Journal of Advanced Concrete Technology最新文献

The accurate numerical solution of a one-dimensional two-component reaction-diffusion equation including a second-order chemical reaction between concrete constituents and carbon dioxide to generate carbonated products was approximated by a simple analytical function which was given as a function of the effective diffusion coefficient of CO₂, the rate constant of CO₂ absorption, and parameters determined by the initial and the boundary conditions of the system. The pseudo-analytical solution thus obtained showed that the depth profile of carbonation shifts in parallel with square-root of time, and the rate constant of carbonation is determined from the location where the amount of the carbonated product is a half of the maximum amount. Comparison of the pseudo-analytical solution with an observed depth profile of concrete carbonation makes it possible to directly extract the rate constant of concrete carbonation that is necessary for predicting the future progress of the carbonation.

Six square steel tube-encased concrete (SC) columns confined by bolted circular thin steel tube were fabricated and tested under cyclical reversed lateral load to investigate their structural behavior. The primary experimental variables included the axial load ratio, the grade of the encased square steel tubes (FB rank and FC rank), the infilling of concrete into the encased steel tube, and the thickness of outer circular bolted thin steel tubes. Experimental results revealed that confinement by the bolted circular thin steel tube with outer-diameter-to-thickness ratio of 189 could ensure sufficient ductility to the SC columns, and the bolted thin steel tube did not rupture until the drift angle of about 0.09 rad. Furthermore, a simple evaluation method for the ultimate flexural strength of SC column section was proposed along with a numerical analytical method to predict the overall behavior of SC columns. The proposed methods can take the confinement effect by the bolted circular steel tube into consideration. Fairly good agreement between the experimental results and the calculated ones verified the reliability and accuracy of the proposed methods.

Paste volumes protected by air voids against frost attacks were estimated using Dirichlet tessellation tiles. Each tile was regarded as an area protected by air voids. The characteristic distance was defined by the largest tile size to reach a cumulative area fraction of 0.95. The significance of this distance was verified by a Monte Carlo test for the simulation of random point patterns. Comparing the characteristic distance and conventional spacing factor, the latter corresponds to the actual distance required for protecting the local region with the highest vulnerability to frost attack. The tessellation model provides the protection characteristic distance without overlaps even in the region of clustered air voids.

This study is a continuation of the published research studies relevant to self-compacting geopolymer mortar (SCGM) prototype using fly ash. The aim of this study was to investigate the effect of fly ash quantity on the rheological properties of SCGM. The flow properties include relative flow area (G_m) and relative funnel speed (R_m) is determined with the variation of fly ash to sand ratio (FA/S), volume of water to powder ratio (Vw/Vp), and superplasticizer to powder ratio (Sp/P). The test results exhibited that the increase in FA/S from 0.5 to 1.0 positively affected the G_m and R_m of SCGM. The maximum G_m of 10.90 and R_m of 1.43 were obtained for the SCGM mix having FA/S of 1.0, Vw/Vp of 1.02, and Sp/P of 3%. Overall, test results exhibited that with an increase in FA/S comparable flow properties of SCGM were achieved even at lower Vw/Vp and Sp/P. The recommended boundary for SCGM is proposed by comparing the experimental test result of this study with previous studies.

Reinforced concrete shear walls are commonly used in buildings to resist lateral loads due to wind and seismic action. They are typically either cast-in-place or precast, with the latter solution used to achieve high construction speed and quality control. At the same time, the main challenge with precast solutions is to ensure appropriate connections between the adjacent walls, as well as the anchorage of the walls in the foundations. A hybrid structural system combining precast and cast-in-place concrete can provide the advantages of both methods such as faster construction, better quality control, improved structural performance, and durability. This study focuses on investigating the shear behaviour of squat hybrid shear walls through full-scale experimental testing. The tests include one conventional cast-in-place wall and one hybrid wall with a pre-wall system (two precast walls) and cast-in-place concrete core. Detailed measurements and kinematic-based modelling are used to develop comprehensive understanding of the behaviour of the test specimens. It is shown that the hybrid method of construction does not affect the stiffness of the walls and results in a slight reduction of shear strength. It is also shown that the three-parameter kinematic theory can be used to predict the shear strength and key deformation components of the tested walls.

Raw fly ash (RFA) was modified by a self-developed new dry energy-saving vertical grinding mill. This was beneficial to improve the particle morphology and distribution of RFA and enhanced its practical applicability in cement and concrete. The physical modification mechanism of RFA was elucidated by simulating the grinding process, and the physical properties and the application performance of modified fly ash (MFA) were characterized. The results demonstrated that the new grinder can effectively realize the physical modification of RFA and significantly improve its properties because the porous structure and the intactness of glass beads of RFA are preserved without being destroyed. The fineness of MFA was improved to the standard of Class I or II fly ash, the 28 d strength activity index of which was increased by 7% to 17%. The fluidity of mortar was also improved by 15 to 34 mm. In addition, the water requirement ratio can be significantly reduced when MFA is used to prepare C30 and C45 concrete. The 28 d strength of concrete with 50% content of MFA was increased by 23.76% and 15.84%, respectively. These results suggest that the grinding process studied here is a novel method for improving the performance of MFA.

