Microstructural Features of Freeze-Thaw Deterioration of Concrete
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TANYA BAKHAREV AND LESLIE J. STRUBLE University of Illinois, Department of Civil Engineering, Urbana, IL. ABSTRACT Microstructural features associated with freeze - thaw deterioration have been studied using scanning electron microscopy and mathematical modelling. Concrete subjected to freezethaw cycles in the laboratory and concrete deteriorated in the field were examined in the SEM (BEI). The microcracks produced after a few freeze-thaw cycles are largely vertical and extend deep (several millimeters) into the specimen. With additional cycles horizontal cracks are produced, connecting the vertical cracks to produce spalling. This crack development probably results from localized expansive stresses produced by hydraulic pressure, as described by Powers, plus localized shear stresses produced by differential volume changes. INTRODUCTION The mechanisms of frost action on hardened cement paste are discussed in the classical works of Powers and Helmuth 1 - 2 .Concrete damage was initially attributed to the 9% volume increase when water crystallizes; if this crystallization pressure exceeds the tensile strength of the paste, cracking results. Hydraulic pressure was subsequently proposed to account for the observed migration of the water during freezing. When it was found that frost -resistant concrete contracts on freezing, the concepts of diffusion of gel water to capillary spaces and osmotic mechanisms were added3 . Investigations of freeze-thaw deterioration have focussed on macroscopic changes, without exploring the process on a microscale. Observation of microstructural characteristics of concrete and cement paste damaged in freezing and thawing cycles
using SEM can help to understand the mechanisms of the deterioration. The present study was undertaken to explore microstructural features of freeze - thaw deterioration of both concrete and neat cement paste. EXPERIMENTAL PROCEDURE Two concrete samples and several pastes were examined. One concrete was prepared in our laboratory with w/c 0.39 (w/c corrected for aggregate absorption), slump 33 mm, and air content 5.2% (measured on the fresh concrete). This concrete was cured at 100% RH and 23 °C for 28 days and had a 28-day compressive strength of 27.6 MPa. The other was a commercial prestressed concrete, similar in composition, slump, and air content, but cured for approximately 12 hours at 70' C prior to detensioning and demolding. This concrete had a specified strength of 48 MPa at 28 days and was 1 year old when subjected to freeze-thaw cycles. The pastes included neat paste, air-entrained paste, and pastes containing silica fume. Two w/c levels were used, 0.4 and 0.5. Three levels of silica fume were used, 0%, 3% and 6% (by mass of cement), and three levels of entrained air, 0%, 4%, and 8% (by volume of paste). All paste samples were cured at 100% RH and 23 *C for 28 days. Samples were exposed to repeated cycles of freezing and thawing using ASTM C-666, Procedure B (freezing in air and thawing in water). The sample size for freeze-thaw exposure was 80 mm x 80
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