The effect of superabsorbent polymers on the cracking behavior due to autogenous shrinkage of cement-based materials

As far as durability is concerned most of the deteriorating mechanisms acting on concrete structures are related to the ingress of aggressive agents inside the structures. Reinforcement corrosion, carbonation and problems during freeze-thaw cycles are related to the ingress of substances such as chlorides, sulfates, carbon dioxide and even water, amongst others. Even before reaching its hardened state, a cement-based composite is subjected to the formation of cracks especially due to the effects of shrinkage during the early ages. The formed porosity of the material can become the perfect path for the ingress of those aggressive agents. The shrinkage phenomena, especially when referred to autogenous and plastic shrinkage are inherent to the hydration process of the cementitious material and (among other factors) are a function of the water-to-cement ratio and curing conditions (temperature and humidity). The use of superabsorbent polymers has proven to be an interesting alternative in the mitigation of shrinkage and reducing shrinkage cracking by means of internal curing. They can also promote sealing and autogenous healing. In this study, different cement pastes and mortars were produced with different combinations of superabsorbent polymers. Their effects on the shrinkage cracking behavior was studied. Both polymers used have already been (individually) studied in previous research and results showed different benefits regarding the crack mitigation and self-sealing and -healing of the samples. In this paper different combinations of those polymers were studied aiming to achieve a composition with optimal properties for the mitigation of autogenous shrinkage. The experimental program of the study was based on the monitoring of autogenous strain and the effect of the polymers in the properties of specimens in the fresh and hardened state. To accomplish that, the hydration and setting time of the mixtures were evaluated by means of ultrasonic measurements (p-wave); the autogenous strain was measured from the final setting time till the age of 7 days following the method described in the ASTM C-1698-09 and mechanical strength tests were also performed

Monitoring the early-age shrinkage cracking of concrete with superabsorbent polymers by means of optical fiber (SOFO) sensors

Concrete structures are subjected to shrinkage since the moment when the water makes contact with the cement. From that moment on, depending on the concrete composition and its curing conditions, chemical, autogenous and drying shrinkage might increase the risk of early-age cracking. Once a crack is formed, it may become the preferential path for the ingress of many aggressive agents inside the structure increasing the probability of damage due to corrosion, carbonation and other mechanisms.The use of superabsorbent polymers (SAPs) has been extensively studied to reduce the shrinkage cracking risk in mortar specimens by acting as internal curing agent to mitigate autogenous shrinkage. In this paper, a commercially available SAP was investigated as shrinkage reducing agentby means of internal curing in concrete specimens. The shrinkage of the concrete was monitored for 28 days with both a demountable mechanical strain gauge (DEMEC) and optical fiber sensors (SOFO).The SAP-containing concrete showed a complete mitigation of autogenous shrinkage during 28 days of measurement, for both methods. The SOFO sensors showed the occurrence of cracks after 7 days for the reference mixture, while for the SAP containing mixture, no crack was identified during the 28 days of measurements

Superabsorbent polymers for smart and durable concrete structures

info:eu-repo/semantics/publishe

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

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

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

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

Ultrasound-guided core needle biopsy for the diagnosis of fibroepithelial breast tumors

OBJETIVO: avaliar a taxa de concordância da biópsia percutânea com agulha grossa guiada por ultrassom seguida pela biópsia excisional em nódulos de mama palpáveis, sugestivos de tumores fibroepiteliais. MÉTODO: estudo retrospectivo que selecionou 70 biópsias com diagnóstico histológico de tumor fibroepitelial em 67 dentre 531 pacientes com lesões mamárias submetidas à biópsia percutânea com agulha grossa guiada por ultrassonografia, com transdutor linear de alta frequência (7.5 MHz), utilizando pistola automática Bard-Magnum e agulha 14 gauge. Foram incluídos os casos com diagnóstico de tumor fibroepitelial na biópsia percutânea ou biópsia excisional. Biópsias com diagnóstico histopatológico de fibroesclerose também foram incluídas no estudo. A força da concordância entre o resultado da biópsia percutânea e da biópsia excisional foi medida pelo coeficiente de Kappa. RESULTADOS: a biópsia excisional revelou 40 casos de fibroadenoma (57,1%), 19 de tumor filoide (27,2%) e 11 de fibroesclerose (15,7%). A taxa de concordância para o fibroadenoma foi substancial (k = 0,68; IC95% = 0,45 - 0,91), quase perfeita para o tumor filoide (k = 0,81; IC95% = 0,57 - 1,0) e moderada para a fibroesclerose (k = 0,58; IC95% = 0,36 - 0,90). CONCLUSÕES: a biópsia percutânea com agulha grossa é propedêutica minimamente invasiva que tem taxas de concordância com a biópsia excisional, de "substancial" a "quase perfeita". A fibroesclerose deve ser considerada no diagnóstico diferencial dos tumores fibroepiteliais.PURPOSE: to evaluate the concordance rate of ultrasound-guided core needle biopsy followed by excisional biopsy in palpable breast lumps, suggestive of fibroepithelial tumors. METHOD: a retrospective study included 70 biopsies with a histological diagnosis of fibroepithelial tumor in 67 out of 531 patients with breast lesions submitted to ultrasound-guided core needle biopsy with a high frequency (7.5 MHz) linear transducer, using an automatic Bard-Magnum gun and a 14-gauge needle. Cases with a diagnosis of fibroepithelial tumor by core needle biopsy or excisional biopsy and with a diagnosis of fibrosclerosis were included in the study. The agreement between the two biopsy methods was assessed using the Kappa coefficient. RESULTS: excisional biopsy revealed 40 cases of fibroadenoma (57.1%), 19 cases of phyllodes tumor (27.2%), and 11 cases of fibrosclerosis (15.7%). The concordance rate for fibroadenoma was substantial (k = 0.68, 95%CI = 0.45 - 0.91), almost perfect for the phyllodes tumor (k = 0.81, 95%CI = 0.57 - 1.0), and moderate for fibrosclerosis (k = 0.58, 95%CI = 0.36 - 0.90). CONCLUSIONS: the core needle biopsy is a minimally invasive method that has "substantial" to "almost perfect" concordance rate with excisional biopsy. Fibrosclerosis should be considered in the differential diagnosis of fibroepithelial tumors