To secure good quality post-installed anchors with a relatively short anchorage length and sufficient pull-out/shear resistance, an anchoring method with enlarged diameter at the end of drilled holes has been developed. Anchors were provided with head plates and fixed into the enlarged holes using non-shrinkage high-strength cementitious grout. Pull-out and shear preliminary tests were conducted to investigate the behavior and evaluate the strength of such anchors set in concrete. Furthermore, an evaluation method, based on the Japanese recommendations for design of composite constructions, was proposed. The evaluated pullout and shear capacities of all tested anchors designed to fail by steel yielding ensured sufficient safety margin as to test results, whereas those of some anchors designed to fail by concrete cone breakout should be reduced.

Post-installed rebar (PIR) is extensively utilized for rehabilitating, strengthening, and retrofitting existing concrete structures, and its anchorage design greatly concerns the failure mode and tensile behavior. PIR anchored in joints or columns generally suffers pressures normal to its anchorage section in one direction, and PIR's failure mode and tensile behavior can be greatly affected. However, limited research on the unilateral pressure effect for PIR has been conducted, with remaining uncertainties on applications and designs for PIR. Thus, this paper carried out the pull-out tests of 38 specimens with various anchorage conditions (20 unilateral pressure specimens, 14 no-lateral pressure specimens, and four bilateral pressure specimens) to investigate the bond behavior for PIR subjected to unilateral pressure. Besides, the effects of concrete strength, rebar diameter, and anchorage length on PIR under unilateral pressure were also considered in the tests. The test results showed that the no-lateral pressure specimens split in the concrete and adhesive layer. In contrast, the unilateral and bilateral pressure specimens occurred two typical failures [adhesive-rebar (A-R) interface failure and adhesive fracture failure]. In addition, the interfacial damage and cracking pattern were discussed in detail. Then, the bond strength and bond slip of specimens were investigated. The result showed that the bond strength under unilateral pressure was greater than that under no-lateral pressure but less than that under bilateral pressure, and there was no obvious change in bond strength while the unilateral pressure increased. Regarding bond slip, it was found that the bond slip increased with the bond strength. This paper performed an experimental investigation on failure modes and cracking patterns for PIR under unilateral pressure. It analyzed the unilateral pressure effect on the bond performance for PIR, raising the safety considerations about PIR applications in load-bearing structures.

Low-field Nuclear Magnetic Resonance (LF-NMR) technique has been attracting increasing concern in nondestructively characterising cement-based materials (CBMs), whose nanoscale pore structure are sensitive to water removal. In order to achieve the multi-exponential inversion of relaxometry data preferred by the interpretation on local pore structure of CBMs, an algorithm incorporating L1 regularisation with capability of yielding sparse solution is developed with the aids of Interior-Point method and various principles for optimising the regularisation parameter. Numerical analyses on representative cases show that, the proposed algorithm equipped with the Morozov discrepancy principle is capable of resolving all artificially designed exponential components of various intensities with satisfactory accuracy and precision, even at relatively low signal-to-noise ratio. When applying to resolve the relaxometry data obtained on a cement paste, the algorithm is good at characterising its pore structure with clear significance and capturing its detailed evolution during curing under hot water with good precision.

Over the past years there has been an increasing trend to use supplementary cementitious materials in concrete to improve its sustainability credentials and durability properties. Perhaps amongst the most popular supplementary cementitious materials is ground granulated blast-furnace slag (GGBS). While the hydration and setting characteristics of mixes with GGBS cured under standard conditions (20°C) has been adequately investigated, the effect of temperature on heat of hydration and setting and the interrelation of these properties has not been evaluated for GGBS containing mixes. In this study, the heat of hydration and setting behaviour of mixes with various levels of GGBS (0, 20, 35, 50 and 70%) cured under elevated temperatures (20, 30, 40, 50 and 60°C) is determined. Elevated curing temperature accelerates the hydration reactions and can significantly reduce the setting time of GGBS-containing mixes. The investigation of heat of hydration at very early ages, can provide an indication of the initial and final setting times of cementitious mixes. The “apparent” activation energy used to characterise temperature sensitivity of cementitious systems, is calculated based on heat of hydration and setting time measurements. It was found that the “apparent” activation energy increases with GGBS content for both heat of hydration and setting behaviour. The value of “apparent” activation energy differs significantly depending on the material property that is considered, such as compressive strength, heat of hydration or setting time.