H.X.D. Lee, H.S. Wong & N.R. Buenfeld

Concrete in service tends to crack as result of tensile stresses from mechanical loading, drying shrinkage or temperature effects. Cracking degrades the water tightness of a structure and accelerates the ingress of aggressive species that cause degradation of concrete structures. Narrow cracks may self-heal to a certain extent. Larger cracks may be controlled by appropriate reinforcement detailing, installation of movement joints, surface coating, resin injection and use of integral water repellents. These methods, although widely used, are not always sufficient.

The aim of this study is to investigate the potential of superabsorbent polymers (SAP) as an admixture to produce concrete with the ability to seal cracks as and when they occur. SAP are unique cross-linked polymers that can absorb a vast amount of liquid and swell substantially to form a soft insoluble gel. Common types of SAP are poly(acrylate) and poly(acrylate-co-acrylamide) that are widely used in personal care products such as nappies.

Other applications of SAP include biomedical bandages, agricultural soil conditioning, waste solidification and meat packaging. The swelling of SAP is highly dependent on pH, ionic content and concentration of the solution. For example, SAP swells significantly in deionised water (~200g/g), but only slightly when added to concrete (~5-20g/g). This unique feature allows SAP to be exploited for sealing cracks in concrete.

Figure 1 shows the basic mechanism. When SAP is added to the concrete mix, it swells only slightly because the mix water reaches a very high pH (~12.5-13) and ionic strength (~0.3M) within minutes of being in contact with cement. As the concrete sets and hardens, the SAP releases water and shrink, leaving behind pores with sizes ranging from tens to hundreds of microns (Figure 1b, c).

The SAP then lie dormant in the microstructure until cracking occurs. Ingress of moisture via cracks causes the SAP to swell again. Fluids with ionic concentration lower than the concrete pore solution, e.g. shallow ground water, triggers the SAP to swell beyond the pore and into the crack. This subsequently blocks the crack and reduces flow.

Measurements of flow through cracked pastes and mortar specimens containing different types and amounts of SAP were carried out. Results show that the flow rate through a 330μm model crack drops significantly when the SAP content is greater than 0.4% vol. (Figure 2). The flow rate and cumulative flow of NaCl solution through cracked cement paste and mortar specimens containing SAP were significantly lower than the control (Figure 2).

Paste specimens containing SAP showed a reduction in the cumulative flow by up to 80%. Mortar specimens also achieved a significant reduction in the cumulative flow, about 85%, despite having a lower SAP content compared to the pastes. The results suggest that poly(acrylate-co-acrylamide) is more effective than poly(acrylate) for crack sealing. Results also suggest that a larger SAP particle size is more beneficial for sealing cracks.

Acknowledgements: This project was supported by EPSRC-DTA and EPSRC Platform Grant for the Concrete Durability Group (EPSRC GR/M97206).

References

• H.X.D. Lee, H.S. Wong & N.R. Buenfeld (2010), Potential of superabsorbent polymer for self-sealing cracks in concrete, Advances in Applied Ceramics, 109, 296-302
• H.X.D. Lee, H.S. Wong & N.R. Buenfeld (2010), Self-sealing cement-based materials using superabsorbent polymers, RILEM Conference on the Use of Superabsorbent Polymers and Other New Additives in Concrete, 15-18 Aug, TU Denmark.
• H.X.D. Lee, H.S. Wong, N.R. Buenfeld (2010), Estimating the swelling ratio of superabsorbent polymers in cement-based materials, RILEM International Conference, Use of superabsorbent polymers and other new additives in concrete, Lyngby, 15-18 Aug.
• H.X.D. Lee (2011) Self-sealing of cracks in cement-based materials using superabsorbent polymers, PhD thesis, Imperial College London.