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

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

Impact of super absorbent polymers on early age behavior of ultra-high performance concrete walls

Early age cracking, a common problem for Ultra-High Performance Concrete (UHPC), is caused by Autogenous Shrinkage (AS) and self-desiccation arising from the chemical shrinkage during the cement hydration reactions when the deformation is restrained. However, to avoid the crack development initiated by AS, several solutions can be adopted; one example is the addition of a promising material, considered as an internal curing agent, the Super Absorbent Polymers (SAP) which limits the capillary depressions that can enhance the formation of the crack. In this study the main goal is to mitigate the shrinkage using SAPs in infrastructure under severe conditions. Therefore, a demonstrator wall was built simulating a typical case with high risk of cracking. With the help of fiber optic SOFO sensors embedded in the wall, real-time deformations are recorded and compared the demountable mechanical strain gauges (DEMEC) measurements to further investigate the behavior of SAPs in real scale infrastructure. The amount of extra water (in SAP) needed to mitigate shrinkage was determined by performing chemical shrinkage tests on different cement paste combinations. Tests of autogenous shrinkage were performed on mortars using corrugated tubes and showed that SAPs reduce to some extent the AS. Under restrained conditions via ring tests, SAP specimens did not crack. Therefore, SAPs were found promising towards mitigating the shrinkage and enhancing the early age behavior of concrete for a better durability

The use of superabsorbent polymers in high performance concrete to mitigate autogenous shrinkage in a large-scale demonstrator

High performance concrete (HPC) is a high strength concrete that undergoes a lot of early-age autogenous shrinkage (AS). If shrinkage is restrained, then micro-cracks arise and threaten the durability of the structure. Superabsorbent polymers (SAPs) can reduce/mitigate the autogenous shrinkage, due to their promising application as internal curing agents. In this paper, large-scale demonstrators were built to investigate the efficiency of SAPs to mitigate autogenous shrinkage in HPC. For this purpose, different measurement techniques were used like embedded fiber optic sensors and demountable mechanical strain gauges, complemented by AS measurements in corrugated tubes and restrained ring tests. The SAP wall showed an AS reduction of 22%, 54%, and 60% at the bottom, middle, and top, respectively, as recorded by the sensors (in comparison with the reference wall (REF)). In the corrugated tubes, mitigation of AS was shown in the SAP mixture, and under restrained conditions, in the ring test, the reference mixture cracked after two days, while the SAP mixture had not cracked at the end of the measurement period (20 days). Cracks were shown on REF wall after one day, while the SAP wall was crack-free. Water flow tests performed on the main crack of the REF wall confirmed that the flow rate is related to the third power of the crack width. All tests showed that SAPs could highly reduce AS in HPC and avoid cracking

Superabsorbent polymers to mitigate autogenous shrinkage in combination with promoted self-healing of high performance cementitious materials

