Holding back the years17 July 2000
It is inevitable that concrete will deteriorate with age but past experience is helping to combat the ageing process. Robert Freer reports
The long term ageing of concrete is a matter of concern and importance to dam engineers, because it can affect the perform-ance and even the stability of a dam. Ageing is a process identified with a deterioration in the properties of the concrete — especially a loss of strength and an increase in permeability. But many of the causes and consequences of deterioration are predictable and some suitable precautions can be taken to minimise the potential damage.
Lessons learnt from the analysis of practical problems can be incorporated into subsequent designs in the expec-tation that in new dams, some ageing effects can be minimised and create fewer problems in the future.
Deterioration of concrete with age can be due to a number of causes. Many dams are built in locations where they experience harsh environmental conditions and extreme temperatures. The effect of a harsh environment, especially the effect of frost, can be exacerbated by the chemical action of water on concrete materials, and by poor design and maintenance or lack of surveillance to detect the early signs of deterioration. Frost penetration, alkali silica reaction, an unsuitable mix design, unsuitable methods of construction and overstressing of the structure can all contribute to the general weakening of the concrete with time.
A study undertaken by an icold committee on the ageing of dams which was published in 1994, found that the major causes of the degradation of concrete dams over time were:
•Chemical reaction of the concrete materials with the environment.
•Loss of strength and durability under permanent and repeated loading.
•Reduced resistance to cycles of freezing and thawing.
Other causes included shrinkage, creep and chemical reactions leading to contraction, the deterioration of structural joints, and the failure of drainage systems. Some of these problems identified by ICOLD can be solved by suitable designs and by proper construction techniques. Other problems can be minimised by appropriate maintenance.
The committee defined ageing as deterioration which is detected five or more years after operation began, and they point out that ageing will be a matter of increasing concern and will affect more dams as time goes by.
The effects of ageing can be influenced and controlled at the design stage. To minimise the risk of deterioration with age, the composition of the concrete is important and the required design, performance characteristics and placing techniques need to be well established.
The conventional structure of a concrete gravity dam consists of a core of low strength, high density concrete with a low cement content and a low heat of hydration to minimise thermal cracking. The facing concrete is designed to be durable with a low permeability.
Ideally, all the concrete should be dense, well compacted, of low water-cement ratio and designed with the correct mix proportions. Strong concrete is in general less permeable and therefore more resistant to chemical attack. A low water-cement ratio increases the resistance to frost attack and attack by seawater. Getting the right balance between strength, permeability, chemical resistance and workability at the construction stage will reduce ageing problems in the future.
For a gravity dam, a typical hearting concrete will have a cement content of 200-250kg/m3 (20 to 30% of which may be pfa) and large size aggregates up to 75mm or 150mm. Facing concrete will typically have a cement content of 300-350kg/m3, a water-cement ratio of 0.5 and characteristic strength of 20-25Mpa. Mixes containing large size aggregates need to be graded accurately so that the smaller stones, down to sand sizes, just overfill the voids between the larger stones. To ensure high resistance to all forms of deterioration, an excess of fine sand particles should be avoided and the concrete should be cured properly.
Effects of water
The chemical reaction of the concrete in contact with organic water, soft water or seawater is the main cause of deterioration which may become progressively worse with age. Both the ground water and reservoir water can be aggressive and this reaction is influenced by the ambient temperature.
Sulphates dissolved in ground water attack the concrete and chemically combine with it to give the concrete a whitish appearance. If there is a continuing flow of water to provide more sulphate, the concrete will deteriorate further by expanding, cracking and spalling. More serious deterioration causes the concrete to become friable.
The chemical effect of reservoir water on concrete depends on the degree of hardness, the amount of free carbon dioxide present and the pH value. Water with a low pH value, due to the presence of mineral acids, is potentially more aggressive than water containing organic acids with a similar pH value. Water with a high pH value and a low value of carbonate hardness may dissolve the lime in the concrete when there is free carbon dioxide present.
