Structural concrete incorporating recycled concrete coarse aggregates Pre‐saturation influence Luís Manuel Madeira Ferreira Extended Abstract Júri Presidente: Prof. Dr. Francisco José Loforte Teixeira Ribeiro Orientador: Prof. Dr. Jorge Manuel Caliço Lopes de Brito Vogais: Dr. António Carlos Bettencourt Simões Ribeiro Setembro, 2007
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Structural concrete incorporating recycled concrete coarse … · be comparable, these mixes must have the same volumetric composition (cement, coarse aggregates and sand), particles
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1. Introduction It is urgent to find alternatives to the disposal in landfills of the increasing volume of construction and demolition waste (CDW) produced annually. Moreover, the continuous use of natural resources without restrictions is unsustainable. This situation makes the reuse of construction and demolition waste imperious. One of the hypothesis with greater potential of reuse is the utilization of construction and demolition waste, namely of concrete rubble, as coarse aggregates in the production of structural concrete. Concrete incorporating recycled concrete coarse aggregates has already been studied by many investigators, even though its properties and performance are far from being entirely known. This text is a summary of an investigation undertaken with the purpose of providing more information about recycled concrete coarse aggregates (RCCA) and concrete incorporating RCCA and in that way helping the redaction of legislation and technical documents which are fundamental to promote the reuse of CDW.
2. Scope and objectives The use of RCCA in new structural concrete faces technological problems associated to the differences between RCCA and natural aggregates (NA). These differences have their origin in the mortar attached to the original NA in RCCA. The adhered mortar affects, besides other properties not evaluated in this investigation, the density and water absorption of RCCA and density, compressive strength, elasticity modulus, shrinkage and water absorption of concrete. Because of the fast and high water absorption of RCCA, their incorporation in concrete without corrective measures, leads to a reduction of the effective W/C ratio of the mix. This situation has consequences on concrete’s workability and mechanical behaviour. The maintenance of workability and effective W/C ratio can be achieved in two different ways. One is to directly add an additional amount of water corresponding to the RCCA’s water absorption and allowing them to absorb it. The second way is to avoid water from being absorbed by RCCA. Since it is impracticable to prevent water from entering the aggregates, it is possible to add RCCA nearly saturated to the mix. This way there would be no water transfers between the RCCA and the cement paste. In this investigation, the effects of both absorption compensation methods on mechanical behaviour and durability of concrete are compared and evaluated. For this purpose, a conventional reference concrete, three mixes with substitution rates of 20, 50 and 100% of natural coarse aggregates (NCA) with RCCA resorting to compensation of the mixing water, and three other mixes with the same substitution rates resorting to pre‐saturation of RCCA have been produced. In order to be comparable, these mixes must have the same volumetric composition (cement, coarse aggregates and sand), particles size distribution and slump. Some authors suggest that saturating aggregates before mixing can be a better solution for controlling RCCA higher water absorption and maintain a good control of concrete properties rather than compensating mixture water (ALAEJOS et al., 2006). However, studies by BARRA & VAZQUEZ (1996) and POON et al. (2004) have demonstrated that a better concrete performance is achieved by using RCCA with a high humidity level compared to dried or saturated. Thus, ETXEBERRIA (2007) recommends an 80% humidity level in RCCA. EXTEBERRIA (2007) and ALAEJOS et al. (2006) suggest that pre‐saturation of RCCA should be done by sprinklers in plant. However, LIMA (1999) refers that by this process it is difficult to guarantee a homogeneous humidity of the aggregates and that it can produce washing‐off of finer particles. The same author suggests as an alternative that pre‐saturation occurs inside the concrete mixer. In both cases, either pre‐saturating RCCA or compensating mixture water, researchers BARRA & VAZQUEZ (1996), SANTOS et al. (2002), POON et al. (2004), MATIAS & BRITO (2005) and GOMES (2007) succeeded in maintaining concrete’s workability.
In what concerns water absorption by RCCA, SANCHEZ (2004) observed that it is quick and that absorptions between 70 and 90% are achieved in only 10 minutes of contact with water.
