Determinación de las brechas de inversión en la región Tacna (2004)

If concrete is to serve the purpose for which it is designed during its intended lifetime, it has to be durable. Unfortunately, many reinforced concrete structures built in the past (particularly, the not-too-distant past) in adverse environments have shown signs of increased structural distress, mainly^† due to chemical attack, causing

† Other factors include ‘abrasive’ actions on concrete surfaces (caused, for example, by machinery and metal tyres) and ‘freezing and thawing’ actions.

deterioration of concrete and corrosion of reinforcing steel. Loss of durability results in a reduced life of the structure. In an attempt to give increased importance to durability considerations, the recent revision of the Code has strengthened the provisions pertaining to durability, by shifting the guidelines from the Appendix (of the earlier Code) to the main body of the Code (Cl. 8), and by enhancing their scope and impact. These changes are in line with other national codes, such as BS 8100 and ACI 318.

Loss of durability in concrete structures is essentially attributable to two classes of factors, viz., external factors and internal factors. The external factors pertain to the type of environment to which the concrete is exposed, whereas the internal factors pertain to characteristics inherent to the built concrete. Primary among the internal factors is the relative permeability of the concrete, as chemical attack can occur only if harmful chemicals can ingress into the concrete. Chemical attack is caused by the ingress of water, oxygen, carbon dioxide, chlorides, sulphates, and other harmful chemicals (borne by surrounding ground or sea water, soil or humid atmosphere). It can also occur due to the presence of deleterious constituents (such as chlorides, sulphates and alkali-reactive aggregate) in the original concrete mix. Concrete members that are relatively thin or have inadequate cover to reinforcement are particularly vulnerable. Lack of good drainage of water to avoid standing pools and rundown of water along exposed surfaces, and cracks in concrete also lead to ingress of water and deterioration of concrete. Impermeability is governed by the constituents and workmanship used in making concrete. Despite the remarkable advances in concrete technology, regrettably, workmanship remains very poor in many construction sites in India, especially of smaller size projects.

Durability in concrete can be realised if the various internal factors are suitably accounted for (or modified), during the design and construction stages, to ensure that the concrete has the desired resistance to the anticipated external factors. Otherwise, the task of repairing and rehabilitating concrete that has been damaged (for want of proper design and quality of construction) can prove to be difficult and expensive.

The most effective ways of providing for increased durability of concrete against chemical attack in a known adverse environment are by:

• reducing permeability by

- providing high grade of concrete - using adequate cement content - using well-graded, dense aggregate - using low water-cement ratio

- using appropriate admixtures (including silica fume) - achieving maximum compaction

- achieving effective curing

- using appropriate surface coatings and impermeable membranes - avoiding sharp corners and locations where compaction is difficult - taking care while designing to minimise possible cracks

• providing direct protection to embedded steel against corrosion by - providing adequate clear cover

- using appropriate corrosion-resistant or coated steel - using sophisticated techniques such as cathodic protection

• providing appropriate type of cement having the desired chemical resistance to sulphates and/or chlorides

• controlling the chloride and sulphate contents in the concrete mix constituents (within the limits specified in Cl. 8.2.5 of the Code)

• avoiding the use of alkali-reactive aggregate

• providing air-entraining admixtures when resistance against freezing and thawing is required

• providing adequate thickness of members

• providing adequate reinforcement designed to contain crack-widths within acceptable limits

• providing adequate drainage on concrete surfaces to avoid water retention (e.g., ‘ponding’ in roof slabs)

2.13.1 Environmental Exposure Conditions and Code Requirements The Code (Cl. 8.2.2.1) identifies five categories of ‘environmental exposure conditions’, viz., ‘mild’, ‘moderate’, ‘severe’, ‘very severe’ and ‘extreme’, in increasing degree of severity. The purpose of this categorisation is mainly to provide a basis for enforcing certain minimum requirements aimed at providing the desired performance related to the severity of exposure. These requirements, having implications in both design and construction of reinforced concrete work, pertain to:

• minimum grade of concrete (varying from M 20 to M 40)

• minimum clear cover to reinforcement (20 mm to 75 mm)

• minimum cement content (300 to 360 kg/m³ for 20mm size aggregates)

• maximum water-cement ratio (0.55 to 0.40)

• acceptable limits of surface width of cracks (0.1 mm to 0.3 mm)

The descriptions of the five categories of environmental exposure, as well as the corresponding specifications for the minimum grade of concrete, ‘nominal cover’^† (minimum clear cover to reinforcement), minimum cement content and maximum free water-cement ratio, for reinforced concrete work, are summarised in Table 2.1^‡. These specifications incorporated in the revised Code constitute perhaps the most significant changes in the Code, having tremendous practical (and economic) implications. These recommendations have been long overdue, and are in line with international practice.

