Query: recycled aggregate

title: Utilization of CO2 curing to enhance the properties of recycled aggregate and prepared concrete: A review
abstract: In recent years, the CO2 curing technique has been developed to enhance the properties of recycled aggregate (RA) and recycled aggregate concrete (RAC) materials that can absorb the CO2 gas and are more sustainable for the construction industry. A body of literature on the CO2 curing of RA and RAC is currently available, but a systematic review is lacking. Therefore, this paper reviewed the published literatures on the use of CO2 curing to enhance the properties of the RA and prepared RAC. The studies on CO2 curing technology, the properties of carbonated recycled aggregate (CRA) and the micro properties, workability, mechanical properties and durability performance of concrete with CRA are respectively reviewed. The results showed that the RA properties, CO2 concentration and pressure, relative humidity and curing time all had a significant impact on the properties of CRA. Carbonation treatment improved the pore structure of RA and reduced its porosity, and the workability, mechanical properties and durability performance of prepared RAC were also improved. In addition, an outlook on the CO2 curing of RA and prepared concrete was presented, and we expected that this work may inform further investigation of the use of CO2 curing to enhance the properties of concrete and cement products.

title: A novel artificial intelligence technique to predict compressive strength of recycled aggregate concrete using ICA XGBoost model
abstract: Recycled aggregate concrete is used as an alternative material in construction engineering, aiming to environmental protection and sustainable development. However, the compressive strength of this concrete material is considered as a crucial parameter and an important concern for construction engineers regarding its application. In the present work, the 28 days compressive strength of recycled aggregate concrete is investigated through four artificial intelligence techniques based on a meta heuristic search of sociopolitical algorithm (i.e. ICA) and XGBoost, called the ICA XGBoost model. Based on performance indices, the optimum among these developed models proved to be ICA XGBoost model. Namely, findings demonstrated that the proposed ICA XGBoost model performed better than the other models (i.e. ICA ANN, ICA SVR, and ICA ANFIS models) in estimating compressive strength of recycled aggregate concrete. The suggested model can be used in construction engineering in order to ensure adequate mechanical performance of the recycled aggregate concrete and allow its safe use for building purposes.

title: Macro and micro properties of multi recycled aggregate concrete
abstract: In a future scenario in which all the concrete is recycled concrete, it will be necessary to recycle the recycled concrete. However, it is known that the recycling of concrete implies a loss of properties. This paper shows an innovative technique, the computerized microtomograph, used to evaluate closed porosity, volume of limestone aggregate fraction and volume of mortar of the multi recycled aggregate concrete, in order to answer the question: how many times it is possible to recycle concrete? First, the source concrete mix using limestone coarse and fine aggregates was characterized. This mix was crushed to obtain a recycled aggregate that was used to manufacture the 1st generation or current recycled aggregate concrete. After the characterization of this 1st generation concrete, and in the same way, a 2nd and a 3rd generation concrete were obtained and characterized, using recycled aggregates from the 1st and 2nd concrete generations respectively. The evaluation by computerized axial tomography allows to know how the successive recycled affect the properties of the concrete. The results show that it is possible to observe the distribution and quantify the aggregate, cement paste and closed porosity contents of the recycled aggregate concrete showing that 3rd generation recycled concrete shows almost twice as much mortar as 1st generation one and demonstrates that it is only possible to recycle the concrete a finite number of times.

title: An evaluation of the recycled aggregate characteristics and the recycled aggregate concrete mechanical properties
abstract: The difference between the properties of recycled aggregate (RA) and natural aggregate (NA) are caused due to the old mortar attached to RA. In this review, the quantitative relationships were represented about the content of old attached mortar and the performance of RA at the material level. In term of the component level, the influence of replacement ratio of RA on the mechanical properties of concrete was summarized. Finally, some researches have focused on ways to improve the properties of aggregate. The results show that the method of stripping old mortar to improve the performance of recycled aggregates was not the only way to promote the application of recycled aggregates. However, by evaluating some simple and economical methods such as controlling the water cement ratio, adjusting the aggregate moisture content and the different mixing method can improve the performance of recycled concrete in order to meet the requirement of concrete quality. Three aspects of the performance prediction, application range and reinforcement method for RAs were illustrated in the article. It was believe that the results could promote the accurate application for RAs with different quality in engineering, thus the application range of RAs was expanded.

