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A new image analysis technique coupled with the power consumption were employed in this paper and proved to be a relevant method to detect the different characteristic times of the mixing evolution, i.e. the cohesion time, the fluidity time and the half-life time of the mixture evolution after fluidity. It was shown that the “effective water-to-powder ratio in the mortar” which excludes from the effective water the quantity of water needed to fill the less compact packing in the vicinity of the coarse aggregates, governed the mixing behaviour before fluidity and the interval of water content where the concrete mixture behaves as a homogenous and fluid granular suspension. It was also shown that the mixing behaviour was adversely affected by the amount of water into the mixture. The fluidity and cohesive times were faster to be obtained when the water proportion was increased. At opposite, it was more difficult to obtain the final consistency for a more fluid mixture, at given super-plasticizer proportioning. As a general remark, the behaviour seemed controlled by the mixture composition before the fluidity point and by the mixture consistency after the fluidity point. To end, the image analysis technique coupled with the mixing power technique can be employed at the end of the mixing to complete the information obtained with the other methods of analysis in order to monitor the consistency of the mixture. But, the use of the cohesion and fluidity times, easy to detect with the texture analysis, could be more efficient monitoring tools from an industrial point of view. [ABSTRACT FROM AUTHOR]

In this work, the recycled concrete aggregate (RCA) friability during mixing was studied in order to better understand the evolution of this material during the mixing process and improve the recycled aggregate concrete mix-design. The influence of some important materials and process parameters was evaluated: initial abrasion resistance and initial moisture of the aggregates, mixer geometry, mixing time and mixing speed. To assess the mixing process effect on the recycled concrete aggregate friability, three different aspects were evaluated; the mass loss (mass of fraction inferior to 2.5 mm) the grading and the angularity evolutions with mixing time of an initially 10–14 mm aggregate. Tests were carried out in two types of laboratory concrete mixers, a planetary 30 l mixer from Skako and an intensive 5 l Erich mixer. The results revealed that in normal laboratory setting of the mixers configuration, the mass loss for natural aggregate (NA) is less than 1% of the coarse aggregate. This percentage reach 3% for good quality recycled concrete aggregate (MDE value of 21) and 5% for lower quality recycled concrete aggregate (MDE value of 27). The mass loss directly depends on the mixing parameters and the degradation of the recycled concrete aggregate drastically increased when the mixing speed was raised to 500 RPM. By analyzing the grading evolution during mixing, it was shown that both cleavage (creation of intermediate size particles) and attrition (creation of small particles) mechanisms influenced the aggregate degradation. However, the configuration of mixing significantly influenced the proportion of attrition and cleavage mechanisms. To complete this work, the angularity evolution showed that recycled concrete aggregate surface becomes smoother and the edges more rounded after mixing. [ABSTRACT FROM AUTHOR]

In recent decades, large amounts of construction and demolition waste (CDW) have been generated and accumulated throughout Europe, which is a challenge to manage and control nowadays. This work shows the results of a study carried out with samples of ceramic recycled aggregates (CRAs) and recycled concrete aggregates (RCAs) mixed with cement (C) in mortars. The main objective of this research is to demonstrate how, by adding CRAs and RCAs to a mixture of cement and natural aggregate (NA), it is possible to develop a high-strength mortar and achieve the best mixing ratio. To achieve these objectives, the characterization of the samples was initially carried out such as XRF, XRD and SEM. Next, tests were carried out on the products obtained, such as the consistency of the fresh mortar and the density of the specimens. Finally, a study of mechanical compressive strength was performed at 7, 28 and 200 days. The results show that although both CRAs and RCAs negatively affect the curing process of the specimens, it is possible to develop mortars with compressive strengths greater than 20 MPa. An obvious increase in mechanical compressive strengths was seen between 7 and 200 days of analysis. The results achieved in this research could be an important guide for the management of CDWs by local industries, thus favouring the development of the circular economy. [ABSTRACT FROM AUTHOR]

The fine portion of crushed old concrete is difficult to be recycled for use in new concrete because it contains old cement paste, which generally has high porosity and low strength. Hence, in practice, the coarse portion is recycled as coarse aggregate and the fine portion is mostly not recycled. Nevertheless, attempts have been made in recent years to recycle the fine portion as waste concrete powder (WCP) by grinding before use. In this research, WCP was used to make pervious concrete. The WCP was added using the paste replacement method (PR method) of replacing an equal volume of cementitious paste. A series of pervious concrete mixes containing 100% recycled coarse aggregate and having different amounts of WCP added were produced for testing of interconnected porosity, water permeability and strength. The results showed that the addition of WCP using the PR method can improve the interconnected porosity by 9% and water permeability by 18%, greatly enhance the strength by 86%, as well as decrease the cement consumption by 10% at the same time. Therefore, the addition of WCP as paste replacement has great potential to be applied to the production of eco-friendly high-performance pervious concrete. [ABSTRACT FROM AUTHOR]

