Mechanical and elastic behaviour of concretes made of recycled-concrete coarse aggregates

In this paper an investigation of mechanical behaviour and elastic properties of recycled-aggregate concretes is presented. These concretes were prepared by alternatively using two different (coarse and finer coarse) recycled-aggregate fractions both made of recycled concrete coming from a recycling plant in which rubble from demolition is collected and suitably treated. Several concrete mixtures were prepared by using only virgin aggregates (as reference), 30% finer coarse recycled aggregate replacing fine gravel and 30% coarse recycled aggregate replacing gravel. Five different water to cement ratios were adopted as: 0.40, 0.45, 0.50, 0.55 and 0.60. Concrete workability was in the slump range of 190–200 mm. Compression tests were carried out after 28 days of wet curing. In addition, concrete elastic modulus and drying shrinkage were evaluated. Results obtained showed that structural concrete up to C32/40 strength class can be manufactured by replacing 30% virgin aggregate with recycled-concrete aggregate. Moreover, a correlation between elastic modulus and compressive strength of recycled-aggregate concrete was found and compared to those reported in the literature. Finally, on the basis of drying shrinkage results, particularly if finer coarse recycled-concrete aggregate is added to the mixture, lower strains could be detected especially for earlier curing time.     

Post-fire residual mechanical properties of concrete made with recycled rubber aggregate

This study investigates the effects of elevated temperatures on the residual mechanical performance of concrete produced with recycled rubber aggregate (RRA). Four different concrete compositions were prepared: a reference concrete (RC) made with natural coarse aggregate and three concrete mixes with replacement rates of 5%, 10% and 15% of natural fine and coarse aggregate by RRA from used tyres. Specimens were exposed for a period of 1 h to temperatures of 400 °C, 600 °C and 800 °C, after being heated in accordance with ISO 834 time–temperature curve. After cooling down to ambient temperature, the compressive strength and the splitting tensile strength were evaluated and compared with reference values obtained prior to fire exposure. For the replacement rates used in the present experiments, the obtained results show that concrete made with recycled rubber aggregate (CRRA) present a thermal response that is roughly similar to that of RC; in addition, although residual mechanical properties of CRRA are noticeably more affected than those of RC, particularly for higher exposure temperatures, the relative reduction should not prevent it from being used in structural applications.     

Experimental behaviour of RACFST stub columns after exposed to high temperatures

The experimental studies on the behaviour of recycled aggregate concrete-filled steel tube (RACFST) stub columns after exposed to high temperatures are reported in this paper. Forty specimens, including 32 RACFST stub columns and 8 normal concrete-filled steel tube (CFST) stub columns as reference, were tested, and the failure pattern, load versus strain relation and ultimate strength of the specimens were presented and analysed. Five types of concrete were produced: one reference concrete with natural aggregates, two concrete mixes with recycled coarse aggregate (RCA) replacement ratios of 50% and 100%, and two concrete mixes with recycled fine aggregate (RFA) replacement ratios of 50% and 100%. The specimens were exposed to 300 °C, 600 °C and 800 °C for 3 h. The test results showed that, due to the existence of the recycled aggregates, the post-fire performance of RACFST stub columns was lower than the corresponding normal CFST specimens under the same maximum temperature suffered, and the RACFST specimens with RCA had a better behaviour than those with RFA under the same recycled aggregate replacement ratio.     

Creep and shrinkage of recycled aggregate concrete

This paper presents the results of experimental research into concrete produced by replacing the natural aggregates with recycled aggregates coming from construction waste and concrete work demolitions. The main aim of this work was to determine creep and shrinkage variations experienced in recycled concrete, made by replacing the main fraction of the natural aggregate with a recycled aggregate coming from waste concrete and comparing it to a control concrete. The substitution percentages were 20%, 50% and 100%. Fine natural aggregate was used in all cases and the amount of cement and water–cement ratio remained constant in the mixture. It was possible to state that the evolution of deformation by shrinkage and creep was similar to a conventional concrete, although the results after a period of 180 days showed the influence of the substitution percentage in the recycled aggregates present in the mixture. In the case when 100% coarse natural aggregate was replaced by recycled aggregate there was an increase in the deformations by creep of 51% and by shrinkage of 70% as compared to those experienced by the control concrete.     

