The ideal situation would be to be able to determine when loss of durability would occur such that acceptable limits were breached, i.e. durability would become a true limit state phenomenon. In practice, the requirements for detailing such as cover are set to ensure that the loss of durability should not occur within the service life of the structure, provided there is good quality assurance on the concrete and on site procedures to ensure that specified cover is met (Clark et al., 1997). The major concerns with loss of durability are corrosion of the reinforcement, alkali-aggregate reaction and sulfate attack. The Concrete Society have issued a Technical Report (Bamforth, 2004) which provides an excellent background to measures which may be taken to enhance durability.
5.1.1 Reinforcement Corrosion
At its extreme, corrosion of the reinforcement will lead to spalling of the concrete cover with the resultant exposure of such reinforcement, together with loss in strength of the structural member as all or part of the cross-sectional area of the rebar is lost. In a less extreme form, unsightly surface rust staining will occur. When the first signs of spalling are observed it will often be too late to combat corrosion and recourse will be needed to expensive remedial repairs or mitigation. In the ultimate case, expensive and often very difficult replacement of the structural member will be necessary. Spalling occurs as the chemical products of the corrosion reaction occupy a larger volume than the original rebar.
Corrosion occurs when the pH value of the concrete reduces from its original high value of around 13 during placing to below 9 at a later stage (Page and Treadaway, 1982). This change in pH is known as a loss in passivity and is due to two possible causes:
(a) Carbonation, and (b) Chloride attack.
5.1.1.1 Carbonation
This is a gradually occurring attack caused by the penetration of acidic gasses (mostly carbon dioxide) and reaction with any free alkali present. This reaction lowers the pH value. Since the mechanism is diffusion controlled, the depth of carbonation is proportional to the square root of time. The covers to reinforcement specified in EN 1992-1-1 are such that for good quality concrete the depth of carbonation during the design service life of the structure should not depassify the concrete between the concrete surface and any rebar. The process of carbonation is illustrated in Fig. 5.1.
The minimum cover cmin,durrequired by EN 1992-1-1 together with the environmental conditions and structure classification applicable to each are summarized in Table 5.1.
The standard structure classification is Class 4 (design life of 50 years). However, the structure classification is adjusted for parameters such as increased design life, higher strength concretes than those noted in Table 5.1, slab type elements, and special quality control. Increasing the design life to 100 years increases the structure classification to two levels higher and the remaining factors reduce the structure classification by one level lower. The actual cover required cminis given by
cmin ¼max½cmin ,b; cmin ,durþcdur,cdur,stcdur,add:10mm ð5:1Þ
where cmin,bis the minimum cover due to bond requirement, cdur, additive safety element (recommended value of zero), cdur,st reduction in cover due to use of stainless steel (recommended value of zero), and cdur,add reduction due to use of 66 Chapter 5 / Durability, Serviceability and Fire
additional protection (recommended value is zero). The minimum cover due to bond cmin,bfor reinforced concrete is bar diameter for separated bars and equivalent diameter given by n¼ / ffiffiffi
pn
b ( 55 mm) where n is the number of bars (from Table 4.2 of EN 1992-1-1). For prestressed concrete, it is the duct diameter for circular post tensioning ducts, twice the diameter of pretensioning strand or 3 times the diameter of indented wire.
The nominal cover cnom used in design is given by
cnom¼cminþcdev ð5:2Þ
The recommended value for cdev is 10 mm, although the code does allow reductions where there is a quality control system (more likely in precast concrete) or where a very accurate device is used for monitoring and any resulting non-conforming items rejected.
The covers specified in Table 5.1 are only adequate when the concrete has the requisite low coefficient of permeability. This can only be ensured if the concrete has a sufficiently high workability to enable the concrete to be properly placed and vibrated. Workability should be controlled through the use of admixtures such as plasticisers or super-plasticisers and NOT by the addition of extra water which may give problems with shrinkage cracking (also causing potential loss in durability
Exposed surface Carbonation zone Aggregate
Reinforcement
(b) Corrosion in carbonated concrete
Carbonation zone Rust
patches
(a) Partially carbonated concrete
FIGURE5.1 Corrosion due to carbonation
unless designed for). It is therefore essential when detailing the reinforcement, especially at laps including those between starter bars and main bars to ensure the concrete can flow. Construction joints which are often at points of high internal forces are also areas where the concrete quality needs considering as honeycombing will give a high coefficient of diffusion and thus allow easy ingress of carbon dioxide or chloride-bearing fluids.
Problems with construction joints can also be eased by kickerless construction methods and the use of retarding agents to delay the initial set of the concrete.
A further influence on achieving acceptable levels of permeability is adequate curing of the concrete. Concrete which is not properly cured is a potential source for concern. The economics of modern construction has forced reductions in times before the formwork is struck. EN 206-1 provides some data on minimum curing times related to ambient conditions, concrete temperature during curing and rate of strength development. The earlier the formwork is struck, the more the surface-free water will evaporate rather than combine chemically with the cement. It is this surface layer which is important to ensure the concrete member is relatively impermeable.
