Durable concrete for infrastructure under severe conditions : smart admixtures, self-responsiveness and nano-additions, Proceedings

The continuously growing world population and wide-spread industrialization increase the need for sustainable infrastructure. The construction industry currently is responsible for an important part of the environmental impacts related to the use of natural resources and energy, the production of waste, and greenhouse gas emissions. To minimize these impacts, our civil engineering structures need to become more long-lasting and smart. Since concrete is the most used construction material, increasing the durability of concrete structures is an important goal in this respect. To obtain such enhanced durability and sustainability, in the last decade several smart admixtures have been developed to impart self-responsiveness to this material, including self-sensing, selfcuring, and self-healing. Carbon nanofibers and nanotubes have been used to make the concrete self-sensing and report when damage is about to occur or has occurred already. Layered double hydroxides can capture aggressive agents intruding into the concrete and can release corrosion inhibitors to prevent damage. Superabsorbent polymers have been developed to provoke internal curing and hence can mitigate autogenous shrinkage cracks; they can also self-seal cracks from intruding liquids and stimulate self-healing through the deposition of calcium carbonate and binder hydration products. Micro- and macro-capsules containing mineral or polymeric healing agents can provide autonomic self-healing properties. With the International Conference on Durable Concrete for Infrastructure under Severe Conditions – smart admixtures, self-responsiveness and nano-additions, we want to offer participants a full overview of the most recent advances in the development of these smart admixtures. The compatibility of the smart admixtures with other concrete components and the effects on fresh and hardened concrete properties are considered. Modelling of the hydration reactions and microstructure formation in the novel durable concrete, of the activation of smart properties, of the service life in specific environments, and of environmental impacts, is of importance as well. Existing and emerging energy technologies also require that these materials perform in more and more extreme operating conditions as they are installed in sub-arctic/arctic areas (low temperatures, ice-abrasion), desert areas (high temperatures), along coast lines (high chloride contents), deep-sea or underground (large temperature gradients and high pressure). Evaluation of the resistance to extreme conditions is also included

Healing of dynamic concrete cracks using acrylate-endcapped polymer precursors

The occurrence of cracks in concrete is inevitable due to its limited tensile strength. Up to now, several approaches have been used to design concrete with self-healing properties. Depending on the type of damage, different healing agents can be used. In case of dynamic cracks in structures exposed to cyclic loads, elastic healing materials can be used to cope with the crack opening and closing movement. In this study, we aimed at determining the most suitable polymer backbone to be applied for the healing of dynamic cracks. For this purpose, different polymeric precursors including siloxane- (PDMS), polypropylene glycol/urethane- (PPG), epoxy- and polyester-based (PE) have been evaluated. The healing capacity was evaluated by determining the regain in water tightness of the healed cracks using water flow tests. The strain capacity of the polymers was assessed after widening of the healed cracks in a stepwise fashion. Comparison of the sealing properties before and after each stepwise elongation shows that a strain of at least 50% could be achieved for epoxy and PDMS-based healing agents. For polyester and PPG-based precursors failure occurred due to polymer detachment. The effect of high alkalinity on the degradation of the polymerized healing agents has been also evaluated with the PPG-based precursor showing the best performance in terms of degradation. and PE the highest mass loss percentage after incubation in concrete pore solution

Use of superabsorbent polymers to mitigate autogenous shrinkage in ultra-high performance concrete

Ultra-high performance concrete (UHPC) with low w/c-ratio is very prone to the formation of cracks due to autogenous shrinkage. These cracks can lead to a decreased durability of the concrete, resulting in higher maintenance and/or repair costs in the future. Superabsorbent polymers (SAPs) can be added to cementitious materials to provide internal curing and as a result reduce or even mitigate this autogenous shrinkage. In this paper, two different types of SAPs were added to cement paste to see their influence on mitigating autogenous shrinkage. One SAP is a commercially available SAP whereas the other SAP is especially developed within the framework of the LORCENIS project by the company ChemStream, with the aim to mitigate autogenous shrinkage and induce self-healing of cracks. The SAPs from ChemStream were based on a copolymerization of sodium vinyl sulfonate (SVS) with 2-acryloylamino-2-methyl-propane-1-sulfonate (NaAMPS) and contained 1.0 mol% N,N’-methylenebisacrylamide (MBA) with respect to the monomer as cross-linker. The commercial SAP from BASF was based on poly(acrylamide-co-acrylic acid). In case SAPs were used, an additional fixed amount of water was added to mitigate autogenous shrinkage. The amount of SAPs used was determined based on their swelling capacity in cement filtrate and in order to obtain the same workability as the reference mixture. The amount of SAPs needed was in the range of 0.2-0.26 m% of the cement weight. To see whether the size of the SAPs plays a role in the efficiency of mitigating autogenous shrinkage, two average particle sizes, namely 40 and 100 µm, were tested. With the used amount of SAPs, a reduction or even complete counteraction of autogenous shrinkage was observed for the cement pastes