Identification of frost-susceptible recycled concrete aggregates for durability of concrete

Demolished concretes to be recycled show great diversity in property with regard to residual potential of resistance to the environmental effects such as frost action, when they are used in a new concrete as aggregate. There seems to exist no appropriate test method fitting specific characteristics of the recycled concrete aggregates with respect to the fair judgement of their unbound state frost durability. This fact rises another important prejudice against performance of recycled concrete aggregates compared to virgin alternatives. This study deals with reliable identification of frost susceptibility of recycled concrete aggregates. An unbound frost durability test with a mild pre-drying procedure (at 50 °C) was performed on recycled concrete aggregates originated from both air-entrained and non-air-entrained type of source concretes. The procedure distinctly identified the non-durable recycled coarse aggregates in parallel to freezing and thawing durability results of the concretes incorporating identical aggregates. Sulfate soundness test gave misleading results in judgement of the unbound state frost durability due to the highly disruptive physicochemical effect of the method on the recycled concrete aggregates, regardless of the air void characteristics of the materials.     

Behaviour of recycled aggregate concrete under drop weight impact load

This paper presents the experimental results of recycled aggregate concrete (RAC) beams prepared with different amount of recycled coarse aggregate (RCA) subjected to low velocity impact. The recycled coarse aggregates are obtained from a demolished RCC culvert. Four concrete mixes with 0%, 25%, 50% and 100% RCA respectively are prepared. With each mix three beam specimens of size 1.15 × 0.1 × 0.15 m are prepared and tested under drop weight impact load. The behavior of the RAC beams are studied in terms of acceleration, strains and support reaction histories under impact load in addition to the physical and mechanical characteristics of RCA and RAC. It is observed that 25% RCA does not influence the strength of concrete. In addition, it is found that for a given impact energy (the energy imparted by the hammer per blow) the reactions and strains of RAC with 50% and 100% RCA are significantly lower and higher respectively than those of normal concrete and RAC with 25% RCA.     

The role of industrial by-products in self-compacting concrete

The development of self-compacting concrete is considered as a milestone achievement in concrete technology due to several advantages. In order to be self-compactable the fresh concrete must show high fluidity besides good cohesiveness. For the purpose of evaluating these properties, several concrete mixtures were prepared with a water to cement ratio of 0.45 in the presence of an acrylic-based superplasticizer at a dosage ranging from 1% to 2% by weight of very fine material fraction (maximum 150 μm). Either limestone powder or fly ash or recycled aggregate powder (that is a powder obtained from the rubble recycling process) were used as mineral addition, in order to assure adequate rheological properties, in terms of cohesiveness, in the self-compacting concretes. Preliminary rheological tests were carried out on cement pastes containing these mineral additions. In some cases, recycled instead of natural aggregate was used by substituting either the coarse or the fine aggregate fraction. The fresh concrete properties were evaluated through slump flow, L-box test and segregation resistance. Compressive strength of concrete was determined at 1, 3, 7 and 28 days of wet curing. Results obtained showed that an optimization of self-compacting concrete mixture seems to be achievable by the simultaneous use of rubble powder and coarse recycled aggregate with improved fresh concrete performance and unchanged concrete mechanical strength.     