The reduction of striking times has been made possible by grinding Ordinary Portland Cements (OPC) very fine, thereby increasing the specific surface area and increasing hydration rates, and turning OPC’s into effectively Rapid Hardening Cements (RHC).
This increase in hydration is also made possible by increasing the proportion of C3A in the cement. Drawbacks to this trend are the resultant increase in heat of hydration and early thermal or shrinkage cracks and that at normal ages the concrete strength becomes potentially high enough to cause spalling problems in fire (Bailey, 2002).
Beeby (1978) has indicated that cracking parallel to the rebar is more harmful in corrosive conditions than that of the normal to the bar, as such cracking allows an TABLE5.1 Minimum Covers cdur,min(mm) for reinforced concrete.
Structure class X0 Unreinforced; reinforced in buildings 10 10 10 10 15 20 C30/37 XC1 Dry; permanently wet low humidity 10 10 10 15 20 25 C30/37 XC2/XC3 Wet, rarely dry; foundations;
external sheltered concrete
10 15 20 25 30 35 C35/45 XC4 Cyclic wetting and drying 15 20 25 30 35 40 C40/50
XD1 Airborne chlorides 20 25 30 35 40 45 C40/50
XS1 Airborne salt near to coast 20 25 30 35 40 45 C40/50 XD2 Swimming pools; industrial chlorides 25 30 35 40 45 50 C40/50 XS2 Permanently submerged in sea 25 30 35 40 45 50 C45/55 XD3 Cyclic due to chloride spray 30 35 40 45 50 55 C45/55 XS3 Marine tidal and splash zones 30 35 40 45 50 55 C45/55 68 Chapter 5 / Durability, Serviceability and Fire
easier path to the surface of the bar. Such parallel cracks are generally due to shrinkage or early thermal movement and can be reduced by the provision of adequate distribution reinforcement. Such reinforcement is more effective when it is of small diameter at relatively low spacing, and may need to exceed the minimum laid down in any design code.
5.1.1.2 Chloride Attack
Some free chloride ions exist in the hydrating cement but in low proportions. The once historically popular use of admixtures containing CaCl2 is now expressly forbidden, but contaminants containing chlorides can still be introduced when sea-dredged aggregates or sea water is used in the production of concrete. Both these should be avoided wherever possible. However, two possible sources of chloride ingress cannot be avoided. These are where the structure is exposed to a marine environment or to de-icing salts on highways. The major problem with both these is chloride ingress through cracks parallel to rebar (Fig. 5.2), and it is thus necessary to control the width of such cracks by explicit calculations.
5.1.2 Alkali-Aggregate Reaction
In this case a chemical reaction occurs between the alkalinity of the pore water (usually Na2O or K2O), due either to alkaline elements within the cement or an external source such as sea water or de-icing salts, and certain types of silica in the aggregate (this is known as Alkali–Silica Reaction, or ASR). The reaction causes a positive volume change with associated microcracking, which will eventually result in loss of cover to the rebar. Most, but not all, British aggregates are not susceptible to this type of reaction. Unfortunately, there is no simple test to determine any such susceptibility, and thus care needs taking to limit the alkali content of the mix, the effective alkali content of the cement itself and where the structural element is exposed to an alkaline environment, consideration should be given to provision of an impermeable surface membrane. The alkali content of a cement can be reduced by using a cement replacement such as Pulverized Fuel Ash (PFA) or Ground Granulated Blast Furnace Slag (GGBFS), both of which are industrial waste Exposed surface Crack
Carbonation zone
Reinforcing bar
Rust scale FIGURE 5.2 Corrosion in cracked concrete
products. Both these replacements slow down the rate of strength gain in the concrete and additionally have a lower heat of hydration, thus reducing early thermal shrinkage and cracking. Recent guidance on ASR has been published by BRE in Digest 330 (BRE, 2004).
5.1.3 Sulfate Attack
This occurs due to attack from free sulfates contained in groundwater or soil, and is therefore generally limited either to buried structures including foundations or structures in an aggressive environment where sulfates are naturally present, e.g.
certain types of sewage, industrial effluent or chemical process plants. It is generally sufficient to combat sulfate attack by using Sulfate-Resisting Portland Cements (SRPC). In these cases, the requirement to ensure the existence of low permeability concrete in the structure is paramount as any ingress of sulfates should be prevented (Dunster and Crammond, 2003).
5.1.4 Taumasite Attack
This phenomenon is caused by a chemical reaction between cement and certain types of clay in the presence of groundwater. It will therefore affect foundations or buried concrete. Fortunately the types of clay involved only occur in limited areas and thus the phenomenon is not widespread (Taumasite Experts Group, 1999). A useful overview of concrete deterioration, methods of overcoming it and specialist techniques for concrete below ground is given in a CIRIA Report (Henderson et al., 2002).