3. Aggregates properties NA and RCCA have been submitted to several tests in order to determine particle size distribution (EN 933‐2), particle density and water absorption (EN 1097‐6), loose bulk density and voids (EN 1097‐3), water content by drying in a ventilated oven (EN 1097‐5) and particles shape by flakiness index (EN 933‐3). The most relevant results obtained are shown in Table 1. Particle size distribution results are shown in Table 2.
Table 1 – Results of tests done in aggregates Property Coarse gravel Fine gravel Sand RCCA
Particle dry density (Mg/m3) 2,6 2,65 2,5 2,3Particle saturated surface‐dried density (Mg/m3) 2,64 2,67 2,55 2,44
Water absorption (%) 1,2 0,7 2,3 5,8Loose bulk density (Mg/m3) 1,33 1,42 1,56 1,14
* Aggregate dried before mixing As expected, RCCA show lower particle density (apparent, dry, and saturated surface‐dried density) and loose bulk density in comparison to natural aggregates. This is due to the lower density of the adhered mortar in comparison to NA and also, for loose bulk density, to a bigger volume of voids between particles in RCCA. The higher porosity of adhered mortar in comparison to NA is responsible for the higher water absorption in RCCA. The flakiness index reveals no difference between RCCA and NA. The values obtained for these properties match results obtained in bibliographic research. Furthermore, two other characterization tests have been done exclusively in RCCA. The first one has been the determination of water absorption with time. This test has had major importance in this campaign since it has allowed quantifying the water absorbed by RCCA during mixing and determining the pre‐saturation time period. It consists in monitoring RCCA’s apparent weight when submerged into water. This test was adapted from the one proposed by LEITE (2001), and later followed by GOMES (2007), in order to take into consideration initial absorption. The water absorption with time curve obtained is shown in Figure 1.
This absorption curve reveals a very high initial absorption. Only 5 minutes after submerging RCCA into water, the absorption is 89,2% of absorption potential. After this period, the absorption is rather slow and with small significance. Considering this, the time period for pre‐saturation of RCCA was established as 5 minutes expecting 90% of water absorption potential to be absorbed. The second test has been the determination of the quantity of adhered mortar in RCCA. The procedure consists in submitting the RCCA to a thermal shock which detaches the adhered mortar in RCCA from the original NA. The resulting particles are sieved and all particles smaller than 4 mm are considered to be part of the adhered mortar. A percentage of 69,4% of adhered mortar relative to dry weight of aggregates has been obtained. This value is clearly higher than values found in consulted bibliography and, considering the corresponding results of other properties, may indicate a good original concrete quality.
4. Concrete mixes and production 4.1. Concrete design Three types of natural limestone aggregates have been used in the production of the conventional reference concrete (RC): coarse gravel, fine gravel and sand as fine aggregates. Due to the “wall effect”, a maximum particle size of 20 mm was established. All the aggregates used were obtained from rock crushing. A fast settling cement type CEM I 42,5R has also been used and no admixtures of any kind have been employed. The reference concrete has been designed by the Faury method for a XC2 ambient according to EN 206 targeting a C30/37 strength class and a slump of 80 ± 10 mm. For recycled aggregates concrete (RAC) two conditions were set for their design. The first one was that the particles size distribution of RAC matched RC. The second condition was that the substitution would only apply to particles with size over 4 mm. Naturally, the recycled aggregates size distribution do not coincide with the RC’s distribution. Therefore it has been necessary to sieve its particles and prepare a matching mix. Aggregates and concrete particle size distribution is shown in Table 2.
Table 2 – Particle size distribution of aggregates and concrete Particle size distribution
With the intention of emphasizing the water absorption condition effects on concrete behaviour, RCCA were previously dried in the oven. Due to its great humidity sensibility and considerable water absorption, sand was also previously dried. This way it has been possible to maintain a better control of water presence in the mix. The substitution of natural coarse aggregates (NCA) by RCCA has been made by replacing a percentage of NCA by its equivalent in volume of RCCA. The composition of each mix produced is
shown in Table 3. It is important to refer that the total amount of water in mixes with the same substitution rate is exactly the same, allowing differences between mixes to be a consequence of RCCA’s water absorption condition only.
