Effect of Stress Amplitude on the Damping of Recycled Aggregate Concrete.
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ABSTRACT:
SUBMITTER: Liang C
PROVIDER: S-EPMC5455513 | biostudies-other | 2015 Aug
REPOSITORIES: biostudies-other
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Project description:At present, many modification methods have been proposed to improve the performance of recycled aggregate concrete (RAC). In this study, tests on the compressive strength and damping properties of modified RAC with the addition of different proportions of recycled coarse aggregate (RCA) (0, 50, 100%), rubber powder (10, 15, 20%), steel fibre (5, 7.5, 10%) and fly ash (15, 20, 5%) are carried out. To elucidate the effect of the modification method on the interfacial transition zone (ITZ) performance of RAC, model ITZ specimens are used for push-out tests. The results show that when the replacement rate of RCA reaches 100%, the loss factor of the RAC is 6.0% higher than that of natural aggregate concrete; however, the compressive strength of the RAC decreases by 22.6%. With the addition of 20% rubber powder, the damping capacity of the modified RAC increases by 213.7%, while the compressive strength of the modified RAC decreases by 47.5%. However, with the addition of steel fibre and fly ash, both the compressive strength and loss factor of the RAC specimens increase. With a steel fibre content of 10 wt%, the compressive strength and loss factor of the RAC increase by 21.9% and 15.2%, respectively. With a fly ash content of 25 wt%, the compressive strength and loss factor of the RAC increase by 8.6% and 6.9%, respectively. This demonstrates that steel fibre and fly ash are effective in improving both the damping properties and compressive strength of RAC, and steel fibre is more effective than fly ash. Two methods were used for modification of the RAC: reinforcing the RCA through impregnation with a 0.5% polyvinyl alcohol (PVA) emulsion and nano-SiO2 solution, and strengthening the RAC integrally through the addition of fly ash as an admixture. Both of these techniques can improve the ITZ bond strength between the RAC and new mortar. Replacing 10% of the cement with fly ash in the new mortar is shown to be the best method to improve the ITZ strength.
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Project description:In this paper, shear strength of fiber reinforced recycled concrete was investigated. A Self Consolidated Concrete (SCC) matrix with 100% coarse recycled aggregate and different types of fibers were used in the study. Steel (3D and 5D), synthetic and hybrid fibers with a volume fraction of 0.75% were added to the concrete matrix to prepare eight beams. In addition, two beams were cast without fibers as control specimens. All beams were prepared without shear reinforcement and were tested to evaluate concrete contribution to the shear capacity. In addition, optical images were captured to allow for full-field displacement measurements using Digital Image Correlation (DIC). The results showed about 23.44-64.48% improvement in the average concrete shear capacity for fiber-reinforced beams when compared to that of the control specimens. The percentage improvement was affected by fiber type and the steel fiber beams achieved the best performance. The addition of the fiber delayed the crack initiation and improved the post-cracking and ductile behavior of all beams. Moreover, the experimental results were compared to those predicted by codes and proposed equations found in the literature for concrete strength with and without fibers.
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Project description:The research reported herein studied the permeability of concrete containing recycled-concrete aggregate (RA), superfine phosphorous slag (PHS), and ground granulated blast-furnace slag (GGBS) with and without stress. Test results showed that the chloride diffusion coefficient of RA concrete (RAC) without external loads decreased with time, and the permeability of RAC is much lower than that of the reference concrete due to the on-going hydration and the pozzolanic reaction provided by the PHS and GGBS additives in the RAC mixture. The permeability of chloride under flexural load is much more sensitive than that under compressive load due to the differences in porosity and cracking pattern. At low compressive stress levels, the permeability of chloride decreased by the closing of pores and microcracks within RAC specimens. However, in a relatively short time the chloride diffusion coefficient and the chloride content increased rapidly with the increase of compressive stress when it exceeded a threshold stress level of approximate 35% of the ultimate compressive strength. Under flexural stress, the chloride transport capability increased with the increase of stress level and time. At high compressive and flexural stress levels, creep had a significant effect on the permeability of chloride in the RAC specimens due to the damage from the nucleation and propagation of microcracks over time. It is apparent that mortar cracking has more of a significant effect on the chloride transport in concrete than cracking in the interfacial transition zone (ITZ).
