Study On The Correlation Of Surface Hardness And Strength Of High Performance Concrete And Its Mechanism

Concrete is the principal element in the infrastructure construction, and the strength controlling of concrete can ensure the safety of the whole structure of buildings. Rebound method is widely applied in concrete strength testing for its simplicity, conveniences,economical efficiency and non-damage to the structure. However, high performance concrete which is mixed with large content of admixtures and high fluidity in recent years, leading to the distinct errors when calculating concrete compressive strength using rebound strength curve. In this paper, the high performance concrete rebound strength curve is established based on the research on multiple factors. This research is of significant importance for concrete quality control, engineering accident prevention and accurate assessment of engineering quality. Effects of water to cement ratio, coarse aggregate size, mineral admixtures, specimen size, curing age, curing method, specimen surface humidity and carbonation depth on the surface hardness were studied.The rebound value, compressive strength and carbonation depth were used as a measurement index to established high performance concrete strength curve. Research indicates that with the decrease of water to cement ratio, the surface hardness of concrete grows slowly in the beginning, then increases rapidly, and finally reaches to a stable level; the surface hardness of concrete with 10% slag is the highest, and the surface hardness decreases with the increase of slag content; when the content of slag and fly ash are the same, the strength and surface hardness of concretes with slag are all higher than concretes with fly ash; the surface hardness concrete under standard curing is higher than that under natural curing; the surface hardness decreased with the increase of surface moisture; carbonation increases the surface hardness of concrete significantly and the hardness increases by more than 40%. The strength curve can satisfy the requirements of area the precision because the average relative error is 8.3% and relative standard deviation is 10.6%. Finally, this paper has applied XRD, SEM, TG-DSC, MIP, XCT technology to analyze the solid phase composition and pore structure in concrete. Results indicate that the hydration degree and hydration products increased while the porosity decreased with the decrease of the water to cement ratio and the increase of age. It is also found that with the surface carbonation of concrete, a large number of holes are refined by the calcium carbonate, thus the structure gets more compact.