title: Utilising CO2 technologies for recycled aggregate concrete: A critical review
abstract: Employment of recycled aggregate within concrete provides great potential for the reduction of landfilling. Unfortunately, recycled aggregate exhibits a high porosity and water absorption and consequently produces a substandard material when compared to the mainstream virgin aggregate concrete. Recently, the injection of CO2 into cementitious materials has been studied, for both improving the overall quality of recycled aggregate concrete as well as permanently chemically converting CO2 into stone. CO2 treatment can permit recycled aggregate concrete to rival virgin aggregate concrete in phycial and mechanical properties. Currently, there are two primary methodologies for the sequestration of CO2 into concrete: (1) carbon conditioning is the injection of CO2 into recycled aggregate; and (2) carbon curing involves sequestering CO2 into new concrete's cement paste. Whilst both technologies permit recycled aggregate concrete for achieving great mechanical property and durability, carbon conditioning provides a practical implementation. Carbon conditioning permits a prompt and complete carbonation of recycled aggregate which enhances the final concrete's mechanical property and durability. This paper provides an insight into the available CO2 technologies for concrete improvement.

title: Properties of self compacting high strength concrete containing multiple use of recycled aggregate
abstract: Advances in recycling can exceed traditional practice in that a non renewable resource can at least become a partially renewable resource. This research attempts to support the cradle to cradle concept by testing the effect of reused recycled concrete aggregates (RRCA) on the fresh, physical, mechanical, and microstructure properties of self compacting high strength concrete (SCHSC) Two generations of recycled aggregate concrete (RAC) are produced; the first and second are created by recycled concrete aggregate (RCA) and by RRCA, respectively. For each generation, two replacement amounts are used (25% and 50% by mass) Results show that using RRCA does not affect the fresh properties of SCHSC, and up to 50% RRCA replacement amount by mass can be adopted for enhancing the mechanical and microstructure properties of reused recycled aggregate concrete (RRAC) Computed tomography indicates that the porosity of RRCA is less than that of RCA.

title: Influence of the incorporation of recycled coarse aggregate on water absorption and chloride penetration into concrete
abstract: Knowledge of the transport properties for recycled aggregate concrete (RAC) exposed to chloride environment plays a critical role in assessing its durability and predicting the service life. In this study, the effect of the incorporation of coarse recycled concrete aggregates (RCAs) on the durability of cement based cementitious materials was experimentally investigated. Two types of RCAs were obtained to produce the RAC with water to binder ratio of 0.33 and 0.39, and different substitutions of RCAs ranging from 0% 30% 50% and 100% by weight were selected. A series of experiments including compressive strength, water and salt absorption test, and rapid chloride migration (RCM) test for RAC with various replacement ratios of RCAs were conducted. The results indicate that the incorporation of RCAs generally decreases the compressive strength, and inversely increases water and chloride transport coefficient compared to those of the control concrete. An increasing trend can be found in the influence of RCAs replacement ratio on the initial coefficient of capillary water absorption, but it inversely decreases as the quality of RCAs is improved. The chloride content and penetration depth in RAC were remarkably increased as RCA content increases and its quality decreases. The linear changing behavior for the penetration depth between water and chloride ion in RAC with different replacement ratios was well exhibited. This finding reveals the retardation effect that the chloride transport process lags behind water migration for the absorption tests of RAC.

