Assessment of the effect of the sample thawing method on the frost resistance of concrete according to the accelerated third method of the State Standard 10060-2012 with Amendment No. 1

Introduction. In connection with the introduction of the Amendment No. 1 to State Standard 10060-2012 "Concretes. Methods for determining frost resistance" the Institute has received an increased number of requests related to the use of various climatic automatic chambers to perform tests to determine the frost resistance of concrete. An analysis of the design solutions and the principle of operation of climate chambers presented on the Russian market has shown that in most chambers the thawing process occurs either with warm air blowing, or is implemented by incomplete immersion of sample containers in a solution of 5 % sodium chloride, i. e. it is a deviation from the regulatory requirements. It should be noted that the unit of measurement for frost resistance (i. e., the freeze-thaw cycle) is a conditional value and the use of such a unit of measurement requires testing under strictly regulated conditions. Various approaches to changing the methods of concrete testing for frost resistance, including the method of samples thawing after freezing, can lead to an unreliable assessment of the quality of concrete, that in real conditions will lead to a decrease of the safety of construction sites.

Aim. Assessment of the effect of various methods of thawing samples during the cycle of freezing and thawing according to the accelerated third method of the State Standard 10060-2012 with Amendment No. 1 on the frost resistance of concrete.

Materials and methods. The results of determining of the frost resistance of concrete according to the accelerated cooling regime of State Standard 10060-2012 with Amendment No. 1 are presented with various methods of samples thawing – under conditions of complete or incomplete immersion of sample containers in a thawing medium – a solution of 5 % sodium chloride and under conditions of blowing sample containers with warm air.

Results. Based on the results of experimental studies, the coefficients of transition (K) from the grade of concrete in terms of frost resistance, determined by thawing under conditions of incomplete immersion of sample containers in a solution of 5 % sodium chloride and blowing warm air into sample containers, to the grade of concrete, determined by the standard method of samples thawing in a bath with a solution of 5 % sodium chloride. The following average K coefficients were obtained for concretes of various grades in terms of frost resistance, respectively, for F₁200–F₁300/F₁400–F₁500: K = 0.7/0.6 when samples are thawed when sample containers are not fully immersed in a solution of 5 % sodium chloride; K = 0.8/0.9 with samples thawing under conditions of blowing samples with warm air.

Conclusions. The determination of the frost resistance of concrete by the accelerated third method with thawing after freezing sample containers in a bath with a solution of 5 % sodium chloride is optimal in terms of the most negative effect on the frost resistance of concrete. At the same time, the method of samples thawing with incomplete immersion in a solution of 5 % sodium chloride turned out to be the most "harsh". The regime of thawing of samples by blowing warm air into sample containers showed a significant variation in the determined parameters – compressive strength and mass. The coefficient of variation of the transition coefficients for this method of thawing significantly exceeds the standard value of 9 %.

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