The technique of recycling entails the transformation of utilized resources to facilitate their subsequent utilization in the production of fresh products that possess more value. The fundamental procedure that is followed during the recycling process involves breaking the demolished concrete to produce minor fragments by exposing them to a series of procedures, such as: For example, the removal of impurities (reinforcements, plastic, wood etc.), includes various selection and classification levels. Many varieties of aggregates can also be processed step by step, requiring time and effort for sorting, breaking, semi-finishing and classifying (previous breaking and subsequent crushing). The detection and removal of pollutants depend on the degree of pollutants and the application for which the recycled ingredients are used. C & D waste can be crushed by various crushers such as jaw crushers, impact crushers, hammer mills and hand hammers [5].
Various recycling procedures have varying effects on the mechanical and physical properties of recycled aggregates. These effects are determined by the efficiency of the processing technique used, and they eventually impact the performance of concrete. Many researchers followed and performed different processing techniques to get the recycled aggregates from C&D wastes. The following are the researchers and their methods adopted in recycling process.
The biggest challenge encountered when employing recycled aggregates as a replacement for conventional aggregates in concrete stems from the concern surrounding water absorption by the recycled aggregates. The inclusion of recycled aggregates results in a decrease in the available water for cement hydration within the concrete mixture. This water absorption problem was addressed by Julia Gonzalez et al.( 2014) in their research and proposed a pre saturation technique to overcome the above-mentioned problem in usage of RA in concrete.
Gonzales et al (2014) exhibits the viability of pre-saturation technology to resolve water absorption problems of recycled aggregates in concrete production. They replace natural aggregates in terms of 20%, 50%, 80% and 100% by recycled aggregates. The water uptake of the aggregates was verified to regulate the required soak time to bring the recycled aggregates to an appropriate moisture level for merger into the mixture. They found that the recycled aggregates attain full saturation after 10 days and reached saturation of up to 56% in the first hour. So they opted only 3 minutes and 5 minutes soaking for which they got 47% and 50% saturation respectively 3 minutes of soaking leads to plastic or soft slump and 5 min soaking showed fluid state consistency in slump. They also observed that, for concrete mixes with aggregates of 3 minutes soaking presented a loss up to 11%- and 5-minutes soaking results in 13% loss compared to control concrete mix with recycled aggregates without pretreatment. Finally, they conclude that, despite the strength loss, all the samples have yielded the target mean compressive strength.
Because of the mortar adhering to the surface of recycled coarse aggregates, their use in concrete is restricted, which increases their porosity and water absorption, resulting in a weaker interface with the new cement mortar. This limitation reduces the strength and mechanical performance of cement concrete containing recycled aggregates.
In order to overcome the above difficulty, Tam et al. (2007) conducted the experiment using three presoaking approaches for the removal of cement mortar adhered to the surface of recycled aggregates. The three presoaking procedures namely remortar HCl, remortar H2SO4; remortar H3PO4 was proposed and compared in their study. They start by soaking the RCA in an acidic solution at 20 C for 24 hours, then they rinse it in pure water to get rid of the acid. The three acidic solvents used in their experiments explicitly hydrochloric acid (HCl), sulfuric acid (H2SO4) and phosphoric acid (H3PO4) with concentration of 0.1 moles. By using above three acidic solvents, they replace natural aggregates by RA in terms of 5%, 10%, 15%, 20%, 25% and 30%. They found that, water absorption, chloride content and PH values of recycled aggregates earlier and later the three presoaking action procedures were within the prescribed code limits. They observed that, the rate of water absorption for RCA are between 3% and 10%, compared to less than 1–5% for natural conventional aggregates.
They found that, 22% improvement with 20% RCA replacement is noted for flexural strength at curing for 14 days and 21% enhancement with 30% RCA replacement for modulus of elasticity when using the remortar Hcl methodology. Finally, the authors conclude that RCA pretreatment should be a more effective technique for increasing RCA quality for the advanced grade application.