In this PhD thesis, the use of superabsorbent polymers (SAPs) to mitigate autogenous shrinkage in combination with promoted self-healing and self-sealing of (high performance) cementitious materials is studied. Therefore, the following four main topics were investigated in a broad research plan: from the level of the micro meter sized SAPs and their influencing characteristics, to a variety of tests on different high performance cementitious materials, with the focus on the mitigation of autogenous shrinkage, in combination with self-sealing and self-healing tests, to finally realize the mentioned effects in a large-scale demonstrator wall. Each of these topics was connected to a research objective: - gaining insight in the link between the characteristics of SAPs and their influence on the properties of cementitious materials; - comparing the efficiency of SAP addition on the mitigation of autogenous shrinkage in cement paste, high performance mortar and high performance concrete; - searching for the ideal combination of SAPs that can both mitigate autogenous shrinkage and promote self-sealing and self-healing in cementitious materials; - measuring the efficiency of SAPs to mitigate autogenous shrinkage in a large-scale high performance concrete demonstrator by means of different test methods. In the following paragraphs, a summary of each research objective and the results obtained is given. Effect of SAP characteristics on cementitious material properties Nowadays, superabsorbent polymers (SAPs) are added to cementitious materials for different reasons: mitigating autogenous shrinkage, modifying the rheology of fresh concrete, self-sealing and selfhealing of cracked concrete, increasing the freeze-thaw resistance of concrete etc. Hence, they can help to increase the durability of concrete structures in different ways. The type, swelling characteristics, kinetics of water uptake and release, amount and particle size of the SAPs will dictate their effectiveness for the envisaged purpose. In this part of the study, SAPs with different chemistry, cross-linking degree, particle sizes and amount of solubles were investigated. By varying these parameters, insight was gained on the influence of each of these parameters on SAP properties such as the swelling capacity. In a next step, the SAPs were implemented in mortar to assess their influence on mortar properties like workability, compressive and flexural strength or hydration kinetics. Based on these results, the ‘ideal’ SAP with tuneable properties for a specific concrete application can be selected. For this purpose, different series of SAPs were tested: two different types of in-house developed SAPs (one sulfonate based and the other based on combinations of acrylic acid (AA) with dimethylaminoethyl methacrylate (DMAEMA), and also two commercially available, poly-acrylate based SAPs, for which efficient use in cementitious materials to mitigate autogenous shrinkage and to promote self-sealing and self-healing of cracks is already reported in literature. A fixed amount of 0.5 m% SAPs by weight of cement was added to a reference mortar mixture with water-to-cement ratio of 0.43. To compensate for the water uptake by the SAPs and hence the reduction in workability, an additional amount of water, equal to 1.5 times the swelling capacity in cement filtrate obtained in a filtration test, was added. Despite this amount of additional water, the initial flow of SAP-containing mixtures was in most cases lower than the value obtained for the reference. However, SAP-containing mixtures were able to keep their flow quite constant during the first hours, whereas the reference mixture showed a loss of flow over time. The swelling capacity of SAPs can be fine-tuned by varyingxviii | the cross-linking degree of the SAPs. The higher the amount of cross-linking, the lower the swelling capacity as the cross-links will impede the SAPs from swelling. By adding more DMAEMA compared to AA, a pH-responsive SAP can be synthesized that swells significantly at pH 7, but swells less at a pH around 12.5, which is the pH of fresh mortar. The hydration of mixtures containing SAPs was retarded compared to the reference mixture and a lower peak of hydration heat was observed. The higher the swelling capacity of the SAP (i.e. the lower the cross-linking degree), the lower the flexural and compressive strength of the mortar, due to the formation of macro-pores. The difference in compressive strength with the reference decreased as a function of the mortar age. When using SAPs with particle sizes in the range of 10 – 600 µm, no significant differences between the studied particle sizes were found concerning the tested mortar properties. With the ease of upscaling in mind, the need to purify the SAPs and to remove the non-cross-linked soluble fraction was further investigated. It was shown that the solubles had no effect on the mortar properties, except for increasing the setting time with almost 100%. SAPs to mitigate autogenous shrinkage in high performance cementitious materials Due to the high amount of fines in combination with a low water-to-cement ratio (<0.35), high performance mortar and concrete are very prone to cracking as a result of autogenous shrinkage (AS), resulting in a decreased durability, integrity and aesthetics of the structure. Superabsorbent polymers (SAPs) can be added as internal curing agents in order to reduce or even mitigate autogenous shrinkage in cementitious materials. At the right time in the hydration process, the SAPs release their stored water to the surrounding matrix, maintaining the relative humidity and preventing the cementitious material from self-desiccation and associated shrinkage cracking. In this second part of the study, the influence of SAP addition on mitigating autogenous shrinkage in cement paste, high performance mortar and high performance concrete was investigated. The studied cement paste had a water-tocement ratio of 0.3 and contains CEM I. The studied high performance mortar and concrete had a water-to-cement ratio of 0.24 and contained CEM III and silica fume. It was also investigated whether the determination of the amount of additional water to compensate for the uptake of mixing water by the SAPs, can be based on the swelling capacity of the SAPs in cement filtrate solution obtained through a filtration test, instead of the more labour and material intensive method of equal workability. By adding SAPs in various cementitious materials such as cement paste, mortar and concrete, it was studied whether SAPs perform differently in the tested materials and if trends and conclusions for one material can be transferred to other materials or not. For this purpose, two different types of SAPs in varying amounts were added to the tested cementitious materials: one SAP with specifically selected properties (cross-linking degree and particle size), made by the Belgian chemical R&D company ChemStream (CS) and a commercially available SAP from BASF. The first SAP was sulfonate based, whereas the latter SAP was poly-acrylate based. To study the effect of the SAP addition on the mitigation of autogenous shrinkage, corrugated tube tests in combination with an automated Vicat needle test in case of cement paste and mortar, and restrained ring tests for concrete were performed. It was studied whether the knee point of the autogenous shrinkage curves can be used as alternative for the Vicat needle test to determine the time-zero for cement paste and mortar mixtures. It was found that the commercially available SAPs from BASF reduced the autogenous shrinkage after 7 days in the studied mortar mixtures with 