Effects of silica fume fineness on mechanical properties of steel fiber reinforced lightweight concretes subjected to ambient and elevated temperatures exposure

This paper presents the effects of silica fume (SF) fineness and fiber aspect ratios of steel fiber on fresh and harden characteristics of high-strength lightweight concrete containing oil palm shell as coarse aggregates. The effect of elevated temperatures on the residual compressive strength of above concretes is also evaluated in this study. Three different SF fineness of 18400, 21000, and 28000m 2 /kg and 2 different aspect ratios of steel fiber of 40 and 80 are considered. Results show that the increase in SF fineness and steel fiber aspect ratio marginally affect the air-dry density of steel fiber reinforced lightweight high-strength concretes, however, the workability is reduced by about 9% to 14% due to increase in SF fineness. The compressive strength of steel fiber reinforced lightweight concretes at all age increases with increase in SF fineness and an improvement of about 37% is observed at 56days by increasing the SF fineness from 18400 to 28000m 2 /kg. Strong correlations are also observed between the strength improvement factor and the SF fineness. Water absorption of above concretes is also reduced by 3% to 14% due to increase of SF fineness from 18400 to 21000 and 28000m 2 /kg. The increase of SF fineness also significantly reduces the residual compressive strength loss at 300°C and 450°C. This loss of residual compressive strength is lower in lightweight concretes containing 16mm long steel fiber than 8 mm long steel fiber. The existing Eurocode model overestimates the residual compressive strength of steel fiber reinforced lightweight concretes containing no SF, however, this discrepancy is significantly reduced with increase in SF fineness

The influence of calcium nitrate as antifreeze admixture on the compressive strength of concrete exposed to low temperatures

Based on ACI 306R-10, the minimum temperature necessary for maintaining concrete hydration and strength gaining is 5 degrees C. If the weather becomes lower than 5 degrees C, some special measures should be taken in order to prevent decrease in the rate of hydration and to prevent fresh concrete from freezing. Most of the cold weather living countries spend annually plenty of money in order to facilitate concrete placing in the cold weather and to extend the construction season. It has been investigated that the behavior of fresh and hardened concrete contained calcium nitrate at different curing temperatures below freezing temperature of water and compare the results with the both control samples. For this reason, calcium nitrate is used at level of 6% by weight of cement dosage in mixes. After casting, one group of samples was cured in the different deepfreezes at -5 degrees C, -10 degrees C, -15 degrees C, and -20 degrees C for 7, 14 and 28 days, and then the same samples were cured in water at (23 +/- 1.7)degrees C for 7, 14, and 28 days. Calcium nitrate increased the compressive strength of concrete between 48-964, 50-721, 29-393 and 24-183%, for -5 degrees C, -10 degrees C, -15 degrees C and -20 degrees C, respectively, when compared to mixes without antifreeze admixtures. The results showed that it is possible to use calcium nitrate as an antifreeze admixture in concrete technology in cold weather concreting without additional precautions. (c) 2013 Elsevier B.V. All rights reserved