Durability of Structural Recycled Aggregate Concrete Subjected to Freeze-Thaw Cycles

Effect of Municipal Solid Waste Slag on the Durability of Cementitious Composites in Terms of Resistance to Freeze-Thaw Cycling

The article presents durability results for cement mortars made with the addition of slag from municipal waste incineration plants as a replacement for natural aggregate. The undertaken durability tests included frost resistance tests and evaluation of strength, microstructure, water absorption and abrasiveness before and after 150 freeze-thaw cycles. The work reveals that MSWI slag in amounts up to 50 vol. % caused deterioration in the workability and water absorption of cement mortars, regardless of the type of cement used. This, in turn, resulted in a reduction in the compressive and flexural strengths of the composites compared to mortars made with sand alone. Nevertheless, the use of a higher grade of cement, CEM I 52.5 R, resulted in an increase in compressive strength and thus increased the mortars' frost and abrasion resistance. In addition, after the induced freeze-thaw cycles, mortars made with MSWI slag showed comparable or higher frost and abrasion resistance than those made using natural aggregate.

Deicer Salt-Scaling Resistance of Concrete Using Recycled Concrete Aggregates Pretreated by Silica Fume Slurry

Concrete wastes such as recycled concrete aggregates (RCA) make up a significant part of construction and demolition waste (C&DW) which can be used to minimize usage of natural aggregates and reduce carbon footprint. This paper studies the salt-scaling resistance of recycled aggregate concrete produced with pretreated RCAs. The test method for evaluating salt-scaling resistance in concrete according to DIN EN 1340: 2003 was performed. Four series of concrete mixes using natural aggregates, RCAs, manually pretreated RCA, and modified RCA in a desiccator were subjected to the different tests in terms of bulk electrical resistance in two directions (X and Y) before and after freeze-thaw cycles, ultrasonic pulse velocity, and weight loss of the surface layer of concrete specimens. Moreover, Scanning Electron Microscopy (SEM) of mixes was conducted and the microstructure of mixes considering the interface transition zone was studied. Results show that after exposure to cycles of freezing and thawing, the quality of concrete regarding ultrasonic pulse velocity did not change. The electrical resistance of specimens decreased significantly in X-direction and slightly in Y-direction after applying freeze-thaw cycles in all mixes. Nevertheless, surface modification of RCAs can increase electrical resistance and improve durability of concrete. SEM images show that the interface transition zone before and after freeze-thaw cycles remained unchanged which means strong bond between aggregate, new mortar, and old mortar. An estimation of the total charge passed indicated that all recycled aggregate concretes can be classified in a safe area and with very low chloride ion penetrability according to ASTM C1202.

Performance of Building Solid Waste Powder in Cement Cementitious Material: A Review

Recycled powder (RP) is a by-product of preparing recycled aggregates from construction waste through debris removal, step-by-step crushing, screening, and mechanical strengthening. It is a fine powder with a particle size of less than 75 μm. Reasonable use of RP can increase the utilization rate of construction waste and reduce dust pollution. This study introduces the current research status of RP. It describes the source of RP; the activation mode of activity; the effect on several aspects, such as early performance and mechanical properties of cement-based materials; and its mechanism of action in light of the research and development. Moreover, the linear regression analysis method was used to obtain the mathematical model between the content of RP and the performance of cement-based materials. The correlation degree between the content of RP and the performance of cement-based materials was obtained based on the gray relation analysis method. It was concluded that the change of the content of RP had the most significant influence on the compressive strength of foamed concrete over 28 d. Finally, some feasible suggestions and prospects for RP are provided.

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.

The Properties and Durability of Self-Leveling and Thixotropic Mortars with Recycled Sand

In recent decades, relevant environmental and economic reasons have driven an increasing interest in using a large amount of recycled aggregate in replacement of natural ones to produce mortar and concrete. The present study aims to investigate the effect of substituting 100% of natural sand with recycled aggregate on fresh properties, mechanical properties, and the durability of a thixotropic and a self-leveling mortar. Recycled aggregate was characterized using X-ray diffractometry and energy-dispersive X-ray spectroscopy. Its morphology was investigated using scanning electron microscopy and automated morphological imaging. Recycled aggregate mortars showed a moderate decline in initial workability, as well as higher shrinkage and porosity than the control ones. The compressive strength of self-leveling mortars produced with recycled aggregate was only 6% lower than mortars produced with natural sand. The gap increased to 40% in the case of thixotropic mortars. The self-leveling recycled aggregate mortar showed equivalent resistance to freeze–thaw cycles and better sulfate resistance than the control one. The thixotropic recycled aggregate mortar showed comparable sulphate resistance and only slightly lower resistance to freeze–thaw cycles than the control one. Their capacity to relief stresses, due to hydraulic pressures and the formation of expansive products, arises from their higher porosity. Thermal stability of the prepared mortars, after a curing period of 90 days, up to 700 °C, was also investigated. A significant decrease in ultrasonic pulse velocity is observed in the 200–400 °C interval for all the mortars, due to the dehydration–dehydroxylation of calcium silicate hydrate. The overall decline in the strength of both the recycled aggregate mortars was comparable to the control ones. The results reported in the present investigation suggest that the selection of high-quality recycled aggregate helps to obtain good-quality mortars when a large amount of natural sand is replaced.

