Workability study of concretes made with recycled mortar aggregate

The use of recycled aggregates (RA) for concrete production usually involves a reduction of the workability as measured by slump. This reduction is mainly due to higher absorption rates, rough-textured surfaces and particles with quite irregular shape from RA. Besides that, the slump test is a static test, whose result is influenced by its own weight and by internal friction between particles. Because of that, recycled concrete workability is not well explained by the slump test. Thus, the main goal of this study is to assess the workability of concrete by varying: (1) workability tests (slump test, flow test and VeBe test); (2) the amount of fine and coarse RAs (0, 50 and 100 %); and, (3) the compensation index of RA absorption rates (60, 70, 80 and 90 %). Concrete compressive strength, at 28 days of age was also evaluated. It was observed that the flow test is the most suitable method for measuring the workability of recycled concrete. The results show that increasing the compensation index of RA absorption rate, there is an improvement of the workability of concrete and a reduction of compressive strength, which is related to the increase of the initial water content in mixtures.     

Durability performance of concrete made with fine recycled concrete aggregates

Fine recycled aggregates are seen as the last choice in recycling for concrete production. Many references quote their detrimental influence on the most important characteristics of concrete: compressive and tensile strength; modulus of elasticity; water absorption; shrinkage; carbonation and chloride penetration. These two last characteristics are fundamental in terms of the long-term durability of reinforced or prestressed concrete. In the experimental research carried out at IST, part of which has already been published, different concrete mixes (with increasing rates of substitution of fine natural aggregates – sand – with fine recycled aggregates from crushed concrete) were prepared and tested. The results were then compared with those for a reference concrete with exactly the same composition and grading curve, but with no recycled aggregates. This paper presents the main results of this research for water absorption by immersion and capillarity, chloride penetration (by means of the chloride migration coefficient), and carbonation resistance, drawing some conclusions on the feasibility of using this type of aggregate in structural concrete, while taking into account any ensuing obvious positive environmental impact.     

Use of recycled concrete aggregate in high-strength concrete

The results of a test programme to study the use of recycled concrete aggregate (RCA) in high-strength, 50 N/mm² or greater, concrete are described. The effects of coarse RCA content on the ceiling strength, bulk engineering and durability properties of such concretes have been established. The results showed that up to 30% coarse RCA had no effect on concrete strength, but therafter there was a gradual reduction as the RCA content increased. A method of accommodating the effects of high RCA content, involving simple adjustment to water/cement ratio of the mix is given. It is shown that high-strength RCA concrete will have equivalent engineering and durability performance to concrete made with natural aggregates, for corresponding 28-day design strengths. The practical implications of the study for concrete construction are discussed.

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Résumé Sont décrits ici les résultats d’une série d’essais destinés à étudier l’utilisation de granulats provenant du recyclage d’éléments en béton (RCA) dans des bétons de haute résistance (50 MPa et plus). Les effets de la teneur en gros granulats recyclés sur la résistance des plafonds et des batiments, ainsi que les propriétés de tels bétons ont été établis. Les résultats ont montré qu’une teneur allant jusqu’à 30% en gros granulats recyclés n’a pas d’effet sur la résistance du béton, mais qu’au dessus de 30%, la résistance diminue progressivement à mesure que la teneur en gros granulats recyclés augmente. Une méthode visant à accommoder les effets dus à une forte proportion de RCA, nécessitant un simple ajustement du rapport eau/ciment dans le mélange, est proposée. Il est prouvé que le béton RCA de haute résistance aura des qualités de résistance et de durabilité équivalentes à celles de bétons constitués de granulats naturels, pour les résistances mécaniques à 28 jours prévues. Les implications pratiques de l’étude sur la réalisation d’ouvrages en béton sont présentées.

     

Short and long-term behavior of structural concrete with recycled concrete aggregate

Recycling concrete construction waste is a promising way towards sustainable construction. Coarse recycled concrete aggregates have been widely studied in recent years, however only few data have been reported on the use of fine recycled aggregates. Moreover, a lack of reliable data on long-term properties of recycled aggregate concrete has to be pointed out.In this paper the effects of both fine and coarse recycled concrete aggregates on short and long-term mechanical and physical properties of new structural concrete are investigated. The studied concrete mixes have been designed by adjusting and selecting the content and grain size distribution of concrete waste with the goal to obtain medium–high compressive strength with high content of recycled aggregates (ranging from 27% to 63.5% of total amount of aggregates).Time-dependent properties, such as shrinkage and creep, combined with porosity measurements and mechanical investigations are reported as fundamental features to assess structural concrete behavior.     