Discussing different approaches for the time-zero as start for autogenous shrinkage in cement pastes containing superabsorbent polymers

Many studies have already been published concerning autogenous shrinkage in cementitious materials. Still, no consensus can be found in the literature regarding the determination of the time-zero to initiate the recording of autogenous shrinkage. With internal curing agents, a correct evaluation of their efficiency depends on an appropriate choice of the time-zero. This study investigates different approaches to estimate the time-zero for cement paste mixtures with and without superabsorbent polymers as internal curing agents. The initial and final setting times were determined by an electronic Vicat and ultrasonic pulse velocity measurements (UPV); the transition point between the fluid and solid state was determined from the autogenous strain curve; the development of the capillary pressure was also studied. The choice of time-zero before the transition point led to higher values of shrinkage strain that should not be taken into account for autogenous shrinkage. A negligible difference was found between the strains when the final setting time and the transition point were taken as time-zero. Considering the artefacts and practical issues involving the different methods, the use of the transition point from the autogenous strain curve is the most suitable technique for determining the time-zero

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

Exploring different choices of 'time zero' in the autogenous shrinkage deformation of cement pastes containing superabsorbent polymers

Shrinkage in concrete structures has been the focus of many studies. Lately a lot of attention has been given to autogenous shrinkage. Although it may not be prominent in ordinary concrete structures, in systems with very low water-to-cement/binder ratio (ultra-high performance concrete for example) it can become a serious issue associated with the cracking of the structure at early age. This type of shrinkage develops due to a reduction in the internal relative humidity of the material and it is also associated to the development of capillary pressure in the pore system due to receding menisci. A big challenge in studying autogenous shrinkage is determining the "time-zero". Given a lack of consensus in literature, this study aimed to investigate the influence of different estimations of time-zero: the final setting time determined by both an electronic Vicat apparatus and ultrasonic measurements; the "knee-point" in the shrinkage curve; and the capillary pressure build-up. Cement pastes with and without superabsorbent polymers (SAPs) were produced with Portland cement CEM III-B 42.5 N and superplasticizer (Glenium 51, 35% conc.). SAPs have proven to be quite effective in the mitigation of autogenous shrinkage as they can act as water reservoirs for the system. Among all methods, the capillary pressure was very suitable for all mixtures. For those containing SAPs no difference was found in picking the time-zero with any method. For the one without SAPs and lower w/c the choice of time-zero based on the setting time led to a different magnitude of autogenous shrinkage deformation in comparison to the other methods, which could be interpreted as an underestimation of the autogenous shrinkage deformation

Real-scale testing of the efficiency of self-healing concrete

After several years of research in the Magnel Laboratory (Belgium) to obtain concrete with self-healing properties, the most promising mechanisms were tested on a larger scale. Instead of small mortar samples with self-healing properties, real-scale self-healing concrete beams (150 mm × 250 mm × 3000 mm) were made and the efficiency of autonomous crack repair was evaluated over time after loading the beams in four-point bending. In addition to a reference beam without self-healing properties, a beam with encapsulated polyurethane and a beam containing superabsorbent polymers were investigated. While for the beam with polyurethane, crack repair is obtained as the healing agent is released as soon as cracks damage the embedded capsules, the superabsorbent polymers absorb water which intrudes into the cracks, immediately blocking the crack through swelling and later on by continued hydration and precipitation of calcite. The efficiency of both self-healing approaches was compared by measuring the reduction in water ingress into the cracks and by measuring the crack width reduction over time

Simulation-aided design of tubular polymeric capsules for self-healing concrete

Polymeric capsules can have an advantage over glass capsules used up to now as proof-of-concept carriers in self-healing concrete. They allow easier processing and afford the possibility to fine tune their mechanical properties. Out of the multiple requirements for capsules used in this context, the capability of rupturing when crossed by a crack in concrete of a typical size is one of the most relevant, as without it no healing agent is released into the crack. This study assessed the fitness of five types of polymeric capsules to fulfill this requirement by using a numerical model to screen the best performing ones and verifying their fitness with experimental methods. Capsules made of a specific type of poly(methyl methacrylate) (PMMA) were considered fit for the intended application, rupturing at average crack sizes of 69 and 128 μm, respectively for a wall thickness of ~0.3 and ~0.7 mm. Thicker walls were considered unfit, as they ruptured for crack sizes much higher than 100 μm. Other types of PMMA used and polylactic acid were equally unfit for the same reason. There was overall good fitting between model output and experimental results and an elongation at break of 1.5% is recommended regarding polymers for this application