Optimisation of using lightweight aggregates in mitigating autogenous deformation of concrete

Experiments were conducted to determine the effects of using dispersed saturated lightweight aggregates (LWAs) as water reservoirs in mitigating the autogenous deformation of high performance concrete and to establish the optimum solutions as a combination of a number of factors affecting the fracture and mechanical characteristics of concrete. For this purpose, in concretes prepared with a constant low water to cement ratio, normal aggregates were replaced by natural LWAs with size fractions of 2–4 mm or 4–8 mm at three different volume fractions such as 10%, 20% and 30% of the total aggregate volume of concrete. The results indicate that the inclusion of fine fraction of LWAs in concrete reduces the autogenous deformation significantly compared to that of the coarse fraction. It is also shown that concretes with fine fraction of LWAs have enhanced fracture and mechanical properties compared to those with coarse fraction of LWAs. Increasing the replacement ratio of LWAs mitigates autogenous deformation, while having an unfavourable effect on fracture and mechanical properties of concrete for both size replacements. A multi-objective simultaneous optimisation technique, in which the response surface method (RSM) is incorporated, is used to optimise the mitigation ratio of autogenous deformation and fracture parameters of high strength concretes in an effort to obtain a more ductile concrete with less autogenous deformation.     

Valorization of coarse rigid polyurethane foam waste in lightweight aggregate concrete

This study examines the mechanical properties and the durability parameters of lightweight aggregate concretes (LWAC) incorporating rigid polyurethane (PUR) foam waste as coarse aggregates (8/20 mm). The influence of both the increasing incorporation of PUR foam waste and the presence of superplasticizer on the workability, bulk density, mass loss, drying shrinkage, compressive strength, dynamic modulus of elasticity, total porosity, gas permeability and chloride diffusion coefficient of the different concretes, has been investigated and analyzed. The results showed that the use of PUR foam waste enabled to reduce by 29–36% the dry density of concrete compared to that of the normal weight concrete (made without foam waste). The reduction of density was due to the increase of total porosity in the lightweight concretes, which also induced higher gas permeability and chloride diffusion coefficient. These negative effects on durability of concrete were lowered by improving the characteristics of the cementitious matrix. The mechanical properties of the LWAC ranged between 8 and 16 MPa for the compressive strength and between 10 and 15 GPa for the dynamic modulus of elasticity; the concrete mixture with the higher performances almost satisfied the mechanical and density criteria of structural lightweight concrete. These results consolidate the idea of the use of PUR foam waste for the manufacture of lightweight aggregate concretes.     

Determining the specific gravities of coarse aggregates utilizing vacuum saturation approach

The current method of specific gravity and absorption of coarse aggregate testing is based on the AASHTO T 85 and ASTM C-127 standards. This approach involves the soaking of the coarse aggregate samples for 15h (AASHTO T 85) and 24 ± 4 h (ASTM C-127), and drying the aggregate to its saturated-surface dry (SSD) state with the aid of a dry absorbent cloth. The attainment of the SSD condition of the coarse aggregate is very subjective, and the total test duration makes it inconvenient for use in construction quality control and quality assurance testing (QC/QA).The objective of this paper is to determine the specific gravity and absorption of coarse aggregates using a new proposed approach utilizing vacuum saturation. In lieu of the conventional soaking period of 24 ± 4 h, this proposed research approach employs the use of 10, 20 and 30 min of vacuum saturation at 30 mm Hg (4.0 kPa) pressure. In this paper, the soaking time is 24 ± 4 h for all the AASHTO method. It is also believed that the 24 ± 4 h shall give better soaking and therefore more accurate test results would be achieved. Vacuum saturating the coarse aggregates aims at removing all the entrapped air within the sample mass, in addition to forcing water into the effective pores of the coarse aggregates. This method is applied to a wide range of coarse aggregates including trap rock, limestone, gravel, steel slag, crushed concrete, and the results are compared statistically with those of AASHTO T 85. Results from the experiments indicate that the vacuum saturation method can replace the AASHTO T 85 for coarse aggregate specific gravity testing at 10, 20 or 30 min of vacuum saturation. A significant finding was that the AASHTO T 85 underestimates the full absorption potential of highly absorptive aggregates when compared to this proposed vacuum saturation approach.     

