Repair of Second-Generation Recycled Fine Aggregate of Waste Concrete from Freeze-Thaw Environment by Carbonation Treatment

The reuse of waste recycled concrete from harsh environments has become a research hotspot in the field of construction.This study investigated the repair effect of carbonation treatment on second-generation recycled fine aggregate(SRFA)obtained from recycled fine aggregate concrete(RFAC)subjected to freeze-thaw(FT)cycles.Before and after carbonation,the properties of SRFA were evaluated.Carbonated second-generation recycled fine aggregate(CSRFA)at five substitution rates(0%,25%,50%,75%,100%)to replace SRFA was used to prepare carbonated second-generation recycled fine aggregate concrete(CSRFAC).The water absorption,porosity and mechanical properties of CSRFAC were tested,and its frost-resisting durability was evaluated.The results showed after carbonation treatment,the physical properties of SRFA was improved and met the requirements of II aggregate.The micro-hardness of the interfacial transition zone and attached mortar in CSRFA was 50.5%and 31.2%higher than that in SRFA,respectively.With the increase of CSRFA replacement rate,the water absorption and porosity of CSRFAC gradually decreased,and the mechanical properties and frost resistance of CSRFAC were gradually improved.Carbonation treatment effectively repairs the damage of SRFA caused by FT cycles and improves its application potential.

Controlled low-strength material incorporating recycled fine aggregate from urban red brick based construction waste

Sixteen controlled low-strength material( CLSM)mixtures with various cement-to-sand( C/Sa) ratios and water-to-solid( W/So) ratios were prepared using recycled fine aggregate from urban red brick based construction waste.The fluidity and bleeding of the fresh CLSM mixtures were measured via the modified test methods, and the hardened CLSM mixtures were then molded to evaluate their compressive strength and durability. The results showthat the fluidity of the fresh CLSM mixtures is 105 to 227 mm with the corresponding bleeding rate of 3. 7% to 15. 5%, which increases with the increase in fluidity. After aging for 28 d,the compressive strength of the hardened CLSM mixtures reaches 1. 15 to 13. 96 M Pa, and their strength can be further enhanced with longer curing ages. Additionally, the strength increases with the increase of the C/Sa ratio, and decreases with the increase of the W/So ratio under the same curing age. Based on the obtained compressive strength, a fitting model for accurately predicting the compressive strength of the CLSM mixtures was established, which takes into account the above two independent variables( C/Sa and W/So ratios).M oreover, the durability of the hardened CLSM mixtures is enhanced for samples with higher C/Sa ratios.

Strength and Chloride Diffusion Behaviour of Three Generations of Repeated Recycled Fine Aggregate Concrete

The feasibility of using different generations of recycled fine aggregate（RFA） in structural concrete in a chloride environment was evaluated by studying the performance of the RFA and the corresponding concrete. The different generations of RFA were recycled by following the cycle of ‘concrete-waste concrete-fine aggregate-concrete＇. The properties of three generations of repeatedly recycled fine aggregate（RRFA） were systematically investigated, and we focused on the compressive strength and splitting tensile strength and chloride ion permeability of the related structural concretes with 25%, 75%, and 100% replacement of natural fine aggregates with RFA. The results indicated that the quality of RRFA presents a trend of slow deterioration, but the overall performance of all RRFA still fulfils the quality requirements of recycled fine aggregate for structural concrete. All RRFA concretes achieved the target compressive strength of 40 MPa after 28 days except for the second generation of the recycled aggregate concrete and the third generation of the recycled aggregate concrete with 100% replacement, and all the concrete mixes achieved the target compressive strength after 90 days. The insights obtained in this study demonstrate the feasibility of using at least three generations of RRFA for the production of normal structural concrete with a design service life of 100 years in a chloride environment.

Mathematical models for properties of mortars with admixtures and recycled fine aggregates from demolished concretes

In order to expand the engineering application of recycle aggregate mortars （RAM） with aggregates from demolished concretes, the models for the properties of RAM and the replacement rate of these recycled fine aggregates were proposed. First, different kinds of mathematical models for the basic properties （compressive strength, water retention rate, and consistency loss） of RAM with two kinds of admixtures, thickening powders （TP） and self-made powdery admixtures （SSCT） designed for RAM, and the replacement rates were established, while the average relative errors and relative standard errors of these models were calculated. Additionally, the models and their error analyses for the curves of drying shrinkage and curing time of RAM ＋ SSCT at different replacement rates were put forward. The results show that polynomial functions should be used to calculate the basic properties of RAM ＋ TP and RAM ＋ SSCT at different replacement rates. In addition, polynonfial functions are the most optimal models for the sharp shrinkage sections in the curves of drying shrinkage-curing time of RAM ＋ SSCT, while exponential functions should be used as the models for the slow shrinkage sections and steady shrinkage sections.

Properties of Cementitious Rendering Mortar Prepared with Recycled Fine Aggregates

The results of an experimental study on investigating the properties of cementitious rendering mortars prepared with a recycled fine aggregate（RA） were presented.The RA was obtained from a recycling plant in which mixed demolition wastes were processed by mechanical crushing,sieving and sorting operations.Two series of rendering mortar mixes were prepared with a constant water/cement and a constant aggregate/cement ratios of 0.55 and 3 respectively.River sand and natural crushed rock fine were originally used in the two series separately,and they were consistently replaced by 25%,50%,75% and 100% by the recycled aggregate.The experimental results showed that mechanical properties,such as compressive strength,flexural strength and modulus of elasticity of the mortars prepared with the RA were lower than the mortars made with the natural aggregates.Nevertheless,the bond strength at the interface between the mortar and masonry bricks determined by the Triplet test was found to be higher for the mortars prepared with the RA.

Effect of waste concrete with different strength on mechanical properties of recycled fine aggregate mortar

The influence of source concrete (SC) with different compression strengths on the workability and mechanical properties of recycled mortar made with river sand substituted by 100% fine recycled concrete aggregates (FRCA) is experimentally investigated. The basic physical performance test shows that with the increase in SC strength, FRCA exhibit lower water absorption and crushing index, meanwhile keeping higher densities. Mechanical property tests, including compressive strength, flexural strength and uniaxial compressive stress-strain tests, show that compressive strength,flexural strength and elasticity modulus of recycled sand mortars increase roughly with the increase in SC strength. The proposed mixture design method demonstrates that all of the components can be kept as the same as those in natural mortar mixture design and FRCA must be pre-wetted before making mortar mixture. Meanwhile, the reuse of higher strength SC can ensure that recycled mortar mixtures are able to achieve similar mechanical performance when compared to natural mortar designs.

Effects of admixture on properties of recycled aggregate mortar

Compared with strengthening the recycled fine aggregate(RFA)from construction and demolition waste,which is time-consuming and complex,adding admixture into the mixtures directly is more efficient and effective to expand the application of recycled aggregate mortar(RAM).The admixture(named as SSC),mixed with sodium hexametaphosphate,sodium ligninsulfonate and citric acid,was directly added into the RAM.First,the compositions and physical properties of the RFA and reference aggregate were studied,respectively.The properties of fresh and hardened mortars were then investigated.The results show that there is a clear difference between the RFA and reference aggregate,and the properties of RAM without SSC are not as good as those of normal mortar.However,the consistency value,water retention rate,compressive strength and setting time values of RAM increase by 5%,7%,66%and 67%,and its consistency loss and density values decrease by 42%and 4%after the SSC is added into the RAM.Therefore,improving the properties of RAM through adding admixture is an effective and efficient approach to expanding its application.