Pepe et al.,[7] investigated alternative processing methods for recycled aggregates in structural concrete and examines their impact on the significant physical and mechanical properties of the resultant aggregates and concrete mixtures. They performed recycling processes in 3 stages i.e particle homogenization then grinding and sieving, finally by doing autogenous cleaning of the aggregates. The primary steps in making RCA include dealing with C&D trash. In addition, a third process was introduced to improve the RCA and the mechanical characteristics of the concrete produced with recycled aggregates. The RCA was ground, sieved, and separated into three size groups before being cleaned mechanically.
Along with this autogenous cleaning, they performed thermal treatment method to evaluate the adhered mortar content of the recycled aggregates. They found that, quantity of absorbed water was decreased from 20–50% by autogenous cleaning. They produced three concrete mixes i.e reference concrete mix, recycled aggregates concrete mix and then cleaned recycled aggregates concrete mix. They observed that, for reference mix, they got 33 MPa of compressive strength in 28 days. The research findings indicated a reduction in the compressive strength of recycled concrete aggregate (RCA) by roughly 20%, but the compressive strength of cleaned RCA exhibited a fall of just 8%.
Kathy Bru et al. (2013)[8] conducted studies evaluating the process of recovering high-grade aggregates from concrete waste can be achieved through the use of a microwave-assisted recycling procedure. This innovative technique exploits the effects of interior heating caused by microwaves on various mineral phases, leading to differential thermal expansion and the creation of stresses.
The presence of internal mechanical stresses gives rise to cracks, particularly at the interfacial transition zone (ITZ) where the aggregates come into contact with the cement paste, leading to a reduction in material strength. RCA was produced from a concrete slab which is 5 months cured and subjected to a jaw crusher with setting of 80 mm. About 6 KW power capacity microwave generated microwave heat is passed on the concrete aggregates in a multimode cavity and then it is crushed in a Hazemag impact crusher with a rotation speed of 290 rpm. They concluded that it was found that the previous microwave attenuation treatment was effective regardless of the type of aggregate. The material's properties were unaffected by the low levels of microwave heating energy used during its incorporation into the concrete.
Some inventive process for procuring a large variety recycled aggregates from concrete recycling was performed by Menard et al. (2013)[9]. They investigated two embrittlement technologies addressed on electrical impulse discharge or microwave heating for the selective disintegration of concrete. They used laboratory-produced concrete samples as a depiction of concrete waste from the concrete mixer and found that both the technologies are efficient for the weakening and release of aggregates from concrete. But they also found that electro hydraulic technique consumes less energy i.e. 1–3 KWht− 1 compared to microwave treatment, which consumes 10–40 KWht− 1 of power for the fragmentation of aggregates from concrete.
Infrared sorting technology is also one of the innovative recycling processes used in many of the European countries for procuring recycled aggregates free from other waste material present in C&D waste. Vegas et al. (2014)[10] investigated on Near Infrared Sorting Technology (NIR) for the improvement in the procurement of quality recycled aggregates from C&D waste. NIR sorting technology works in 3 stages. At the first stage, the NIR sensor detects the fed material from the conveyor belt and in the second stage, sensors detect the sorted out material and send command to control unit to blow out the suitable things at the end of the belt. The flow jets are used to separate good quality aggregates and discarding materials, which were stored in separate chamber. Then in the third stage, sored materials were sorted out based on grain size and specific gravity. They observed that, NIR mechanism achieved better efficiency when sorting of gypsum, it was one of the main contaminations in the C&D waste. And also they found that, there will be decrease in the impact on the environment in terms of sulphate leaching is achieved using this NIR – sorting technology.
Aggregates of recycled concrete are most often produced from two fields. One with cubes of prefabricated elements, processed after the tests, and the second - with demolished concrete structures. In the former case, the resulting aggregates were comparatively clean, and only cement paste was attached on them. But in the latter case, the aggregates were adulterated with bricks, tiles, dust of sand, wood, plastic, cardboard and other metals (Akash et al. 2011)[11]. The quality of the RCA depends mainly on the processing methods, but the properties of the RCA were primarily governed by the proportion of cement and water in the native concrete from which the RCA was acquired.