Influence of mineral additions on the performance of 100% recycled aggregate concrete

A judicious use of resources, by using by-products and waste materials, and a lower environmental impact, by reducing carbon dioxide emission and virgin aggregate extraction, allow to approach sustainable building development. Recycled aggregate concrete (RAC) containing supplementary cementitious materials (SCM), if satisfactory concrete properties are achieved, can be an example of such sustainable construction materials.In this work concrete specimens were manufactured by completely replacing fine and coarse aggregates with recycled aggregates from a rubble recycling plant. Also RAC with fly ash (RA + FA) or silica fume (RA + SF) were studied.Concrete properties were evaluated by means of compressive strength and modulus of elasticity in the first experimental part. In the second experimental part, compressive and tensile splitting strength, dynamic modulus of elasticity, drying shrinkage, reinforcing bond strength, carbonation, chloride penetration were studied. Satisfactory concrete properties can be developed with recycled fine and coarse aggregates with proper selection and proportioning of the concrete materials.     

Concrete building blocks made with recycled demolition aggregate

A study undertaken at the University of Liverpool has investigated the potential for using recycled demolition aggregate in the manufacture of precast concrete building blocks. Recycled aggregates derived from construction and demolition waste (C&DW) can be used to replace quarried limestone aggregate, usually used in coarse (6 mm) and fine (4 mm-to-dust) gradings. The manufacturing process used in factories, for large-scale production, involves a “vibro-compaction” casting procedure, using a relatively dry concrete mix with low cement content (≈100 kg/m³). Trials in the laboratory successfully replicated the manufacturing process using a specially modified electric hammer drill to compact the concrete mix into oversize steel moulds to produce blocks of the same physical and mechanical properties as the commercial blocks. This enabled investigations of the effect of partially replacing newly quarried with recycled demolition aggregate on the compressive strength of building blocks to be carried out in the laboratory. Levels of replacement of newly quarried with recycled demolition aggregate have been determined that will not have significant detrimental effect on the mechanical properties. Factory trials showed that there were no practical problems with the use of recycled demolition aggregate in the manufacture of building blocks. The factory strengths obtained confirmed that the replacement levels selected, based on the laboratory work, did not cause any significant strength reduction, i.e. there was no requirement to increase the cement content to maintain the required strength, and therefore there would be no additional cost to the manufacturers if they were to use recycled demolition aggregate for their routine concrete building block production.     

Use of recycled demolition aggregate in precast products,phase II: Concrete paving blocks

A study undertaken at the University of Liverpool has investigated the potential for using construction and demolition waste (C&DW) as aggregate in the manufacture of a range of precast concrete products, i.e. building and paving blocks and pavement flags. Phase II, which is reported here, investigated concrete paving blocks. Recycled demolition aggregate can be used to replace newly quarried limestone aggregate, usually used in coarse (6 mm) and fine (4 mm-to-dust) gradings. The first objective, as was the case with concrete building blocks, was to replicate the process used by industry in fabricating concrete paving blocks in the laboratory. The compaction technique used involved vibration and pressure at the same time, i.e. a vibro-compaction technique. An electric hammer used previously for building blocks was not sufficient for adequate compaction of paving blocks. Adequate compaction could only be achieved by using the electric hammer while the specimens were on a vibrating table. The experimental work involved two main series of tests, i.e. paving blocks made with concrete- and masonry-derived aggregate. Variables that were investigated were level of replacement of (a) coarse aggregate only, (b) fine aggregate only, and (c) both coarse and fine aggregate. Investigation of mechanical properties, i.e. compressive and tensile splitting strength, of paving blocks made with recycled demolition aggregate determined levels of replacement which produced similar mechanical properties to paving blocks made with newly quarried aggregates. This had to be achieved without an increase in the cement content. The results from this research programme indicate that recycled demolition aggregate can be used for this new higher value market and therefore may encourage demolition contractors to develop crushing and screening facilities for this.     

