Cathodic protection of structures with ASR

Alkali-silica reactivity (ASR) is a condition where certain silicateous aggregates are susceptible to the alkalinity in the concrete and react to form silica gel. Silica gel is used as a desiccant because it absorbs moisture. As it absorbs moisture it swells. ASR is characterized by map cracking , and weeping of the gel as a white efflorescence from the cracks (BRE Digest 330,1999). 

One way of controlling ASR at the mix design stage is to limit the maximum alkali content of the mix. This can be done by controlling the chemistry of the cement powder or by blending the cement. In these cases marginally susceptible aggregates can be used. 

It has been shown earlier that cathodic protection creates hydroxyl ions and will also attract positive ions such as sodium and potassium to the steel. This will increase the alkalinity around the reinforcing bar. In principle this could cause ASR or accelerate ASR in susceptible mixes. This has been demonstrated in the laboratory. However, there are no recorded cases of ASR being caused or accelerated by CP in field structures. In a review of field structures by SHRP, a structure with ASR showed no acceleration in the ASR in areas where CP was applied. The issue is also discussed in Mietz (1998). In the European standard on cathodic protection BSEN 12696(2000) Appendix A.5 states that cathodic protection applied in accordance with this standard has been demonstrated to have no influence on alkali silica reaction/alkali aggregate reaction (ASR/AAR).  

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