Assessing relationships among properties of demolished concrete, recycled aggregate and recycled aggregate concrete using regression analysis

Recycled demolished concrete (DC) as recycled aggregate (RA) and recycled aggregate concrete (RAC) is generally suitable for most construction applications. Low-grade applications, including sub-base and roadwork, have been implemented in many countries; however, higher-grade activities are rarely considered. This paper examines relationships among DC characteristics, properties of their RA and strength of their RAC using regression analysis. Ten samples collected from demolition sites are examined. The results show strong correlation among the DC samples, properties of RA and RAC. It should be highlighted that inferior quality of DC will lower the quality of RA and thus their RAC. Prediction of RAC strength is also formulated from the DC characteristics and the RA properties. From that, the RAC performance from DC and RA can be estimated. In addition, RAC design requirements can also be developed at the initial stage of concrete demolition. Recommendations are also given to improve the future concreting practice.     

Effect of recycled coarse aggregate on damage of recycled concrete

This study evaluates the possibility of measuring the damage of the recycled concrete. In this way, two conventional concretes with a w/c ratio of 0.55 and 0.65 were designed. Based on them, six recycled concretes with different percentages of replacement of natural coarse aggregates with recycled coarse aggregate (20, 50 and 100%) were obtained. To take into account the high absorption capacity of the recycled aggregates, before using them they were pre-wetted for 10 min. The results concluded that scalar damage mechanics (based on the variations of the elastic modulus) and volumetric strains curves can be use to quantify the damage of the recycled concrete. The results from both approaches indicated that the damage to concrete depended on the percentage of replacement, increasing with higher replacement percentages. Additionally, values of the damage, that are quantified using the critical stress and according to the scalar damage mechanics, are given.     

Properties of self-compacting concrete prepared with recycled glass aggregate

The effects of recycled glass (RG) cullet on fresh and hardened properties of self-compacting concrete (SCC) were investigated. RG was used to replace river sand (in proportions of 10%, 20% and 30%), and 10 mm granite (5%, 10% and 15%) in making the SCC concrete mixes. Fly ash was used in the concrete mixes to suppress the potential alkali-silica reaction. The experimental results showed that the slump flow, blocking ratio, air content of the RG–SCC mixes increased with increasing recycled glass content. The compressive strength, tensile splitting strength and static modulus of elasticity of the RG–SCC mixes were decreased with an increase in recycled glass aggregate content. Moreover, the resistance to chloride ion penetration increased and the drying shrinkage of the RG–SCC mixes decreased when the recycled glass content increased. The results showed that it is feasible to produce SCC with recycled glass cullet.     

Restrained shrinkage cracking of recycled aggregate concrete

The increase in drying shrinkage and decrease in tensile properties of concrete proportioned with recycled concrete aggregate (RCA) can result in a high risk of cracking under restrained conditions. However, the reduction of the modulus of elasticity of such concrete, can lead to greater stress relaxation and reduction in cracking potential. An experimental program was undertaken to evaluate the effect of using RCA at high substitution rates of 50 and 100% (by vol.) on the cracking potential under restrained conditions. Four different types of coarse RCA, two binder types, and water-to-cementitious materials ratio (w/cm) of 0.37 and 0.40 were considered in the study. Mechanical properties, drying shrinkage, and cracking potential using the ring test were investigated. Test results indicated no cracking up to 35 days in the case of the reference mixture and the concrete prepared with 50% RCA replacement. The 28-day stress rate of such mixtures were limited to 0.12 MPa/day. Depending on the RCA type, the incorporation of 100% coarse RCA in a binary system made with 0.40 w/cm increased the 35-day cracking potential to up to 74%, with values of stress rate ranging from 0.25 to 0.34 MPa/day. The mixtures proportioned with 100% RCA developed tensile creep coefficient of 0.34–0.78 at the time of cracking compared to 0.34–0.36 for the reference concrete at the same age. However, greater elastic concrete strain and lower tensile strength resulted in reduced time to cracking at 100% RCA replacement, which was 9.0–11.0 days.     

