Concrete precast rejects – treatment, characterization, and application in new concrete as recycled aggregate

This article evaluates the feasibility of using concrete waste from precast production as recycled aggregate in concrete. The processing of concrete waste employed a jaw-hammer crusher and a sieve, producing three types of Recycled Concrete Aggregate (RCA). After their characterization, RCA was incorporated in two types of concrete used in the precast factory: a flowing concrete (FC), slump 220 mm, employed in columns and beams; and the second one, a no-slump extruded concrete (NSC), used for hollow core slabs. X-Ray diffraction, thermogravimetric analysis and scanning electronic microscopy showed phases from the hydrated cement paste and the original aggregate. Results of mechanical performance showed that RCA did not influence the compressive strength but influenced other properties such as water absorption and modulus of elasticity. Lastly, it was concluded that RCA obtained in the precast factory showed great potential to be used within the factory production process.

Influence of internal curing on the properties of non-fibrous high strength concrete—A critical review

The demand for the construction of high-strength concrete in the civil engineering zone is growing, particularly in the last couple of years, due to the construction of sustainable and economic buildings with an extraordinary slim design. Concrete curing in construction is an operative manner and essential to provide that concrete structures meet future performance and durability. High-strength concrete has a low water-to-binder ratio; proper concrete curing is important to ensure its planned performance and durability. Conventionally, exterior curing applied after placing and casting concrete stays warm and moist to provide continued cement hydration. Lately, theoretically and experimentally comprehends that internal curing is an important tool to provide additional moisture in the concrete to enhance cement's hydration. Internal curing of high-strength concrete is an active technique to lessen or even remove autogenous shrinkage and effects on chemical shrinkage, dry shrinkage, etc. Most studies recently have emphasized that a reduction in high strength concrete mixtures' shrinkage is due to internal curing, and the compressive strength can increase higher in mixtures with LWA or SAP than in mixtures without this agent rising degree of hydration by providing extra water in the hydrated cement paste. However, the use of internal curing leads to improving the durability of high-strength concrete.

The Examination of Hydrated Cement Paste Made of CEM III/A 42,5 N-LH/HSR/NA under the Influence of Urea Solution

The production of urea, used inter alia in agriculture, is increasing. Therefore, urea hydrolysis products are expected in groundwater. Due to lack of new research on the influence of urea on the technical condition of concrete structures, the changes that this compound may cause to hardened cement paste were initially check. After 11 months of immersion of hardened cement paste in 20% CO(NH2)2 solution, tests were conducted at different depths of penetration. A pH of 11.97 was recorded in the first layer with a thickness of 0.5 mm, and the pH of the innermost layer was 12.48. The decalcification process and the formation of predominantly secondary calcite in the edge layers were confirmed using XRD, SEM, and analytical methods. No nitrogen phase was formed, but the deeper was the layer, more wollastonite was present. Moreover, up to a depth of about 20 mm, the sample was mechanically weak-breakable by the force of the hands. The examination of the filtrate’s conductivity, leachable calcium content, and pH along the way of urea diffusion confirmed changes in the examined material. When analyzing the technical condition of concrete treated with urea, pH could be an indicator due to the possibility of buffer reactions.

Elucidating the Effect of Accelerated Carbonation on Porosity and Mechanical Properties of Hydrated Portland Cement Paste Using X-Ray Tomography and Advanced Micromechanical Testing

Carbonation of hydrated cement paste (HCP) causes numerous chemo–mechanical changes in the microstructure, e.g., porosity, strength, elastic modulus, and permeability, which have a significant influence on the durability of concrete structures. Due to its complexity, much is still not understood about the process of carbonation of HCP. The current study aims to reveal the changes in porosity and micromechanical properties caused by carbonation using micro-beam specimens with a cross-section of 500 μm × 500 μm. X-ray computed tomography and micro-beam bending tests were performed on both noncarbonated and carbonated HCP micro-beams for porosity characterization and micromechanical property measurements, respectively. The experimental results show that the carbonation decreases the total porosity and increases micromechanical properties of the HCP micro-beams under the accelerated carbonation. The correlation study revealed that both the flexural strength and elastic modulus increase linearly with decreasing porosity.