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Project description:Pervious concrete is considered to be porous concrete because of its pore structure and excellent permeability. In general, larger porosity will increase the permeability coefficient, but will significantly decrease the compressive strength. The effects of water-cement ratio, fiber types, and fiber content on the permeability coefficient, porosity, compressive strength, and flexural strength were investigated. The pore tortuosity of the pervious concrete was determined by volumetric analysis and two-dimensional cross-sectional image analysis. The concept and calculation method of porosity tortuosity were further proposed. Results show that the permeability coefficient of the pervious concrete is the most suitable with a water-cement ratio of 0.30; the water permeability of the pervious concrete is influenced by fiber diameter. The permeability coefficient of pervious concrete with polypropylene thick fiber (PPTF) is greater than that with copper coated steel fiber (CCF) and the polypropylene fiber (PPF). The permeability coefficient is related to tortuosity and porosity, but when porosity is the same, the permeability coefficient may be different. Finally, general relations between the permeability coefficient and porosity tortuosity are constructed.
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Project description:In attempting to enhance the mechanical properties of recycled concrete after high temperature and solve the problem of large stacking of lithium slag (LS), this paper proposes lithium slag recycled concrete (LSRAC). In this research, LS was used to replace part of the cement (γL = 10%, 20%, and 30%), recycled coarse aggregate (RCA) completely replaced the natural aggregate (γR = 100%), and the heated temperatures were 200°C, 400°C, and 600°C. This paper carried out the heating test and the strength tests. The test results indicated, for the same heating temperature, the loss of strength of LSRAC was less than that of RAC and the compressive strengths and splitting strength of LSRAC with 20% lithium slag replacement rate were improved by 33.9%, 36.5% and 34.5%, respectively. The increase in flexural strength of LSRAC with 10% lithium slag dosage reached 24.1%. The results indicate LSRAC can effectively improve the bearing capacity of structural concrete subject to high temperature. The strength retention equations of LSRAC were established by comparing the strengths of 20°C. The calculation results of the strength retention formula for post-high-temperature LSRAC matched the measured results well. Therefore, this paper provided reliable experimental basis and theoretical guidance for on-site rescue, post-disaster assessment and reinforcement of RAC used for pavement base and public facilities constructions, and the eco-friendly way for sustainable development.
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Project description:Compressive and flexural strength are the crucial properties of a material. The strength of recycled aggregate concrete (RAC) is comparatively lower than that of natural aggregate concrete. Several factors, including the recycled aggregate replacement ratio, parent concrete strength, water-cement ratio, water absorption, density of the recycled aggregate, etc., affect the RAC's strength. Several studies have been performed to study the impact of these factors individually. However, it is challenging to examine their combined impact on the strength of RAC through experimental investigations. Experimental studies involve casting, curing, and testing samples, for which substantial effort, price, and time are needed. For rapid and cost-effective research, it is critical to apply new methods to the stated purpose. In this research, the compressive and flexural strengths of RAC were predicted using ensemble machine learning methods, including gradient boosting and random forest. Twelve input factors were used in the dataset, and their influence on the strength of RAC was analyzed. The models were validated and compared using correlation coefficients (R2), variance between predicted and experimental results, statistical tests, and k-fold analysis. The random forest approach outperformed gradient boosting in anticipating the strength of RAC, with an R2 of 0.91 and 0.86 for compressive and flexural strength, respectively. The models' decreased error values, such as mean absolute error (MAE) and root-mean-square error (RMSE), confirmed the higher precision of the random forest models. The MAE values for the random forest models were 4.19 MPa and 0.56 MPa, whereas the MAE values for the gradient boosting models were 4.78 MPa and 0.64 MPa, for compressive and flexural strengths, respectively. Machine learning technologies will benefit the construction sector by facilitating the evaluation of material properties in a quick and cost-effective manner.
