Adhered Mortar Research Articles

Mix Design Method for Self-Compacting Recycled Aggregate Concrete and Its Microstructural Investigation by Considering Adhered Mortar in Aggregate

AbstractThis article is on the development of a mix design method for self-compacting recycled aggregate concrete (SCRAC) by considering the adhered mortar content (AMC) on the surface of the aggre...

Numerical analysis of the failure of recycled aggregate concrete by considering the random composition of old attached mortar

Considering the randomness in properties of old adhered mortar (OM) around recycled aggregate (RA), a statistical analysis on its composition has been performed. The random effect of OM on the fracture behaviour of recycled aggregate concrete (RAC) at the mesoscopic scale has then been studied. This numerical approach explicitly takes into account an effective interphase (EI) between RA and cement mortar, which is composed of unknown OM and new forming mortar. Based on the identified component fraction, the elastic and fracture properties of EI have been calculated. Corresponding numerical results agree with the experimental ones. Numerical damage localization fields show that local crack patterns of RAC are different from those of normal concrete due not only to the fragility of the OM in EI but also to the brittleness of aggregate issued from demolition. The direct effect of OM properties on the macroscopic mechanical characteristics of RAC is explicitly demonstrated. The established numerical model provides the possibility to predict fracture behaviour of RAC by performing a certain number of experimental tests.

The use of recycled concrete aggregate for concrete production incorporating calcined clay as pozzolanic admixture

Concrete is the most widely used construction material thereby required a lot of raw resources to keep up with its demand. This leads to depletion of raw resources as well as a continuous build-up of demolished concrete wastes (DCW). In most developing nations like Nigeria, the usual way of disposing these DCW is landfilling. However, recycling has become a viable options for conserving the raw resources and reducing greenhouse gases. This study examines the alternative way of reusing concrete wastes for production of fresh structural concrete. The DCW was obtained from the demolition of an existing concrete building and were treated by soaking in water to help reduce the amount of adhered mortar. A mix ratio of 1:1:2 was adopted with a water-cement ratio of 0.25 to attain a relatively high strength concrete and superplasticizer was added to ensure workable mixes. The recycled aggregates (RCA) were used as partial and complete substitute for natural coarse aggregates in percentages of 0, 20, 40, 60, 80 and 100%, while calcined clay was added as pozzolanic admixture into the mixes at 15% and 20% partial replacement for cement. Tests carried out include the physical characterization of the constituent materials and the compressive strength of the hardened concretes were determined after 7, 14 and 28 days of curing in water using cubes of 100 mm × 100 mm × 100 mm. Results showed that the recycled concrete aggregates has high water absorption due to its porous interface. Moreso, results clearly indicate a continual decreasing trend in the compressive strength as the percentages of the RCA increases, however, a significant increase in compressive strength was observed when calcined clay was added as substitute for cement thereby improving the strength development of concrete.

Appraisal of Processing Techniques for Recycled Aggregates in Concrete

Due to depletion of natural aggregates the need for the usage of recycled aggregate in concrete has gained significance. In this regard, the present study is an attempt to evaluate the performance of M20 grade of concrete made with 100% recycled aggregate processed using various techniques. Handpicked aggregate from concrete rubble is used to prepare recycled aggregate concrete. The use of chemical admixture is mandatory to compensate the extra water (3 to 6%) required by the RCA (Recycled Aggregates). RCA treated with calcite mineral precipitating bacterial suspension enhances its surface permeability. RCA exposed to different acid concentrations improved the surface of the aggregate with the removal of the loosely adhered mortar. In thermal – mechanical method the recycled aggregates from rubble are heated in microwave to 300°C to remove adhered mortar from the aggregate and placed in a rotating drum containing iron balls. In chemical–mechanical method the recycled aggregate is exposed to Na2So4 and is subjected to freeze-thaw cycles to create mechanical to separate adhered mortar from RCA. In acid soaking beneficiation method the mortar around RCA is removed by immersing them in 5% HCl and H2SO4 for 24 hours. All the above mentioned recycled aggregate processing techniques are however to be tested in full scale to study the efficiency of these treatment techniques. Compressive strength and water absorption capacities of various concrete samples made with recycled aggregate prepared using above discussed processing techniques are evaluated.

