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Concrete Petrography Test

posted Jul 4, 2018, 7:04 AM by jeffery jim

In one of the recent structural repair projects which i am assisting enforced the concrete petrography test. At first, i believe this test have very limited prospect in telling the story about in-situ concrete since the strength of concrete take precedence in determining its limit state rather than to review its chemical composition.

From my geological and metamorphic database as well as rheology of concrete understanding, i can quickly identify sources for Alkali-Aggregate Reaction (AAR) and Akali-Silica Reaction (ASR) within the state. True enough, the factual finding report highlighted what i have expected from chemical reaction under microscopic analysis. No ASR and no AAR reaction found within the samples which need specialized interpretation from a lab in Singapore.

The deterioration of reinforced concrete members are divided into two groups; the concrete itself and the reinforcement bars within. Both types of delamination lead to the reduction of strength and robustness of a concrete structure. For a period of time, the assumption of the coring of in-situ samples suffice the understanding of its strength apart from the tell-tale signs of its physical degradation. Is it so?

No, there are several other issues in regards to concrete health which are not tackled or highlighted on core samples. Phenolphthalein used for carbonation depth test only tell half of the actual story about the robustness of the concrete. This test only indicate the depth of carbonation penetration but it was never design to explore and making empirical comparison. This never explain the result of pore and capillary sizes and the relationship of water ingress as well as the instantaneous Calcium-Silicate-Hydrate (C-S-H) gel reaction during the initial stage of curing of concrete/cement paste and morphs through period of time.

The finding through petrography test put the related issues as precedents in remedial or rehabilitation works consideration when determining the best principal and method based on BS EN 1504. In this case, the actual issue of concrete delamination is not due to the material and workmanship but the poor comprehension on concrete design mix.

I summarized the finding as the case of high free cement/water ratio which allows aggressive substances to transgress and deteriorate the concrete. As for the best remedial, i reckon methods by instating passivity of the reinforcement bars and moisture control of the concrete.

Therefore, it is a good practice to allow for the provision of petrography test to be conducted as it is vital in providing the full story of the concrete (chemical) deterioration. Often, most tests conducted only guided by concrete strength which substantiate the ultimate limit state instead of furnishing understanding on the superstructure's serviceability limit state. The rest of the test are concentrated on the steel reinforcement bars' state of corrosion.

All this while, we have placed too much emphasis on the tension components and missed out on the importance of compression components of reinforced concrete structural members.