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Project description:Currently the investigation on recycled cement concrete aggregate has been widely conducted, while the understanding of the recycled polymer concrete aggregate is still limited. This study aims to fill this knowledge gap through the experimental investigation on mechanical and durability performance. Specifically, the remolded polyurethane stabilized Pisha sandstone was collected as the recycled polymer concrete aggregate. The remolded Pisha sandstone was then applied to re-prepare the polyurethane-based composites. After that, the mechanical performance of the prepared composites was first examined with unconfined and triaxial compressive tests. The results indicated that the Pisha sandstone reduces the composite's compressive strength. The reduction is caused by the remained polyurethane material on the surface of the remolded aggregate, which reduces its bond strength with the new polyurethane material. Aiming at this issue, this study applied the ethylene-vinyl acetate (EVA) to enhance the bond performance between the polyurethane and remolded sandstone. The test results indicated both the unconfined and triaxle compressive strength of the polyurethane composites were enhanced with the added EVA content. Furthermore, the durability performance of the EVA-modified composites were examined through freeze-thaw and wet-dry cycle tests. The test results indicated the EVA could enhance the polyurethane composites' resistance to both wet-dry and freeze-thaw cycles. Overall, the modification with EVA can compensate for the strength loss of polyurethane composites because of the applied remolded aggregate and enhance its sustainability.
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Project description:The undesirable properties of conventional recycled fine aggregate (RFA) often limit its application in the construction industry. To overcome this challenge, a method for preparing completely recycled fine aggregate (CRFA), which crushes all concrete waste only into fine aggregate, was proposed. The obtained CRFA had high apparent density, and its water absorption was lower than that of the conventional RFA. To take advantage of the CRFA, this paper introduced the modified packing density method for the CRFA concrete mix design. The modified packing density method took account of the powder with a particle size of smaller than 75 ?m in the CRFA and balanced both the void ratio and the specific surface area of the aggregate system. Concrete (grade C55) was prepared using the CRFA to validate the feasibility of the proposed method. The unit price of the prepared CRFA concrete was around 12.7% lower than that of the natural aggregate concrete. Additionally, the proposed procedure for the concrete mixture design could recycle all concrete waste into the new concrete and replace all the natural fine aggregate in the concrete mixture.
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Project description:Previous literature indicates a decrease in the mechanical properties of various concrete types that contain recycled aggregates (RA), due to their porosity and to their interface of transition zone (ITZ). However, other components of the RA concrete microstructure have not yet been explored, such as the modification of the new paste (NP) with respect to a reference concrete. This paper deals with the microstructure of the new paste of self-compacting concrete (SCC) for different levels of RA. The water to binder ratio (w/b) was kept constant for all concrete mixtures, and equal to 0.5. The SCC mixtures were prepared with percentages of coarse RA of 0%, 30%, 50% and 100%. Mercury intrusion porosimetry test (MIP) and scanning electron microscope (SEM) observations were conducted on the new paste of each concrete. The results indicated that the porosity of the new paste presents a significant variation for replacement percentages of 50% and 100% with respect to NP0 and NP30. However, RA contributed to the refinement of the pore structure of the new paste. The amount of macrospores the diameter of which is in the 50-10,000 nm range was reduced to 20% for NP50 and NP100, while it was about 30% for NP0 and NP30, attributed to the water released by RA. Compressive strength loss for SCC50 and SCC100 concretes are both influenced by porosity of RA, and by the NP porosity. The latter is similar for these two concretes with the 26% increase compared to a reference concrete.
| S-EPMC7254211 | biostudies-literature
Project description:Since high quality natural aggregates are becoming scarce, it is important that industrial recycled products and by-products are used as aggregates for concrete. In Japan, the use of recycled aggregate (RG) is encouraged. Since, strength and durability of recycled aggregate concrete is lower than that of normal aggregate concrete, the use of recycled aggregate has not been significant. In order to improve physical properties of concrete using recycled coarse aggregate, blast furnace slag sand has been proposed. Recently, blast furnace slag sand is expected to improve durability, freezing, and thawing damage of concrete in Japan. Properties of fresh and hardened concrete bleeding, compressive strength, and resistance to freezing and thawing which are caused by the rapid freezing and thawing test using liquid nitrogen is a high loader than the JIS A 1148 A method that were investigated. As a result, concrete using treated low-class recycled coarse aggregate and 50% or 30% replacement of crushed sand with blast furnace slag sand showed the best results, in terms of bleeding, resistance to freezing and thawing.
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