THE INFLUENCE OF CURING CONDITIONS ON THE MECHANICAL PERFORMANCE OF CONCRETE MADE WITH RECYCLED CONCRETE WASTE

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1 THE INFLUENCE OF CURING CONDITIONS ON THE MECHANICAL PERFORMANCE OF CONCRETE MADE WITH RECYCLED CONCRETE WASTE Brito, J. de (1) and Fonseca, N. (1) (1) Instituto Superior Técnico / ICIST, Technical University of Lisbon, Portugal ABSTRACT The research on the use of Construction and Demolition Waste as recycled aggregates (namely crushed concrete) for the production of new concrete has by now fairly established the feasibility of this environmentally friendly use of otherwise deleterious waste. However, contrary to conventional concrete, concrete made with recycled concrete has not been the subject of large-scale use and some of the problems already solved in conventional concrete are still the source of some distrust due to lack of knowledge when recycled aggregates concrete (RAC) is mentioned. One such issue concerns curing conditions. These conditions greatly affect the performance of concrete made on site and some potential users of RAC wander whether RAC is more susceptible to far-from-ideal curing conditions. The paper presents the main results of an experimental campaign (Master dissertation) concerning the influence of different curing conditions on the mechanical performance of concrete made with recycled aggregates (crushed concrete). The properties analyzed include compressive strength, splitting tensile strength, modulus of elasticity, and abrasion resistance. The general conclusion in terms of mechanical performance is that RAC are affected by curing conditions similarly to conventional concrete. Keywords: concrete, recycled aggregates, curing conditions, mechanical performance. 1. INTRODUCTION This paper presents the main results of an experimental campaign (Master dissertation of the second author [1]) concerning the influence of different curing conditions on the mechanical performance of concrete made with recycled aggregates (crushed concrete). 1.1 Preliminary remarks The use of recycled aggregates (RA) in concrete opens a whole new range of possibilities in reusing materials in construction. Reuse of waste concrete as RA in new concretes is beneficial from the view point of environmental protection and preservation of resources. This could be an important breakthrough for society in its endeavour towards sustainable development. Preceding studies were mainly engaged in the processing of demolished concrete, mix proportion design, mechanical properties, durability aspects and improvements. Recently, structural performances and economic aspects of using recycled aggregate concrete have also 365

2 been analysed. Some previous research results on the mechanical behaviour of recycled aggregate concrete (RAC) have been reviewed by Hansen [2] and de Brito [3]. It was shown that, in fact, none of those works indicated that RAC is unsuitable for structural applications. Recent investigation on the performance of concrete made with recycled concrete fine aggregates [4] and recycled concrete coarse aggregates[5], as well as on the influence of the pre-saturation of recycled concrete coarse aggregates [6], has given positive results, which further supports and encourages the possibilities of applying RAC in civil engineering structures. 1.2 Scope and methodology of the investigation The mechanical behaviour of RAC depends on the characteristics of the RA, mix proportions and curing conditions. Despite the fact that there are several studies concerning the first two, there is a lack of information regarding the influence of the curing conditions on RAC properties. As such, this research aims at assessing the influence of the curing conditions on the mechanical characteristics of recycled concrete coarse aggregate concrete (RCCAC), as well as evaluating the effect of the incorporation of recycled concrete coarse aggregates (RCCA) on the properties of the concrete. Compressive strength, splitting tensile strength, elasticity modulus and abrasion resistance are investigated. Proper curing maintains a suitably warm and moist environment for the development of hydration products, thus reducing the porosity in hydrated cement paste and increasing the density of the concrete s microstructure. The hydration products extend from the surfaces of cement grains, and the volume of pores decreases due to proper curing under appropriate temperature and moisture conditions. If a concrete is not well cured, particularly at an early age, it will not gain the desired properties and durability due to a lower degree of hydration, and will undergo irreparable loss [7]. Researching international and national experimental campaigns was the primary stage of this investigation. The collected information constituted a repository which refers the most important properties of the aggregates, the experimental test results, and the conclusions of each campaign. A common observation made in all the different works on this matter is that of a generalized worsening of the mechanical properties of the RAC, with the increase of the substitution rate of natural aggregates (NA) by RA, when compared with natural aggregate concrete (NAC) (concrete produced with NA only or regular concrete). After this step, the experimental program was planned and executed. The RCCA and NA (fine and coarse) were analysed, but their results were not listed in detail in this abstract. Four different concrete mixes were produced, along with four different curing methods. In order to establish a legitimate assessment between different mixes and/or different curing conditions, fresh concrete tests analyses were carried out, so as to maintain the same slump and workability. After the curing period, the hardened concrete tests were performed. Subsequent to this stage, the experimental results were analysed and discussed in detail. Correlations were established between the properties of the RCCAC and the density and water absorption of the aggregates, the substitution rate of natural coarse aggregate (NCA) by RCCA, and the curing conditions as well. 2. EXPERIMENTAL PROGRAM 2.1 Materials The materials used in the experimental program were: 366