97% compared to the reference without SAPs, whereas in the studied cement paste, the autogenous shrinkage after 7 days was completely mitigated and even expansion occurred for series with this type of SAP. Although the mixtures with CS SAPs did not show complete mitigation of the autogenous shrinkage, the shrinkage was significantly reduced for all cement paste mixtures: increasing the amount of CS SAPs from 0.257 m% to 0.38 m% and 0.57 m%, led to a reduction in the autogenous shrinkage after 7 days with 80%, 85% and 89% respectively compared to the reference without SAPs. For CS SAPs in high performance mortar, the| xix reductions for the same amount of SAPs were 19%, 20% and 70% respectively compared to the reference without SAPs. The higher the amount of SAPs added, the larger the reduction in autogenous shrinkage as more internal curing water is available and a high relative humidity can be sustained preventing the mixture from self-desiccation. The addition of BASF SAPs to the concrete poured into the rings for restrained shrinkage tests reduced the occurring strains significantly (-88%) compared to the reference and prevented the rings from cracking. Although the addition of the CS SAPs did not prevent the concrete rings from cracking, the moment of cracking was delayed and lower strains compared to the reference were observed. These two effects became more pronounced the more SAPs were added to the concrete. For the tested cement pastes and mortars, the amount of extra water that should be added to compensate for the water uptake by the SAPs, can be determined based on the swelling capacity in cement filtrate after 10 minutes obtained by the filtration method, as it led to the same results as obtained with the method of equal workability. The time-zero of cement paste, based on the time when the knee point appears in the curve representing the autogenous shrinkage, corresponded well with the time of final setting determined by a Vicat needle test. Despite some negative effects on the workability and compressive strength, this study showed that the addition of a correct amount of SAPs to cementitious materials can reduce or even completely mitigate autogenous shrinkage, leading to more durable and aesthetically more appealing concrete structures. Ideal combination of SAPs to mitigate shrinkage and to promote self-healing and self-sealing The use of superabsorbent polymers (SAPs) to either mitigate autogenous shrinkage or to stimulate self-healing and self-sealing in cementitious materials is already reported frequently in literature, as it will lead to more durable concrete structures with lowered repair and maintenance costs. However, few studies focus on finding an optimal amount or combination of SAPs that can do both. In the third part of this study, SAPs were added to high performance mortar (w/c = 0.24) to study the influence on both mitigating autogenous shrinkage and promoting self-healing and self-sealing of cracks at the same time. For this purpose, different amounts, types, sizes and combinations of SAPs were added to mortar mixtures. The healing and sealing efficiency of cracks with an initial crack width of about 150 µm was studied by means of microscopic measurements in combination with water permeability tests. The autogenous shrinkage was measured by corrugated tube tests in combination with the final setting time determined by an automated Vicat needle test. Also the compressive and flexural strength and the air void content was measured for each mixture. When (combinations of) SAPs were added to the high performance mortar, a significant reduction in autogenous strain was measured for all series and for some series even complete shrinkage mitigation was obtained after 7 days of measuring. Although none of the studied SAP-series showed complete self-healing of all the 150 µm cracks, stimulated autogenous healing was obtained leading to partial closure of the cracks. Also the initial water flow was reduced compared to the reference series without SAPs due to the self-sealing effect of the swollen SAPs. However, the addition of SAPs had a significant negative effect on the flexural and compressive strength due to the formation of macro-pores in the hardened state, which can cause a limitation on practical applications. Depending on the type of concrete and the envisioned application, it must be examined whether the positive effects of the addition of SAPs, namely the mitigation of autogenous shrinkage, the stimulated self-healing leading to partial crack closure and a decreased initial water flow, outweigh the reduction in compressive and flexural strength. However, the results in this study showed that by making combinations of SAPs, promising results for both mitigating autogenous shrinkage and promoting self-healing and self-sealing in cementitious materials can be obtained.xx | Demonstrating the SAP efficiency in a high-performance concrete wall In the last part of this study, two large-scale demonstrator walls were built to investigate the efficiency of SAPs to mitigate autogenous shrinkage and to promote the self-healing and self-sealing of cracks in HPC: one reference wall without SAPs (REF) and a second wall containing 0.3 m% by weight of cement of commercial BASF SAPs (SAP). The demonstrator consisted of a wall cast on a slab. This setup was chosen to restrain the shrinkage at the slab-wall connection in order to create shrinkage cracks at the bottom of the wall, based on numerical simulations. Different shrinkage measurement techniques were used and their results were compared: the use of embedded fibre optic sensors and the use of demountable mechanical strain gauges complemented by autogenous shrinkage measurements in corrugated tubes and restrained ring tests. Also compressive strength tests, air void analysis and water flow tests were performed for both types of concrete. In the corrugated tube test, complete mitigation of autogenous shrinkage was shown in the SAP mixture, compared to an average shrinkage of -550 µm/m in the reference mixture after 20 days in the same test setup. Under restrained conditions in the ring test, the reference concrete cracked after 2 days, while the SAP-containing concrete had not cracked at the end of the measuring period (20 days) and showed significant lower strains compared to the reference ring. The wall containing SAPs showed an autogenous shrinkage reduction of 22%, 54% and 60% at the bottom, middle and top as recorded by the embedded sensors, in comparison with the reference wall after 120 days of measuring. Cracks appeared in the reference wall after one day, while the SAP wall remained crack-free during the whole measuring period of 120 days. The addition of 0.3 m% SAPs by weight of cement led to a doubling of the air content compared to the reference mixture, due to the formation of macro-pores upon shrinkage of the SAPs in the hardened state. It was found that a 1% increase of the air content of the concrete resulted in a 5% decrease of the compressive strength. The compressive strength at 28 days of the SAP containing mixture was 11.5% lower than the value measured for reference concrete, but could still be classified as HPC. All executed tests showed that SAPs can largely reduce autogenous shrinkage in HPC and avoid cracking. To conclude, the addition (of a combination) of superabsorbent polymers (SAPs) with tailored parameters, can efficiently reduce and even mitigate autogenous shrinkage in high performance cementitious materials, in combination with a promoted self-healing and self-sealing of cracks. In this way, more durable structures with lower maintenance and repair costs can be built