It may be noted that in the same structure, different members may be subject to different categories of exposure. For example, a reinforced concrete building located in a port city (such as Chennai or Mumbai) would be exposed to a coastal environment, which qualifies to be categorised as ‘severe’ (or ‘very severe’, in case it is very near the beach, exposed to sea water spray). However, for concrete members located well inside the building (excepting foundations), sheltered from direct rain and aggressive atmospheric environment, the exposure category may be lowered by one level of severity; i.e., from ‘severe’ to ‘moderate’ (or ‘very severe’ to ‘severe’).

† Additional cover requirements pertaining to fire resistance are given in Table 16A of the Code.

‡ The requirements for plain concrete (given in Table 5 of the Code) are not shown here.

Table 2.1 Exposure conditions and requirements for RC work with normal aggregate (20 mm nominal size)

Exposur

Sheltered from severe rain or freezing whilst wet, or Exposed to condensation and rain, or

Continuously under water, or In contact with or buried under non-aggressive soil or ground alternate wetting and drying or occasional freezing whilst wet

Exposed to sea water spray, corrosive fumes or severe freezing whilst wet, or

In contact with or buried under aggressive sub-soil or ground water

M 35 50 340 0.45

Extreme

Members in tidal zone, or Members in direct contact with liquid/solid aggressive

chemicals

M 40 75 360 0.40

* can be reduced to 15 mm, if the bar diameter is less than 12 mm.

** can be reduced by 5mm, if M 35 or higher grade is used.

Accordingly, corresponding to the ‘severe’ category, the roof^↕ slab must be (at least) of M 30 grade concrete and its reinforcement should have a minimum clear cover of 45 mm. These values may be compared to M 15 grade and 15 mm cover hitherto adopted in design practice, as per IS 456 (1978). The increase in capital investment on account of the substantial increase in slab thickness and enhanced grade of concrete may appear to be drastic, but should be weighed against the significant gain in terms of prolonged maintenance-free life of the structure.

2.13.2 Permeability of Concrete

As mentioned earlier, reducing the permeability of concrete is perhaps the most effective way of enhancing durability. Impermeability is also a major serviceability requirement — particularly in water tanks, sewage tanks, gas purifiers, pipes and pressure vessels. In ordinary construction, roof slabs need to be impermeable against the ingress of rain water.

Permeability of concrete is directly related to the porosity of the cement paste, the distribution of capillary pores and the presence of micro-cracks (induced by shrinkage effects, tensile stresses, etc.). The main factors influencing capillary porosity are the water-cement ratio and the degree of hydration. The use of a low water-cement ratio, adequate cement and effective curing contribute significantly to reduced permeability. The steps to be taken to reduce permeability were listed in the previous Section. In addition, it is essential for the concrete to be dense; this requires the use of well-graded, dense aggregate and good compaction. For given aggregates, the cement content should be sufficient to provide adequate workability with a low water-cement ratio so that concrete can be completely compacted with the means available. The use of appropriate chemical admixtures (such as superplasticisers) can facilitate working with a reduced water-cement ratio, and the use of mineral admixtures such as silica fume can contribute to making a dense concrete with reduced porosity.

Provision of appropriate tested surface coatings and impermeable membranes also provide additional protection in extreme situations.

2.13.3 Chemical Attack on Concrete

The main sources of chemical attack, causing deterioration of concrete are sulphates, sea water (containing chlorides, sulphates, etc.), acids and alkali-aggregate reaction.

Sulphate Attack

Sulphates present in the soil or in ground (or sea) water attack hardened concrete that is relatively permeable. Sulphates of sodium and potassium, and magnesium in particular, react with calcium hydroxide and C3A to form calcium sulphate (‘gypsum’) and calcium sulphoaluminate (‘ettringite’) — which occupy a greater volume than the compounds they replace [Ref. 2.3]. This leads to expansion and disruption (cracking or disintegration) of hardened concrete. Sulphate-attacked

↕ In the case of intermediate floor slabs, the grade of concrete can be reduced to M 25 and the clear cover to 30 mm, corresponding to ‘moderate’ exposure.

concrete has a characteristic whitish appearance (‘efflorescence’ — due to leaching of calcium hydroxide), and is prone to cracking and spalling of concrete.