3 2-4 December 29, São Paulo, Brazil Recycled concrete coarse aggregates (RCCA): the RA were produced in a laboratory, using a concrete jaw crusher; the primary concrete was industrially manufactured and cast in situ at the laboratory (Table 1); Natural aggregates (NA): the NA (limestone) were provided by the primary fresh concrete supplier, in order to be identical to the ones used in the RA primary concrete (Table 1); Cement: ordinary CEM II A-L R Portland cement was used; the cement required was collected from the same batch of production of RA primary concrete to avoid adding any further variables; Water: tap water was used for mixing and curing. Table 1. Aggregates main properties Property Particle dry density (kg/dm3) Particle saturated surface-dried density (kg/dm3) Water absorption Loose bulk density (kg/dm3) Water content Shape index Los Angeles coefficient Coarse Fine Granule Coars e gravel gravel sand Fine sand RCCA Figure 1. Water absorption evolution with age. 2.2 Mix design Four different concrete mixes were produced: a conventional reference concretee (NAC) and three recycled aggregates concretes (RAC) with substitution rates, by volume, of 2%, 5% and 1% of NCA by RCCA. All concrete mixes (NAC and RAC) were prepared based on an effective water/binder ratio of.43 and were balanced to have a slump of 8 ± 1 mm. 367

4 The proportions of the materials were determined on the basis of absolute volume of the constituents. The details of NAC s composition are given in Table 2. The mix process followed by Ferreira [6] was used in his research, not only because it guarantees a mix as uniform as possible and a fine envelopment of the aggregates in the cement paste, but also because it respects the water compensation method requisites. The process started off by mixing coarse aggregates, water, with the concrete mixer running. At this stage, the pre-saturation process is in progress, allowing the aggregates to freely absorb water. Then, 9 seconds later cement followed by sand were added to the mix, in a 3 seconds interval and the concrete mixer was left running for another 18 seconds. In total, the mix process took about 3 seconds or 5 minutes. This way, it was assured that, at the end of the mix process, the RCCA absorb about 9% of its full water absorption capacity. Table 2. Composition of natural aggregate concrete (NAC) NCA NAC Sand 1.62 Sand 2.22 Cement.144 Vwater.192 Vvoids Curing conditions The test specimens were subjected to four types of curing conditions, namely: laboratory conditions curing (LCC); outer environment curing (OEC); wet chamber curing (WCC); water immersion curing (WIC). Regular tap water was used in WIC and the curing temperature was maintained at 16.3 ºC. The WCC specimens were kept under a relative humidity of 1% and 2. ºC temperature. In the case of OEC, the specimens were exposed to the weather, without any kind of protection, and were continuously monitored with a thermo-hygrometer. Similarly to that, the LCC specimens were preserved in laboratory, but protected from harsh weather changes. 2.4 Testing of aggregates The particle size distribution was determined in accordance with EN [8] and EN [9]. The particle density and water absorption were measured following EN [1]. The bulk density was determined in accordance with EN [11]. The aggregates 368

