Estimation of insitu strength of concrete

Abstract Twelve concretes of low, medium and high strength, with and without a fly ash and with and without a proprietary superplasticizer were designed. From each concrete 375 × 375 × 150 mm slabs and 100 × 200 mm cylinders were cast. The slabs were stored in out-door exposure conditions and 75 × 150 mm cores were extracted after 28, 91 days and 6 months duration. The moulded cylinders were cured in a fog room, control room maintained at 45% R.H. and placed alongside the slabs. The concrete strength is known to be highly variable and depends on a great many factors. Nonetheless, the results suggest that the insitu strength can be estimated by dividing the strength of the fog cured cylinders by a factor of 1.25. Similarly, for most concretes it can also be assumed that the core strength is equal to the strength of the field-cured cylinders. In spring/summer-like ambient conditions, the core strength is 80% of the strength of the standard cured cylinders instead of 85% required by some codes of practice.