Axial tensile softening characteristics of PVA fiber concrete cured in super absorbent polymer under temperature effects

In the field of practical engineering, concrete faces several key challenges, including low tensile strength, susceptibility to cracking, and risks such as high temperatures after fire that can further degrade its performance. These issues significantly affect the safety and durability of concrete in engineering structures. Traditional curing methods often fall short in providing comprehensive curing effect for concrete. Therefore, incorporating Superabsorbent Polymers (SAP) as an internal curing material for concrete enhances early-age curing effects. Simultaneously, the addition of Polyvinyl Alcohol (PVA) fiber composite materials improves concrete's tensile strength. This combination allows for the full utilization of SAP's hydration enhancement and shrinkage reduction properties while addressing the shortcomings in tensile strength and crack resistance.Currently, there are fewer research results on the synergistic effects of SAP and PVA fibers in concrete, although there have been a large number of studies on the effects of SAP and PVA fibers on concrete properties individually. Meanwhile, the axial tensile softening characteristics of concrete with PVA fibers cured within SAP at different temperatures, as well as the effects of fiber admixture and temperature on the mode of axial tensile fracture, fracture energy, and intrinsic fracture mechanism have not yet been explored in the existing research work. Based on this, this study conducted uniaxial tensile tests of PVA fiber-reinforced concrete cured in SAP, analyzed the change of stress-displacement curves and softening characteristics at different temperatures and fiber admixtures, thoroughly investigated the fracture energy and the intrinsic fracture mechanism of fiber concrete, and explored the synergistic effect of PVA fibers and SAP on the toughening and crack-resisting mechanism of concrete, with a view to providing a new method for the engineering application and damage detection of PVA fiber-reinforced concrete cured in SAP. In order to provide theoretical support for the engineering application and damage detection of SAP cured PVA fiber reinforced concrete.

Axial tensile test adopts MTS322 electrohydraulic servo universal test machine. The specimen is bonded to the steel plate with an epoxy resin adhesive with strength up to 10 MPa. Because the tensile strength of concrete is far less than the strength of epoxy resin adhesive, the specimen will not be damaged at the bond surface. In order to ensure that there is no biasing phenomenon in the axial tensile test, a specially designed ball head connection is used to connect the loading steel plate and the actuator of the testing machine. In order to obtain the descending segment of the tensile loading, the displacement control method was utilized with the loading rate 3.5×10-4mm/s. The type of heating device is BLMT-1800B. The temperature conditions included room temperature (25°C), 200℃, 300℃ and 400℃.

In general, due to the effects of high temperature, the tensile strength of concrete specimens continuously decrease with increasing temperature. The peak tensile strain increases with rising temperature, which is attributed to the increase in concrete ductility caused by high temperatures. The variation in the elastic modulus of the specimens is similar to that of tensile strength, decreasing continuously with increasing temperature. This means that the specimens gradually lose their ability to resist elastic deformation at high temperatures, ultimately leading to complete failure.At room temperature, PVA fibers can effectively enhance the elasticity and tensile strength of concrete while reducing cracks and internal damage. However, at high temperatures, PVA fibers have a negative impact on the strength and elastic modulus of concrete, possibly due to a weakening of the connection between the fibers and the cement matrix. Particularly when the temperature exceeds 230°C, the fibers begin to melt, leading to a decrease in the tensile strength of concrete and an acceleration of damage. Compared to ordinary concrete, SAP and PVA fibers do not significantly improve the peak strain of concrete, except for the SP-0.20% specimens, this may be attributed to the large quantity of PVA fibers enhancing the material's ductility.The fracture energy of concrete specimens generally decreases as the temperature increases. The fracture energy of the specimen increases with fibre incorporation, this may because PVA fibers can still enhance ductility and crack resistance at room temperature and 200°C, absorbing and dissipating some energy and reducing the brittleness of the specimens. However, excessive fiber content can lead to a decrease in the mechanical properties of concrete, resulting in reduced fracture energy. Therefore, the addition of an appropriate amount of PVA fibers can increase the fracture energy of concrete, improving its crack resistance and toughness. The characteristic length of the specimens ranges from 970.6mm to 2110.2mm, and it initially increases and then decreases with an increase in PVA fiber content. This indicates that the crack resistance effect of the fibers is more significant within a certain range of fiber content. The value of the critical crack length of the specimens generally increases with the rise in temperature. When the Hordijk and Li model is used to fit the softening segment curve, it is found that the fitting degree R value is high, indicating that the test curve and the model curve agree well, and with the temperature increases, the falling segment model curve almost completely coincides with the test curve, showing obvious temperature softening effect. However, the fitting effect of the model at 25°C is slightly subpar, because the brittleness of concrete without fiber is relatively high, and the displacement mutation under the bearing load will adversely affect the fracture energy, critical crack length calculation and model parameter fitting. By Analysing the pattern of fitted parameters, it is found that Hordijk model can better describe the decreasing trend of softening section after peak.Conclusions: The main conclusions of this paper are summarized as following:(1) The primary failure mode of concrete under tension is mainly characterized by expansion and penetration at the interface between aggregate and mortar. The temperature effect significantly influences the inherent fracture mechanism of Super Absorbent Polymer (SAP) internally cured Polyvinyl Alcohol (PVA) fiber-reinforced concrete.(2) The temperature effect results in a significant softening behavior in the post-peak segment of the stress-strain curve of concrete, while the addition of PVA fibers at room temperature does not significantly affect the residual stress variation in the specimens after the peak.(3) The fracture energy of concrete specimens generally decreases with the increase of temperature. At the same temperature, the characteristic length of the specimens increases first and then decreases with the fiber content, reaching its maximum at a 0.05% dosage. The critical crack length of the concrete gradually increases with the increase of temperature.(4) By comparing model parameters, it was found that compared to the Li model, the Hordijk model provides a higher fitting accuracy and better describes the influence of changes in inherent mechanisms of SAP internally cured PVA fiber-reinforced concrete on the softening characteristics of axial tensile test curves under different conditions.(5) The fitted model parameters can provide corresponding numerical predictions and experimental basis for stability analysis and crack calculation of similar fiber-reinforced concrete structures under different temperatures. However, the established softening model still has certain limitations, and further research is needed to explore its applicability to other types of fiber-reinforced concrete under different conditions.