Modeling green recycled aggregate concrete using machine learning and variance-based sensitivity analysis

Objective: The objective is to determine the effect of treated RCA on the mechanical strength of hardened concrete, and determine the influence of treated RCA on water transport within the matrix of hardened concrete. Aim: The study aims to use pozzolanic concentration of cement and RHA slurry to pretreat coarse RCA for full replacement of NCA for sustainable concrete production. Background: One of the common modes of disposal of construction waste is strained landfills. This study proposes a constructive way of recovering the coarse aggregates of recycled aggregates from laboratory concrete waste taken through a focused treatment process with pozzolan. The study aims to replace the proportions of the cement slurry to treat and replace with rice husk ash (RHA) for the modification of recycled concrete aggregate (RCA) and replace fully natural coarse aggregate (NCA) for sustainable concrete production. One of the main challenges of recycled concrete aggregate (RCA) is the fractured surface, which often results in reduced strength and loss of concrete durability. The strength of concrete with RCA is principally influenced by the fractured hardened paste attached to the surface, predominantly characterized by voids and gaps resulting from its production using a mechanical method. High water penetration within and around the aggregate matrix due to the presence of micro- and macro-cracks results in strength and durability deterioration of concrete. Methods: This study addresses these problems by specifying aggregate treatment with a pozzolanic slurry of cement and RHA. This was conducted in two (2) phases; first, to determine the optimum proportion of cement slurry concentration (0, 20, 40, 60, 80, and 100%), and second, to increase the silicate formation in cement slurry by substituting the proportions of cement with RHA (0, 5, 10, and 15%). The performance of concrete was evaluated by mechanical strength (compressive and split tensile strength) at 7, 28, and 56 days and water transport within the concrete (surface suction and submergence) at 28 and 56 days of curing age. Results: The use of cement slurry treatment at various concentrations generally increased compressive strength at 28 days compared to untreated RCA. The findings show that at a 20% concentration of cement slurry, the increase in compressive strength is 38.4%. Upon replacement of cement in treatment slurry of RCA with 5% RHA, the increase in compressive strength was 61%, while the split tensile strength increased by 10.2%. The amount of water absorbed was observed after 30 minutes of full submergence in water, indicating that samples with 20% cement concentration at 28 days resulted in the lowest water absorption of 2.01%. However, with the replacement of 5% RHA, the water absorption slightly increased to 2.3%, but still less than the requirement of 3.5%. The corresponding test results for water suction by capillarity, initial and secondary sorptivity coefficients (Si and Ss) were found to be 0.0330mm/√sec. (1.98mm/√hr.) and 7.95×10−4mm/√sec. (0.04mm/√hr.) respectively, all less than 6mm/√hr requirement. This improved performance was attributed to the increased strength and structure of the interfacial transition zone (ITZ) around the RCA. Conclusion: Based on the results of mechanical strength and water transport, characteristics of the new concrete were improved when coarse RCAs were pretreated with 20% pozzolanic concentration containing cement and RHA. The use of a pozzolanic treatment slurry containing 15% cement and 5% RHA to treat coarse RCA can produce concrete with characteristics similar to those of natural concrete aggregates. This study presents a methodological utilization and improvement of RCA wastes for field application.

Due to the depletion of natural aggregates and raw materials for contemporary construction, the construction and demolition waste existing in old concrete structures is an imperative problem. Cutting down on exploiting natural aggregates and reusing construction and demolition waste in the concrete industry are solutions to this problem. This paper investigated the replacement of natural coarse aggregate (NCA) with recycled concrete aggregates (RCA) with different ratios (0%, 50%, 75%, and 100%) in producing self-compacting concrete (SCC). Different components of supplementary cementitious materials (SCMs), such as nano-silica (NS), fly ash (FA), and metakaolin (MK), as well as PVA fibers, were incorporated into the SCC mixtures. The fresh properties (slump flow, V-funnel, and L-box test) and hardened properties (compressive strength, splitting tensile strength, and flexural strength), as well as the flexural behavior of SCC beams (load-carrying capacity, crack pattern, mid-span deflection, and flexural stiffness), were studied for all SCC mixes. The results of fresh and hardened concrete confirmed that it is possible to produce SCC with a 100% replacement of RCA with minimal effects on the concrete properties. The mixture of SCC with 100% RCA replacement, 20% MK, and 22% FA was the optimum mixture with acceptable fresh properties that complied with the EFNARC specifications. For 100% RCA replacement, the compressive strength was reduced by 8.20%, and the ultimate load and flexural stiffness increased by 3.20 and 16.25%, respectively, compared with the control mixture.