The attached mortar is the most prominent feature of RCA, it makes the RCA more porous heterogeneous and less dense. The amount of attached mortar in RCA ranges from 25–60%, depending on the size of the aggregate. 20% of adhered mortar is present in RCA for aggregates size range of 20 to 30 mm has been observed by many researchers (Behera et al. 2014)[5]. Generally, RCA obtained from crushed concrete comprise of 30–35% of old cement pastes and 65–70% by volume of coarse and fine natural aggregates. The density of RCA can vary from 2200 to 2400 kg/m3, and the capacity of water absorption can vary from 5 to 15% (Poon et al. 2004). Sheen et al (2013), revealed that the bulk density of recycled aggregate ranged from 18.32–19.27% which was lower than natural coarse aggregate and also they observed that, the specific gravity was also 5.70–14.07% lower than natural aggregate. They attribute the above results to lower density and higher absorption (5.04–7.54%) rate of recycled aggregates.
It was found that the resulting RCA from the C&D waste has lower density than the natural aggregates but it was within the standard limits for a coarse aggregate. It was observed that, shape of the RCA was round and less flaky than the natural aggregates, and the utmost difference was in absorption of water, where RCA values were four times greater than natural coarse aggregates. The original cement paste's higher porosity and the sand's adherence to RCA are mostly responsible for this (Limbachiya, 2010)[12].
Water absorption and dry density on a saturated surface (SSD) are two crucial properties that determine the quality of the aggregate. The significant water absorption capacity of recycled concrete aggregates (RCA) necessitates consideration in the concrete mixture. This is because a portion of the water introduced into the mixture will be absorbed by the pores present in the recycled aggregates, resulting in a reduction in the workability of the concrete. This effect can be compensated by the pretreatment process called presoaking and the surface saturated density should be considered while mixing concrete [13].
Concrete mixtures with RCA can be designed in the similar way as conventional concrete with natural aggregates, but the additional water absorption should be accounted while calculating the amount of total water content. The compressive strength of concrete using recycled aggregates is influenced by various parameters, such as the degree of replacement with recycled concrete aggregates (RCA), the water-to-cement ratio (w/c), the moisture condition of the RCA, and the physical and mechanical characteristics of the RCA. Mainly, the crushing strength and the impact resistance of the aggregates determines the resistance offered by the concrete against the compressive load. Experimental studies have revealed that the compressive strength of recycled aggregate concrete (RAC) is significantly impacted by the percentage of substitution of the RCA at the same water-cement ratio. According to Behera et al. (2014), there is typically a decrease in compressive strength of concrete when natural aggregates are partially replaced with other materials, with reductions ranging from 12–40%.
An extensive research work on production properties and applications of recycled aggregates was undertaken by Limbachiya (2010)[12]. He uses commercial recycling plant that consists of primary jaw, secondary cone crushers and screening for the production of RCA from C&D waste. It was observed that recycled aggregates were appearing to be coarser, rougher and porous compared to natural aggregates and also water absorption values of RCA was found to be 4 times higher than normal aggregates. The author examines the influence of RCA on concrete that meets the requirements for a variety of applications, such as foundations, pavers and reinforced or prestressed concrete in mild and moderate environments. The concrete mixes containing more than 50% of RCA were observed to exhibit unfavorable properties such as harshness, low cohesion, and bleeding. To overcome this problem, it is suggested to use coarse pulverized fuel ash as filler material. The results show that up to 30% of the coarse RCA has no effect on the strength of the concrete and on the initial surface absorption, but subsequently the strength is gradually reduced with increase in RCA content. The results show a slight difference in the relative characteristics of natural aggregates and RCA concrete mixes for flexural strength and modulus of elasticity.
Many developing nations exhibit a hesitancy to use recycled aggregates into the construction of structural components for tall buildings. But some of the researchers went one step ahead and have used recycled RCA in concrete i.e second time recycling of the recycled aggregate concrete and they called it as second generation recycled aggregate concrete (R-RCA). Concrete with aggregates derived from closed-loop recycled aggregates concrete was studied for its physical and mechanical qualities by Marie and Quiasrawi (2012). They made three mixes, one with 100% natural aggregates, second with replacement of conventional aggregates by RCA obtained by the first-generation recycling in terms of 0–20%.