5 resistance to abrasion was measured by the Los Angeles loss test following LNEC E-237 [12]. The water content was determined in accordance with EN [13]. The shape index was measured following EN [14]. Water absorption in time was determined following the methodology established by Ferreira [6]. 2.5 Testing of fresh concrete The fresh concrete was produced using a revolving drum concrete mixer. Immediately after the mixing, it was tested for slump and density. The slump was determined according to Abrams slump test following EN [15]. The concrete s fresh density was measured according to EN [16]. 2.6 Testing of hardened concrete The 7, 28 and 56-day compressive strength of the concrete was determined in accordance with EN [17]. The 28-day tensile splitting strength was measured following EN [18]. Young s modulus / elasticity modulus in compression was measured following LNEC E-397 [19]. The abrasion resistance was determined by Böhme s grinding wheel wear test, in accordance with DIN 5218 [2]. 3. RESULTS AND DISCUSSION 3.1 Compressive strength The development of compressive strength with age is illustrated in Figure 2. The test results at 7, 28 and 56-day are presented in Figure 3. With all curing methods, concrete s compressive strength increased with age. The averages of the compressive strength (f cm ) at 7, 28 and 56-day are, respectively, 42.8, 49.8 and 51.6 MPa. Generally speaking, after 7 days of curing the specimens revealed 8% of their 56-day compressive strength and 95% after 28 days. Detailed results are given in Table 3, namely average results, standard deviations and relative variations referred to NAC. 369

6 OEC Table 3. Compressive strength of concrete, at different curing conditions and ages fcm 7 7-day 28-day 56-day fcm 28 fcm 56 NAC RAC RAC5 RAC1 LCC fcm 7 7-day 28-day 56-day fcm 28 fcm 56 NAC RAC RAC5 RAC1 WCC fcm 7 7-day 28-day 56-day fcm 28 fcm 56 NAC RAC RAC5 RAC1 WIC fcm 7 7-day 28-day 56-day fcm 28 fcm 56 NAC RAC RAC5 RAC

7 2-4 December 29, São Paulo, Brazil It was expected that compressive strength would decrease linearly with the substitution of NCA by RCCA. This, however, was not the case. In fact, the compressivee strength of all the different concretes typologies differs by no more that 7.5%, in relation to NAC. Therefore, no distinct relation can be established between the compressive strength and the proportion of RA in the concrete mix. For the same reason, RCCAC do not seem to be more, or less, affected by curing conditions than conventional concrete. Figure 2. Compressive strength evolution with age. OEC LCC WCC WIC f cm 46 Concrete age = 7 days Ratio of NCA substitution by RCCA 55 Concrete age = 28 days Ratio of NCA substitution by RCCA 55 Concrete age = 56 days Ratio of NCA substitution by RCCA Figure 3. Variation of the compressivee strength with the ratio of NCA substitution by RCCA. It is considered that the properties of the RCCA are similar to the concrete s binder matrix, thus not constituting a weak spot. Therefore, the particle size distribution, shape and surface texture, have a huge effect on the concrete s compressive strength. The upper values of compressive strength for RAC can be explained by the higher porosity and roughness of the RCCA, which balance their lesser strength. 371

8 3.2 Splitting tensile strength The results for the 28-day splitting tensile strength of concrete are presented in Figure 4, and Table OEC LCC WCC WIC Splitting tensile strength Ratio of NCA substitution by RCCA Figure 4. Variation of the splitting tensile strength with the ratio of NAC substitution by RCCA. Generally, the 28-day splitting tensile strength decreased with the improvement of the incorporation of RCCA. It varied from 2.37 to 3.88 MPa for different RCCA incorporation percentages and curing methods. All RAC1 typologies exhibit the lower values of splitting tensile strength, with the exception of WCC specimens that reveal a slightly higher value than NAC-WCC. Table 4. Split tensile strength of concrete, at different curing conditions and ages fctm 28 OEC fctm 28 LCC NAC RAC RAC RAC WCC WIC fctm 28 fctm 28 NAC RAC RAC RAC The tensile strength results are very inconstant and, as a result, the correlation coefficients are not acceptable. Nevertheless, the OEC specimens results reveal a very good determination coefficient (R2 =.88). 372

