts with concrete and deteriorates it

For the design purpose of concrete, aggressive environment is defined depending on the severity of presence of chemicals stated above.

Preventive measures for corrosion of reinforcement

• Use minimum water cement ratio.
• Avoid entrapped air and segregation during construction.
• Avoid plastic and drying shrinkage cracking of concrete by taking adequate care in designing concrete mix and by proper construction practices especially curing.
• The concrete mix should have good workability and cohesiveness, so that it can be placed and compacted properly.
• Protective coating on steel.
• Another very important preventing measure is to use blended cement containing fly ash or slag.

Type of Cement	Chloride Diffusion Sq.cm / S × 108
OPC	4.47
Pozzolana Cement (70% OPC & 30% Fly ash)	1.47
Slag Cement (35% OPC & 65% slag)	0.41
Sulphate Resistant Cement	10.00

The table shows clearly that Pozzolana or Fly Ash cement Prevent chloride entering into concrete.