Effective W/C ratio 0,50 0,50 0,50 0,50Cement paste/aggregates ratio 0,33 0,34 0,35 0,36
4.2. Presaturation The first concern about the pre‐saturation method was that it corresponded to a practical and executable procedure that could be applied when producing concrete in a plant and especially at a construction site. Consequently, it was determined that pre‐saturation would occur inside the mixer, while working, mixing the total amount of water and the total amount of RAC used in the concrete. It was also predefined that pre‐saturation would last sufficiently long for the aggregates to reach a stable humidity level. The 5 minutes period of pre‐saturation, corresponding to a 90% humidity level of RCCA, was elected because it corresponds to a small variation period and does not reach saturation. This way, it was intended to guarantee that there would be no water devolution from the aggregate to the paste. Also, by choosing a short period of pre‐saturation it is possible to prevent NA’s absorption, even though it is low. After the pre‐saturation process, the mixing procedure follows the steps described in point 4.3. At the end of the mixing procedure, immediately before the slump test, RCCA had been a total of 10 minutes in contact with water. Taking into consideration the water absorption with time results, in the hypothesis of a continuing absorption of water by RCCA, they would reach a maximum humidity level of 93%. The 3% difference for the non pre‐saturated RCCA should have little influence on the slump test. A remark must be made that water absorption continues long after mixing procedure. Thus there is no difference in W/C ratio between mixes with pre‐saturated aggregates and those with water compensation. 4.3. Mixing Concrete mixing has been done in a vertical axe mixer with 50 litres capacity. The first stage has consisted in the mixing, with the mixer working, of coarse aggregates, cement and water for 90 seconds. In the mixes produced resorting to pre‐saturation of RAC the first stage has consisted in adding cement (and eventually remaining coarse and fine gravel) to the pre‐saturation mixture of RAC and the total amount of water. After this, in a period of 30 seconds, the sand was added. Then, a final 180 seconds mixing period has been allowed. In total, the mixing procedure has been done in 300 seconds, or 5 minutes. The mixes produced are identified as shown in Table 4. The moulds and concrete needs for each test are presented in Table 5. For each concrete mix, 45 litres of concrete have been produced, except for B50 for which two mixes have been produced. The making and curing of concrete specimens has followed EN 12390‐2.
Absorption by immersion 28 4 cubes (100x100x100 mm) ‐*Absorption by capillarity 28 4 prisms (100x100x200 mm) 8,00
Total 39,74* The same cubes were used in determining concrete’s density and water absorption by immersion
5. Results In fresh concrete, the slump test (EN 12350‐2) has been performed and density (EN 12350‐6) has been determined. In hardened concrete, tests have been performed to evaluate compressive strength (EN 12390‐3), density (EN 12390‐7), elasticity modulus (LNEC E397), shrinkage (LNEC E398 / UNE 83‐318‐94), water absorption by immersion (LNEC E394) and water absorption by capillarity (LNEC E393). Moreover, the mixes’ surface has been observed in a magnifying glass. Before presenting results from the tests mentioned above, it must be pointed that the primary objective of this investigation was to evaluate the RAC pre‐saturation effect on mechanical behaviour and durability of concrete. Therefore, in this summary, only this issue will be focused. However, a brief note is made to comment the expectable differences between RC and RAC. These differences are observed in all tests undertaken and show similar trends to other investigations. The differences between RAC and RC are, in all cases except for autogeneous shrinkage, more evident the higher RCCA incorporation rate, and are listed below:
• lower density of fresh and hardened concrete in RAC; • lower compressive strength in RAC at both ages of 7 and 28 days; faster initial compressive
strength in RAC, with slower evolution between the ages of 7 and 28 days; • lower modulus of elasticity in RAC, with higher variation relative to RC than in compressive
strength; • higher drying shrinkage but lower autogeneous shrinkage; • higher water absorption by immersion and by capillarity.