Application of a weighted Grey-Taguchi method for optimizing recycled aggregate concrete mixtures

Assessment of the optimal mixture is an important issue to obtain desired quality. This paper integrates grey relational analysis and an objective weighting technique into the Taguchi method to propose the weighted Grey-Taguchi method. This method can be employed to assess the optimal mixture with multiple responses. In the application of this method, water/cement ratio, volume ratio of recycled coarse aggregate, replacement by river sand, content of crushed brick, and cleanliness of aggregate are selected as control factors with responses of slump, slump-flow, resistivity (7-day, 14-day, 28-day), ultrasonic pulse velocity (7-day, 14-day, 28-day), and compressive strength (7-day, 14-day, 28-day) to assess the optimal mixture of recycled aggregate concrete. Results demonstrate and verify that the optimal mixture has a water/cement ratio of 0.5, a volume fraction of recycled coarse aggregate of 42.0%, 100% replacement of river sand, 0% crushed brick, and water-washed aggregates.     

Creep and drying shrinkage characteristics of concrete produced with coarse recycled concrete aggregate

Laboratory tests are performed to investigate the effects of a new method of mixture proportioning on the creep and shrinkage characteristics of concrete made with recycled concrete aggregate (RCA). In this method, RCA is treated as a two component composite material consisting of residual mortar and natural aggregate; accordingly, when proportioning the concrete mixture, the relative amount and properties of each component are individually considered. The test variables include the mixture proportioning method, and the aggregate type. The results show that the amounts of creep and shrinkage in concretes made with coarse RCA, and proportioned by the new method, are comparable to, or even lower than, those in similar concretes made entirely with natural aggregates. Furthermore, it is demonstrated that by applying the proposed “residual mortar factor” to the existing ACI and CEB methods for calculating creep or shrinkage of conventional concrete, these methods could be also applied to predict the creep and shrinkage of RCA-concrete.     

Properties of recycled aggregate concrete made with recycled aggregates with different amounts of old adhered mortars

This paper presents the experimental results of a study on comparing the difference in properties of recycled aggregates (RAs) with varying amounts of old adhered mortar obtained from different sources and evaluating the influence of the different RAs on the mechanical and durability properties of recycled aggregate concrete (RAC). Four concrete mixes (one with natural aggregate and three others with recycled aggregates) with 28 day target compressive strength varying from 30 MPa to 80 MPa are designed by using each RA to fully replace NA. The properties of RAC are also modeled by using the artificial neural networks (ANN) method.The experimental results show that the performance of RAs from different sources varied greatly and RA of good quality can be used to produce high strength concrete with hardened properties comparable to those of the corresponding natural aggregate concrete (NAC). The comparison of the predicted results based on the ANN models and the experimental values indicated that the ANN method could be used to evaluate the properties of RAC made with RAs derived from different sources. This will facilitate the wider application of RA in concrete.     

Variation in mechanical properties of natural and recycled aggregate concrete as related to the strength of their binding mortar

Experimental work was performed to study the effect of binding mortar strength on the mechanical properties of recycled natural aggregate concrete mixes as well as reference corresponding natural aggregate concrete mixes. The moduli of elasticity of both NAC and RAC were found to be higher than that of corresponding mortar by about 40% and 10% respectively, for all compressive strengths investigated. It was possible to reach compressive strength for RAC of 53.5 MPa. The ratios of compressive strength of NAC or RAC to that of mortar varied between (1.05–1.56) and (1.02–1.26) respectively, these ratios decreased with the increase in compressive strength. Also from the results of compressive strength, it was found that the ratios cylinder/cube compressive strengths of RAC and mortar were smaller than those of NAC. The ranges of values obtained were (0.71–0.84) and (0.69–0.75) for RAC and mortar respectively, while for NAC this ratio ranged between (0.81–0.92), these values were obtained for compressive strengths ranging between 15 to 55 MPa. It was found that it is better to relate the cylinder/cube strength ratio to the modulus of elasticity of the concrete or mortar rather than to its compressive strength. The flexural strength showed an opposite trend, the ratios of NAC and RAC to that of mortar ranged between (0.72–0.95)% and (0.61–0.80)% respectively. These ratios increased with the decrease in compressive strength of mortars. On the other hand, the splitting tensile strength of NAC was higher than that of RAC and mortar for all strength levels investigated. The ratio of NAC to mortar splitting tensile strength ranged between (1.13–1.69), while this ratio for RAC ranged between (0.87–1.36). Finally, several regressions were developed that can relate the mechanical properties of the three materials investigated.     

