Key Engineering Materials Vols. 629-630

Abstract: High-performance concrete (HPC) will undergo severe damage under fire conditions. It is well known that vapor pressure induced by high temperatures plays an important role in the damaging process. Therefore, the determination of the moisture distribution evolution in concrete is essential to the damage analysis of heated HPC. This paper presents a numerical method for the prediction of the moisture distribution evolution in HPC under fire conditions. In the method, the vapor pressure and the moisture transport induced by the vapor pressure gradient are analyzed. The effect of the thermal decomposition on the moisture distribution and the effects of the slippage flow and the water saturation degree on the permeability are considered. The proposed method is applied to the moisture distribution analysis of a concrete cube with 90% initial moisture content under fire conditions and can be further used for the analysis of the thermal damage of heated HPC.

Abstract: After being subjected to different elevated temperatures, ranging between 200 °C and 800 °C, the flexural strength, matrix mass loss rate and water absorption of polypropylene (PP) macro-fiber reinforced high strength concrete (HSC) were investigated. Moreover, the internal damage of concrete was analyzed by the ultrasonic non-destructive testing technology. The results indicate that PP macro-fiber in HSC has an adverse effect on flexural strength, while the synergistic effect of hybrid fibers (PP micro-fiber plus PP macro-fiber) can minimize this effect. Compared with PP micro-fiber, PP macro-fiber is more effective in increasing the matrix mass loss rate and water absorption of HSC. However, if the dosage of PP macro-fiber is too high, the pressure relief channels formed by fibers melt will be too coarse, and the total porosity of HSC will be increased significantly. Finally, a mathematical model relating the damage degree to temperature was established based on the non-linear fitting of the experimental data.

Abstract: This paper studied the cascade use of industrial waste slag as the cementitious material, the fine aggregate and the coarse aggregate, assisted by XRD, SEM and other microscopic test methods. The results that the system had the best volume stability when the steel slag addition of 40%. When the cement mortar prepared by 10% steel slag fine aggregate, and mixed with 20% steel slag powder and 20% blast furnace slag powder, the mortar construction performance and shrink resistant performance is excellent. On the basis of concrete double mixing 25% steel slag aggregate and 30% steel slag powder, compound mixing 20% blast furnace slag powder, the durable properties of concrete are also excellent.

Abstract: The synergy of hybrid fibers allows for an enhanced concrete composite performance at a lower fiber volume fraction as compared to other types of fiber-reinforced concrete. This paper outlines the development process and properties of a new concrete composite termed high-performance green hybrid fiber-reinforced concrete (HP-G-HyFRC). Steel and polyvinyl alcohol (PVA) fibers were used as discontinuous reinforcement of the composite. Up to 60% of cement by mass was replaced by industrial wastes comprising slag and fly ash. At water-binder ratio of 0.25 and with the presence of coarse aggregates, careful proportioning of the mix constituents allows for a composite that is highly flowable. At a combined fiber volume fraction of only 1.65%, the composite also exhibits a deflection hardening behavior which is known to be beneficial for both serviceability and durability of structures. The composite was proposed to be used in an innovative double skin façade (DSF) system consisting of 30 mm air gap in between two thin HP-G-HyFRC skins with no main reinforcing rebars. It was shown that the DSF system alone allows for about 7.6% reduction of cooling energy in buildings.

Abstract: Circulating fluidized bed combustion (CFBC) fly ash is a promising admixture for construction and building materials due to its pozzolanic activity and self-cementitious property. In this study, CFBC fly ash and coal-fired fly ash were used in Portland cement to investigate the pozzolanic and cementitious characteristics of CFBC fly ash and the properties of cement-based composites. Tests show that CFBC fly ash has the potential instead of cementing materials and as an alternative of pozzolan. In fresh specimens, the initial setting time of mortars increases with the increasing amount of cement replacement by CFBC fly ash and coal-fire fly ash. In harden specimens, adding CFBC fly ash to replace OPC reduces the compressive strength. Meanwhile, CFBC fly ash would results in a higher length change when adding over 30%. Based on the results, the amount of CFBC fly ash replacement cement was recommended to be limited below 20%.

