Incorporation Of Recycled Aggregates Research Articles

Reliability‐based recommendations for EN1992 carbonation cover design of concrete with coarse recycled concrete aggregates

AbstractResearch argues in favor of using recycled aggregates in structural concrete. However, lack of specific clauses in standards for recycled aggregate concrete design hinders the use of recycled aggregate concrete by the industry. Recycled aggregate concrete‐specific design clauses are hindered because conventional research must be complemented with experiments directed towards stochastic modeling, so that reliability concepts may evaluate whether the probabilities of failure of design comply with the expectations of Society or whether specific design rules are needed. This article compares the within‐batch variability of carbonation under accelerated conditions of four natural aggregate concrete and four analogue recycled aggregate concrete mixes and analyses the reliability of Eurocode 2 cover design for carbonation exposure. In general, the incorporation of coarse recycled concrete aggregates worsened the mean durability behavior of concrete, including resistance to carbonation. However, the variability of carbonation depth was not meaningfully affected by the recycled aggregates used (coarse recycled aggregates produced from concrete waste) and the reliability of concrete cover design only decreased due to the effect of the recycled aggregates on the mean resistance to carbonation. A 5 mm increase of concrete cover was found to more than offset the detrimental influence of full incorporation of recycled aggregates on the reliability of concrete cover design.

Discrete Element Simulation Analysis of Biaxial Mechanical Properties of Concrete with Large-Size Recycled Aggregate

Concrete made with large-size recycled aggregates is a new kind of recycled concrete, where the size of the recycled aggregate used is 25–80 mm, which is generally three times that of conventional aggregate. Thus, its composition and mechanical properties are different from that of conventional recycled concrete and can be applied in large-volume structures. In this study, recycled aggregate generated in two stages with randomly distributed gravels and mortar was used to replace the conventional recycled aggregate model, to observe the internal stress state and cracking of the large-size recycled aggregate. This paper also investigated the mechanical properties, such as the compressive strength, crack morphology, and stress–strain curve, of concrete with large-size recycled aggregates under different confining pressures and recycled aggregate incorporation ratios. Through this research, it was found that when compared with conventional concrete, under the confining pressure, the strength of large-size recycled aggregate concrete did not decrease significantly at the same stress state, moreover, the stiffness was increased. Confining pressure has a significant influence on the strength of large-size recycled aggregate cocrete.

Uncertainty of shear resistance models: Influence of recycled concrete aggregate on beams with and without shear reinforcement

The model uncertainty of the shear resistance equations of three design codes {the current Eurocode 2 (2004), Model Code (2010), and the final draft of the Eurocode 2 (2020)} was investigated for coarse natural and recycled aggregate concrete beams. Databases of beams with and without shear reinforcement were made with clearly defined criteria. The statistics of the model uncertainty of natural and recycled concrete beams were compared and it was found that recycled aggregate incorporation has detrimental effects on the model uncertainty of shear design. Surprisingly, recycled concrete aggregate beams designed following the current version of Eurocode 2 (2004) are safer than those designed using the other codes. This is due to the shear resistance equations of the latter overestimating the aggregate interlock of recycled aggregate. A preliminary partial factor was proposed, offsetting the influence of recycled aggregate on the safety of beam designs. The database of beams with shear reinforcement lacks representativeness but hinted that recycled aggregate incorporation also reduces the safety of this type of shear design. The paper finishes presenting suggestions of experiments that would complement the current knowledge on this topic.

Uncertainty Models of Reinforced Concrete Beams in Bending: Code Comparison and Recycled Aggregate Incorporation

AbstractThe bias factor of the Eurocode 2 [CEN (European Committee for Standardization) (2008). Eurocode 2: Design of Concrete Structures–Part 1-1: General Rules and Rules for Buildings] and ACI 31...

