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    • Influence of free water content on the compressive mechanical behaviour of cement mortar under high strain rate

      Jikai Zhou Xudong Chen Longqiang Wu Xiaowei Kan

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      The effect of free water content upon the compressive mechanical behaviour of cement mortar under high loading rate was studied. The uniaxial rapid compressive loading testing of a total of 30 specimens, nominally 37 mm in diameter and 18.5 mm in height, with five different saturations (0%, 25%, 50%, 75% and 100%, respectively) were executed in this paper. The technique ‘Split Hopkinson pressure bar’ (SHPB) was used. The impact velocity was 10 m/s with the corresponding strain rate as 102/s. Water-cement ratio of 0.5 was used. The compressive behaviour of the materials was measured in terms of the maximum stress, Young’s modulus, critical strain at maximum stress and ultimate strain at failure. The data obtained from test indicates that the similarity exists in the shape of strain–stress curves of cement mortars with different water content, the upward section of the stress–strain curve shows bilinear characteristics, while the descending stage (softening state) is almost linear. The dynamic compressive strength of cement mortar increased with the decreasing of water content, the dynamic compressive strength of the saturated specimens was 23% lower than that of the totally dry specimens. With an increase in water content, the Young’s modulus first increases and then decreases, the Young’s modulus of the saturated specimens was 23% lower than that of the totally dry specimens. No significant changes occurred in the critical and ultimate strain value as the water content is changed.

    • Mechanical properties of concrete to cyclic uniaxial tensile loading using variable waveforms

      YUZHI CHEN XUDONG CHEN JINGWU BU

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      The mechanical properties of concrete under cyclic tensile loading using square waveform, sine waveform and ramp waveform are studied. The experiments are performed on a closed-loop electro-hydraulic servo-controlled material testing system (MTS). The axial strain, dissipated energy per loading cycle, the damage evolution law and deformation modulus are mainly studied. The results show that the three-stageevolution law of axial strain and damage variable of concrete under ramp waveform and sine waveform are more obvious than those under the square waveform. The dissipated energy changes at different stages of fatigue life.At the beginning and end of the fatigue life, the rate of dissipated energy is higher than that at the medium stage of the fatigue time, which is attributed to the formation of cracks. The evolution of deformation modulus of concrete subjected to cyclic tensile loading using three loading waveforms also shows three stages: fast increase in the damage—increase at a slow constant rate—and accelerated increase in damage until failure.

    • Dynamic compressive and splitting tensile properties of concrete containing recycled tyre rubber under high strain rates

      GUO YANG XUDONG CHEN WEIHONG XUAN YUZHI CHEN

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      In order to raise the efficiency of resource utilization, recycling waste rubber particles into concrete as aggregate has been widely accepted. When the size and content of the rubber particles are appropriate, rubberized concrete can achieve many excellent properties. This study investigated the impact of rubberreplacement on dynamic compressive and splitting tensile properties of concrete. The split Hopkinson pressure bar tests of rubberized concrete containing 5%, 10%, 15% and 20% volume replacement for sand were completed. The failure modes, stress curves and dynamic strength values of rubberized concrete under high strain rates were recorded. The results reveal that the dynamic compressive and splitting tensile strength of rubberized concrete decrease with increasing rubber content. Meanwhile, peak strain increases with increasing rubber content. Dynamic increase factors (DIFs) of compressive and splitting tensile strength also were calculated, where rubberized concrete shows a stronger strain rate sensitivity. The analysis of specific energy absorption illustrates that rubberized concrete with 15% rubber replacement has the best impact toughness. In addition, ratios of dynamic compressive–tensile strength of rubberized concrete were calculated, which are between 3.82 and 5.39.

    • Experimental study on cyclic tensile behaviour of air-entrained concrete after frost damage

      JIAYI YUAN XUDONG CHEN SHENGSHAN GUO

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      To investigate fatigue tensile behaviour of air-entrained concrete after the freeze–thaw damage, fatigue tensile tests with four different loading paths were conducted on air-entrained concrete after 0, 100, 200 and 400 freeze–thaw cycles. The four different loading paths contained the monotonic (M) test where theenvelope stress–strain curve was obtained, the cycles with constant strain increment (CSI) test where the variation of elastic modulus on the whole stress–strain curve was studied, the cycles to variable maximum strain amplitude (VMS) test where the low-cycle fatigue behaviour at different strain levels was analysed and the cycles with CMS’ test, which was designed to analyse the post-peak behaviour of the specimens. Experimental results indicated that the properties of the air-entrained concrete basically remained unchanged under 200freeze–thaw cycles, including the mass loss rate, tensile strength, elastic modulus and the dissipated energy per unit volume. While the freeze–thaw cycles increased over the critical value, the energy resulted from the cyclic load was not released from the materials and accumulated inside the materials fast. Energy accumulation directly led to the deterioration of the air-entrained concrete. To observe the pore structure of the air-entrained concrete, the scanning electron microscope test (SEM) was also adopted in this paper.

    • Experimental study on the pH for activating ground granulated blast-furnace slag activity at different temperatures

      PING LI JIANHUI TANG YIN BAI XUDONG CHEN JUNJUN CHEN

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      In order to explore the initial pH values of activator solution needed for early activation of slag at different temperatures, the effects of curing temperatures (5, 20 and 35°C) and pH (12.10, 12.55, 13.02 and 13.58) on the activation and hydration characteristics of ground granulated blast-furnace slag (GGBFS) wereinvestigated. Sodium hydroxide (NaOH) was used as the alkaline activator. The compressive strength and non evaporable water content of GGBFS paste cured for 1, 3, 7, 14 days were determined. The hydration characteristics of slag pastes at different temperatures and pH were analyzed by XRD and SEM. The results showed that the pH value required for slag activation decreased with increasing temperature. Based on the 3 days strength of slag paste, the pH values required for activation of slag activity at 5, 20 and 35°C were 13.58, 13.02and 12.10, respectively. With the high temperature and high pH of solution, a dense calcium-rich product formed on the surface of slag particles, which suppressed the further slag reaction. With the low temperature and highpH of solution, a layer of network product was found on the surface of the slag particles, and the later strength developed rapidly as the further hydration was not prevented. This study provides a reference for the application of alkali-activated slag.

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