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Technologies that can contribute to the reduction of greenhouse gases are mandatory, and those based on solar energy are good candidates to achieve this. In this sense, massive solar thermal collectors are suitable technologies for supplying the primary energy demand of buildings. To design these devices, it is necessary to fully understand the physics of the problem before proposing any new optimized solution. This review aims to briefly summarize significant aspects regarding the current state of development of these solar technologies. Attention is paid to works devoted to experimental studies to analyze the behavior of these systems, as well as numerical models to predict the physics of the problem. Furthermore, the future directions and prospects in the field of massive collectors are briefly described. The main novelty of this review is to provide a comprehensive overview that summarizes the works done so far in the field over the past 30 years, which allows the reader to delve deeper into the topic. According to the reviewed works, it can be concluded that these technologies can contribute to the reduction of greenhouse gases while at the same time being excellent examples of the integration of solar energy devices with buildings. [ABSTRACT FROM AUTHOR]

Thermal loading, especially in fire scenarios, challenges the safety and long-term durability of concrete structures. The resulting heat propagation within the structure is governed by the heat conduction equation, which can be difficult to solve analytically because of the nonlinearity related to the thermophysical properties of concrete. A semi-analytical approach for the transient nonlinear heat conduction problem in concrete structures was established in the present work. The nonlinearity related to the temperature-dependent thermal conductivity, mass density, and specific heat capacity of heated concrete was taken into consideration. A Taylor series approximate solution was first established within a small neighborhood, employing the Boltzmann transformation in combination with the mean value theorem. Thereafter, it was extended to the whole domain by utilizing the Bernstein polynomial. The semi-analytical approach was validated by comparing it with the numerical results of two independent Finite Element simulations of nonlinear heat conduction along concrete plates, subjected to either moderate or fierce thermal loading. Absolute values of the relative errors are smaller than 5 % . The validated semi-analytical approach was further applied to prediction of the temporal evolution of the temperature field of a scaled model of a subway station, subjected to fire disaster. The nonlinearities, related to the time-dependent surface temperature and the temperature-dependent thermophysical properties of concrete, were taken into consideration. The predictions agree well with the experimental measurements. The established semi-analytical approach exhibits good accuracy and stability, providing insight into the interaction between the thermophysical properties of concrete in the heat conduction process. [ABSTRACT FROM AUTHOR]

Highlights: [•] The concrete industry is responsible for about 10% of the greenhouse gas emissions. [•] The production of concrete with recycled constituents can reduce such an impact. [•] The influence of the aggregate replacement on the hydration process is investigated. [•] The relationship between concrete strength and hydration degree is shown. [•] The influence of initial moisture conditions of aggregates is particularly addressed. [Copyright &y& Elsevier]

Abstract: This paper presents a sound theoretical formulation and an effective numerical implementation of a heat-flow and hydration model for concrete hardening. The model is based on the Fourier equation of heat flow with the adiabatic hydration curve employed as a reference for simulating the hydration heat source. The proposed formulations are based on a consistent scheme for the partial differential equation and its boundary and starting conditions. The hydration kinetics is simulated through the Arrhenius approach. Formulations for the compressive strength and the elastic modulus are provided and the maturity function is also considered. A finite difference numerical solution is derived with a forward explicit time integration in the time–space domain. The numerical solution is designed as a stepwise “recipe” specifically conceived to be easily implemented by means of either a high-level programming language or even a spreadsheet tool. Experimental temperature measurements for two different mixtures, under adiabatic and semi-adiabatic conditions, are used for validating the proposed model. The adiabatic and semi-adiabatic temperature simulations show good agreement with the experimental data for both concrete mixtures. The degree of hydration could be simulated and used as the fundamental parameter for scrutinising the evolution of the compressive strength. Particularly, a linear trend between the compressive strength and the degree of hydration and the maturity was figured out. [Copyright &y& Elsevier]

As sustainability is becoming a fundamental requirement for all modern industrial activities, upcoming environmental research efforts should be intended at both enhancing the efficiency in the use of raw materials and reducing the carbon footprint. The present research activity has been launched at the Federal University of Rio de Janeiro (UFRJ, Brazil) for demonstrating the feasibility of “ecological concrete” for structural use, characterised by a significant replacement of natural aggregates with recycled ones, which are produced from processing Construction Demolition Waste (CDW). Particularly, this work explores alternative processing procedures and investigates their influence on the relevant physical and mechanical properties of the resulting aggregates and concrete mixtures. Therefore, particle size distribution, bulk density attached mortar content as well as the associated water absorption capacity of recycled aggregates were monitored to scrutinise the effect of such processing procedures. Subsequently, a series of concrete batches was produced for evaluating the influence that alternative processing procedures for recycled aggregates have on the main mechanical performance in a fresh and hardened state. Results reported in this paper show the feasibility of “autogenous cleaning”, which removes surface impurities and reduces particle heterogeneities, usually characterising the morphology of a recycled aggregate matrix. It shows that the considered cleaning procedure significantly reduces the gap between the performance of recycled aggregate concrete (RAC) and ordinary ones, both in terms of workability at the fresh state and strength at the hardened state of concrete mixtures. [ABSTRACT FROM AUTHOR]