Experimental investigation on the variability of the main mechanical properties of concrete produced with coarse recycled concrete aggregates

Research on the variability of the properties of recycled aggregate concrete is lacking and is necessary for the development of reliability analyses and code calibration procedures. This paper presents an experimental programme on the within-batch variability of the compressive strength, Young’s modulus, and splitting tensile strength of several recycled and natural aggregate concrete mixes. The influence of the recycled concrete aggregates on the mechanical properties and variability of concrete is analysed and discussed and benchmarks with standard predictions for the variability of natural aggregate concrete are made. It was found that full recycled aggregate concrete incorporation did not increase the variability of any of the properties tested, but intermediate ratios of recycled aggregate incorporation did. The properties of high-strength concrete mixes were more variable than that of all other mixes, irrespective of recycled aggregate incorporation. All properties of all compositions were suitably modelled by normal distributions. The coarse recycled aggregates were sourced from concrete waste.

Durability and shrinkage of concrete with CDW as recycled aggregates: Benefits from superplasticizer’s incorporation and influence of CDW composition

The shrinkage and durability properties of a total of 34 concrete mixes with recycled aggregates produced from different untreated construction and demolition waste (CDW) were tested. The effect of a polycarboxylic superplasticizer on the enhancement of these properties is presented, discussed, and compared with the findings of studies on concrete whose recycled aggregates are sourced from concrete. All properties were significantly affected by recycled aggregate incorporation and this effect was strongly dependent on the properties of the recycled aggregates of each specific source. The superplasticizer was less efficient in mixes with CDW than when the recycled aggregates were produced from concrete, the most common source of recycled aggregates in experimental works, despite untreated CDW being more practical and desirable from an industrial and environmental perspective.

Unstabilized and stabilized compressed earth blocks with partial incorporation of recycled aggregates

ABSTRACTThis article aims to characterize the physical, mechanical, and water-resistant behavior of unstabilized and stabilized compressed earth blocks (CEB) produced with partial incorporation of recycled fine aggregates from construction debris. The objective was to produce an alternative-earth construction material with lower embodied energy. To this end, different types of common CEB were covered, namely: unstabilized CEB; stabilized CEB with 8% cement; stabilized CEB with 4% cement and 4% lime. CEB were characterized in terms of density, thermal conductivity, compressive and tensile strength, water absorption, permeability, and resistance to water erosion from drip and spray tests. The influence of moisture content was also analyzed. Lime stabilization had little effect on the early age mechanical and durability behavior of CEB. Design coefficients are suggested for common cement-stabilized CEB under different environmental conditions. A minimum of 4% cement content was sufficient to produce water-resistant CEB with partial incorporation of recycled aggregates.

Structural, material, mechanical and durability properties and behaviour of recycled aggregates concrete

Sustainability is a pressing concern of the 21st century. Social development in most countries is raising awareness of the needs of future generations and legislation to protect the present and future quality of life. These needs are compelling various sectors, including construction, to find sustainable solutions. This paper investigates the feasibility of incorporating high-quality recycled concrete aggregates in new concrete, providing a recycling option for precast rejects. The massification of recycled aggregate incorporation would decrease the environmental impacts of the concrete industry, for instance due to reduced landfill disposals, less quarry mining and shortened transport distances. The study presented in this paper concerns the material, mechanical, durability and short-term structural behaviour of concrete specimens and structures made with recycled aggregates sourced from high-quality concrete elements. This paper intends to provide a holistic view of the study, from the procurement and crushing process that generated the aggregates to the conclusions of the different experiments performed. The results of this experimental campaign are benchmarked with other investigations and the importance of the quality of the RA is evaluated. This paper concerns the first full-scale experiments made on recycled aggregates concrete structures. Also, some of the tests performed were the first of their type made on recycled aggregates concrete.

