Monitoring the reduction in shrinkage cracking of mortars containing superabsorbent polymers

Ultra-high performance concrete (UHPC) is characterized by a low water-to-cement ratio, leading to improved durability and mechanical properties. However, the risk for autogenous shrinkage and cracking due to restrained shrinkage increases, which may affect the durability of UHPC as cracks form pathways for ingress of aggressive liquids and gases. These negative features can be prevented by the use of superabsorbent polymers (SAPs) in the mixture. SAPs reduce autogenous shrinkage by means of internal curing: they will absorb water during the hydration process and release it again to the cementitious matrix when water shortage arises. In this way, hydration can continue and shrinkage is diminished

Assessment of the effect of nanosilica on the mechanical performance and durability of cementitious materials

Over the last years, nanotechnology is getting more attractive and nanomaterials are being used more commonly in construction industry. One of these materials is nanosilica: the nano-sized, engineered form of silica fume. The replacement of cement by these nanoparticles is said to enhance both the mechanical performance and the durability of the concrete material. In this paper colloidal silica will be used, which is nanosilica in solution. A characterization of mortar mixtures containing different amounts of silica is done and a comparison is made with respect to a reference mixture

Elastic wave monitoring of cementitious mixtures including internal curing mechanisms

The mitigation of autogenous shrinkage in cementitious materials by internal curing has been widely studied. By the inclusion of water reservoirs, in form of saturated lightweight aggregates or superabsorbent polymers, additional water is provided to the hydrating matrix. The onset of water release is of high importance and determines the efficiency of the internal curing mechanism. However, the monitoring of it poses problems as it is a process that takes place in the microstructure. Using acoustic emission (AE) sensors, the internal curing process is monitored, revealing its initiation and intensity, as well as the duration. In addition, AE is able to capture the water evaporation from saturated specimens. By ultrasonic testing, differences in the hydration kinetics are observed imposed by the different methods of internal curing. The results presented in this paper show the sensitivity of combined AE and ultrasound experiments to various fundamental mechanisms taking place inside cementitious materials and demonstrate the ability of acoustic emission to evaluate internal curing in a non-destructive and easily implementable way

The contribution of elastic wave NDT to the characterization of modern cementitious media

To mitigate autogenous shrinkage in cementitious materials and simultaneously preserve the material’s mechanical performance, superabsorbent polymers and nanosilica are included in the mixture design. The use of the specific additives influences both the hydration process and the hardened microstructure, while autogenous healing of cracks can be stimulated. These three stages are monitored by means of non-destructive testing, showing the sensitivity of elastic waves to the occurring phenomena. Whereas the action of the superabsorbent polymers was evidenced by acoustic emission, the use of ultrasound revealed the differences in the developed microstructure and the self-healing of cracks by a comparison with more commonly performed mechanical tests. The ability of NDT to determine these various features renders it a promising measuring method for future characterization of innovative cementitious materials

The influence of superabsorbent polymers and nanosilica on the hydration process and microstructure of cementitious mixtures

Superabsorbent polymers (SAPs) are known to mitigate the development of autogenous shrinkage in cementitious mixtures with a low water-to-cement ratio. Moreover, the addition of SAPs promotes the self-healing ability of cracks. A drawback of using SAPs lies in the formation of macropores when the polymers release their absorbed water, leading to a reduction of the mechanical properties. Therefore, a supplementary material was introduced together with SAPs, being nanosilica, in order to obtain an identical compressive strength with respect to the reference material without additives. The exact cause of the similar compressive behaviour lies in the modification of the hydration process and subsequent microstructural development by both SAPs and nanosilica. Within the present study, the effect of SAPs and nanosilica on the hydration progress and the hardened properties is assessed. By means of isothermal calorimetry, the hydration kinetics were monitored. Subsequently, the quantity of hydration products formed was determined by thermogravimetric analysis and scanning electron microscopy, revealing an increased amount of hydrates for both SAP and nanosilica blends. An assessment of the pore size distribution was made using mercury intrusion porosimetry and demonstrated the increased porosity for SAP mixtures. A correlation between microstructure and the compressive strength displayed its influence on the mechanical behaviour

Superabsorbent polymers and nanosilica for mitigation of autogenous shrinkage and promotion of self-healing of cementitious materials