Interfacial interactions in concretes with silica fume and SBR latex

This paper deals with the effect of silica fume and styrene-butadiene latex (SBR) on the microstructure of the interfacial transition zone (ITZ) between Portland cement paste and aggregates (basalt). Scanning electron microscope (SEM) equipped with energy dispersive X-ray analysis system (EDX) was used to determine the ITZ thickness. In the plain concrete a marked ITZ around the aggregate particles (55 μm) was observed, while in concretes with silica fume or latex SBR the ITZ was less pronounced (35–40 μm). However, better results were observed in concretes with silica fume and latex SBR (20–25 μm).     

Evaluation of strain gage lengths for testing limestone and granite aggregate concretes

An experimental program was conducted to determine an appropriate length of strain gage for use in testing limestone and granite aggregate concretes. Variables in the test program included type and size of aggregate, and length of strain gage. Test specimens consisted of concrete cylinders fitted with a mechanical dial gage, and with multiple electrical resistance strain gages ranging from 120 mm (4.7 in.) to 5 mm (0.2 in.) in length. Cylinders were subjected to compression loading. Strains reported by the strain gages were compared to the strains calculated using dial gage readings. Recommendations are given for appropriate length of strain gages to be used with limestone aggregate and granite aggregate concretes.     

Effects of crushed glass cullet sizes,casting methods and pozzolanic materials on ASR of concrete blocks

The aim of this study is to investigate the influence of using different particle sizes of recycled glass, casting methods and pozzolanic materials in reducing the expansion due to alkali-silica reaction (ASR) of concrete blocks prepared with the use of crushed glass as fine aggregate. In this work, 25 × 25 × 285 mm mortar bar specimens were prepared using conventional wet-mixed and dry-mixed methods. Except for the control mortar bar, all the specimens were prepared by completely replacing river sand with different particle sizes of recycled glass. In addition, the influence of fly ash (PFA) and metakaolin (MK) content on the reduction of ASR expansion was also investigated. The flexural strength of the mortar bar specimens before and after they had been exposed to 1N NaOH solution was determined to complement the results of ASR expansion test. SEM was performed to examine the microstructure as well as nature of the cement binder-glass interfacial zone. The results reveal that ASR expansion reduced with reducing particle size of glass used. For the same given mix proportion, the dry-mixed method resulted in 44% less expansion when compared with the wet-mixed method. Both PFA and MK were demonstrated to be able to significantly reduce ASR expansion of the concrete glass blocks.     

Modeling of some properties of the crushed tile concretes exposed to elevated temperatures

In this study, artificial neural network (ANN) and fuzzy logic (FL) models have been developed for predicting the compressive strength (f_c) and dynamic modulus of elasticity (E_d) of the crushed tile concretes (CTC) exposed to elevated temperatures. Some relationships are established between chosen inputs and outputs by developing and testing a multi-layered feed forward ANN and FL trained with the back-propagation algorithm. First of these relationships is established between the outputs as f_c of CTC after being exposed to elevated temperatures and the inputs as exposed temperature (T), crushed tile aggregate (CT) and crushed stone II (CSII) contents of concrete. The second one is the relationship between E_d of concretes and the same inputs. In this aim, concrete specimens are produced by CT replacing 16–31.5 mm coarse aggregate at the ratios of 0%, 10%, 25%, 50%, 75% and 100%. Concrete specimens are exposed to 20, 150, 300, 400, 600, 900 and 1200 °C high temperatures corresponding TS EN 1363-1 after an initial 28 day curing period. After heating, the specimens are slowly air-cooled to the room temperature and then E_d and f_c of concretes were determined. Experimental results are also predicted by constructing models in ANN and FL methods. In the models, the training and testing results have shown that ANN and FL methods have strong potential for predicting the f_c and E_d of crushed tile concretes exposed to elevated temperatures.     