Dynamic characterization of full-scale structures made with recycled coarse aggregates

The incorporation of recycled aggregates produced from concrete waste is an option that could reduce the consumption of natural resources and landfill disposal, with significant environmental gains. Several investigations have been conducted to study the viability of this use of concrete waste, with a focus on the material, mechanical and durability related behaviour of recycled aggregates concrete. The structural behaviour has not been properly studied to date, despite some investigations already performed on two-dimensional scaled-down frame structures and on concrete elements, like beams and column–beam joints. To encourage the use of recycled aggregates concrete, the authors conducted experiments on four full-scale recycled aggregates concrete structures with various recycled aggregates incorporation ratios. Here, the dynamic characterization tests made on the structures are described and analysed. The concrete Young's modulus of finite elements models of the test-structures was calibrated with data from the tests, namely the natural frequencies associated with each modal shape. It was proven that the pattern of response of the different structures was the same and that the aggregates used had a relatively small influence on the Young's modulus. One of the concrete mixes included a superplasticizer and the use of this admixture resulted in a concrete structure with higher stiffness than all the other mixes, including a reference mix with no recycled aggregates. To the authors' best knowledge, this is the first study regarding the dynamic behaviour of full-scale recycled aggregates concrete structures and the dynamic properties of recycled aggregates concrete.

In situ materials characterization of full-scale recycled aggregates concrete structures

Aiming at characterizing the behaviour of concrete structures made with coarse aggregates recycled from crushing concrete elements from the precast industry, extensive testing was performed in four full-scale recycled concrete structures, made with various recycled aggregate ratios. This paper concerns the in situ concrete characterization tests carried out. The purpose was to study the use of the coarse fraction of the recycled aggregates (RCA) at different replacement ratios and the possibility of including a superplastizer (SP) in concrete mixes, as a means of replicating the properties of a conventional concrete (RC) mix. The results showed that the behaviour of the concrete mixes produced with these aggregates, originated from the precasting industry, i.e. with high quality, is almost unaffected by their incorporation. Only the results of the ultrasonic pulse velocity (UPV) tests were affected by RCA incorporation, but the loss was lower than the one reported in the literature. This study concluded that these RCA have, in fact, high quality and are suitable for the production of recycled aggregate concrete to be used in structural elements. The SP more than offset the effects of RCA incorporation on the concrete mixes. To the authors’ best knowledge this is the first investigation concerning the in situ concrete properties of full-scale recycled aggregate concrete structures.

Abrasion resistance of concrete made with recycled aggregates

Global sustainable development will require using every resource to its maximum potential. The potential of construction and demolition waste's inert fraction can be maximised if it is reused as recycled aggregates (RA) for concrete production. However, there are technical risks involved in this technique, particularly due to lack of knowledge in terms of the long-term behaviour of concrete made with these aggregates. Abrasion resistance is one of the least known properties of hardened concrete but nevertheless one of the most important for certain applications (hydraulic structures and floors/pavements). Hardly any research has been published on this property for concrete made with RA. In this paper, the results of four experimental research projects supervised by the author are put together to draw conclusions on this property. It is concluded that there is no loss of abrasion resistance with the incorporation of RAs and that there are reasons to believe that it even improves, even though the quantification of that improvement needs further investigation to be accurate.

Chloride Penetration and Binding in Recycled Concrete

Recycled concrete use is necessary in order to decrease the environmental impact of the construction industry. Coarse aggregates from crushed concrete may lead to relatively high porosity in the resultant concrete. For this reason, recycled concrete may seem detrimental to the durability of reinforced concrete structures, particularly when they are exposed to marine environments. The objective of this paper is to analyze the influence of coarse aggregates from concrete on the chloride penetration rate and binding capacity of concrete under marine exposure. Concrete specimens were molded and exposed to a natural marine atmosphere. After exposure periods of 6, 12, and 18 months, ingress profiles of total and water soluble chlorides were measured. Results indicate that recycled aggregate incorporation causes two opposed effects on concrete: It increases its chloride penetration rate and chloride binding capacity. Therefore, water soluble chloride contents in conventional and recycled concrete series have similar values.