Tailor-made Recycled Aggregate Concrete
The effects of CO2 concentration on changes in chemical phases and microscopic characteristics for fly ash (FA) blended cement pastes were investigated in this study. Several microscopic test methods, including X-ray diffraction (XRD), thermo-gravimetric analysis (TGA), ²⁹Si nuclear magnetic resonance (²⁹Si NMR) and scanning electron microscope (SEM), were used to characterize the chemical compositions and microscopic features. The XRD results showed that the precipitation of allotropic calcium carbonate (CC̅) includes calcite (c), aragonite (a) and vaterite (v). The ratio of c/(a + v) was around 0.6 under 3% and 20% CO2, while more percentage of calcite was generated under 100% CO2 (c/(a + v) = 0.79). The precipitation of more calcite than vaterite and aragonite happened with the CO2 concentration elevated to 100%. TGA analysis indicated that the total content of CC̅ was similar under all accelerated conditions and higher than that under natural carbonation. Additionally, in the ²⁹Si NMR spectra, more C-S-H (about 70%) was decalcified after accelerated carbonation compared with natural carbonation (54.1%). The decalcification degree was the same for 3% and 20% CO2 and showed the highest value under 100% CO2. The microstructure changes characterized by SEM observation exhibited denser microstructure after carbonation with the formation of CC̅ but no apparent difference was observed with different CO2 concentrations based on the SEM pictures. Compared with the carbonation of ordinary Portland cement (OPC) paste, the carbonation of FA blended cement paste was more inclined to precipitate as calcite than vaterite and aragonite and caused a lower decalcification degree of C-S-H. Overall, similar to OPC paste, the carbonation results obtained in natural and accelerated conditions for FA blended cement pastes were different and the conditions between 3% and 20% CO2 were similar while 100% CO2 showed different results.
To investigate the effect of different CO2 concentrations on the carbonation results of slag blended cement pastes, carbonation experiments under natural (0.03% CO2) and accelerated conditions (3, 20, and 100% CO2) were investigated with various microscopic testing methods, including X-ray diffraction (XRD), 29Si magic angle spinning nuclear magnetic resonance (29Si MAS NMR) and scanning electron microscopy (SEM). The XRD results indicated that the major polymorphs of CaCO3 after carbonation were calcite and vaterite. The values of the calcite/(aragonite + vaterite) (c/(a + v)) ratios were almost the same in all carbonation conditions. Additionally, NMR results showed that the decalcification degree of C-S-H gel exposed to 0.03% CO2 was less than that exposed to accelerated carbonation; under accelerated conditions, it increased from 83.1 to 84.2% when the CO2 concentration improved from 3% to 100%. In SEM observations, the microstructures after accelerated carbonation were denser than those under natural carbonation but showed minor differences between different CO2 concentrations. In conclusion, for cement pastes blended with 20% slag, a higher CO2 concentration (above 3%) led to products different from those produced under natural carbonation. A further increase in CO2 concentration showed limited variation in generated carbonation products.
In the context of a circular economy, the recycling becomes more than more important. In the construction industry, the wastes from the destructions of old structures are significant. The recycling of the old concretes can contribute to reduce the extraction of the new natural resources (gravel, sand) and to reduce the waste deposit areas. Numerous studies have already been performed to investigate different aspects of recycled aggregate concretes (RAC). Several publications have also done the detailed reviews on certain topics of RAC (such as mechanical properties, mix proportioning); however, still few articles have done the global synthesis on different aspects of RAC. This paper presents a rapid state-of-the-art of numerous topics related to RAC: from the recycling techniques of old-concrete-aggregates, the mix proportioning, the mechanical properties, the durability, the structural behavior and the fire resistance. The number of existing studies synthesized is relatively important (about 170 publications). The aim of the present paper is to provide a rapid overview about existing scientific investigations on RAC, which can offer a good guidance to researchers and engineers who are entering to this area.
The combined advantages of wastes in high-performance concrete production are currently popular topics of investigation. This study aims to examine the effect of foundry sand waste (FDW) as a fine aggregate replacement (30 and 50%wt) on the properties of self-consolidating concrete (SCC) mixed with untreated rice husk ash (RHA) as a cement replacement (10 and 20%wt) with controlling water-binder materials ratios (w/b) of 0.35 and 0.45. The workability and strength characteristics of SCC are considered. The results indicate that the increase in FDW amount affects the SCC with RHA, increases the required superplasticizer and setting times compared to the control SCC, and decreases the ensity and slump flow loss. The incorporation of RHA and FDW decreases the filling and passing ability of the SCC. Based on the workability requirements specified by EFNARC guidelines, SCC mixtures exhibited acceptable V-funnel performance at replacement levels of RHA and FDW not more than 10 and 30 wt% and achieved the highest compressive and splitting strengths.
This paper presents a proposed uniaxial damaged plastic constitutive relation of recycled aggregate concrete (RAC) based on the experimental studies. A total of five groups of RAC specimens with different recycled coarse aggregate (RCA) replacement percentages of 0, 25%, 50%, 75%, and 100%, respectively, are tested under both monotonic loading and cyclic loading. The effect of the RCA replacement percentage is thoroughly investigated on a variety of mechanical properties, including the compressive strength, the peak strain, and the elastic modulus. Based on the test results, a uniaxial damage plastic constitutive relation of the RAC is proposed within the continuous thermodynamics framework. After validated by the experimental results, the proposed damaged plastic constitutive relation of the RAC is applied to perform nonlinear analysis of the RAC columns under cyclic loading, which provides close predictions of the hysteresis behavior of the RAC columns.
This paper has focused on the structural performance of recycled aggregate concrete (RAC) under both cyclic and monotonic loading. RAC specimens with different recycled coarse aggregate (RCA) replacement percentages of 0%, 25%, 50%, 75% and 100% were cast and tested. The compressive stress-strain relationship and the failure mode were investigated for each RCA replacement ratio. The effects of the RCA replacement percentage on the compressive mechanical properties of the RAC specimens including the strength, elastic modulus, peak strain, ultimate strain and Poisson's ratio were also studied. The RAC specimens have shown similar failure characteristics regardless of monotonic or cyclic loading. In addition, the compression skeleton curves of the RAC specimens under cyclic loading agree well with those under monotonic loading. Based on the experimental results, the characteristic points pertaining to the hysteresis loop were defined and their relations were established. Furthermore, the constitutive equations of the RAC as well as its simplified form were proposed and applied in numerical simulations of RAC columns and frames under cyclic loading. The proposed constitutive equations have shown promising accuracy in predicting the hysteresis performance of RAC on both component and structural levels.