The influence of superabsorbent polymers (SAPs) on autogenous shrinkage in cement paste, mortar and concrete

Due to the high amount of fines in combination with a low water-to-cement ratio (<0.35), high performance mortar and concrete are very prone to cracking as a result of autogenous shrinkage, resulting in a decreased durability, integrity and aesthetics of the structure. Superabsorbent polymers (SAPs) can be added as internal curing agents to reduce the autogenous shrinkage in cementitious materials. In this paper, the influence of SAP addition on mitigating autogenous shrinkage in cement paste, high performance mortar and high performance concrete is investigated. Two different types of SAPs were added in varying amounts: one sulfonate based SAP with specifically selected properties, and a commercially available poly-acrylate based SAP. To study the effect of the SAP addition on the mitigation of autogenous shrinkage, corrugated tube tests in case of cement paste and mortar, and restrained ring tests for concrete were performed. The poly-acrylate based SAPs reduced the autogenous shrinkage after 7 days in the mortar mixtures with 97% compared to the reference without SAPs, whereas in the cement paste, the autogenous shrinkage after 7 days was completely mitigated. Although the mixtures with sulfonate based SAPs did not show complete mitigation of the autogenous shrinkage, the shrinkage was significantly reduced for all cement pastes: increasing the amount of SAPs from 0.257 m% to 0.38 m% and 0.57 m% by weight of cement, lead to a reduction in the autogenous shrinkage after 7 days with 80%, 85% and 89% respectively. In mortar the reductions for the same amounts of SAPs were 19%, 20% and 70% respectively. Considering restrained shrinkage ring tests, the poly-acrylate based SAPs reduced the occurring strains significantly (-88%) compared to the reference and prevented the rings from cracking. For the sulfonate based SAPs, the moment of cracking was delayed and lower strains compared to the reference were observed

Severe sulfuric acid attack on self-compacting concrete with granulometrically optimized blast-furnace slag-comparison of different test methods

This article belongs to the Special Issue Self-Healing and Smart Cementitious Construction Materials.The corrosion by severe sulfuric acid attack at pH 2 of two self-compacting concrete (SCC) types that are based on ordinary Portland cement (OPC) and granulometrically optimized blast-furnace slag cement was evaluated by three complementary tests that were performed in different research institutes. The use of SCC is a smart and promising solution to improve the performance of concrete in an aggressive environment, especially regarding ready-mixed concrete applications, since good compaction is less dependent on workmanship. The relevance and practical advantages of the different test protocols and the influence of the experimental parameters are discussed. It appears that the frequency of renewing the acid solution during the exposure period is the main parameter that influences the mass loss and the rate of degradation, while the sample geometry and the ratio between the volume of solution and concrete surface area had no clear influence. Nevertheless, there was reasonable agreement between the methods regarding the magnitude of the concrete degradation (resulting in a mass loss of about 2.5 kg/m² in six months time). The use of granulometrically optimized slag cement provided a moderate increase of the concrete resistance against acid attack, and this practice might be recommended in order to increase the durability of structures exposed to sulfuric acid media. The fact that the difference in comparison with SCC-OPC was rather limited shows that the influence of the cement type becomes less relevant in the case of concrete with low w/c ratio and optimized concrete technology.This work was funded by the European Union´s H2020 grant agreement ID 685445 under the LORCENIS Project (https://www.sintef.no/projectweb/lorcenis/).Peer reviewe

Chloride penetration and carbonation of concrete with superabsorbent polymers

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Chloride penetration and carbonation of concrete with superabsorbent polymers

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