Abstract: This paper presents an experimental research on the influence of defects of recycled coarse aggregate on mechanical properties of recycled aggregate concrete (RAC). Concretes at two water/binder ratios (0.255 and 0.586) were broken into recycled aggregate (RA). A type of thermal treatment, i.e. heating to target temperature 620 °C, which was maintained for 3 hr, was employed to remove mortar from gravel in RA. Tests were conducted on RA and natural aggregate to measure their water absorption and crushing values, and on RAC and natural aggregate concrete (NAC) to measure compressive strength, tensile splitting strength, and fracture energy. The experimental results revealed that both gravel damage and mortar attached can significantly influence the water absorption and crushing value of RA, as well as strength and fracture energy of RAC. The mechanical properties RAC were obviously lower than those of NAC at an identical mix proportion. Moreover the removal of mortar caused additional change in mechanical properties of RA, which might be a decrease in mechanical properties in the case of low water/binder ratio, but might be an increase in mechanical properties in the case of high water/binder ratio. As to RAC at a low water/binder ratio, gravel damage was a main factor governing the decrease in fracture energy, which means a decrease in cracking resistance of hardened RAC, whereas the mortar attached had only a slight influence on fracture energy. However, as to RAC at a high water/binder ratio, mortar attached could significantly cause a greater decrease in cracking resistance of RAC than that caused by gravel damage.

Abstract: This paper presents an original research on the influence of defects in recycled aggregate (RA) on mechanical properties of recycled aggregate concrete (RAC), including compressive strength, splitting tensile strength, fracture energy and elastic modulus. Six types of concretes, with the water to binder ratios (W/B) of 0.26 and 0.60, were prepared using nature aggregate (NA), RA and recycled aggregate treated by 3 mol/L (RA-H). Mechanical properties of RAC was inferior to that of NAC, and treated RA by sulfuric acid solution could improve the mechanical properties. Attached mortar in RA was the main factor resulting in the decrease of mechanical properties of RAC with 0.26 W/B, and for the RAC with 0.60 W/B, the effect of aggregate damage was more significant than that of attached mortar.

Abstract: This paper investigates a set of nonlinear numerical studies slab subjected to blast loading. A three-dimensional finite element model is developed using ABAQUS by emphasizing on using concrete with recycled concrete aggregate and crushed clay bricks (RCA and CCB) to promote the strength of slab against blast loading. Different charge weights of 0.2–0.55 kg equivalent weight of TNT at a 0.4 m standoff above the slabs were considered as variables in order to evaluate damage levels and define the relations among these variables with respect to the residual strength of slab after blast. The maximum deflection and spalling ratio of the specimens were verified with the experimental data. The results corroborate that using recycled concrete aggregate and crushed clay bricks improve the strength of slabs against blast loading. In addition, the best fraction of this type of materials has been illustrated.

Abstract: Utilizing construction waste, mainly waste concrete, to produce recycled cement, provides possibility for realizing the closed circulation between concrete and cement, which is an effective way for sustainable development of cement and concrete industry. Waste clay brick (WCB) and its surface-coated waste mortar are inevitably introduced into the raw meals while waste concrete is used in cement production. So on the basis of the composition characteristics of construction waste, considering of optimizing the performance of recycled cement and improving the adaptability of recycled cement to complex raw material, it's necessary to study the effect of raw meal mixing waste concrete and WCB on sintering process and property of recycled cement clinker. Mixing waste concrete and WCB in three different proportions, the burnability of raw meal, clinker mineral composition, clinker mineral morphology and hydration characteristics of recycled cement have been studied in this paper. The results indicate, the presence of WCB is beneficial for the generation and development of silicate minerals; with increasing proportion of WCB, the content of f-CaO in recycled cement clinker rises, the structure of C-S-H gel from hydration product presents higher densification and the compression strength of 3d and 28d curing period are enhanced.

Abstract: Traditional PHC pile production adopts two-step curing with the disadvantages of high energy consumption, considerable carbon emissions, and easy cracking during construction. In addition, non-autoclaved pipe pile studies involve problems such as adoption of naphthalene based superplasticizer and lengthy coated curing duration, thus leading to their few applications. In order to solve the problems, the study on non-autoclaved PHC pile was conducted to systematically study the impacts of curing system, water-binder ratio, and admixture composition on concrete strength, ratio of tensile and compressive strength, and chloride ion resistance at different ages. Study results show that through the use of a polycarboxylate superplasticizer, an optimal 8-hour curing system and concrete mix ratio, the strength of non-autoclaved concrete reached 93.2MPa within one day, with the ratio of tensile and compressive strength of 6.07%, the coulomb electric flux of 85C, and the chloride migration coefficient of 0.54×10^-12m²/s.