Effect of silica fume on durability of self-compacting concrete made with waste recycled concrete aggregates

Due to adhered mortar in recycled concrete aggregates, some of the properties of concrete made with these types of aggregates, such as water absorption, porosity, electrical resistivity, and chloride ion penetration, may be affected. In this study, silica fume was used as a part of cementitious materials to improve the properties of self-compacting concrete (SCC), which were made with fine and coarse recycled aggregates. Three series of mixtures were made in the laboratory. In the first and second series, coarse recycled aggregates with replacement of 25%, 50%, 75% and 100% were used with and without silica fume. In the third series, 25% of fine recycled aggregates were replaced with fine natural aggregates. Slump flow and J-ring tests were considered for fresh state of SCC. The compressive strength, water absorption, ultrasonic pulse velocity, electrical resistivity, and chloride ion penetration tests were performed for hardened concrete. The use of silica fume improved the properties of fresh SCC. The results showed that silica fume can reduce water absorption and porosity. The silica fume showed that can resulted in a significant increase in electrical resistivity. On the other hand, the replacement of 25% of recycled aggregates did not have a significant effect on electrical resistivity, the electrical resistivity decreased by increasing the amount of replacement. Silica fume was very effective in controlling chloride ion penetration and reducing total charge passed. Silica fume also controlled the temperature of the solutions used during the test which indicates the control of the movement of ions in the penetration into concrete.

Influence of Surface Treatment of Recycled Aggregates on Mechanical Properties and Bond Strength of Self-Compacting Concrete

In the last decade there has been a massive growth for development of concrete infrastructures all around the world. Take into account environmental concerns, concrete technology should direct efforts toward assuring development and fabrication of sustainable and resilient concrete. For this purpose, incorporation of recycled concrete aggregate in concrete products particularly self-compacting concrete (SCC) for structural and non-structural application would be significant achievement. In this study the fresh and hardened properties of SCC prepared by substituting natural aggregates (NA) with recycled coarse aggregates (RCA). In addition, bonding behaviour of reinforced RCA-SCC for structural application was investigated. Moreover, surface treatment of RCA using lithium silicate solution was proposed to investigate its feasibility to improve the fresh and hardened properties of SCC as well as its bonding strength. The mechanical properties including compressive strength, tensile strength and elastic modulus of SCC mixes using untreated RCA and treated RCA (TRCA) were investigated. The results showed an improvement in performance of SCC mixes made with TRCA in compare with the untreated samples. The bond behaviour between SCC made with RCA and steel reinforcement was studied and the relationship between the brittleness and bonding of SCC mixes using untreated RCA and TRCA determined. The effect of surface treatment on the interfacial transition zone (ITZ) between adhered mortar and RCA studied using scanning electron microscope (SEM). It was determined that the treatment of RCA improved the bond at the ITZ through densification. The results gave experimental evidence of the suitability of RCA-SCC for structural use and application in reinforced concrete.

Properties of recycled concrete aggregates strengthened by different types of pozzolan slurry

