Crack mitigation in concrete : superabsorbent polymers as key to success?

Cracking is a major concern in building applications. Cracks may arise from shrinkage, freeze/thawing and/or structural stresses, amongst others. Several solutions can be found but superabsorbent polymers (SAPs) seem to be interesting to counteract these problems. At an early age, the absorbed water by the SAPs may be used to mitigate autogenous and plastic shrinkage. The formed macro pores may increase the freeze/thaw resistance. The swelling upon water ingress may seal a crack from intruding fluids and may regain the overall water-tightness. The latter water may promote autogenous healing. The use of superabsorbent polymers is thus very interesting. This review paper summarizes the current research and gives a critical note towards the use of superabsorbent polymers in cementitious materials

Utility of pH-sensitive superabsorbent polymers in concrete repair

The largest issue with concrete is that cracks can occur due to its relatively low tensile strength. These cracks can generate an entrance for harmful compounds which are dissolved in fluids and gases and endanger the durability of concrete. The cost for crack repair is very high. Alternatively, introducing a polymer during concrete mixing can create a self-sealing material. Fresh cement pore solution possesses a pH value of 12.8, but when a crack occurs, the pH drops to 9 - 10 or even lower, depending on the environment. At this lower pH value, the swelling degree of the hydrogel incorporated must be sufficiently high in order to fill up the crack. As a result, a cross-linked pH-sensitive copolymer of acrylic acid and acrylamide has been synthesized in the present work. The chemical structure has been characterized and sorption and desorption effects have been investigated using dynamic vapour sorption experiments. In addition, a swelling curve was established over the entire pH-range (pH 1-13). Interestingly, the hydrogel developed possessed a maximal swelling capacity of more than 400 times its own weight. Next, water permeability and flexural and compressive strength tests were performed on these samples. The significant decrease in water permeability of hydrogel containing cracked concrete relative to the cracked reference concrete is a quantitative indication of the sealing capacity of the applied hydrogel. The uptake of mixing water by the hydrogel will reduce the effective water/cement ratio of the cementitious matrix. This water will then be released later-on and will cause internal curing. In the present work, experiments have been performed using additional mixing water. Additional mixing water resulted in a higher apparent water/cement factor during internal curing and, together with macro-pore formation, in a lower strength. The results indicate that the polymer developed can be promising to introduce crack-sealing potential in concrete

Superabsorbent polymers : a review on the characteristics and applications of synthetic, polysaccharide-based, semi-synthetic and ‘smart’ derivatives

The current review provides an overview of different types of superabsorbent polymers (SAPs) together with appropriate strategies elaborated to enable their synthesis. The main focus will be on polysaccharide-based, semi-synthetic and 'smart' SAPs along with their derivatives. SAPs have already shown their use in a plethora of applications including diapers, the biomedical field, agriculture, etc. The different polymer classification possibilities are discussed, as well as the classification of the constituting building blocks. The main part of SAPs still has a synthetic origin. However, as they are often not biocompatible, biodegradable or renewable, natural SAPs based on polysaccharides have gained increasing interest. Due to the low solubility of synthetic polymers, purification problems or the need for organic solvents, a trend has emerged towards combining polysaccharides with synthetic monomers to create semi-synthetic, hybrid SAPs for specialized applications with fine-tuned properties including wound dressings, fertilizers or self-healing concrete. These specialized, semi-synthetic SAPs offer strong potential for a series of applications in the future. However, future research in this respect is still needed to optimize homogeneity and to increase gel fractions. A final part of this review includes 'smart' SAPs such as SAPs with a T-, electro- and pH-sensitivity. These 'smart' SAPs are especially becoming useful for certain biomedical applications such as drug release for which an in vivo location can be targeted. The use of 'smart', semi-synthetic SAPs with fine-tuned characteristics combining the best characteristics of both synthetic and natural SAPs, offer the greatest potential for the future

Mitigating autogenous shrinkage by means of superabsorbent polymers : effect on concrete properties

(Ultra-)high performance concrete ((U)HPC) is very prone to autogenous shrinkage cracking. These cracks can create preferential pathways for the ingress of harmful substances which can facilitate the corrosion process of the steel reinforcement, resulting in a decreased durability and structural integrity of the concrete structure. Superabsorbent polymers (SAPs) can reduce or even mitigate autogenous shrinkage as they absorb water in the fresh concrete mix and provide it to the cement particles at the right moment in the hydration process, acting as internal curing agent for the concrete. To study the mitigation of autogenous shrinkage by SAPs, five different superabsorbent polymers based on the copolymerization of acrylic acid (AA) with dimethylaminoethyl methacrylate (DMAEMA) were synthesized at Ghent University. This paper focusses on the compatibility tests aiming at evaluating the effect of these SAPs on initial flow and slump life (rheology), hydration kinetics (reactivity) and mechanical properties (3, 7 and 28 days strength). The most promising SAPs will be further studied on their effect to mitigate autogenous shrinkage

Pore structure of mortars containing limestone powder and natural pozzolan assessed through mercury intrusion porosimetry and dynamic vapour sorption

Pore structure characterization is a key aspect when studying the durability of cementitious materials. When supplementary cementitious materials (SCMs) are used changes in pore structure are expected, and the complexity of its analysis is increased. The purpose of this paper is to describe the pore structure variation of mortars with two types of SCMs: natural pozzolan from volcanic origin (NP), and limestone powder (LP). We tested mixes with cement replacements (in weight) of 20 % and 40% by NP, and 10 % and 20% by LP. To analyse the pore structure, two widely accepted and complementary techniques were applied: dynamic water vapour sorption (DVS) and mercury intrusion porosimetry (MIP). With the DVS data, the Barret-Joyner-Halenda (BJH) model was used for pore size distribution assessment. Calculations with the Dubinin-Radushkevich (DR) model were also made for the smallest pore size range. Tests were performed at 28 and 90 days. MIP and DVS allowed evaluating the effect of the studied SCMs on different pore size ranges. Both techniques provided comprehensive information over a wide range of pore sizes. The mix with 40 % of NP had the best evolution, showing a significant volume decrease in the mesopore range