9 The RAC specimens kept in OEC conditions appear to be more harmed by this curing method than regular concrete. The LCC and WIC curing methods exhibit a similar development with the increase of the incorporation of RCCA; therefore, they do not seem to be more, or less, affected by the curing conditions than conventional concrete. On the other hand, WCC specimens reveal an unusual variation, with splitting tensile strength increasing with the NCA substitution by RCCA. It must be noted that the correlation in this curing condition is very low (R2 =.16), for which reason no clear conclusion can be reached. 3.3 Elasticity modulus The results for the elasticity modulus in compression of concrete are presented in Table 5 and Figure 5 The modulus of elasticity decreased, with the increase in the incorporation of RCCA. It varied from 3.6 to 43.4 GPa for different RCCA incorporation ratios and curing methods. The LCC specimens display the lowest elasticity modulus values. In view of the fact that this curing method involved only minor relative humidity, this reduction is related to moisture movement from the specimens. Since moisture moved out with the increase in age and the concrete specimens were dried with increasing exposure lengths, the microstructure of concrete remained porous and resulted in a lower modulus of elasticity. Table 4. Modulus of elasticity of concrete, at different curing conditions and ages EC 28 (GPa) OEC (GPa) EC 28 (GPa) LCC (GPa) NAC RAC RAC RAC EC 28 (GPa) WCC (GPa) EC 28 (GPa) WIC (GPa) NAC RAC RAC RAC The variation of the LCC specimens elasticity modulus (Figure 4) suggests that RCCAC is less affected by this curing condition than regular concrete (decrease with RCCA incorporation still exists, but with a slighter rate). However, the divergence is minimal and not clear. The remaining curing conditions (OEC, WCC and WIC) had all included elevated relative humidity and all exhibit extremely similar correlations with RCCA incorporation. Therefore, they do not seem to be more, or less, affected by the curing conditions than conventional concrete. 373

10 46 OEC LCC WCC WIC 44 Elasticity modulus (GPa) Ratio of NCA substitution by RCCA Figure 5. Variation of the elasticity modulus with the ratio of NAC substitution by RCCA. 3.4 Abrasion resistance The results for the abrasion resistance of concrete are presented in Figure OEC LCC WCC WIC 1.1 l BAR / l BR Ratio of NCA substitution by RCCA Figure 6. Variation of the abrasion resistance with the ratio of NAC substitution by RCCA. Since curing conditions strongly affect concrete s surface layer, it is noted that the test specimens (71x71x5 mm 3 ) were obtained by sawing larger concrete cubes (1 mm edge) after curing. This was done in order to prevent the concrete s surface finishing from being a variable in the test. Thus, the test surface is the cutting surface itself, i.e., an internal plane of the concrete element, composed by aggregates and binder mix, and not an outer surface. The irregular variation of abrasion resistance values, in all curing conditions, does not allow the establishment of a clear relation between this property and the incorporation of RCCA. Excluding the RAC5-WCC, which reveals a 1% higher wear than NAC, the abrasion resistance of all other concretes typologies differs by no more than 5.4%, in relation to NAC, which is not statistically significant from an experimental point of view. On the other hand, it must be noted that all RAC1 specimens reveal the lowest loss of thickness and subsequently higher abrasion resistance. For that reason, it can be concluded that the incorporation of RCCA leads to a better performance, in what concerns abrasion 374

11 resistance. This can be explained by the better connections established between the binder and the RCCA, in view of their higher porosity. In what concerns the curing conditions influence, no clear conclusion can be reached. Nevertheless, the lower values of variations suggest that RCCAC do not appear to be affected any differently than conventional concrete. These results indicated that the performance of mixes incorporating RA is comparable to the concrete mix in which 1% NCA was used. 4. CONCLUSIONS The use of RAC should always take into consideration that they have, in most cases, a lower performance when compared to conventional concrete. Still, RCCAC can acquire adequate quality as structural concrete. The following conclusions can be drawn based on the experimental results and the respective discussion of the study: Compressive strength does not seem to be affected by RCCA incorporation or by different curing conditions, when compared with conventional concrete; Splitting tensile strength decreases with the increase in RCCA incorporation. Recycled concrete specimens maintained in OEC conditions seem to be more harmed than specimens of conventional concrete; Elasticity modulus decreases with the increase of RCCA percentage. Recycled concrete s specimens maintained in LCC seem to be slightly affected by RCCA incorporation. Such behaviour may be justified by the low relative humidity conditions present inside the laboratory, which caused a deficient hydration of the cement, leading to weaker bonding between RCCA and the matrix with greater porosity. The elasticity modulus of RCCAC kept in the other curing conditions (OEC, WCC and WIC) does not seem to be more, or less, affected than conventional concrete; Abrasion resistance test values reveal an erratic variation; therefore, no correlation can be established. Nevertheless, all RAC1 typologies present the lowest wear. The lower values for variation suggest that the performance of mixes incorporating RA is comparable to conventional concrete, independently of curing conditions. While this field presents many possibilities (and necessities) of investigation if the behaviour of recycled aggregates concretes is to be fully understood, it can be concluded from the results of this experimental study that these aggregates do indeed reveal a potential for being used in the production of structural concrete. ACKOWLEDGEMENTS The authors thankfully acknowledge the support of the ICIST Research Institute from IST, Technical University of Lisbon and of FCT (Foundation for Science and Technology). REFERENCES [1] Fonseca, N. 'Structural concrete with incorporated recycled concrete coarse aggregates: Influence of the curing conditions on the mechanical behaviour' (in Portuguese), MSc Dissertation in Civil Engineering (IST, Lisbon, 29). [2] Hansen, T. 'Recycling of demolished concrete and masonry', in 'Demolition and Reuse of Concrete', Report of technical committee 37-DRC (Taylor & Francis, London, 1992). 375