Alkali silica reaction
The effect of alkali silica reaction (ASR) has been reported and studied extensively during the last 20 or 30 years, and although the effects can be conspicuously damaging in a concrete structure, the occurrences are concentrated in certain local areas. ASR causes the whole mass of concrete to expand and this is evident by cracking in a close but random pattern. Continued deterioration without treatment may cause the structure to disintegrate.
ASR will occur if all three of the following factors are present:
•High moisture levels.
•A critical percentage of reactive silica.
If one of these factors is absent, then the reaction will not occur.
The alkali level of Portland cement is now specified within the range of 0.4-1.0% and the use of admixtures, especially ground blast furnace slag and pulverised fuel ash, appears to be beneficial. Present design criteria recommend that concrete should be designed with less than 0.6% reactive alkali and a total reactive alkali content of 3kg or less per cubic metre.
Concrete with an internal relative humidity of less than 75% is not likely to be affected by ASR, but this humidity will only occur in particularly dry and ventilated structures. In most dams, the relative humidity will be higher than this. Therefore dams, and indeed any water retaining concrete structures, are particularly vulnerable to ASR on account of the prevailing dampness.
The reactive silica is contributed to by the aggregate in the concrete. Aggregates from modern quarries are tested, but aggregates used in some of the older dams came from local quarries which are not normally used for concrete aggregates, and in such cases it is more difficult to identify the source of reactive silica if any problems arise.
Methods for repairing structures damaged by ASR are limited to trying to contain the expansion of the concrete. One example is by using post-tensioned cables.
Most dams in northern Europe and the Alps will be affected by frost, and if the consequences of frost action are not properly considered in the design stage and during construction, the concrete will be damaged and will deteriorate rapidly.
The damage is caused when the water in the pores of the concrete freezes and expands creating disruptive internal pressures which break down the concrete paste. The aggregates are not usually affected.
For freezing to occur, water must be present (dry concrete does not suffer damage when frozen) and freezing does not start until the temperature is about
-3°C or -4°C. Crystals of ice, once formed, continue to grow and attract water from surrounding concrete or ground surface. Ice particles often form in laminations and lead to surface spalling or even disintegration of the concrete.
The best way to prevent frost damage is to prevent water getting into the concrete initially and this can be achieved if the concrete has low permeability. Low permeability requires a low water-cement ratio and also a careful selection of material sizes to ensure that the necessary workability can be achieved with the low water content.
One method of reducing the potential damage to concrete by frost is by using air entrainment. An air entraining agent improves the workability of the fresh concrete and means that the water can be reduced, which makes the concrete less porous.
Air entrainment is intended to create bubbles or voids in the concrete which are discontinuous. Any ice which forms in these microscopic voids in the cement paste can only grow to a limited size, thus reducing the risk of frost damage to the concrete. In contrast, the voids caused by excessive water are more likely to be continuous, and this encourages the growth of larger crystals which cause more damage to the concrete, and are more likely to cause spalling of the surface. Where the concrete is initially of poor quality, there is little that can be done to protect it from frost damage, except to try and prevent water entering the concrete. For example, to prevent spalling on the downstream face of a dam it is important to try and seal the upstream face to reduce seepage through the structure.
Indicators of ageing
The signs of ageing in concrete dams can be detected by an intelligent observation of the dam condition. A summary of these indications are given in the 1996 CIRIA publication: Engineering guide to the safety of concrete and masonry dam structure in the UK, by MJ Kennard, CL Owens and RA Reader. The indicators’ causes and effects are listed in: Table 7.3 Surveillance indicators of possible defects. Perhaps the most practical advice for the dam designer is given in a report published by ICOLD in December 1983 entitled: Deterioration of Dams and Reservoirs. The report says degradation of exposed faces of concrete with age is considered inevitable, so dams should be designed and built with a suitable thickness and profile to allow for this ageing process.