DEVELOPMENT OF A LABORATORY TECHNIQUE TO QUANTIFY CONCRETE CURING QUALITY

EPHRAIM SENBETTA, Purdue University

Abstract

Proper curing of newly placed concrete is essential if desirable qualities and expected durability of the hardened concrete are to be realized. Curing is particularly important for concrete structures such as pavements that have a large surface area. Review of the literature indicates that there is no satisfactory method by which the effectiveness of a certain curing method or material and the resulting quality of the concrete can be evaluated during or after the curing period. The indirect test methods used to elvaluate the effectiveness of liquid membrane forming and sheet-like curing aids have been a subject of controversy owing to their inconsistency and lack of reproducibility. This investigation was concerned with devising a direct, reliable, and sensitive test method by which the effectiveness of the curing of concrete can be evaluated. Being able to distinguish between well and poorly cured concrete by examining physical characteristics exhibited by the concrete in question is essential to control the quality of the curing process during construction and to identify curing related problems in concrete that is experiencing difficulty. In this study mortar was used rather than concrete in order to maximize the amount of cement paste and to reduce the effect of the size of the aggregates on the relatively small test specimens. The approach used consisted of making mortar slabs and subjecting them to widely different but controlled curing conditions that were thought to produce samples with different degrees of curing. The next step was determining appropriate test methods that are sensitive to small changes in the properties of the paste, as affected by the curing. The selected test methods were used on the samples, and a procedure was developed by which quantitative distinctions among the samples could be made. The test methods selected were: The absorptivity test, which is indicative of the pore structure of the paste, non-evaporable water determination, which is an indicator of the extent of hydration of the cement, and the abrasion test (ASTM C418), which is a measure of the strength development of the surface region. The absorptivity test and the abrasion test were found to be sensitive and reproducible indicators of the quality of the mortar samples. On the other hand, the non-evaporable water test did not produce the expected results. The suggested method for distinguishing between adequate and inadequate curing involves changes in the absorptivity of the mortar in going from the surface to deep into the slab. The test is quick and easy and reflects changes in the pore structure of the paste dependent on the severity of the exposure conditions and the extent of curing. Characteristic curves that show changes in the absorptivity of the mortar samples with depth were produced. The curves indicated significantly large changes in absorptivity in the case of poorly cured samples, and for the well cured samples the changes were negligible The dividing line between adequate and inadequate curing was based on differences between the absorptivity values at depths of 1 and 6 cm. It is suggested that a difference of (LESSTHEQ) 3.7 x 10('-6) cm('2)/sec is indicative of adequate curing, and a difference of (GREATERTHEQ) 5.5 x 10('-6) cm('2)/sec corresponds to poor curing. The abrasion test results were in agreement with those of the absorptivity test, and there was good correlation between the two test results. (')

Degree

Ph.D.

Subject Area

Civil engineering

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