Reuse of incineration fly ashes and reaction ashes for manufacturing lightweight aggregate

This paper reports the result of the investigation on manufacturing lightweight aggregate by incorporating municipal solid waste (MSW) incineration fly ashes and reaction ashes with reservoir sediments. The investigation was first performed in a laboratory scale to assess the effects of the composition and the firing conditions on the properties of the resulting aggregate. Afterward, a big amount of aggregates was manufactured in a pilot scale using a commercially available rotary kiln. Physical properties of the synthetic aggregates were subsequently assessed. In addition, compressive strength of the concrete made from the manufactured aggregates was experimentally measured. The investigation shows that the analysis results for the MSW incineration fly ashes and reaction ashes are not in the limits of the expandable region of Riley’s ternary diagram due to the low content of SiO₂. Therefore, they can only be used as additives. The proper content for MSW incineration ashes should not exceed 30%, except compositional adjustment using oxide constituents. The particle density of the manufactured aggregates using a commercially available rotary kiln was 0.99 g/cm³, which is significantly lower than normal density aggregate. Moreover, its dry loose bulk density is 593 kg/m³, which meets the requirements of ASTM C 330 with bulk density less than 880 kg/m³ for coarse aggregate. On the other hand, the results of toxicity test meet the Taiwan Environmental Regulatory requirements, which demonstrate that the aggregate thus fabricated is non-hazardous for construction use.     

CDW recycled aggregate renderings: Part II – Analysis of the effect of materials finer than 75 μm under accelerated aging performance

Quality control of the performance of renderings made up of construction and demolition waste (CDW) recycled aggregates needs to be improved as CDW recycling can prove to be an alternative to waste disposal in developing countries. This experimental work focuses the effectiveness of a mix design method to control and analyze the recycled aggregate composition influence on the performance of mortars and renderings. Leveling time in the placement of renderings was also studied. The mix design method of mortars takes into account two parameters: the “aggregates and plasticizing materials to cement ratio” and “the total materials finer than 75 μm” in the dry mortar. In Part I of this study [Construction and Building Materials, submitted to publishing] the basic properties of mortars of several mixes were analyzed for a constant cement content around 155 ± 10 kg/m³. The performance of renderings is the scope of Part II of this paper. The initial bond strength and visible drying cracks under laboratory conditions were first analyzed for renderings applied on masonry panels with two times of leveling during placement. After five months, accelerated aging of renderings was carried out. Ten wetting–drying cycles upon thermal shock, from 80 °C to laboratory room temperature, were applied to the masonry panels, and cracks were assessed for each cycle. Bond tensile strength was not affected by the thermal shock, but additional cracks were seen on the renderings. The mix design parameters of the mortars and their hardened state properties were related to the cracking of the renderings. The results show that the parameters “total materials finer than 75 μm” and “aggregate to cement ratio” can be used for the mix design of mortars with recycled CDW aggregates. The increase in tensile strength and the reduction in the content of total fines smaller than 75 μm have proved efficient parameters to control cracking of renderings under thermal shock. Leveling time during rendering placement was a secondary parameter for cracking behavior.     

Use of fine glass as ASR inhibitor in glass aggregate mortars

Demand for recycled glass has considerably decreased in recent years, particularly for mixed glass. Glass is cheaper to store than to recycle, as conditioners require expenses for the recycling process. In order to provide a sustainable solution to glass storage, a potential and incentive way would be to reuse this type of glass in concretes. Depending on the size of the glass particles used in concrete, two antagonistic behaviours can be observed: alkali–silica reaction, which involves negative effects, and pozzolanic reaction, improving the properties of concrete. The work undertaken here dealt with the use of fine particles of glass and glass aggregates in mortars, either separately or combined. Two parameters based on standardised tests were studied: pozzolanic assessment by mechanical tests on mortar samples and alkali-reactive aggregate characteristics and fines inhibitor evaluations by monitoring of dimensional changes. It is shown that there is no need to use glass in the form of fines since no swelling due to alkali–silica reaction is recorded when the diameter of the glass grains is less than 1 mm. Besides, fine glass powders having specific surface areas ranging from 180 to 540 m²/kg reduce the expansions of mortars subjected to alkali–silica reaction (especially when glass aggregates of diameters larger than 1 mm are used).