Abstract Recycled concrete aggregate (RCA) retrieved from construction and demolition concrete wastes is widely used as an alternative to natural aggregate (NA) in the construction industry. However, the weak adhered mortar adversely affects the properties of RCA, which has a detrimental effect on the performance of resultant concrete. This paper aims to strengthen the weak adhered mortar and improve the properties of RCA by using an economic and environmentally friendly technique. Soaking RCA in different types of pozzolan slurries (i.e., fly ash, silica fume, and nano-silica fume) at different dosages and soaking times is the proposed technique to enhance the RCA quality. The physical and mechanical properties of RCA were comprehensively assessed before and after the surface treatment. Meanwhile, the surface morphology of RCA was characterized by using scanning electron microscope with energy dispersive spectroscopy (SEM-EDS) to examine the effect of this technique on the micro properties of RCA surface. Results indicate that the pozzolan slurry soaking methods can generally help to strengthen the weak adhered mortar and thus enhance the RCA properties. On trialing various pozzolan slurries and treatment duration, it is found that the 40% FA&C and 3% NSF slurries with 4 h soaking time can render an obvious effect, e.g. with more than 50–55% lowering of water absorption, 10–11% increasing particle densities, respectively. 40% FA&SF, 40% FA&C and 3% NSF slurries with 4 h soaking time exhibited a comparable improving performance, e.g. with 31, 35, 33% lowering of abrasion values, respectively. Obvious microstructure enhancement was also observed in the treated RCA where surface homogeneity and reduced Ca/Si ratio are found. The physical properties of RCA including water absorption and porosity are strongly correlated with the mechanical and durability characteristics in terms of resistance to crushing and abrasion values through conducting the proposed treatment technique.

Shear Behaviour of Reinforced Self-Compacting Concrete Beams Made with Treated Recycled Aggregate

In this paper, an experimental work was conducted to investigate the possibility of improving the structural performance of reinforced self-compacting concrete (SCC) beams cast with 100% treated and untreated recycled aggregate (RA). RA was first exposed to a simple treatment method to reduce the amount of its adhered mortar and to improve its performance by the impregnation in polyvinyl alcohol (PVA) polymer solution. After completing RA treatments, rectangular simply supported reinforced SCC beams cast with RA, treated recycled aggregate (TRA) as well as normal aggregate (NA), were prepared and tested under two-point loading up to failure. Half of the prepared beams were tested to evaluate the shear behaviour of normal strength (NS) SCC mixes, while the other half was tested for high strength (HS) SCC mixes. The results were evaluated with regards to load deflection response, ultimate failure load, first crack load, and cracking pattern. The main experimental results demonstrated that using treated RA considerably improved the shear capacity of reinforced SCC beams in comparison with that of untreated RA. Based on the ACI 318-14 and Euro codes, the shear strength values showed that the treated RA beams were considered more conservative compared to the RA beams in both strength grades.

Mechanical performance of shotcrete produced with recycled coarse aggregates from concrete

Abstract This research intends to analyse the mechanical performance of shotcrete with coarse recycled concrete aggregates (CRCA). Four replacement ratios of coarse natural aggregate with CRCA were studied: 0%, 20%, 50% and 100%. A dry mix-process was used in the production of shotcrete. A considerable reduction of the rebound effect with the incorporation of recycled aggregates was observed. In terms of hardened properties, a reduction was observed in compressive strength, splitting tensile strength, modulus of elasticity and ultrasound pulse velocity. The adhered mortar in CRCA is the reason for their greater deformability and lower strength, which were reflected in a decline of the properties of shotcrete. However, the abrasion resistance increased with the incorporation of recycled aggregates, which can be explained by its roughness and higher porosity, allowing better bond with the new cement paste. The adhesion strength in the various compositions was not affected by the incorporation of these aggregates.

Improving the mechanical and durability performance of recycled concrete aggregate-bearing mortar mixtures by using binary and ternary cementitious systems

