Influence of microcracks on the transport properties of concrete

• Main Author: Wu, Zhigen
• Other Authors: Buenfeld, Nick ; Wong, Hong
• Published: Imperial College London 2014
• Subjects: 624.1
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.684241

This research aims to understand the influence of microcracks on the transport properties of concrete. Concretes, mortars and pastes (w/c: 0.35 and 0.50; curing ages: 3, 28 and 90 days) were conditioned to equilibrium by drying at 105°C, 50°C (7%RH), 21°C (33%RH) or by stepwise drying at 21°C (93%RH -> 3%RH) to produce varying degrees of microcracking prior to transport tests. The characteristics of microcracks were measured using fluorescence microscopy and image analysis. The data were used to understand factors influencing microcracks and to quantify the influence of microcracks on the transport properties. All samples exhibited drying-induced microcracks perpendicular to the exposed surface. Most of the microcracks had widths < 10 μm and lengths < 100 μm. Increasing the severity of drying caused a significant increase in the degree of microcracking, O2 diffusivity, O2 permeability and sorptivity. Furthermore, a significant size effect was observed. Samples with smaller ratio of sample thickness to maximum size of aggregate had more severe microcracking and consequently higher O2 permeability. Image analysis showed that microcracks undergo closure when the sample is confined. This significantly decreased O2 permeability and the effect was greater for samples with a higher degree of microcracking. Image analysis showed that the initial water absorption is dominated by rapid absorption into microcracks. Subsequent uptake then occurs through non-cracked regions at a much lower rate. This produced non-linear behaviour in the water absorption-time½ plot that deviates from classical unsaturated flow theory. However, the study found that O2 diffusivity and electrical conductivity were relatively insensitive to microcracking. Micro-cracked samples were reconditioned at gradually increasing humidity to study the influence of rewetting on transport properties and to isolate the effect of microcracking from water content. Moisture hysteresis was observed when the samples were rewetted. Results showed that, following rewetting, the effect of microcracks on transport properties is insignificant when the comparison is made between samples of similar degree of saturation. This may be due to condensation and self-healing of microcracks that occur upon rewetting. The implications of the findings from this study on the impact of drying-inducing microcracking on the durability of most real concrete structures are discussed.