Carbonation of concrete made with high amount of fly ash and recycled concrete aggregates for utilization of CO2

The concrete building industry has to adhere to stringent sustainability standards. In previous investigations, the utilization of recycled concrete aggregates (RCA) from construction and demolition waste (CDW) or crumb rubber (CR) from waste tires in producing sustainable self-compacted concrete (SCC) was well studied; however, the combined effect of RCA and CR on the performance of SCC is scarce. Therefore, the main purpose of this study is to evaluate the feasibility of developing SCC mixtures containing both RCA and CR and their influence on the performance of SCC. Three distinct series of SCC mixes were designed so that the water-to-binder ratio and total binder content were kept at 0.32 and 505 kg/m3. In the first series, the effect of RCA content (0%, 50%, and 100%) in place of natural coarse aggregate (NCA) on the performance of SCC mixtures was studied. Moreover, the effect of various CR contents (0%, 10%, 20%, 30%, and 40%) as a sand substitution and 50% RCA was investigated in the second series. In the third series, the influence of CR inclusion and 100% RCA was utilized to promote the sustainability of SCC. The fresh behavior of the mixtures was assessed by measuring followability, viscosity, and segregation resistance. Density, water absorption, and apparent porosity for all mixes were also examined. The mechanical properties were assessed for compressive, splitting tensile, and flexural strengths. The findings show that the inclusion of RCA and CR has an adverse impact on the fresh, physical, and mechanical properties of SCC, excluding segregation resistance. However, it is possible to produce a SCC using 100% RCA and 40% CR with a slump flow diameter of 605 (SF1 class) and a compressive strength of greater than 31 MPa, which is applicable for structural applications. Moreover, the ACI and Euro code CEB-FIP proposed models can predict the mechanical properties of the developed SCC made with recycled materials. This research could contribute to a more sustainable SCC by substituting recycled materials for natural aggregates.

This paper presents experimental results of two-stage mixing approach on properties of concrete made with recycled concrete aggregates. Concretes with recycled concrete coarse aggregates have poorer workability and lower mechanical strength than concretes with natural coarse aggregate. Concrete mixtures lose workability in early time due to their increased capillary porosity and presence of interfacial transition zone (ITZ). It consists of ettringite and calcium hydroxide. Due to the existence of two weaknesses in the ITZ, the performance of recycled aggregate concrete is much lower than that of natural aggregate concrete. When formation time of cement-based materials in concrete with recycled concrete aggregate, new appear in addition to the old ones - between recycled concrete aggregate and a new mortar part. Interfacial transition zone is less durable than the interfacial transition zone in recycled concrete aggregate itself. The article describes the method of using two-stage mixing approach (TSMA) with research of the time of formation of concrete made with recycled concrete aggregates. Concrete mixture prepared by two-stage mixing approach showed difference in time formation in early stages, without a sharp loss of workability in contrast to the concrete mixtures with normal mixing. The samples made by TSMA has higher strength than the samples made by normal mixing. The compressive strength in 3, 7, and 28 days increased by 16%, 8%, and 16%, respectively, compared with the samples made by normal mixing. The obtained data showed the positive influence of two-stage mixing on loss of workability in early time and strength of the concrete.