title: Effects of combined usage of GGBS and fly ash on workability and mechanical properties of alkali activated geopolymer concrete with recycled aggregate
abstract: Geopolymer binder and recycled aggregates are considered as the two important sustainable ingredients in concrete production. Ground granulated blast furnace slag (GGBS) and fly ash combination based geopolymeric recycled aggregates concretes (GRAC) were prepared as a substitution of normal concrete in order to reduce the consumption of ordinary Portland cement and natural aggregates as well as to make use of demolished waste concrete. The aim of this study is to investigate the combined effects of GGBS and fly ash considering water binder (W/B) ratio on the fresh and hardened properties of alkali activated GRAC. The slump, setting time, compressive strength, stress strain relation, elastic modulus, Poisson's ratio, toughness and failure mode of alkali activated GGBS and fly ash based GRAC containing 100% recycled coarse aggregates were experimentally examined. In addition, the hydration mechanisms were studied by XRD and SEM. Finally, the failure mechanism of this kind of novel green concrete subjected to compression was revealed. The results show that GGBS and fly ash exhibits a superior synergetic effect on the performance of GRAC, i.e. fly ash and GGBS are mainly responsible for the workability and mechanical properties, respectively. GGBS/fly ash ratio has a significant influence on the fresh and hardened properties of GRAC. Moreover, the effect of W/B ratio on GRAC strongly depends on the GGBS/fly ash ratio. GGBS content at less than 25% was found to slightly influence the consistency and compressive strength of GRAC. The combination of 50% GGBS and 50% fly ash with a 0.5 W/B ratio could provide excellent mechanical performance and workability for GRAC.

title: Axial stress strain behavior of macro synthetic fiber reinforced recycled aggregate concrete
abstract: This study aims to investigate the axial stress strain behavior of macro synthetic fiber reinforced recycled aggregate concrete. Concrete cylinders reinforced with macro synthetic fibers were tested under axial compression, with the variation of three different replacement ratios of recycled aggregates (i.e. 0, 50 and 100% and three different dosages of macro polypropylene fibers (i.e. 0, 0.5 and 1% of volume of recycled aggregate concrete) A comparative study of the existing stress strain models for steel fiber reinforced normal and recycled aggregate concrete with the test results indicates that the stress strain behavior of steel fiber reinforced normal and recycled aggregate concrete can be well predicted by these existing models. No stress strain model for macro synthetic fiber reinforced normal and recycled aggregate concrete has been developed. Based on the test results, a stress strain model is developed in this work by modifying the parameters of best performing stress strain model for steel fiber reinforced normal aggregate concrete. The proposed model can effectively predict the stress strain behavior of both steel and macro synthetic fiber reinforced normal and recycled aggregate concrete. Test results show that the peak stress, peak strain and ultimate strain of concrete specimens increase with the increase in fiber dosage and the addition of fibers has a better effect on recycled aggregate concrete and as compared to normal aggregate concrete.

title: A review of life cycle assessment of recycled aggregate concrete
abstract: Recycled aggregate has emerged expecting to address raw materials shortage in construction sector by converting waste concrete into resource. Many researchers adopted life cycle assessment (LCA) to evaluate and compare environmental impact of recycled aggregate concrete (RAC) and natural aggregate concrete (NAC) However, there are numbers of problems in applying the LCA in RAC, a unified treatment of the subject is still lacking. By reviewing existing corresponding literature, this paper identified the existing problems of applying the LCA method in RAC from 5 aspects, including mixture design method, selection of functional units, allocation of inventory, CO2 uptake during the life cycle of concrete and transport distance of recycled aggregate. Review results indicate that when analyzing the environmental impact of RAC by LCA, a clear indication of the mixture design methods of RAC should be given at first. Second, a benchmark method to define the functional unit of the LCA of RAC is urgently needed to make different LCA studies comparable. A method that combining the effect of quality, mass and market price on the allocation procedure of recycled aggregate should be further investigated. Besides, ignoring CO2 uptake in the use stage of RAC will lead to an overestimated CO2 emission of RAC. In addition, a critical distance for recycled aggregate is existed when comparing the environmental impact between NAC and RAC. At last, some contents of recycled aggregate concrete for further research have been proposed.