From High Strength to High Performance Concrete

Durability of concrete is now becoming the major concern for engineers as concrete may deteriorate with time in several ways. The most common durability failures in an outdoor climate are due to reinforcement corrosion or frost attack.

In special environments concrete may suffer from chemical attack by various substances such as sulfates, acids, soft water etc. causing disintegration or expansion.

Durability failure may also occur because of internal expansion from concrete constituents that are swelling, usually because of a reaction product absorbing water.

The concept of ‘durability’ is difficult to quantify. Durability may be ‘good’ or ‘better’, but such a description has no meaning without a proper definition.

Durability is not a property of a concrete material, or a concrete structure, but ‘behavior’, a performance, of a concrete structure in a certain exposure condition.

Therefore, in the last few years the emphasis of construction industry has shifted from high strength to high-performance concrete.

The need to build durable structures is felt not only form the point of view of economy, but also for the conservation of resources, energy, and environment.

Technical Requirements for Durability of Concrete

In the fourth revision of the IS 456:2000, special consideration has been given to the durability of concrete structures.

IS 456:2000, suggests following technical requirement for RCC construction in connection with blended cements and durability, with reference to attack by deleterious agents, like sulphate attack, alkali aggregate reaction and carbonation.

1. One of the major properties of concrete affecting its durability is its permeability.

The more permeable a concrete is the less durable it will be.

Because when concrete is permeable it helps to the ingress of deleterious agent that affects both concrete and embedded reinforcement.

2. The factors which influence the durability of concrete are

• environment,
• cover to embedded steel,
• type and quality of construction materials,
• cement content and
• water-cement ratio,
• workmanship during compaction
• curing, and
• shape and size of the member.

3. The general environment, to which the concrete will be exposed during its working life, is classified into five levels of severity, namely

• Mild,
• Moderate,
• Severe,
• Very severe, and
• Extreme

The table shown below specifies different levels of severity with respect to its exposure condition.

Table-1
Sl. No	Type of environment	Exposure condition
1	Mild	Concrete surfaces protected against weather or aggressive conditions, except those situated in coastal area.
2	Moderate	Concrete surfaces sheltered from severe rain or freezing whilst wetConcrete exposed to condensation and rain Concrete continuously under water.    Concrete in contact or buried under nonaggressive soil/ground water Concrete surfaces sheltered from saturated salt air in coastal area
3	Severe	Concrete surfaces exposed to severe rain, alternate wetting and drying or occasional freezing whilst wet or severe condensationConcrete completely immersed in sea water    Concrete exposed to coastal environment
4	Vere Severe	Concrete surfaces exposed to sea water spray, corrosive fumes, or severe freezing conditions whilst wetConcrete in contact with or buried under aggressive sub-soil/ground water
5	Extreme	Surface of members in tidal zoneMembers in direct contact with liquid/ solid aggressive chemicals

4. Suitable air-entraining admixtures can be used to improve durability of RCC where freezing and thawing actions happens under wet conditions.

5. For concrete exposed to different sulphate concentrations, recommendations given in IS-456 should be followed.

There is a table in IS-456 (Table-4) which specifies type of cement to be used, maximum water cement ratio and minimum cement quantity to be used for different concentration of sulphate.

6. The minimum values for the nominal cover to meet the durability requirements is shown in Table-2.

Table-2
Exposure	Nominal concrete cover in mm not less than
Mild	20
Moderate	30
Severe	45
Very Severe	50
Extreme	75

7. Table 3 & table 4 shown below shows appropriate values for the minimum cement content and the maximum free water cement ratio of the concrete, for different exposure conditions for PCC & RCC respectively.

Table -3
Sl. No.	Environmental Exposure Condition	Plain Concrete
Minimum Cement Content (kg/m3)	Maximum Free Water-Cement Ratio	Minimum Grade of Concrete
1	Mild	220	0.60	—
2	Moderate	240	0.60	M15
3	Severe	250	0.50	M20
4	Very Severe	260	0.45	M20
5	Extreme	280	0.40	M25

Table -4
Sl. No.	Environmental Exposure Condition	Reinforced Concrete
Minimum Cement Content (kg/m3)	Maximum Free Water-Cement Ratio	Minimum Grade of Concrete
1	Mild	300	0.55	M20
2	Moderate	300	0.50	M25
3	Severe	320	0.45	M30
4	Very Severe	340	0.45	M35
5	Extreme	360	0.40	M40

8. The upper limit of the cement content, not including fly ash (FA) and ground granulated blast furnace slag (GGBS), has kept at 450kg/m3.

Consideration has been made to the increased risk of cracking due to drying shrinkage in thin sections or early thermal cracking and the increased risk of damage due to alkali aggregate reaction, at the higher cement contents.

9. The total amount of chloride content (as Cl) in the concrete, at the time of placing is shown in Table-5

Table-5
Type or Use of Concrete	Maximum Total Acid Soluble Chloride Content Expressed as kg/m3 of concrete
Concrete containing metal and steam cured at elevated temperature and pre-stressed concrete	0.4
Reinforced concrete or plain concrete containing embedded metal	0.6
Concrete not containing embedded 3.0 metal or any material quiring protection from chloride	3.0

10. The maximum total water-soluble sulphate content of the concrete mix, expressed as SO3, has been specified as 4 percent by mass of the cement in the mix.

11. As a precaution against alkali aggregate reaction, recommendations have been given on the constituent materials like use of non-reactive aggregates and low alkali Portland cement (durability by extending the hydration of cement.)

12. Recommendations have been given on the concrete constructions in sea water or directly exposed along the seacoast, with respect to the grade of the concrete, type of cement, mix design and the use of precast members.

The durability of concrete incorporates, besides strength, its capacity to resist the penetration of the internal and external deteriorating factors.

• sulphate attack,
• chloride attack manifested in the corrosion of the reinforcement,
• carbonation,
• alkali-aggregate reaction,
• freezing and thawing,

If all the above factors are considered during design stage, then concrete will give a satisfactory performance during the economic life, for which it is designed.