Concrete Containing Recycled Concrete Aggregate with Modified Surface

The usage of recycled aggregate (RA) for concrete works is spreading for gaining wide acceptance around the world, and total amount of RA utilized in concrete industry increases day-by-day. RA is used in many concrete products, such as concrete interlocking blocks, pavements, etc. One of the physical causes of concrete deterioration is surface wear or loss of mass for concrete products which are especially exposed to vehicular traffic (i.e., industrial floors and pavements). Besides, the loss of mass by concrete surface wear becomes the primary factor of one or more processes of chemical deterioration as a result of an increase in permeability due to mass loss of the concrete surface wear. The main goal of this research is to investigate the abrasion behavior of recycled aggregate concrete (RAC) with/without silica fume (SF) related with the mechanical and the physical properties of RAC. In this research, lower quality RA (<15 MPa) is utilized as recycled coarse aggregate (RCA1 and RCA2) and recycled fine aggregate (RFA) with 25, 25, and 50% replacement ratios, respectively, in concrete mixes. As a result, RCA2 with/without SF gives better performance than RCA1 and RFA if abrasion resistance is considered only. Besides, RFA utilization in concrete decreases the abrasion performance of RAC. Also the relations of compressive strength-abrasion loss and elasticity modulus-abrasion loss are examined, and high regression coefficient (R) for the relations is found. Keywords: abrasion resistance, mechanical properties, physical properties, recycled aggregate, silica fume

Effect of lime filler on RCA incorporated bituminous mixture

The paper presents the results of an investigation conducted to study the strength properties of pervious concrete containing coarse recycled concrete aggregates (RCA). In this study, 13 pervious concrete mixes were prepared with the use of RCA replacing coarse natural aggregates (NA) by 0, 25, 50, 75 and 100%. Keeping the water–cement (w/c) ratio constant, Viscosity Modified Agent (VMA) was added at 0.8% by weight of binder content. Being environmentally advantageous, 30% Fly Ash (FA) has been added with 70% Ordinary Portland Cement (PC) in the binder of control mix. Despite being environmentally safe product, metakaolin (MK) also tends to enhance the properties of concrete containing RCA. Therefore, keeping the PC content 70% for all mixes, FA was replaced with MK at two different levels of 5% and 10% to improve the properties of pervious concrete made with RCA. Specimens in the form of cubes, cylinders and beams were prepared for studying the mechanical properties at different ages of curing. The test results showed that there was significant loss in compressive, splitting tensile and flexural strength of pervious concrete while increasing the percentage of RCA replacing NA. Addition of 5% MK compensated the decrement noticed in the strength properties after replacement of NA with RCA. The mix with 25% RCA and 5% MK was having almost same compressive strength as that of reference mix. 10% MK further enhanced the strength properties and it was concluded that loss of compressive strength in pervious concrete due to addition of 50% RCA can be compensated by the addition of 10% MK.

The Efficiency of Steel Slag and Recycled Concrete Aggregate on the Strength Properties of Concrete

The construction industry strongly relies on concrete and clay bricks for various applications. The escalating demand for these materials, driven by rapid population growth, has led to resource depletion and increased construction and demolition waste (CDW). Recycling CDW into construction materials, particularly in the form of recycled concrete aggregates (RCAs) and recycled brick aggregates (RBAs), has emerged as a promising solution. This study deals with the structural performance of concrete incorporating RCAs and RBAs. The experimental program encompasses material characterization, concrete mix design, and several tests to assess density, compressive strength, bond behavior, and flexural properties. The results indicate that the replacement of fine natural aggregate (NA) with fine RCAs or RBAs has a negligible impact on density, while the partial replacement of coarse NAs with RAs yields modest reductions in compressive strength. Notably, the bond strength between steel rebar and concrete is influenced by the type and content of RA, with specimens containing RCAs exhibiting a higher bond strength than those with RBAs. Empirical models used to predict bond strength generally align with experimental results, with conservative predictions by some models, such as ACI 318, and overestimation by others, such as models proposed by AS-3600 and CEB-FIB. The flexural tests of beams highlight the variation in stiffness and load-bearing capacity with the proportion of NAs replaced by RAs. While beams with 50% NA replacement demonstrate comparable performance to control beams, those with 100% RA replacement exhibit lower cracking and yielding stiffness. Cracking patterns in beams with RAs differ from control beams, with RA-containing beams showing more cracks and an altered crack distribution. The findings underscore the feasibility of using recycled aggregates in construction, with partial NA replacement offering a balance between sustainable material usage and desired structural properties.

