Influence of Citric Acid on the Fundamental Properties of CO2 Cured Magnesium Oxysulfate Paste.

Magnesium oxysulfate (MOS), mainly composed of magnesium oxide and magnesium sulfate, is a kind of gas-hardening cementing material with low energy consumption and CO2 emissions. In order to develop environment-friendly cement-based materials, MOS needs to be studied systematically. The paper mainly investigates the influence of citric acid (a retarder) on the working and mechanical properties of MOS paste. In this study, the setting time of fresh MOS paste is determined. The flexural and compressive strengths of hardened specimens exposed to the environment of water dry-wet (D-W) alternations, freeze-thaw (F-T) cycles, and sulfate D-W alternations are investigated. Furthermore, the drying shrinkage (D-S) rate of MOS paste is tested for 3 days and 28 days. The specimens are cured in standard or CO2 curing environments. A scanning electron microscope energy spectrum (SEM-EDS) is obtained to analyze the morphology of hydration products. Results show that citric acid can increase the setting time of MOS paste. The citric acid and CO2 curing have a positive effect on the mechanical strengths and the resistance to erosion by water, F-T cycles, and sulfate D-W alternations. The D-S rate decreased in relation to the increasing dosages of citric acid and increased with CO2 curing. MOS with 0.3% of the total binder material mass shows the best erosion resistance. As observed in the results of SEM-EDS, the CO2 curing and the citric acid can make the hydration products denser.

Experimental and Simulation Study of the Fracture Instability Behavior in Polypropylene Fiber-Reinforced Concrete.

This study investigated the fracture characteristics of plain concrete and polypropylene fiber-reinforced concrete (PFRC) using pre-notched three-point bending beam tests with the digital speckle correlation method (DSCM). Then, the fracture instability behavior of the two types of beams was simulated in finite elements based on the plastic damage model and the cohesion model, for which the applicability was assessed. Furthermore, the stability of the Big Gang Mountain Dam made from plain concrete or PFRC subjected to the earth-quake loading was simulated with the plastic damage model. The results show that the limiting length of the non-local deformation zone can be used as an indicator of instability damage in a concrete structure. The simulation results of the plastic damage model agreed well with the local deformation in the pre-notched three-point bending beam test obtained from the DSCM. The plastic damage model was found to be capable of describing the residual strength phenomenon, which the cohesive model was not capable of. The damage evolution regions of the PFRC dam are strictly constrained in some regions without the occurrence of the local deformation band across the dam, and PFRC can dramatically reduce the failure risk under earthquake loading. The numerical solution proves that PFRC is an advisable material for avoiding failure in concrete dams.

Experimental Studies on the Seismic Performance of Prefabricated Circular Hollow Bridge Piers Constructed with PVA Fiber Concrete.

To investigate the seismic performance of prefabricated circular hollow piers with socket and slot connection, eight 1/3.5-scale specimens constructed with polyvinyl alcohol (PVA) fiber at the pier body were tested. The main test variables included the axial compression ratio, grade of pier concrete, shear-span ratio, and stirrup ratio. The seismic performance of prefabricated circular hollow piers was studied and analyzed from the aspects of the failure phenomenon, hysteresis curve, bearing capacity, ductility index, and energy dissipation capacity. The test and analysis results showed that all specimens suffered from flexural shear failure, and the increase in axial compression ratio and stirrup ratio would lead to more significant spalling of the concrete at the bottom of the specimen, but the existence of PVA fiber would improve this phenomenon. In a certain range, the increase in axial compression ratio, stirrup ratio, and the decrease in shear span ratio can improve the bearing capacity of the specimens. However, an excessive axial compression ratio would easily lead to a decrease in the ductility of the specimens. The increase in the stirrup ratio and shear-span ratio caused by the change in height can improve the energy dissipation characteristics of the specimen. On this basis, an effective shear-bearing capacity model of the plastic hinge area of prefabricated circular hollow piers was proposed, and the prediction effects of specific shear capacity models on test specimens were compared.

Experimental Study on the Mechanical Properties and Compression Size Effect of Recycled Aggregate Concrete.

To explore the basic mechanical properties and size effects of recycled aggregate concrete (RAC) with different substitution ratios of coarse recycled concrete aggregates (CRCAs) to replace natural coarse aggregates (NCA), the failure modes and mechanical parameters of RAC under different loading conditions including compression, splitting tensile resistance and direct shear were compared and analyzed. The conclusions drawn are as follows: the failure mechanisms of concrete with different substitution ratios of CRCAs are similar; with the increase in substitution ratio, the peak compressive stress and peak tensile stress of RAC decrease gradually, the splitting limit displacement decreases, and the splitting tensile modulus slightly increases; with the increase in the concrete cube's side length, the peak compressive stress of RAC declines gradually, but the integrity after compression is gradually improved; and the increase in the substitution ratio of the recycled aggregate reduces the impact of the size effect on the peak compressive stress of RAC. Furthermore, an influence equation of the coupling effect of the substitution ratio and size effect on the peak compressive stress of RAC was quantitatively established. The research results are of great significance for the engineering application of RAC and the strength selection of RAC structure design.

An Experimental Study of Dynamic Compression Performance of Self-Compacting Concrete.

To investigate the dynamic performance of self-compacting concrete (SCC), the dynamic uniaxial compression tests at eight different loading strain rates were performed on the ordinary concrete and SCC cubic specimens. Based on the tests, the compression failure patterns and stress-strain curves of both kinds of concrete were obtained. The results show that SCC performs more brittle than ordinary concrete by showing the diagonal crack failure pattern of SCC at a high strain rate. Besides, with the increase of loading strain rate, the peak compressive stress of SCC is slightly lower than that of ordinary concrete, but the increase of elastic modulus is slightly higher than that of ordinary concrete. The peak compressive strains of the two kinds of concrete are discrete under the influence of loading strain rate, thus putting forward the relation equation for the loading strain rate and peak compressive stress increase coefficient of the two kinds of concrete. Besides, based on the theory of elastic-plastic damage and considering the dynamic extension of damage, the dynamic constitutive relation with good applicability between ordinary concrete and SCC was established.

The Application of Electrical Parameters to Reflect the Hydration Process of Cement Paste with Rice Husk Ash.

This paper aims to study the electrical parameters (electrical resistivity and alternating current (AC) impedance spectroscopy) of cement paste with rice husk ash (RHA). The water to cement (Mass ratio of water to cement (w/c)) ratios of the paste in this study varied from 0.4 to 0.5. The mass ratio of rice husk ash in each w/c ratio of specimens ranged from 0% to 15% by t mass of cement. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to determine the microstructures of specimens. Moreover, the slump flow and plastic viscosity of fresh paste were determined. The results indicated that with the increasing dosage of RHA, the fluidity decreased, while the plastic viscosity increased. Meanwhile, a high w/c ratio led to a low plastic viscosity and high slump flow. The electrical resistivity of RHA cement paste gradually ascended with the increasing curing period. The conduction of specimens intricately changed by mixing RHA, a reasonable equivalent circuit was selected to describe the conduction mechanism by AC impedance spectroscopy. Additionally, the results of XRD and SEM showed that RHA could effectively promote the hydration process as well as decrease the size and number of cracks in hardened cement paste.