Author: Li, Long
Title: Dynamic mechanical behavior of recycled aggregate concrete
Degree: Ph.D.
Year: 2018
Subject: Hong Kong Polytechnic University -- Dissertations
Aggregates (Building materials) -- Recycling
Concrete -- Recycling
Pages: xxiv, 220 pages : color illustrations
Language: English
Abstract: Using recycled aggregate concrete (RAC) is generally considered as one of the most effective measures to solve the problem of waste concrete. At present, most of the researches on RAC are aboutitsstatic mechanical behaviors, andonlya few are about its dynamic mechanical behaviors. In this study, the dynamic mechanical behaviors of RAC were studied. Firstly, based on cylindrical RAC specimens and modeled recycled aggregate concrete (MRAC) specimens, the dynamic mechanical behaviors of RAC under uniaxial compressive loadings at low strain rates (10⁻⁵/s to 10⁻¹/s) were experimentally studied. The strain-rate sensitivity of RAC and the influence of recycled coarse aggregate (RCA) replacement percentage, the static strength, and the moisture condition were studied. The results showed that with the increase in strain rate, the peak stress and elastic modulus of RAC increased, the peak strain fluctuated around a constant value, while the failure pattern had no obvious difference. The strain-rate sensitivity of RAC with lower static strength was more significant. The strain-rate sensitivity of RAC with 100% RCA was larger than that of natural aggregate concrete (NAC). There was no significant difference between the strain-rate sensitivity of RAC under wet and air-dry conditions. Secondly, based on the cylindrical RAC specimens, the dynamic mechanical behaviors of RAC under uniaxial compressive loadings at high strain rates (10¹/s ~ 10²/s) were experimentally studied by using a Split Hopkinson Pressure Bar (SHPB). The strain-rate sensitivity of RAC and the influences of the RCA replacement percentage and moisture condition were explored. The results showed that at high strain rates, the peak stress and elastic modulus of RAC increased with the increase in strain rate and the increase was more significant than that at low strain rates, the peak strain fluctuated around a constant value with the increase in strain rate, there were more fractured aggregates than that at low strain rates, the strain-rate sensitivity of RAC was slightly larger when RCA replacement percentage was higher, and the moisture condition had no significant influence on the strain-rate sensitivity of RAC.
Thirdly, MRAC specimens were used to study the effect of carbonation on the mesoscopic properties of RCA and the static mechanical behavior of RAC. The effect of using carbonated RCA on the dynamic mechanical behaviors of RAC at low strain rates was studied based on the cylindrical specimens. The results showed that the old ITZ and old mortar in RCA were enhanced after carbonation, the static and dynamic peak stress and elastic modulus of RAC with carbonated RCA increased, while the strain-rate sensitivity of RAC with carbonated RCA decreased. Based on the test results, the relation between the dynamic increase factor of peak strain (DIFf) of RAC and the strain rate ranging from 10⁻⁵/s to 10²/s was established, and the mechanism of strain-rate sensitivity of RAC was discussed. It showed that Stefan effect, inertia force effect and cracking development were not the main cause of the strain-rate sensitivity of RAC at low strain rates; at high strain rates, Stefan effect and transverse inertia effect had no significant influence on the strain-rate sensitivity of RAC, while longitudinal inertia effect had some influence on the strain-rate sensitivity of RAC but it was still not the leading factor.The strain-rate dependence of crack propagation resistance of meso-phase materials may be the leading factor of strain-rate sensitivity of RAC when the strain rate varied from 10⁻⁵/s to 10²/s. In addition, a static and dynamic model of RAC was established, which explained the reason why the strain-rate sensitivity of RAC with 100% RCA was larger than that of NAC. Finally, finite element model based on the MRAC was established. Considering the strain-rate sensitivity of all meso-phase materials (i.e. mortar, ITZ, and aggregate), the overall strain-rate sensitivity of RAC was studied. The influences of the strain-rate sensitivity of each meso-phase material on the overall strain-rate sensitivity of RAC were also studied. Moreover, the influence of RCA replacement percentage and the strength of the new mortar and the old mortar on the strain-rate sensitivity of RAC were studied. The simulation results showed that the peak stress and elastic modulus of RAC increased almost linearly with the increase in strain rate, and the increasing of elastic modulus was more uniform. When compared with ITZ and aggregate, the strain-rate sensitivity of RAC were mainly decided by the strain-rate sensitivity of the mortar in RAC. There was no clear relationship between the RCA replacement percentage and the strain-rate sensitivity of peak stress, while the strain-rate sensitivity of elastic modulus was larger when the RCA replacement percentage was larger. When the strength of the new mortar or the old mortar was lower, the strain-rate sensitivity of elastic modulus was greater, but the strain-rate sensitivity of peak stress did not show a strict increase.
Access: open access

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