高强再生骨料混凝土的配制及性能研究

采用再生粗骨料配制强度在50MPa或更大的高强再生骨料混凝土,并对其变形能力和耐久性进行测定,为高强再生骨料混凝土在工程上的应用提供理论和实验基础。通过一系列的抗压实验确定再生粗骨料的强度极限,并通过对水灰比的调整,使配制的高强再生骨料混凝土在强度上达到设计值,并以再生粗骨料取代率为0、30%、50%、80%和100%的高强再生骨料混凝土为研究对象进行实验。当再生粗骨料取代率为30%时,对再生混凝土的强度影响不大;之后混凝土强度随再生骨料的增加而降低。高强再生骨料混凝土与天然混凝土在耐久性上具有相似的性能,可以将高强再生混凝土应用于工程中。     

Influence of field recycled coarse aggregate on properties of concrete

This paper investigates the influence of different amounts of recycled coarse aggregates obtained from a demolished RCC culvert 15 years old on the properties of recycled aggregate concrete (RAC). A new term called “coarse aggregate replacement ratio (CRR)” is introduced and is defined as the ratio of weight of recycled coarse aggregate to the total weight of coarse aggregate in a concrete mix. To analyze the behaviour of concrete in both the fresh and hardened state, a coarse aggregate replacement ratio of 0, 0.25, 0.50 and 1.0 are adopted in the concrete mixes. The properties namely compressive and indirect tensile strengths, modulus of elasticity, water absorption, volume of voids, density of hardened concrete and depth of chloride penetration are studied. From the experimental results it is observed that the concrete cured in air after 7 days of wet curing shows better strength than concrete cured completely under water for 28 days for all coarse aggregate replacement ratios. The volume of voids and water absorption of recycled aggregate concrete are 2.61 and 1.82% higher than those of normal concrete due to the high absorption capacity of old mortar adhered to recycled aggregates. The relationships among compressive strength, tensile strengths and modulus of elasticity are developed and verified with the models reported in the literature for both normal and recycled aggregate concrete. In addition, the non-destructive testing parameters such as rebound number and UPV (Ultrasonic pulse velocity) are reported. The study demonstrates the potential use of field recycled coarse aggregates (RCA) in concrete.     

Behaviour of recycled aggregate concrete filled stainless steel stub columns

Past research indicates that recycled aggregate concrete (RAC) could be successfully used in concrete-filled steel tubular (CFST) columns. Their yielded performance is almost as good as that of the traditional CFST columns. In addition, as a comparatively new construction material, stainless steel can be used to replace carbon steel for enhancing the durability and ductility of CFST columns. With an aim to combine the advantages of both RAC and stainless steel, RAC is proposed in this paper to be used as a filling material for stainless steel tubes. A test program is introduced in this paper to investigate the behaviour of RAC-filled stainless steel stub columns. For comparison purposes, reference specimens with carbon steel tubes are also tested. In the end, finite element analysis is conducted to simulate the current test results and those reported in the literature.     

Failure processes of modeled recycled aggregate concrete under uniaxial compression

In order to investigate the failure processes of Recycled Aggregate Concrete (RAC), cracking behavior of modeled RAC specimens under compressive loading was investigated using Digital Image Correlation (DIC). Strain and displacement contour maps were produced to analyze the cracks’ initiation and propagation during loading. The testing results indicate that the discrepancy between the elastic moduli of coarse aggregates and mortar matrix significantly influences the mechanical properties and crack patterns of the modeled materials. It is found that the failure process is related to the relative strength of coarse aggregate and mortar matrix. For modeled RAC, the first bond cracks appear around both the old and new interfacial transition zones (ITZ), and then propagate into the old and new mortar matrix by connecting each other. The observation implies that the initiations and propagations of microcracks are different between RAC and Natural Aggregate Concrete (NAC). The findings in this investigation are useful to improve the mechanical properties of RAC by optimizing the mix proportion.