Resistance to sulphate attack can be improved by the use of special cements such as PSC, SRPC, HAC and SC [refer Section 2.2.1], and by reducing the permeability of concrete. Recommendations regarding the choice of type of cement, minimum cement content and maximum water-cement ratio, for exposure to different concentrations of sulphates (expressed as SO3) in soil and ground water are given in Table 4 of the Code. The Code recommends the use of Portland slag cement (PSC) with slag content more than 50 percent, and in cases of extreme sulphate concentration, recommends the use of supersulphated cement (SC)^† and sulphate resistant cement (SRC). The specified minimum cement content is in the range 280 to 400 kg/m³ and the maximum free water-cement ratio is in the range 0.55 to 0.40 respectively for increasing concentrations of sulphates.

Sea Water Attack

Sea water contains chlorides in addition to sulphates — the combination of which results in a gradual increase in porosity and a consequent decrease in strength. The same measures used to prevent sulphate attack are applicable here. However, the use of sulphate resistant cement (SRC) is not recommended. The Code recommends the use of OPC with C3A content in the range 5–8 percent and the use of blast furnace slag cement (PSC). In particular, low permeability is highly desirable. The inclusion of silica fume admixture can contribute to the making of the densest possible concrete. The use of soft or porous aggregate should be avoided. Concrete shall be at least M30 Grade in case of reinforced concrete. The use of a higher cement content (of at least 350 kg/m³) above the low-tide water level, along with a lower water-cement ratio (of about 0.40) is recommended, owing to the extreme severity of exposure in this region, where construction joints should also be avoided [Ref. 2.30].

Adequate cover to reinforcement and other measures to prevent corrosion are of special importance to prevent chloride attack on reinforcement.

Alkali-Aggregate Reaction

Some aggregates containing reactive silica are prone to reaction with alkalis (Na2O and K2O) in the cement paste. The reaction, however, is possible only in the presence of a high moisture content within the concrete. The reaction eventually leads to expansion, cracking and disruption of concrete, although the occurrence of damage may be delayed, sometimes by five years or so [Ref. 2.3]. Care must be taken to avoid the use of such aggregates for concreting. Also, the use of low alkali OPC and inclusion of pozzolana are recommended by the Code (Cl. 8.2.5.4). The Code also recommends measures to reduce the degree of saturation of concrete during service by the use of impermeable membranes in situations where the possibility of alkali-aggregate reaction is suspected.

† However, use of supersulphated cement in situations where the ambient temperature exceeds 40 °C is discouraged.

2.13.4 Corrosion of Reinforcing Steel

The mechanism of corrosion of reinforcing steel, embedded in concrete, is attributed to electrochemical action. Differences in the electrochemical potential on the steel surface results in the formation of anodic and cathodic regions, which are connected by some salt solution acting as an electrolyte.

Steel in freshly cast concrete is generally free from corrosion because of the formation of a thin protective film of iron oxide due to the strongly alkaline environment produced by the hydration of cement. This passive protection is broken when the pH value of the regions adjoining the steel falls below about 9. This can occur either by carbonation (reaction of carbon dioxide in the atmosphere with the alkalis in the cement paste) or by the ingress of soluble chlorides. The extent of carbonation penetration or chloride penetration depends, to a great extent, on the permeability of concrete in the cover region. The increased cover stipulated in the recent code revision will contribute to increased protection against corrosion, provided, of course, the cover concrete is of good quality (low permeability).

However, it may be noted that increased cover also contributes to increased flexural crack-widths [Ref. 2.31], and it is necessary to contain the cracking by suitable reinforcement design and detailing [refer Chapter 10].

The electrochemical process of corrosion takes place in the presence of the electrolytic solution and water and oxygen. The consequent formation and accumulation of rust can result in a significant increase in the volume of steel and a loss of strength; the swelling pressures cause cracking and spalling of concrete, thereby allowing further ingress of carbonation or chloride penetration. Unless remedial measures are quickly adopted, corrosion is likely to propagate and lead eventually to structural failure. Cathodic protection is the most effective (although expensive) way of arresting corrosion [Ref. 2.32].

Prevention is easier (and less costly) than cure. If it is known in advance that the structure is to be located in an adverse environment, the designer should aim for structural durability at the design stage itself, by adopting suitable measures such as:

⇒ control crack widths in reinforced concrete by suitable design [refer Chapter 10], or by resorting to partial or full prestressing;

⇒ provide increased cover to reinforcement [refer Cl. 26.4.2 of the Code];

⇒ ensure low permeability by specifying optimum cement content, minimum water-cement ratio, proper compaction and curing;

⇒ specify the use of special corrosion-resistant steel or fusion bonded epoxy steel;

⇒ use of special cements;

⇒ use of cathodic protection.