H.S. Wong

Potential of superabsorbent polymer for self-sealing cracks in concrete

Abstract Abstract The potential of conventional superabsorbent polymers (SAPs) based on partially neutralised acrylate and acrylate/acrylamide copolymers as an admixture for self-sealing of cracks in concrete is investigated. SAPs are cross-linked polymers that can absorb a disproportionately large amount of liquid and swell substantially to form a soft and insoluble gel. However, their swelling capacity is highly dependent on the alkalinity and ionic content of the solution. These characteristics may be exploited for self-sealing cracks in concrete. In this preliminary study, the mechanism involved is described and tests performed to determine the swelling ratios of SAP in various solutions including synthetic pore solution, groundwater and seawater are reported. Transport testing found that the flowrate through a 340 μm wide model crack is reduced substantially by using less than 1 vol.-% SAP. The reswelling capacity of SAP in cement paste and the effect of SAP on cement paste microstructure were investigated by microscopy.

3D imaging of cement‐based materials at submicron resolution by combining laser scanning confocal microscopy with serial sectioning

In this paper, we present a new method to reconstruct large volumes of nontransparent porous materials at submicron resolution. The proposed method combines fluorescence laser scanning confocal microscopy with serial sectioning to produce a series of overlapping confocal z‐stacks, which are then aligned and stitched based on phase correlation. The method can be extended in the XY plane to further increase the overall image volume. Resolution of the reconstructed image volume does not degrade with increase in sample size. We have used the method to image cementitious materials, hardened cement paste and concrete and the results obtained show that the method is reliable. Possible applications of the method such as three‐dimensional characterization of the pores and microcracks in hardened concrete, three‐dimensional particle shape characterization of cementitious materials and three‐dimensional characterization of other porous materials such as rocks and bioceramics are discussed.

Effect of confining pressure and microcracks on mass transport properties of concrete

Abstract This paper investigates the effect of low confining pressure on transport properties of cement based materials and establishes if it can be used to study the influence of microcracks on transport. Oxygen diffusivity and permeability of paste and concrete (w/c ratios: 0·35 and 0·50; curing ages: 3 and 28 days) were measured at increasing confining pressures up to 1·9 MPa (4–8% of 3 day compressive strength). Before transport testing, samples were subjected to gentle stepwise drying at 21°C or severe oven drying at 105°C to induce microcracking. Microcracks were quantified using fluorescence microscopy and image analysis. Permeability decreased significantly with increasing confining pressure and this was more significant for samples with a greater degree of microcracking. Image analysis shows that microcracks undergo partial closure when confined, but the total accessible porosity was not significantly affected. Implications of these results with respect to the influence of microcracks on transport properties are discussed.

Modelling the Effect of Microcracks on the Transport Properties of Concrete in Three Dimensions

This paper investigates the effect of microcracks on transport properties of concrete using a three-dimensional model. Concrete is idealised as consisting of aggregate particles (1-10 mm, 60% vol.), cement paste matrix and microcracks with widths of 1-50 µm. Using aligned meshing, aggregate particles are discretised with tetrahedral elements and the microcracks are incorporated as interface elements. The microcracks are either bond cracks at the aggregate-paste interface or matrix cracks that span nearest neighbouring aggregate particles. A finite-element model is applied to simulate diffusion and permeation. The model is used to perform a sensitivity analysis to examine the effect of microcrack width, density and percolation. It is found that the effect of microcracks is more pronounced for concretes with denser matrix. Furthermore, the effect of microcracks in non-percolated networks increases up to a finite limit, the value of which is independent of crack width but depends on other crack characteristics and transport property of the matrix. In all cases, microcracking has a greater effect on permeation than diffusion and this is more pronounced for percolated crack networks.

Defining clogging potential for permeable concrete

Permeable concrete is used to reduce urban flooding as it allows water to flow through normally impermeable infrastructure. It is prone to clogging by particulate matter and predicting the long-term performance of permeable concrete is challenging as there is currently no reliable means of characterising clogging potential. This paper reports on the performance of a range of laboratory-prepared and commercial permeable concretes, close packed glass spheres and aggregate particles of varying size, exposed to different clogging methods to understand this phenomena. New methods were developed to study clogging and define clogging potential. The tests involved applying flowing water containing sand and/or clay in cycles, and measuring the change in permeability. Substantial permeability reductions were observed in all samples, particularly when exposed to sand and clay simultaneously. Three methods were used to define clogging potential based on measuring the initial permeability decay, half-life cycle and number of cycles to full clogging. We show for the first time strong linear correlations between these parameters for a wide range of samples, indicating their use for service-life prediction.