Influence of the Geosynthetic Type and Compaction Conditions on the Pullout Behaviour of Geosynthetics Embedded in Recycled Construction and Demolition Materials

The effects of the climate change that the planet has been experiencing, and the growing awareness of citizens that natural resources are finite, highlight the inevitability of making society more sustainable. Since the construction industry is responsible for a high consumption of natural resources and it simultaneously produces high volumes of waste, it is of great importance to investigate the feasibility of using construction and demolition (C&D) wastes as alternatives to common natural materials. This paper investigates the feasibility of using fine-grain recycled C&D wastes as backfill material of geosynthetic reinforced steep slopes, through a laboratory study focused mainly on the pullout behaviour of two geosynthetics embedded in these alternative materials. The influence of the geosynthetic type, moisture content and compaction degree of the recycled C&D material on the pullout behaviour is assessed and discussed. The physical and mechanical characterization of the filling material is also presented. The pullout test results have pointed out that, although the two geosynthetics have similar tensile strength, the pullout resistance of the geogrid is higher than that of the geotextile and is achieved at lower frontal displacements. While the reduction of the compaction moisture content below the optimum value induced a slight decrease in the geogrid pullout resistance (ranging from 5% to 7%), conversely the pullout capacity of the geotextile increased up to 22%. The compaction degree of the recycled C&D material had the expected effect on the geotextile pullout resistance, reflected in an increase of about 20% when the degree of compaction rose from 80% to 90%. However, the expected trend was not observed on the geogrid pullout behaviour. The pullout interaction coefficient tended to decrease with the variation of the compaction moisture content around the optimum value (maximum decrease of 33% and 16% for the geogrid and the geotextile, respectively) and with an increase in the vertical confining pressure from 10 kPa up to 50 kPa (decrease around 25%). The average value of the pullout interaction coefficient, fb, ranged from 0.61 to 1.09 for the geogrid and from 0.67 to 1.25 for the geotextile. From all these findings it can be concluded that recycled C&D materials can be seen as an environmentally friendly alternatives to the natural resources commonly used in the construction of geosynthetic-reinforced embankments.

Influence of Wetting and Drying Cycles on Physical and Mechanical Behavior of Recycled Aggregate Concrete

Recently, concerns have been rising about the impact of increasing the depletion of natural resources and the relevant generation of construction and demolition waste, on the environment and economy. Therefore, several efforts have been made to promote sustainable efficiency in the construction industry and the use of recycled aggregates derived from concrete debris for new concrete mixtures (leading to so-called recycled aggregate concrete, RAC) is one of the most promising solutions. Unfortunately, there are still gaps in knowledge regarding the durability performances of RAC. In this study, we investigate durability of structural RAC subjected to wet-dry cycles. We analyze the results of an experimental campaign aimed at evaluating the degradation process induced by wetting and drying cycles on the key physical and mechanical properties of normal- and high-strength concrete, produced with coarse recycled concrete aggregates (RCAs) of different sizes and origins. On the basis of the results we propose a degradation law for wetting and drying cycles, which explicitly makes a possible correlation between the initial concrete porosity, directly related to the specific properties of the RCAs and the resulting level of damage obtained in RAC samples.

Improvement of the Durability of Recycled Masonry Aggregate Concrete

The use of recycled masonry aggregate for concrete is mostly limited by the worse properties in comparison with natural aggregate. For these reasons it is necessary to find ways to improve the quality of recycled masonry aggregate concrete and make it more durable. One possibility is utilization of crystalline admixture which was verified in this study by laboratory measurements of key material properties and durability. The positive influence of mineral admixture was proved for freeze-thaw resistance. The positive impact to carbonation resistance was not unambiguous. In conclusion, the laboratory evaluation shows how to improve the durability of recycled masonry aggregate concrete, however, it is necessary to investigate more about this topic.