12 [3] de Brito, J. 'Recycled aggregates and their influence on concrete s properties' (in Portuguese). Public lecture within the full professorship in Civil Engineering pre-admission examination (IST, Lisbon, 25). [4] Evangelista, L. 'Performance of concrete made with fine recycled concrete aggregates' (in Portuguese), MSc Dissertation in Civil Engineering (IST, Lisbon, 27). [5] Gomes, M. 'Structural concrete with incorporation of concrete, ceramic and mortar recycled aggregates' (in Portuguese). MSc Dissertation in Civil Engineering (IST, Lisbon, 27). [6] Ferreira, L. 'Structural concrete with incorporation of coarse recycled concrete aggregates: Influence of the pre-saturation' (in Portuguese), MSc Dissertation in Civil Engineering (IST, Lisbon, 27). [7] Raman, S., Safiuddin, M.D. and Zain, M. 'Effect of different curing methods on the properties of microsilica concrete', Australian Journal of Basic and Applied Sciences 1 (2) (27) [8] EN 'Tests for geometrical properties of aggregates. Part 1: Determination of particle size distribution. Sieving method' (in Portuguese) (IPQ, Lisbon, Portugal, 1997). [9] EN 'Tests for geometrical properties of aggregates. Part 2: Determination of particle size distribution. Test sieves, nominal size of apertures' (in Portuguese) (IPQ, Lisbon, Portugal, 1995). [1] EN 'Tests for mechanical and physical properties of aggregates. Part 6: Determination of particle density and water absorption' (in Portuguese) (IPQ, Lisbon, Portugal, 2). [11] EN 'Tests for mechanical and physical properties of aggregates. Part 3: Determination of loose bulk density and voids' (in Portuguese) (IPQ, Lisbon, Portugal, 1998). [12] LNEC E-237 'Aggregates: Los Angeles abrasion test' (in Portuguese) (LNEC, Lisbon, Portugal, 197). [13] EN 'Tests for mechanical and physical properties of aggregates. Part 5: Determination of the water content by drying in a ventilated oven' (in Portuguese) (IPQ, Lisbon, Portugal, 28). [14] EN 'Tests for geometrical properties of aggregates. Part 4: Determination of particle shape. Shape index' (in Portuguese) (IPQ, Lisbon, Portugal, 28). [15] EN 'Testing fresh concrete. Part 2: Slump test' (in Portuguese) (IPQ, Lisbon, Portugal, 1999). [16] EN 'Testing fresh concrete. Part 6: Density' (in Portuguese) (IPQ, Lisbon, Portugal, 1999). [17] EN 'Testing hardened concrete. Part 3: Compressive strength of test specimens' (in Portuguese) (IPQ, Lisbon, Portugal, 21). [18] EN 'Testing hardened concrete. Part 6: Tensile splitting strength of test specimens' (in Portuguese) (IPQ, Lisbon, Portugal, 2). [19] LNEC E-397 'Concrete: Determination of elastic modulus in compression' (in Portuguese) (LNEC, Lisbon, Portugal, 1993). [2] DIN 5218 'Testing of inorganic non-metallic materials: Wear test with the grinding wheel according to Böhme' (in German) (DIN, Germany, 22). 376