To find the energy required during the mixing process of self-compacting concrete in a ready-mixed concrete plant and correlate the results with the yield stress of concrete. Power consumption required during the mixing of concrete is measured with a wattmeter connected to the mixing unit's power supply. A coaxial cylinder viscometer is used to measure the yield stress of concrete. The clamp meter measures the power when the impeller rotates inside the coaxial cylinder viscometer, which is filled with concrete. When the impeller rotates in a coaxial cylinder filled with concrete, the power is measured by a clamp meter. Torque is obtained through the power relationship, which is an essential factor in determining the yield stress. The cost of a rheometer is so high that all construction industries, research institutions, and researchers cannot measure rheological parameters. Nowadays, all rheometers are automated; hence, the cost is very high. Tattersall's approach of power requirement in mixing the concrete and calculating the yield stress reduces the complexity in determining the rheological parameter. [ABSTRACT FROM AUTHOR]

With the rapid development of urbanization, many new buildings are erected, and old ones are demolished and/or recycled. Thus, the reuse of building materials and improvements in reuse efficiency have become hot research topics. In recent years, scholars around the world have worked on improving recycle aggregates in concrete and broadening the scope of applications of recycled concrete. This paper reviews the findings of research on the effects of recycled fine aggregates (RFAs) on the permeability, drying shrinkage, carbonation, chloride ion penetration, acid resistance, and freeze–thaw resistance of concrete. The results show that the content of old mortar and the quality of recycled concrete are closely related to the durability of prepared RFA concrete. For example, the drying shrinkage value with a 100% RFA replacement rate is twice that of normal concrete, and the depth of carbonation increases by approximately 110%. Moreover, the durability of RFA concrete decreases as the RFA replacement rate and the water–cement ratio improve. Fortunately, the use of zeolite materials such as fly ash, silica fume, and meta kaolin as surface coatings for RFAs or as external admixtures for RFA concrete had a positive effect on durability. Furthermore, the proper mixing methods and/or recycled aggregates with optimized moisture content can further improve the durability of RFA concrete. [ABSTRACT FROM AUTHOR]

Concrete mixing can lead to mechanical degradation of aggregates, particularly when dealing with recycled concrete aggregates. In this work, the attrition of such materials during mixing is studied by means of experiments and simulations. The effect of the presence of fines, water addition, flow configuration of the mixer (co- or counter-current) and impeller frequency is discussed. Experiments were performed in a laboratory Eirich mixer. Discrete element numerical simulations (DEM) were performed on the same geometry by mimicking the behaviour of the material and, in particular, the cohesion induced by water and the cement paste using either Hertz–Mindlin or Hertz–Mindlin with Johnson–Kendall–Roberts (JKR) contact laws. The combination of the collision energy spectra extracted from the DEM simulations and an attrition model allowed the prediction of the mass loss due to attrition in 1-min experiments. Semi-quantitative agreement was observed between experiments and simulations, with a mean relative error of 26.4%. These showed that higher mass losses resulted from operation at the highest impeller speeds, co-current operation, and also with the wet aggregate. Mixing of the agglomerate in the concrete mix resulted in a significant reduction in attrition when compared to mixing aggregates alone. With further validation, the proposed simulation approach can become a valuable tool in the optimization of mixing by allowing the effects of material, machine and process variables to be studied on the mass loss due to attrition. [ABSTRACT FROM AUTHOR]

Our research focused on the influence of fillers obtained from crushed waste materials on the selected properties of concrete composites. The used waste materials were sourced from the production of ceramic tiles, ceramic pots, and sanitary ceramics. We evaluated concretes modified with the addition of 10% (by mass of cement) of different fillers. The properties, including the air content in the fresh concrete mix, consistency, compressive strength, and freeze-thaw resistance were examined. The evaluation of the freeze-thaw resistance was carried out by testing the concrete with the direct method for 150 cycles of freezing and thawing. The characteristics of the concrete porosity structure were assessed using automated digital image analysis. Concretes modified by coarse and fine fillers were characterized by different improvements in the mechanical properties and resistance to cycles of freezing and thawing. Composites with the addition of coarse fillers did not show any significant changes in comparison to the control concrete. An automated digital image analysis of the pore distribution in concrete proved to be an effective tool for the assessment of the freeze–thaw resistance of the concretes in question. [ABSTRACT FROM AUTHOR]