Improvement of Compressive Strength of Concrete by a Two-phase Mixing Method

Abstract

The variation in the strength of concrete and allied products may be attributed to high variation in the properties of aggregate used, quality of cement, environmental conditions, and batching procedures (mixing method). It's been noted, that batching error accounts for the larger percentage of variability of concrete strength. Thus, this project, the advance preparation (premixing) of "Cement Paste" and "Grout" separately, which is then blended with aggregate to produce concrete otherwise called "Two-Phase Mixing Method", was investigated. The objectives of premixing is to eliminate undissolved cement particles, enhance hydration, coat the surface of all aggregate particles with the cement paste or grout, and to blend all the ingredients of concrete into a uniform mass. A total of 90 cubes of 15x15x15cm cube mould were cast, with 18 as control sample cubes for conventional concrete mixing and 36 cubes each for the Two-phase concrete premixing (Cement Paste and Grout). Concrete mixtures of three water-cement ratios (0.40, 0.45, and 0.50) were included in the experimental program. The quantity and quality of cement, aggregate and additive was kept constant with a mix proportion of 1:2:4. Each cube also contains 1.29kg of cement and 0.026kg of superplasticizer (representing 2% of cement weight) for all concrete mixture. Laboratory investigation also included two mixing speeds (25rpm and 32rpm) and Seven (7) to Twenty-eight (28) curing days. Results show that premixing of Cement Paste concrete has the highest compressive strength with 33.78N/mm², which represents 21.9% increase over that of the control sample with 27.71N/mm² for 0.40 water/cement ratio at 28days. While Grout premixing method only managed a 1.05% increase with 28.00N/mm². Also, 32rpm mixing speed recorded about 16% to 19% increase in compressive strength over 25rmp mixing speed.