Environmental concerns arising from the generation of huge amount of construction and demolition waste requires recycling this material, which would otherwise be sent to landfill. The mechanical and durability performance of mortar mixtures containing recycled concrete (RC) aggregate was investigated in this study. Although it is known that the adhered mortar creates a porous and a weak additional interfacial transition zone in recycled aggregate-bearing mixture, the dependence of the behavior on the type of new matrix should be searched in detail. For this purpose, compressive strength, ultrasound pulse velocity, water absorption, chloride ion penetration, freeze-thaw and sulfate resistance as well as drying shrinkage tests were conducted on mortar mixtures containing either natural aggregate (sand) or recycled concrete aggregate. Scanning electron micrographs and optical microscope images were obtained on specimens exposed to sulfate attack. The mineral admixtures used in the study included silica fume (SF), metakaolin (MK) and a Class C fly ash (FA). In addition to the control mixture including no mineral admixture (PC), silica fume- and metakaolin-incorporating binary systems (PC-SF and PC-MK) were prepared. Besides, two ternary systems, i.e., PC-SF-FA and PC-MK-FA were also designed. The results indicated a gradual strength gain beyond 28 days and reduction in initial shrinkage values in RC-bearing mixtures, compared to those of the sand-bearing mixtures. Besides, the porous character of RC helped reducing the internal pressure and resultant damage related with expanding water and swelling ettringite crystals in freeze-thaw and sulfate attack tests, respectively. The exact effect of recycled aggregate on mortar properties was found to be greatly dependent on the type of the cementitious system of the new mix.

Strength Properties of Recycled Aggregate Concrete Treated with Low Concentration Acetic Acid

Recycled concrete aggregates have inferior qualities compared with the natural aggregates, mainly attributed due to the porous nature of the attached cement mortar. In previous researches treatment of recycled aggregates are done using concentrated nitric acid, hydrochloric acid and sulphuric acid but they showed less view on acetic acid which is eco-friendly and also removes more adhered mortar in the recycled aggregates. The aim of this investigation is to determine the strength characteristics of Recycled Concrete Aggregates(RCA) which have been treated using low concentration acetic acid solution (1% solution and with immersion duration of 1 and 3 days). The treated aggregates are tested for compressive strength, split tensile strength, water absorption, impact strength and bond strength for a curing period of about 7 and 28 days.

Understanding variability in recycled aggregate concrete mechanical properties through numerical simulation and statistical evaluation

This paper investigates the effects of adhered mortar content on the mechanical properties of recycled aggregate concrete (RAC) systems using two-dimensional finite element analysis of RAC specimens subjected to uniaxial compression. Sensitivity and statistical analyses of RAC systems were conducted to explore how individual material stiffnesses (aggregate, mortar matrix, adhered mortar, new Interfacial Transition Zone (ITZ), and old ITZ) and adhered mortar contents (2, 10, 20 and 50%) influence RAC mechanical performance. In total, 128 simulation results were performed to understand variability in stress development, damage progression, compressive strength, and elastic modulus of RAC systems. Statistical inferences on the effects of variability in material stiffness and adhered mortar content were made based on frequency distributions, probability density functions, Pareto charts, main effects plots, and bivariate contour plots. Numerical results showed that compressive strength and elastic modulus decreased with increasing adhered mortar contents, while the strain corresponding to compressive softening increased with adhered mortar contents. Statistical results showed that compressive strength was most significantly influenced by aggregate stiffness and mortar matrix stiffness. Strain localizations were observed near the aggregate boundaries due to large material stiffness discontinuities in the RAC meso-level structure. RAC elastic modulus and ultimate compressive strain were mainly governed by the stiffness of the mortar matrix. Based on the numerical results, bivariate contour plots were developed to understand how variations in material stiffness and adhered mortar content influence the strength and stiffness of RAC systems.

Durability of recycled aggregate concrete – A review

Recycling of waste concrete has become an important issue worldwide due to the continued increase of construction wastes. Also, the growing global construction activities urge to find sustainable resources to replace natural materials for the production of concrete. In the past few decades, many researches have been carried out on the use of recycled aggregate (RA) derived from construction and demolition wastes to produce concrete products. This paper reviews the previous findings on the effects of use of RA on durability of concrete. In general, the amount of adhered mortar and the quality of the original concrete have a significant effect on the properties of resulting concrete. The increase of RA content and w/c ratio results in poorer durability of concrete. In comparison, the negative effect of recycled fine aggregate is more obvious than that of recycled coarse aggregate. The use of pozzolanic materials either for surface coating of RA or intermixed within the concrete are effective and feasible to improve the overall durability of concrete. Recent researches on CO2 treatment indicate that it can enhance the properties of RA and durability of concrete significantly.

