A number of factors can affect compressive strength test results. Some of the more common include errors in proportioning, retempering, aggregate quality, construction practices such as consolidation, and the testing itself. At DRP we rely on standardized test procedures to help investigate low strength test results and have the experience to bring rational, clear explanations for their causes.
Sample preparation matters. Photograph of the polished surface of a cylinder showing irregular distribution of coarse aggregate.
Reflected light photomicrograph of low w/cm mortar coatings (red arrows) on an aggregate particle. Such coatings are typical of late water additions, or retempering, which can diminish strength.
Excessive air is another source of low compressive strengths. This concrete contained 16% air, was well below the specified strength at 28 days and had to be removed and replaced.
Cross-polarized transmitted light photomicrograph showing coarse portlandite crystals (red boxes) in low-strength concrete which had a high w/c.
Photograph of polished surface from a core that shows severe gap grading.
Photograph of polished surface of a core showing consolidation void (red arrows) near the top of a pavement core.