The increasing generation of construction and demolition waste, combined with the rapid depletion of natural resources, has made it imperative to explore sustainable alternatives in the construction industry. One such alternative is the use of Recycled Concrete Aggregates (RCA) in the production of structural concrete. This research, titled "Waste to Strength: Investigating Recycled Concrete Aggregates for Sustainable Construction", aims to assess the viability of using recycled aggregates as a partial or full replacement for natural coarse aggregates in M30 grade concrete. While recycled aggregate concrete (RAC) has been studied extensively for its compressive strength, limited attention has been given to its long-term durability, especially in reinforced concrete structures. This study focuses on both the mechanical and durability performance of RAC. The primary objectives include evaluating the compressive strength of M30 grade concrete made with recycled aggregates and assessing its resistance to chloride ion penetration through the Rapid Chloride Permeability Test (RCPT). The study emphasizes the dual benefits of using RCA reducing environmental degradation caused by demolition waste and conserving natural aggregates while ensuring that the structural integrity of the concrete remains within acceptable limits. The findings contribute to the growing need for sustainable construction materials and promote the use of recycled waste in load-bearing applications. Keywords: Recycled Concrete Aggregate (RCA), Recycled Aggregate Concrete (RAC), Demolition Waste, Sustainable Construction, Compressive Strength, Durability, Chloride Penetration, M30 Grade Concrete, Environmental Impact, Circular Economy in Construction.

Properties of recycled aggregate concrete prepared with scattering-filling coarse aggregate process

The paper presents results of an investigation conducted to quantify the flexural fatigue performance of concrete in which the Coarse Natural Aggregates (NA) were systematically replaced with Coarse Recycled Concrete Aggregates (RCA). Concrete mix containing 100% NA was also tested for comparison purpose. The compressive strength results of 100% RCA showed maximum reduction followed by the concrete made with 50% RCA. The fatigue test data of concrete made with 50% and 100% RCA and 100% NA was obtained by conducting flexural fatigue tests on beam specimens of 100×100×500 mm size. The specimens were tested using 100 kN MTS Servo-controlled actuator under four-point flexural fatigue loads applied at frequency of 10 Hz. Static flexural tests were also conducted to facilitate fatigue testing. The maximum flexural strength of 5.10 MPa was observed by the concrete mix containing 0% RCA (100% NA) followed by 4.89 MPa and 4.53 MPa for 50% RCA and 100% RCA respectively. To predict the flexural fatigue strength of concrete made with RCA, the materials coefficients of fatigue strength prediction models have been obtained. The S-N-Pf models for three concrete mixes have also been generated from the fatigue test data analytically. Two-Million cycles fatigue strength has also been estimated which was coming out to be 50% and 56% of the actual flexural strength for concrete mixes containing 50% and 100% RCA respectively.

The use of cement to stabilize soft clay and construction sludge is common for achieving suitable soil properties in construction. To reduce costs and promote recycling, recycled concrete aggregate (RCA) from construction and demolition waste has been adopted as a cement substitute. However, the presence of hexavalent chromium (Cr(VI)) in RCA raises environmental concerns. This research investigates the mechanical properties and Cr(VI) leaching behaviors of cement-stabilized clay SC-RCA mixtures with different RCA contents (0%, 20%, 40%, and 60% by weight) as well as SC and RCA individually. Laboratory experiments including Proctor compaction, hydraulic conductivity tests, batch leaching tests (BLT), and tank leaching tests (TLT) were performed. Results show that adding RCA improves compaction and decreases hydraulic conductivity, with the lowest hydraulic conductivity at 40% RCA and minimum void ratio at 60% RCA. This is because RCAs come into contact with each other and form structures at the content of 60% RCA, causing the SC matrix to become trapped in voids between the RCA structures, resulting in ineffective compaction and increased porosity of the SC matrix. Higher RCA content leads to increased Cr(VI) leaching in BLT, but TLTs showed negligible Cr(VI) leaching. Calcium (Ca) was measured as a trace chemical to determine effective diffusion coefficients (Deff), which were higher for specimens with 60% RCA, since the enhanced pore connectivity of well-contacted RCA particles led to an increase in K, facilitating more direct pathways and quicker diffusion of Ca through the SC-RCA matrix. The 40% RCA mixture exhibited the lowest Deff, suggesting that it improved compaction and limited chemical leaching.