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ASR and Granite

posted Sep 6, 2021, 7:50 AM by jeffery jim
This morning, a query came in asking about the relationship of granite and rhyolite and concrete or cement in alkali-silica reaction or ASR.

Well, ASR is an outcome when cement or the binding matrices mixed with aggregates which supposedly form compound called concrete, produces another byproduct which is the substantly high amount of silica gel.
The production of microcrystalline as well as alite and belite which significantly affect the mass or volume during concrete curing will produce pores or worst voids. Silica gel loss it masses over time and concrete is filled with void which is normal. ASR on the other hand produces gel which expand between the aggregate and binding matrices which result in expansion and cracks.

ASR reduces strength and affect physical properties moduli. When condition aggravated, the concrete structure will succumb grazzing cracks and reduces durability when it is impregnated after loosing cover and passivity after it is plagued by delayed ettringite.

Normally, cement is less likely the culprit when it is certified to comply required standards. I don't trust standards and test! I did petrographical examination and found cement to be culprit as well and did not conform BS EN 197 and MS 522 where minor constituents is more than 5%.

Granitic rocks such as granite and rhyolite usually caused ASR. It is not due to its geological property per se but very much depending on their chemical maturity which most geologists could understand from element tracing.

Should we avoid granite? How can we avoid the use of granite when JKR specification require the use of granite for medium grade concrete (G40) and substructure?

The reason for this requirement in the fatally obsolete JKR specification is in order to produce high grade concrete by capitalizing on compressive strength of granite with low strength binder which concrete rated at 32.5N. Nowadays, we have cement rated at 52.5R and obviously we don't need granite as part of concrete components. It is acceptable for sandstone or mudstone, as long as it is matured enough with high compressive strength.

To avoid conflict with JKR, I suggest the use of admixture, mainly plasticizer. This will reduce the use of water and cement/water ratio and yet achieved the same level of workability at 0.42 w/c ratio as 0.5. It is a bit tricky when it comes to tremie application. One have to do multiple trial mixes just to have a perfect one. My site have more than 50 trial mixes for QAQC and in refining the mix for application.

So, I hope this helps you to understand ASR and the use of granite in construction

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