Introduction
Reinforced concrete structures are currently the most widely used structural form. However, a large number of concrete structures deteriorate with premature failure under the action of environmental factors. Numerous studies and engineering investigations have reported that cracking and expansion of microcracks in concrete structures under various conditions provide a channel for the diffusion of erosive media, which is the most important factor leading to the deterioration of concrete and thus reducing the durability of the structure. Shrinkage of concrete is an important causative factor for its cracking, and the rational use of mineral admixtures can effectively reduce the shrinkage, which is an important way to improve the crack resistance of concrete. Moreover, concrete is subjected to continuous tensile stress caused by contraction and temperature difference during the setting process. It has been proved that improving the early tensile creep of concrete can relax the tensile stress, and thus effectively inhibit the cracking of concrete. In this study, a design method of low shrinkage high creep cementitious materials was realized by adjusting the mineral admixture amount, mineral admixture fineness and water-cement ratio. The enhancement of cracking resistance was finnally obtained as expected.
Methods
P·II 42.5 cement, Class II fly ash, S95 slag, ultrafine fly ash and S115 slag were selected for this test. The mineral admixtures were used in a double mixing mode combining coarse and fine particles, i.e., double mixing of S95 slag with ultrafine fly ash, and double mixing of S115 slag with Class II fly ash, with the range of mineral admixtures to the total cementitious materials from 50% to 80%. The substitution rate of each component changed with a increment of 10%.Three series of tests were included in this work: a full-age shrinkage test, an ultra-early tensile creep test, and a cracking test. The full-age shrinkage test apparatus was used to test the autogenous shrinkage from casting to 3 d and the drying shrinkage from 3 d to 28 d of cementitious materials. The ultra-early tensile device was used to test the tensile strain of cementitious materials from initial to final setting. This device used a polyurethane mold to hold the specimen and transfer the tensile load. The creep displacement was monitored by a laser displacement sensor. A low-elasticity circular restrained cracking mold was used to monitor the restraining stresses due to autogenous shrinkage from casting to demolding, as well as the restraining stresses caused by drying shrinkage after demolding. The mold was made of polyvinyl chloride resin plastic material. A pair of vertically placed strain gauges were pasted on the inner surface of the inner ring to detect the stresses.
Results and discussion
Shrinkage test results show that at constant total replacement rate of mineral admixtures, the mixing group of ultrafine fly ash and S95 slag is smaller than the mixing group of Class II fly ash and S115 slag. The reason is that the activity of the slag is higher than that of the fly ash. Using of smaller fineness slag and larger fineness fly ash in combination is beneficial to decrease the difference in the reaction rate of these two materials, leading to a reduction in the porosity. When the total replacement rate of mineral admixture is 60%, the effect of improving the particle grade matching to reduce the shrinkage of the cement stone reaches maximum. In addition, the shrinkage of the cement stone increased with increasing water-cement ratio for all different mineral admixture substitution rates. Ultra-early tensile creep test results show that under relatively high total substitution rate (60%–80%), the ultra-early creep of cementite with different particle gradation shows obvious differences. Compared with the mixing group of Class II fly ash and S115 slag, the mixing group of ultra-fine fly ash and S95 slag exhibits larger ultra-early creep and later cracking. Contrary to the shrinkage law, under relatively high total substitution rate (60%–80%), increasing the substitution rate of ultrafine fly ash while simultaneously decreasing the substitution rate of S95 slag, the ultra-early creep is shown to increase first and then decrease. Increasing the water-cement ratio slightly increases the ultra-early specific creep and delays the cracking time. The use of low shrinkage high creep cement stone can effectively reduce the internal stress growth rate, resulting in shrinkage stress relax and cracking time delay.
Conclusions
Experimental studies are carried out on the early age shrinkage, creep and cracking properties of cementitious materials. The following conclusions are drawn. The cement stone using S95 slag with coarse particles and ultrafine fly ash with fine particles in combination exhibits characteristics of low shrinkage and high creep. When the total replacement rate of mineral admixture is in the range of 60%–80%, increasing the admixture of ultrafine fly ash while simultaneously decreasing the admixture of S95 slag, the shrinkage of specimens is observed to decrease first and then increase. While the ultra-early creep is characterized by increasing first and then decreasing. The use of low shrinkage high creep cement stone can effectively reduce the growth rate of the internal stress and delay the cracking time.