Multiple recycled aggregate properties analysed by X-ray microtomography

This paper presents a novel technique used to analyse the volume of adhered mortar to the recycled aggregate. A computerized microtomograph (μCT) device was used to evaluate the volume of the aggregate, the volume of natural aggregate and the volume of adhered mortar. To this end, a natural aggregate has been characterized, using the μCT, with which a source concrete has been produced. Subsequently, the source concrete has been crushed to obtain a first cycle recycled aggregate. After the characterization of the first-generation of recycled aggregate, a new source concrete has been made with it to be subsequently crushed again obtaining a second-generation recycled aggregates. In the same way a third-generation recycled aggregate has been obtained and has been equally characterized. The results show that the compaction capacity of the aggregate is reduced after successive recycling. It has been possible to quantify how much the closed porosity of the recycled aggregate decreases with the number of times it is recycled. The loss of natural aggregate and increase of the volume of adhered mortar have also been evaluated using this technique.

Characterization of pervious concrete with blended natural aggregate and recycled concrete aggregates

Abstract The utilization of recycled coarse aggregates (RCA) from construction and demolition wastes to produce green concrete serves as a sustainable solution with manifold environmental benefits. This study aims to widen the potential uses of RCA to fabricate pervious concrete for non-structural applications. The mechanical and surface properties, permeability, and greenhouse gases assessment of pervious concrete made with blended normal granite aggregates and RCA were investigated in this work. RCA replacement levels of 20%, 40%, 60%, 80% and 100% were used. Experimental results show that the RCA mixes have lower mechanical properties. Microscopic analyses show that the lower strength of RCA mixes was attributed to the failure path of RCA specimens which occurred at the weaker adhered mortar on RCA. Despite the drop in strength, all RCA-mixes attained the minimum BS EN 1338 requirements in terms of skid and abrasion resistance. In the mix with 100% RCA, the waste content in the pervious concrete mix was 87% by weight and 73% by volume. The greenhouse gases assessment also shows that the CO2 emission of this 100% RCA mix is 24% lower than the control mix.

Thixotropy and interlayer bond strength of self-compacting recycled concrete

This work is focused on assessing the thixotropy of self-compacting recycled concrete (SCRC) and on evaluating the interlayer bond strength. To do so, four SCRC mixes with 0%, 20%, 50%, and 100% of recycled coarse aggregate (by volume) were studied. This aggregate was used in dry-state conditions and an extra quantity of water was added during mixing in order to compensate its absorption.Three testing methods were used to assess the degree of thixotropy of SCRC: structural breakdown curves at various rotational speeds, hysteresis loop flow curves and yield stress at rest. To evaluate the effect of the structural build-up at rest on SCRC interlayer bond strength, two methods were used: flexural tests and water permeability tests.The results indicate that the increase of thixotropy and interlayer bond strength with the replacement percentage is due to the difference in the effective w/c ratio, result of the non–compensated water absorption, to the higher amount of fines in the recycled aggregates and generated from the old adhered mortar and also to the higher internal friction of recycled aggregates. Moreover, as water absorption is compensated in the mixing protocol, changes over time in the effective w/c ratio are negligible. Therefore, the thixotropic changing rate is similar in all studied mixes.

