International Concrete Abstracts Portal

Ultra-high-performance concrete (UHPC) features tensile strain-hardening behavior and a high compressive strength. Existing studies on the shear behavior of UHPC structural members have been focused on prestressed UHPC girders. More experimental data of the shear behavior of non-prestressed UHPC beams are necessary to quantify the safety margin of shear designs for structures. Moreover, while the UHPC members in most studies did not contain coarse aggregate to strengthen their microstructure, the inclusion of coarse aggregate has been shown to substantially reduce the autogenous shrinkage and enhance the elastic modulus for UHPC materials, which is beneficial for structural applications of UHPC. This study experimentally investigated the shear failure behavior of eighteen non-prestressed rectangular UHPC beams. The experimental variables included the volume fraction of fibers, shear span-to-depth ratio of the beams, and coarse aggregate. The detailed shear failure responses of the UHPC beams were discussed in terms of the damage pattern, shear modulus, shear strength, shear strain, and strain energy. The test results showed that the inclusion of coarse aggregate increased the beam shear strength, and its enhancement became more significant with a higher volume fraction of fibers and a lower shear span-to-depth ratio of the beam. In addition to the experimental investigation, a shear strength model for non-prestressed rectangular UHPC beams that accounts for the interactive effect of the key design parameters was developed. An experimental database of the shear strength of the UHPC beams in existing studies was established to assess the performance of the proposed model. It was shown that the proposed model reasonably predicted the shear strength of the UHPC beams in the database with a higher accuracy and lower scatter compared to the results of existing models.

This paper examines the influence of harvested fly ash on the properties of mortar and concrete. Class F and harvested fly ash were used at the substitution rate of 20% by weight of Portland cement. The investigated properties included heat release, consistency, setting time, compressive strength at different testing ages, absorption, the volume of permeable voids, surface resistivity, and drying shrinkage. The results revealed that the harvested fly ash produced the lowest released heat of hydration and longest setting times. Mixtures containing harvested fly ash displayed lower strength at all curing ages. Compared to traditional fly ash, harvested fly ash showed inferior transport properties for both absorption rate, permeable voids, and surface resistivity. Mixtures containing harvested fly ash showed comparable 120-day drying shrinkage when compared with the companion mortars made with traditional fly ash.

The management of municipal solid waste incineration fly ash (MSWI FA) has become a critical issue as its generation increases rapidly along with the global population growth. In this study, MSWI FA was treated via water-washing, and then the untreated and water-treated MSWI FAs (RFA and WFA) were blended with mainstream supplementary cementitious materials (SCMs), including ground granulated blast-furnace slag (GS), silica fume (SF), and limestone powder (LS). The MSWI FASCMblends were used as a cement replacement in a mortar. The content of MSWI FAs was set at 10% (by weight of binder) for all mortar mixtures. The content of GS and LS was also set at 10%, while the SF content was 2.5%. Flowability, setting time, isothermal calorimetry, compressive strength, and free-drying shrinkage tests were performed. The results showed that mortars containing raw (untreated) fly ash (RFA) had reduced strength, whereas mortars containing water-treated fly ash (WFA) displayed comparable or even higher strength than the control mortar (made with 100% cement) after 28 days. While mortars containing RFA showed increased drying shrinkage, mortars containing WFA exhibited diminutive or no increase in drying shrinkage when compared to the control mortar. Based on the test results, the mixture with a cement:WFA:GS ratio of 80:10:10 was the optimal binder for concrete applications.

Serviceability checks in Reinforced Concrete (RC) elements involves the verification of crack width mainly aimed to limit the exposure of the steel reinforcement to corrosion and chemical attack and, thus, improve durability. Classical approaches for assessing the crack width in RC elements provide the calculation of two terms: 1) the average crack spacing, and 2) the average difference between the strain in the steel reinforcement and in the concrete in tension referred to the average crack spacing. A similar approach can be assumed valid also for RC elements strengthened with externally bonded Fiber Reinforced Polymer (FRP) materials, taking into account the additional tension stiffening effect provided by the external reinforcement.

This paper presents the comparisons of some existing code formulations for predicting crack spacing and crack width in RC elements with the experimental results of a database collected by the Authors and concerning tests on RC beams and ties externally bonded with different types and configurations of FRP materials. The paper is mainly aimed to check the reliability of the existing equations provided by codes in order to address the future assessment of reliable design provisions for cracking verifications in RC elements strengthened with FRP materials. The comparisons have evidenced, indeed, some useful issues for the design provisions: 1) larger scatter in the predictions of crack width than in crack spacing and, in particular, for ties, 2) limited effect of shrinkage on crack width, 3) necessity of taking into account the external reinforcement in crack spacing formulations, 4) good reliability of mechanical models for calculating cracks width.

This paper presents experimental and theoretical investigations on the residual tensile and bond response of polypara-phenylene-benzo-bisthiazole (PBO) fabric reinforced cementitious matrix (FRCM) composites after the exposure to elevated temperatures ranging between 20 °C [68 ºF] and 300 °C [572 ºF]. Experimental results obtained from direct tensile (DT) and single-lap direct shear (DS) tests carried out respectively on PBO FRCM specimens and PBO FRCM-concrete elements were reported and discussed. Overall, specimens exposed to temperatures up to 200 °C [392 ºF] did not present significant reductions of both bond and tensile properties. This result can be attributed to the thermal shrinkage underwent by the inorganic matrix, which may enhance the bond between the fibers and the matrix. On the other hand, when the specimens were heated at 300 °C [572 ºF], marked reductions were observed, primarily stemming from the degradation of both mechanical properties of the FRCM constituent materials and the fiber-to-matrix bond. Subsequently, the experimental results were used for the following purposes: (i) to assess whether the Aveston–Cooper–Kelly (ACK) theory is able to describe the tensile behavior of FRCM materials at elevated temperatures; (ii) to define temperature-dependent local bond stress vs. slip law and (iii) to evaluate the ability of degradation models to simulate the variation with temperature of the FRCM tensile and bond properties. The results obtained from the theoretical analyses showed that, for all the tested temperature, the relative differences between predicted and experimental results are very low, confirming the accuracy of the proposed approaches.