If a structure is suspected to suffer from alkali-silica reaction, the following actions may be necessary:
· Conduct a forensic evaluation to determine whether ASR has contributed to the observed distress and whether any other deterioration mechanisms are operating ± i.e. diagnose the cause(s) of distress.
· Evaluate the condition of the structure to confirm whether the structural integrity or serviceability have been negatively affected by ASR.
· Conduct a prognosis to determine the potential for further reaction and expansion.
· Evaluate strategies for repairing the concrete and preventing or retarding further reaction and expansion.
Guidelines on the evaluation and management of structures affected by ASR have been published by the Canadian Standards Association (2000) and CANMET (Fournier et al., 2004).
If it has been determined that ASR is a major contributor to the deterioration of the concrete and that there is significant potential for further reaction and expansion, few options may be considered to mitigate the reaction. However, the existence of other deterioration processes needs to be determined and, if present, should be dealt with by the repair strategy. If the condition of the structure has deteriorated to the extent that it is no longer structurally safe or it does not adequately perform its required function, these shortcomings have to be addressed. For example, if the concrete is severely cracked and embedded steel reinforcement has begun to corrode, the repair strategy should include methods for preventing further corrosion and crack sealing to protect the steel, in addition to methods for retarding reaction and expansion due to ASR. Methods for mitigating the effects of ASR can be divided into two categories: (i) dealing with the symptoms of distress and (ii) dealing with the cause of distress. Methods for mitigating the symptoms include filling cracks, cutting joints to allow further expansion to take place, thereby relieving internal stresses within the concrete or pressures on adjacent members or structures, and providing restraint to further expansion.
Caulking cracks with an epoxy grout (or similar compound) can help protect embedded reinforcement and reinstate the integrity of the cracked concrete. However, it will not retard the rate of reaction and expansion, and new cracks will inevitably form with time if the reaction is allowed to proceed. Cutting joints to allow for expansion to take place has been used in a number of hydraulic structures, with the principal aim in these cases being to relieve stresses on embedded mechanical equipment such as sluice gates or turbines. Joints can also be cut to isolate expanding structures from adjacent structures or to relieve internal stresses in pavements. Providing space for expansion does not deal with the reaction, and it is likely that the expansion and cracking will increase because of the reduction in restraint. Providing restraint in the form of rock anchors or post-tensioned tendons has also been used in hydraulic structures to prevent unwanted expansion and distortion of the structure. Fibre- reinforced plastics have been used to wrap elements such as columns. The only two practical means for addressing the cause of damage (i.e., to retard or prevent further reaction) are either to dry the concrete to eliminate the moisture required to sustain ASR or to change the nature of the reaction by introducing lithium compounds.
Silane sealers have been used successfully to reduce the relative humidity in ASR-affected concrete piers, railway sleepers (Oberholster, 1992) and median barriers (Berube et al., 2002). Silanes applied to concrete render the surface of the concrete hydrophobic and prevent the ingress of liquid water into the concrete. However, water vapor can still pass through the layer, reducing the moisture content and hence reducing the relative humidity with time. Figure 7.7 shows a photograph of treated and untreated sections of a barrier wall in Quebec.
Lithium compounds have been used to treat existing structures suffering from ASR, although the efficacy of such treatments has yet to be corroborated. A detailed review of lithium treatment for ASR control was recently published by the US Federal Highway Administration (Folliard et al., 2003). A number of pavements have been treated by applying a solution of lithium hydroxide or lithium nitrate topically to the pavement surface; testing has demonstrated that the penetration of lithium from a topical treatment is limited to the upper 25 to 50 mm of the pavement (Thomas and Stokes, 2004). However, it can be argued that the extent of deterioration is often more severe in the surface layer and that retarding the reaction in that area will still extend the service life of the pavement. Alkali concentrations can occur in the surface layer due to the migration and evaporation of water, and the application of deicing salts. Techniques have been developed to increase the rate and depth of lithium penetration; these include electrochemical migration and vacuum impregnation (Folliard et al., 2003; Thomas and Stokes, 2004). These techniques have been applied to a few structures in the field, but there is a need for information on their effectiveness in promoting lithium penetration and suppressing expansion due to ASR.
