A comparison between cube strengths and in situ concrete strength development

摘要

This paper describes field trials carried out in Hong Kong which aimed to examine the relationship between cube and core strengths of in situ concrete. The variability of core strengths within structural elements and strength variations of such elements with age were also examined. Controlled experiments on full-scale trial walls indicated very good compaction (99.2% relative density) and good relative strengths (97% mean relative strength; 86% characteristic relative strength) for the cores relative to cube strengths both at 28 days. The trial wall panels were generally uniform in strength from top to bottom and side to side, with low within-panel coefficients of variation. In situ trials, which were representative of normal site practice, still achieved very good compaction (98.9% relative density). For wall elements, the relative strengths were very reasonable (88% mean relative strength; 76% characteristic relative strength). The slab elements produced less good results (78% mean relative strength; 63% characteristic relative strength). The reasons for these results are discussed in the paper. The effect of age on the in situ strength of the concrete was difficult to detect. There was a trend of increasing strength with age but the increase was generally small, producing an increase of about 8% at 2 years when compared with the strength at 28 days.