Continuously rising demands for more durable and robust materials require the adaptation of the construction industry. Moreover, the increasing concern about global warming results in challenges regarding the extensive use of Portland cement, being the source of around 8% of the world’s carbon dioxide emissions. Over the last years, a shift towards the use of (ultra)-high-performance concrete is noticed, combining an improved mechanical performance and higher durability, which allows to extend a structure’s lifetime and to reduce the amount of repairs needed. These types of concrete are characterized by a low waterto- cement ratio, meaning that complete hydration of cement particles is unfeasible. As hydration proceeds, the limited amount of water present in these cementitious mixtures induces self-desiccation, leading to autogenous shrinkage. When this shrinkage is restrained, tensile stresses develop inside the material. As autogenous shrinkage occurs at early age and the tensile strength of concrete is relatively low, the risk of cracking is strongly increased compared to conventional concrete, which may reduce the durability and compromise the future mechanical performance. To reduce autogenous shrinkage and limit the risk of shrinkage cracking, an internal curing mechanism was introduced inside the cementitious mixtures by means of superabsorbent polymers (SAPs). SAPs are able to absorb and retain large amounts of water. When included during mixing, they absorb part of the available water. When selfdesiccation occurs, associated with a decrease in relative humidity and rising capillary pressure, the stored water is released by the SAPs. This internal curing eect limits the drop in relative humidity and induces continued hydration, mitigating autogenous shrinkage. Within this research, mortar and concrete mixtures with a water-to-binder ratio of 0.35 were examined. To obtain internal curing, a quantity of 0.2% of SAPs by weight of the binder was included, together with an additional amount of 26 grams of water per gram of SAP. The internal curing efficiency of the SAPs was evaluated by means of restrained shrinkage ring tests. The set-up allowed for a comparison between various mixtures of both the shrinkage strains and the age of cracking. Results showed that upon addition of SAPs to mortar and concrete blends shrinkage could be reduced up to 70% when comparing identical curing ages. Also, no visible fracture was observed during at least one month of curing in SAP mixtures, whereas fracture occurred within two weeks of curing for mortars without SAP addition. Despite the beneficial effect of SAP addition for mitigation of autogenous shrinkage, the release of their absorbed water creates macropores inside the hardened cementitious matrix, which negatively influence the mechanical properties. For this reason, it was chosen to partially substitute cement by a nanomaterial, which would counterbalance the lowered mechanical performance. Astudy on different nanoparticles was performed and demonstrated the benefits of incorporating nanosilica, acting as nucleation sites for the hydration products and showing an early pozzolanic reaction. Aprofound characterization of various cementitious mixtures was carried out and revealed the eects of nanosilica and SAPs on the micro- and macrostructural properties. The addition of 0.2% of SAPs largely increased the pore volume and decreased the density of the cementitious material. Caused by the presence of macropores, the compressive strength reduced up to 15% in comparison to the reference material. On the contrary, the substitution of 2% of cement by nanosilica generated a pore refinement as well as an improvement of the compressive strength by 12%. By combining both SAPs and nanosilica, a similar compressive strength of the mortar was obtained with respect to the reference material. Additionally, the development of autogenous shrinkage in these mixtures was reduced in comparison to the reference material, similar to the addition of SAPs solely. In case cracking does occur, due to shrinkage or other causes, the presence of SAPs inside the cracks promotes the self-sealing and self-healing ability of the cementitious material. By absorption of moisture from the environment, the swelling of the SAPs blocks the cracks, meaning that aggressive substances are no longer able to enter the material. The subsequent release of water promotes the further hydration of unhydrated cement particles inside the cracks and the crystallization of calcium carbonate. The self-sealing and self-healing efficiency were evaluated by means of water permeability experiments and mechanical loading and reloading, respectively. During the water permeability tests, an immediate sealing effect was noticed upon the inclusion of SAPs. Furthermore, a decrease in water permeability was noticed over time as specimens were cured in wet-dry cycles, signifying crack closure. The latter was confirmed by microscopic analysis, showing up to 100% closure of crack widths below 150 m after 28 days of wetdry curing. Regarding the assessment of the mechanical restoration, only a minimal effect of SAP addition was noticed after 14 days of wet-dry curing. The inclusion of nanosilica to SAP mixtures did however show an additional improvement of the crack closure over time, caused by the pozzolanic nature of the nanoparticles. Therefore, it could be concluded that a cementitious mixture was designed that benefitted from the SAPs’ internal curing action and improvement of self-healing, without a reduction of the mechanical performance. The results obtained throughout this research demonstrated the internal curing by the SAPs, leading to the mitigation of autogenous shrinkage, and the promotion of crack closure, based on various measuring techniques. In order to characterize the curing and healing processes and gain a better understanding and real time information of the mechanism, elastic wave set-ups were implemented, based both on passive (acoustic emission) and active (ultrasound) monitoring. The internal curing action induced by the SAPs, releasing their absorbed water upon selfdesiccation, was revealed by the use of acoustic emission. As the moment of water release is highly important to effectively reduce autogenous shrinkage, the use of acoustic emission paves the way for reliable, noninvasive and easily applicable monitoring of this delicate microstructural process. Additionally, the improved self-sealing and self-healing ability of mixtures with SAPs was examined by ultrasound measurements. The adopted technique for the monitoring of the crack openings confirmed the closure of cracks by the restoration of the wave velocity and attenuation over time and provided a non-destructive measuring method for selfhealing evaluation, closely related to the crack closure data