As for the third mix, they replaced the natural aggregates with R-RCA aggregates acquired by recycling the second-generation aggregates in terms of 20%. RCA was attained by crunching earlier tested concrete samples in a laboratory for the production of first-generation concrete. The second-generation concrete, known as R-RCA, was produced through the process of grinding the first laboratory-tested first-generation concrete components. Both the initial and subsequent iterations of concrete demonstrate decreased workability when compared to conventional concrete. They discovered that both the first- and second-generation concretes had negative effects on the compressive strength, tensile strength and absorption compared to normal concrete. However, second generation concrete shows superior values than the first-generation concrete.
The incorporation of mineral admixtures like silica fume, GGBS, fly ash and metakaolin improves the microstructure of the recycled aggregates based concrete. The impact of combining mineral admixtures with recycled aggregates in Recycled Aggregate Concrete (RAC) was investigated by Limbachiya et al. (2012) and Kou et al. (2012). It was observed that the mineral additives perform as a micro filler and fill the ITZ between the surface of the aggregate and the matrix. Based on the findings, it is evident that enhancing the density of RAC can lead to enhanced strength and durability, achieved by facilitating the development of secondary C-S-H gel. This gel helps to fill the open pores and unfilled capillary pores in the cured concrete, thereby reducing its porosity and increasing its strength.
Amnon ketz (2004)[14] examined two methods of treatments for the enhancement of quality of recycled aggregate. One is by impregnating solution of silica fume and another method is by ultrasonic cleaning. He prepared silica fume solution by mixing 1Kg of non-condensed silica fume in 10 liter of water and the solution is mixed with 1% super plasticizer in a Hobart mixer. Then the RA was soaked for 24 hours in the silica fume solution. In the second method, the aggregates were submerged in the ultrasonic bath (US) with a huge quantity of water and treated for 10 minutes, then the water was exchanged with clean water and again aggregates were cleaned for 10 minutes. He found that treatment with silica fume brings about an increase in compressive strength of 23 to 33% and 15% after 7 and 28 days of curing respectively. Ultrasonic treatment resulted in a modest increase of about 7%, with no significant variance between early and late age. They also observed that impregnation with silica fume solution develops both the mechanical properties of RAC and ITZ between the RA and the new cement matrix.
The effects of pozzolanic admixtures applied to the surface of recycled aggregates on the RCA's qualities were studied by Kong et al. (2010). Using pozzolanic materials including Fly Ash, Silica Fume, and Blast Furnace Slag, they investigated two different crushing procedures and a two-stage mixing methodology. Recycled aggregate concrete (RAC) with varying pozzolanic material mixtures was tested for its fresh and hardened qualities. The researchers noted that by utilizing a new mixing technique and coating the recycled coarse aggregates with pozzolanic material, they were able to achieve significant flow properties and strength in the resulting concrete, as well as a dense and well-defined Interfacial Transition Zone (ITZ) structure.
Zhao et al. (2012)[15] evaluated the effect of pre coating the recycled aggregates by different combination of cement with pozzalanic materials. They used Portland cement and sulfoaluminate cement along with silica fume, GGBS and fly ash. They studied different combinations of above materials with different w/c ratio ranging from 0.5 to 1.2. The recycled aggregates were added to the suspension with uniform agitation and quenched for 5 to 10 minutes and then dried in natural wind. Higher physical performance was observed for RA coated with 0.035 mm resulted from the solution of w/c ratio of 0.8, which indicates that optimum thickness of coating results in better mechanical properties. Further it is found that, aggregates coated with sulfoaluminate cement with fly ash have got compressive strength of 70 Mpa.
Other researchers investigated several novel approaches to limit the disadvantages associated with the use of RA in RAC [16]. It has been found that the undesirable effects can be diminished to some extent by modified addition of various ingredients to the concrete. Using a cement slurry mounted on the surface of the RA to seal cracks and voids, Tam et al. (2007)(Tam, Tam, and Le 2007) developed a two-stage mixing procedure and observed the development that results in an increased ITZ in the premix phase. The outcomes demonstrated the viability of a two-stage blending strategy in enhancing the RAC's toughened properties.