Investigating Probable Causes of Cracks & Defects in Concrete

a. Summary
Concrete placed about four years ago at a Subdivision in Houston has sustained extensive cracks and some surface scaling. Stakeholders are interested in determining the cause of these defects. From the petrographic evaluation the principal cause of the cracks is due to the coarse aggregate used.

Petrographic Analysis

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Aggregate chipping of in the course of mixing leaving voids in hardened concrete.

b. Discussion of Results
The concrete examined appear to have sufficient ingredients in the right proportions, namely, the coarse and fine aggregates, the cement and fly ash content. Variable water content was noted in the four samples examined.
The unusual cracks, as shown by two photos taken from the field and shown in the Appendix, is due primarily to the coarse aggregate used. The coarse aggregate is gravel; it is hard rock as seen in the preparation of the sample for the thin section. But the aggregate is weathered with inherent cracks in them and thus have a propensity to chip off sections of the aggregate during the mixing, particularly during the latter stages of the mixing when the paste has thickened to the point where it cannot flow to fill the voids created. The aggregates used do not conform to standards for optimum engineering performance.