Improvement of the Durability of Recycled Masonry Aggregate Concrete

Design of Performance-Based Concrete Using Sand Reclaimed from Construction and Demolition Waste-Comparative Study of Czechia and India

The main goal and novelty of this study is to show the transferability of practices and experiences with the use of reclaimed sand worldwide in the case in two different regions, the Czech Republic and India, which is necessary for both regions due to the sand availability (Czech Republic) and illegal sand mining including criminal offences (India). Due to the deteriorating environmental impacts associated with sand mining, finding substitution possibilities for natural sand is becoming more important worldwide. It is realized that the reuse of construction demolition waste concrete is inevitable in the pursuit of circular concrete and cleaner production, envisioned by the United Nations (UN) as the attainment of ensuring sustainable consumption and production patterns (Sustainable Development Goal 12-SDG 12) with an inclusive approach of partnerships to achieve the goal (Sustainable Development Goal 17-SDG 17) for the validation of results. The basic material properties of reclaimed sand were examined, and its impact on the physical, mechanical, and durability properties of concrete with complete replacement of sand was evaluated. Generally, a slight decline in properties of concrete with fine recycled aggregate was found. No significant decrease was found from usage possibility in the point of view of its utilization in specific structures and conditions. The research shows the slight differences of results between the Czech and Indian investigations, which are not essential for the transferability of the results.

Effect of Composite Impregnation on Properties of Recycled Coarse Aggregate and Recycled Aggregate Concrete

To improve the properties of recycled aggregate concrete, single and composite impregnation treatments were carried out on recycled coarse aggregates with sodium silicate solution, silane slurry, and polyvinyl alcohol solution. The effects of the three chemical modifiers and different impregnation methods on the apparent density, water absorption, and crushing index of recycled coarse aggregates, as well as the basic properties of recycled aggregate concrete, were investigated. Additionally, the microstructure of the surface of recycled coarse aggregate and the interior of recycled aggregate concrete were analyzed by scanning electron microscopy. The experimental results show that the water absorption of recycled coarse aggregate soaked in polyvinyl alcohol solution decreases most significantly, reaching 64.56%. Only the combination of sodium silicate and silane impregnation produces a positive compounding effect, with a significant increase in the apparent density and a significant reduction in the crushing index of the recycled coarse aggregate. Compared with untreated concrete, the slump, compressive strength, splitting tensile strength, and flexural strength of recycled aggregate concrete prepared by sodium silicate and silane composite impregnation are increased by 9.8%, 26.53%, 21.70%, and 14.72%, respectively. The microstructure analysis shows that the composite impregnation treatment of sodium silicate and silane is most conducive to filling the cracks and holes on the surface of recycled coarse aggregate, which makes the interfacial transition zone of recycled aggregate concrete more compact and the structure more stable.

The Structural Use of Recycled Aggregate Concrete for Renovation of Massive External Walls of Czech Fortification

The use of recycled aggregate concrete is mainly negatively affected by its poorer mechanical and long-term properties. However, there are few structural applications for which recycled aggregates can be used. In this case study, the possibility of use as massive external reinforcement wall is verified. For this structural application, the most important characteristics are freeze–thaw resistance, and carbonation resistance and then the mechanical properties such as compressive strength. Durability characteristics of the materials have been tested and improved in the study. The mechanical properties and durability of recycled aggregated concrete have been verified and crystalline mixture has been used to improve durability. The specific structural application of the massive external reinforcement wall is for the renovation of the Czech WW2 concrete fortification, which is one of the most important cultural heritages of the Czech Republic of the 20th century. However, these buildings have not yet been professionally rebuilt, but this research project aims to change this trend. The thickness of the bunker wall is between 0.5 and 3.5 m (depending on the type of bunker) which leads to a huge amount of concrete and primary resources consumption; however, the security function is not necessary today, so the reconstruction could be provided by recycled aggregate concrete. The results showed a positive effect of the crystalline mixture on the essential properties of recycled aggregate concrete. Recycled aggregate concrete with a complete replacement of aggregate by recycled concrete or masonry aggregate is possible to use for the reconstruction of the Czech WW2 concrete fortification and save natural aggregate as a primary resource.

Effect of Mixed Recycled Aggregate on the Mechanical Strength and Microstructure of Concrete under Different Water Cement Ratios

Mixed recycled aggregate (MRA) is a kind of recycled aggregate containing discarded bricks and other impurities that is inferior to ordinary recycled concrete aggregate. To study the effect of MRA in concrete, specimens with 100% MRA under different water–cement ratios (W/C) of 0.50, 0.42, 0.36 and 0.30 were prepared, and the mechanical properties and microstructure were tested. Results show that compared with ordinary concrete, the compressive strength of mixed recycled aggregate concrete (MRAC) with the same W/C was reduced by more than 50% at 28 days, but the axial compression ratio was relatively high, reaching over 0.87. Affected by the high water absorption of MRA, the hydration rate of cement slowed, which was beneficial to the long-term development of the properties of MRAC. An appropriate increase in cement content could strengthen MRA and densify the pore structure of MRAC. The research results of this article prove that MRA has high utilization value and could be used to prepare MRAC with application potential using optimal gradation, which is of positive significance for promoting the consumption of construction waste.