Evaluation of self-healing by a combination of ultrasonic measurements and 3D numerical simulations

Self-healing concrete became an attractive resolution to costly and labour-intensive manual repairs. Up to now, the regain in mechanical performance is generally assessed using destructive tests, which are not suited for in-situ measurements, nor for monitoring purposes. Hence, ultrasound was adopted, combining a non-intrusive character together with a direct correlation to the elastic properties. Ultrasound has shown its potential to evaluate repair and self-healing processes in literature. The wave velocity provides a direct link to the global E-modulus. However, the healing layer cannot be separated from the intact material that is included in the investigated area. Therefore, ultrasonic measurements are combined with 3D numerical wave simulations. Through a comparison between experiments and simulations, an estimation of the elastic properties of the healing layer was performed. Furthermore, a method to evaluate the stiffness and the filling ratio of healed layers within the crack is proposed, based on wave velocity and amplitude

Self-healing evaluation through ultrasonic measurements and 3D numerical simulations

Self-healing cementitious materials have gained attention as a resolution to costly and labourintensive manual repairs. Up to now, the regain in mechanical properties after healing is mostly evaluated through destructive tests, which are neither applicable for in-situ measurements, nor allow to monitor the healing evolution. Thus, a non-intrusive measuring technique is in demand, which could be found in the application of ultrasonic measurements (elastic waves in the ultrasonic frequency range). In this study, a method for evaluating mechanical recovery after healing is investigated by means of elastic waves. It comprises an assessment of the healing ability of mortars by experiments and numerical analysis. Experimental results show the decrease in wave velocities and amplitudes due to the presence of cracks, while upon healing both values are partially restored. To isolate the healing layer from the intact mortar around it, 3D numerical simulations are performed. A comparison between experimental and numerical results enables the determination of elastic moduli of the healing products filled in cracks. Further, a method to evaluate the stiffness and the filling ratio of healed layers at the crack is proposed, based on characteristics of elastic waves (wave velocity and amplitude)

Influence of ultrasonic frequency on the evaluation of self-healing and repair in concrete

Self-healing and repair of cementitious media entails restoration of mechanical properties. However, testing of the effectiveness of the process is not straightforward. Microscopy, and computed tomography can potentially verify the deposition of healing/repairing material in the crack, while other tests like water permeability evaluate the “sealing” properties. However, they do not supply information on the mechanical performance of the healed or repaired layer. It is only possible to check mechanical properties by re-loading, but this cannot be used in-situ while the monitoring of continuous healing cannot take place on the same specimen even in laboratory since the measurement is destructive. This is the research gap that ultrasound can fill. Since elastic waves physically propagate through the material, they gather information on the elastic properties of the different constituents. Ultrasound has been recently used to monitor the healing and repair effectiveness in cementitious materials and structures. The present paper addresses the importance of the applied frequency in different modalities. In one-sided measurements, the wavelength defines the Rayleigh wave penetration and therefore a modification of the frequency defines the depth of material that can be characterized. On the other hand, in through transmission, the wavelength defines essentially the resolution of the technique. This becomes very important for heterogeneous materials and specifically, for crack interfaces at various conditions, like totally empty -acting as discontinuities-, having bridging points between the sides, and partially or fully healed

Self-healing assessment of cementitious mortars through ultrasonic monitoring

info:eu-repo/semantics/inPres