Variation of in-place concrete strength in structures

The variation of in-place strength in a structure is due to within-batch va riation, batch-to-batch variation, systematic within-member strength variat ion, and systematic between-member strength variation. Batch-to-batch varia tion is particularly significant for cast-in-place structures and may eithe r inflate the within-member variation if each member is cast from many batc hes or inflate the between-member variation if each member is cast from a s ingle batch. Values of coefficients of variation that represent the overall variation of the in-place concrete strength in a structure vary from 7 per cent for one member cast from one batch of concrete to 13 percent for a str ucture consisting of many members cast from many batches of cast-in-place c oncrete. Multiple regression analysts techniques are used to assess the systematic v ariation of the strength of concretes in laboratory specimens cast from one batch of concrete. Statistically significant systematic strength variation is detected over the height of 32 of 43 columns with average strengths fro m 2200 to 5200 psi. Typically, the top region was 3 to 14 percent weaker th an the region in the middle, and the bottom region was 3 to 9 percent stron ger than the region in the middle. Significant systematic variation of the in-place strength is also detected in 20 of 26 beams, blocks, slabs, and wa lls with average strengths from 2200 to 17,000 psi. Investigation of ultras onic pulse velocity and pull-off test data from building columns and bridge girders corroborates the findings of the investigation of elements cast in the laboratory.