Effect Of Inorganic Salts On The Early Age Shrinkange Of Concrete

In order to ensure the normal construction of concrete in winter at cold regions, workers often add salts to concrete as early strength agent and antifreeze, the actual construction works find that the addition of inorganic salts make shrinkage cracking serious. This article selected the following four commonly used inorganic salts: Na₂SO₄, Ca(NO₂)₂, NaNO₂, Ca(NO₃)₂, used the traditional dial gauge testing method, and studied the effect of single and complex inorganic salts on the early-aged shrinkage of the concrete, including 3 day's autogenous shrinkage and 56-aged drying shrinkage, so as to guide the application of inorganic salts in practical engineering.The article set the time when shrinkage rate arrived its maximum as"zero time"of autogenous shrinkage. Through this research, the test found the following conclusions:The hardening time and zero time of concrete with salts advanced. When the w/c is 0.3, autogenous shrinkage of samples with inorganic salts generally increased. For samples with single salt, the autogenous shrinkage increased with more salt in a range. And more over this range will make the autogenous shrinkage reduce a little. The research of samples with salts suggested the effect of Na₂SO₄ was the same as it was in samples with single salt. But the autogenous shrinkage of samples with NaNO₂ decreased. The adding of Ca(NO₂)₂ made autogenous shrinkage decreased in a range, but samples with more shank more. When Ca(NO₃)₂ was added with NaNO₂ and Na₂SO₄, the autogenous shrinkage increased with the dosage. When it was added with Na₂SO₄ and Ca(NO₂)₂, the autogenous shrinkage of concrete samples having Ca(NO₃)₂ increased, but samples with more Ca(NO₃)₂ shrank less. When w/c was 0.5, samples without Na₂SO₄ had larger autogenous shrinkage, other compenents expanded at the early age.When the w/c was 0.3, the adding of single salt would make drying shrinkage larger. A restricted dosage increased drying shrinkage, but more dosage would reduce shrinkage a little.The drying shrinkage of samples with salts decreased a little. The adding of Na₂SO₄ had the greatest impact on the shrinkage rate, NaNO₂ and Ca(NO₂)₂ nexted, and Ca(NO₃)₂ was the smallest role. For samples with w/c of 0.5, the shrinkage didn't increase obviously. For samples being added with salts, the effect of different salt on the drying shrinkage was related to the age, and also they were all small.This paper set the time when shrinkage rate arrived its maximum as"zero time"of autogenous shrinkage. In order to evaluate the reasonableness of this method, this paper selected groups of samples for mercury test, whose water-cement ratio is 0.3, and got the pore-size distribution of 3 day-aged samples. The study of autogenous shrinkage and micro-pore structure showed that greater autogenous shrinkage corresponded to more pores with diameter between zero and 50nm. So it is reasonable to set the peak time of shrinkage rate as zero time of autogenous shrinkage.