Effect of Mortar Reduction in Recycled Aggregates Used in Concrete

Background:Following a global environmental concern, concrete manufactured with recycled materials has been widely studied. The reuse of concrete as raw material can reduce the amount of debris and the amount of natural resources needed minimizing the environmental impact.Objective:One of the fundamental issues when using recycled aggregates is the mortar that remains adhered to their surfaces. The effect of this adhered mortar on aggregates, obtained from pavement demolition debris, used in recycled concrete is studied.Method:A mechanical abrasion process is used to reduce the amount of mortar in different degrees from the recycled aggregates that will be used to replace a 40 % of natural coarse aggregates in structural concrete. The mechanical behavior is studied through the compressive and flexural strength of the material and compared with the values obtained for a control concrete, with no recycled aggregates.Results:The abrasion process is proven to be effective eliminating the adhered mortar to the aggregates and the results show that a medium abrasion level, around 200rev, improves significantly the mechanical properties of the recycled concrete, increasing its compressive strength.Conclusion:The percentage of recycled aggregates used in structural concrete can be increased if these are treated with simple mechanical abrasion. The residuals of the eliminated mortar can be also reused as cement replacement, maximizing the material reuse.

Enhancing mechanical and durability properties of recycled aggregate concrete

The main difference between Recycled Concrete Aggregate (RCA) and Natural Aggregate (NA) is the mortar adhered on RCA. This paper presents the effect of RCA to concrete and a treatment method utilized to improve the properties of RCA, by reducing the amount of the adhered mortar, and therefore the properties of the Recycled Aggregate Concrete (RAC). Mineral admixtures were used as partial replacement of cement. Three types of coarse RCA and two types of mineral admixtures were used (fly ash and silica fume). In addition, the RCA were employed as internal curing (IC) agents in concrete mixtures to assess their effectiveness in enhancing the properties of concrete. The mechanical properties and durability of RAC were improved using the proposed methodologies. Cost analysis showed that RAC mixtures could be less expensive than NA mixtures.

Recycled aggregate concrete incorporating fly ash: Comparative study on particle packing and conventional method

In the present study, particle packing method (PPM) of mix proportioning is applied in a novel way for recycled aggregate concrete (RAC) incorporating fly ash as a partial replacement of cement. Lesser fresh mortar content was achieved by PPM which resolves the major concern of excess adhered mortar in RAC. Theoretical packing density (PD) obtained using compaction-interaction packing model was compared with experimental PD. 18% and 28% reduction in cement content was observed for RAC with 20% and 30% fly ash replacement respectively due to PPM mix proportions as compared to conventional method at water-binder ratio of 0.45. Compressive strength, flexural strength and modulus of elasticity of RAC with fly ash showed comparable results with natural aggregate concrete in PPM. The rate of long term compressive strength gain was about 11%–20% and 30% in RAC and natural aggregate concrete (NAC) respectively. Split tensile strength and modulus of elasticity in RAC was comparable in both the conventional as well as PPM methods. Flexural strength was improved in RAC incorporating fly ash mix due to PPM. Statistical analysis showed better tensile strength for RAC as compared to NAC up to the characteristic compressive strength of 25MPa. Poor tensile behaviour was reported for high strength recycled aggregate concrete. The results suggest that, the particle packing method can be used in RAC with fully replaced recycled aggregates incorporating fly ash (up to 30% replacement) for sustainable construction practices.

Improving of Recycled Aggregate Quality by Thermal-mechanical-chemical Process

To save natural resources and promote the sustainable development of construction industry, the use recycled coarse aggregate (RCA) from waste concrete has been encouraged recently. However, it is widely accepted that RCA concrete has lower performance about 15% to 20% than that of natural aggregate concrete. The adhered mortar on RCA surfaces causes lower quality of RCA produced through conventional recycling process. Therefore, it is necessary to provide higher quality of RCA from waste concrete by detaching old mortar as much as possible; so that the quality of RCA concrete can be improved as well. This research proposes a thermal-mechanical-chemical process to produce better properties of RCA from waste concrete. Then, new concrete was produced by utilizing the RCA. Results indicate that the physical properties of RCA are close to natural aggregate and meet the requirement of Indonesian Standard for concrete aggregate. Furthermore, the mechanical performance of RCA concrete produced by the proposed method has better mechanical properties to that of conventional RCA. It is about 3-8% lower than the mechanical properties of natural coarse aggregate concrete.