In the Netherlands, beside cement replacement with more green cement types, there is also an urgent need for alternative materials for natural sand in new concrete in order to make it circular. Furthermore, the recyclers have raised questions regarding upscaling and the potential of fine recycled concrete aggregates (fRCA) in structural concrete elements since the availability of recycled construction rubble is increasing. The variations in their chemical and physical properties and lack of standards for their quality evaluation is the main reason for not yet using fRCA in new concrete. In this paper, an in-depth characterization of different fRCA is performed in order to define their chemical properties. The properties can be eventually related to concrete mix design and performance (next step), so that fRCA can be classified as a material that can be used in more advanced applications. This is achieved with a multi-level chemical characterization of individual and total fractions (0-0.25 mm, 0.25-4 mm and 0-4 mm) for finding type and content of the original sand and cement phases and potential contamination of selected fractions. The tests include quantification of element composition with X-ray fluorescence (XRF), qualitative and quantitative phase analysis with X-ray diffraction (XRD) and Rietveld method. In addition, cement paste content, chlorides and sulfates of each type of fRCA was measured in order to evaluate contamination of studied material. It was shown that fRCA from different origins have similar chemical and mineralogical composition and contain comparative chloride content. The chemical composition testing can provide a first line control regarding composition and potential contamination of fRCA. After that, it can be decided which additional tests are necessary to be done in order to evaluate the suitability of fRCA for replacement of primary natural fine aggregates in new concrete.@en

Fine recycled concrete aggregates (fRCA, 0–4 mm) are produced from demolished concrete structures and consist of natural aggregates and old cement mortar. The presence of old cement mortar has detrimental effect on the fresh properties and strength of new concrete. This study aims to investigate the working mechanism and effectiveness of different methods for the optimization of mortar mixtures with fRCA. Three streams of fRCA were considered in the approach. As reference material, river sand was used. The river sand was replaced at 0 wt%, 25 wt% and 50 wt% with fRCA. The use of tailor-made superplasticizers (SP's), drying of fRCA, modified content of 0–0.250 mm, modified mixing sequence, increase of cement content were investigated. Once the mortar mixtures were optimized, the reaction kinetics was investigated with isothermal calorimetry. SP was applied to prevent use of additional water and to maintain mix consistency. When the river sand was replaced at 25 wt% with fRCA, no extra cement was needed. The air content of mortars with fRCA was up to 18 %, due to some unforeseen effects. The results indicated that using as received, agglomerated and unwashed fRCA may have a negative effect on the working mechanism of SP leading to high air content in the fresh mortars. Using dried fRCA has substantially decreased air content in mortars. In addition to drying of fRCA, change of mixing sequence has equal or even superior importance to reduction of air content. As a result, the compressive strength was comparable to reference mix with river sand at 25 wt% replacement level, however, the strength of mortars with 50 wt% fRCA was reduced despite that the cement paste content was increased. The use of fRCA did not affect the kinetics and degree of cement hydration in mortars with 25 wt% fRCA. The positive side of this is that the fRCA can be considered as non-reactive.@en

In circular concrete design, beside cement replacement with more environmentally friendly cement types, there is also an urgent need for sand replacement with fine recycled concrete aggregates (fRCA). The variations in physical and chemical properties of fRCA and lack of standards for their quality evaluation are the main reasons for not yet using fRCA in new concrete. In this study, an in-depth characterization of different Dutch fRCA is performed in order to examine suitability of fRCA as an alternative material for river sand and define indicators for fRCA quality. These indicators eventually can be related to concrete mix design and performance, so that fRCA can be classified as a material that can be used in structural concrete elements. This is achieved with physical, chemical and mineralogical characterization of individual and total fractions (0–0.250 mm, 0.250–4 mm and 0–4 mm). The physical properties such as grading, density, surface area, water absorption and cement paste content of fRCA were tested. The chemical analyses include quantification of element composition with X-ray fluorescence spectrometry (XRF) and carbonate content with thermogravimetry and mass spectrometry (TG-MS). Potential contamination (chlorides and sulfates) and reactivity of selected fractions were evaluated. In addition, qualitative and quantitative phase analyses with X-ray diffraction (XRD) combined with Rietveld refinement method were performed and supported by optical polarizing-and-fluorescence microscopic (PFM) study. Based on combined experimental approaches, characteristic quality indicators were defined for fRCA. These indicators showed that fRCA were uncontaminated and nonreactive. Despite fRCA were from different origins, they had similar chemical and mineralogical composition and contained comparative chloride content. In contrast, the content and surface area of fine fraction (0–0.250 mm) and particle size distribution of fRCA varied with the source. With this it can be assumed that fRCA will have different effect on the properties of the new concrete.

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This paper discusses the state-of-the-art of the fine recycled concrete aggregates (fRCA), focusing on their physical and chemical properties, engineering properties and durability of concretes with fRCA. Based on the systematic review of the published literature, it is impossible to deduce without any further research the guidelines and tools to introduce the widespread application of the fRCA in new concrete whilst keeping the cement contents at least the same or preferably lower. Namely, what is still missing is knowledge on key physico-chemical properties and their relation to the quality of the concrete mix and the concrete performance. This paper sets the foundations for better understanding the quality of fRCA obtained either from parent concrete specifically produced in the laboratory, with controlled crushing and sieving of the recycled aggregates or from field structures. By comparing properties of fRCA with properties of fine natural aggregates, the key limiting properties of fRCA are identified as the high water absorption of fRCA, moisture state of fRCA, agglomeration of particles and adhered mortar. As such, continuous quality of fRCA is hard to be obtained, even though they may be more continuous in terms of chemistry. Advanced characterization techniques and concrete technology tools are needed to account for limiting properties of fRCA in concrete mix design.

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With the recent developments in modelling the shear behaviour of reinforcement concrete (RC) members without transverse reinforcement, the contribution of aggregate interlock has been more and more recognized as one of the critical components in the shear resistance of RC members. This paper presents a study on the aggregate interlock action utilizing laser scanned crack surfaces including the shear crack surface of a 1.2 m concrete beam. The study extends the classic aggregate interlock model proposed by Walraven to enable the possibility of employing modern measurement technology.@en