Concrete strength reduction due to over compaction

The purpose of this study was to evaluate the effect of over-compaction on the strength of concrete with regards to commercial and industry practices. Normal concrete, which is commonly used in construction was chosen. Samples were cast using a varying vibration time and testing was performed to determine concrete compressive strength, air content, density and a measure for segregation. These parameters were used to determine the relative sensitivity of concrete to under- versus over-compaction. If segregation was present, it was hypothesized the results would indicate a mathematical relationship between the segregation and strength reduction. This relationship would allow for determination of an optimum vibration time range. It was found in this study there was no segregation or strength reduction in vibrated samples. This result proved the concrete was far more sensitive to under-compaction, with overcompaction not being a practical concern. It was also determined that concrete compressive strength and vibration time had a basically logarithmic relationship and 30 s of continuous internal vibration provided the most efficient result of approximately 92% of maximum strength. The study recommends adjustment of standard concrete vibration practice to accommodate the more efficient 30 s duration. Alternatively, a 1.4x 'vibration factor' can be applied during design of compressive strength of concrete to accommodate the current 5-15 s vibration time window.