The good performance of concrete with 20% RCCA incorporation rate produced resorting to compensation of mixture water must also be mentioned showing very similar behaviour to RC. 5.1. Slump Slump test results represented in Figure 2 show that the target slump of 80 ± 10 mm is achieved in all mixes and that no differences are revealed between mixes with pre‐saturated RCCA and those with water compensation. Thus, it may be concluded that concrete’s workability, evaluated by the slump
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5.3. Compressive strength The average compressive strength of 59 MPa in RC, which corresponds to a characteristic compressive strength of 50,1 MPa, includes it in the C40/50 class. Even though higher than the C30/37 target class, this class is favourable to emphasize differences between mixes with pre‐saturated RCCA and those with water compensation. At both ages of 7 and 28 days (Figure 5 and Figure 6) mixes with pre‐saturated RCCA have lower compressive strength than mixes with water compensation. This is possibly due to a “nailing effect”. This effect is produced by the penetration of cement paste inside superficial porous of aggregate particles.
Figure 5 – Compressive strength at 7 days versus RCCA incorporation rate
Figure 6 – Compressive strength at 28 days versus RCCA incorporation rate
Considering the high compressive strength of the mixes produced, it is predictable that fracture occurs in the interfacial transition zone between aggregates and cement paste, or even through the aggregates, if the compressive strength is too high. Before mixing, pre‐saturated RCCA present not only a high level of humidity but also water on the surface and in the interior of surface porous. This situation may impair the penetration of the cement paste into the pores leading to a decrease of the “nailing effect” and, consequently, to a weaker interfacial transition zone between cement paste and RCCA. The difference between mixes with pre‐saturated RCCA and those with water compensation is attenuated with the increase of RCCA’s incorporation rate which may be due to an increase of weak zones in concrete. 5.4. Elasticity modulus Elasticity modulus results, represented in Figure 7, show that, for RCCA incorporation rates of 20 and 50%, mixes with pre‐saturated RCCA have lower modulus of elasticity. The exception for mixes with total substitution of NCA by RCCA may be due to the fewer valid tests for B100, since this mix showed higher compressive strength than B100PS. So, it is not possible to conclude on the evolution of the difference in elasticity modulus with the incorporation rate of RCCA.
Figure 7 – Elasticity modulus versus RCCA incorporation rate
5.5. Shrinkage Drying shrinkage test specimens have been cured in climatic chamber at 20°C and 50% humidity. However, due to a technical problem with the weathering chamber, B100 and B100PS have had to be cured in ambient conditions. For autogeneous shrinkage measure, test specimens have been cured for 28 days in humid chamber. Drying shrinkage results represented in Figure 8 and Figure 9 show a much higher shrinkage in mixes with pre‐saturated RCCA. This trend is supported by a higher weight loss of B100PS comparing to B100 during drying (4,4% against 3,9%). Also, in Figure 8, it is seen that the difference between B50PS and B50 is bigger than between B20PS and B20 suggesting that for a higher RCCA substitution rate the difference between mixes with pre‐saturated RCC and mixes with water compensation increases. The lower “nailing effect”, reducing friction in the interfacial transition zone between RCCA and the cement paste, the higher porosity, allowing an easier water exit from the interior of concrete, and the lower elasticity modulus of mixes with pre‐saturated RCCA are the causes of these results.
Figure 8 – Drying shrinkage evolution of mixes cured in the weathering chamber with concrete’s age
Figure 9 – Drying shrinkage evolution of mixes cured in ambient condition with concrete’s age
Obtained results for autogeneous shrinkage represented in Figure 10 do not allow concluding on the effect of RCCA’s pre‐saturation.
Figure 10 – Autogeneous shrinkage evolution with concrete’s age
5.6. Water absorption by immersion As seen in Figure 11, water absorption by immersion is slightly higher in mixes with pre‐saturated RCCA. The only point outside this trend for the RCCA incorporation rate of 50% may be due to an error during the test of one of the two B50 mixes which presents much higher absorption.
Figure 11 – Water absorption by immersion versus RCCA incorporation rate
5.7. Water absorption by capillarity The results from the water absorption by capillarity test, represented in Figure 12, point out a clearly higher absorption for mixes with pre‐saturated RCCA. It is also seen that the difference for mixes with water compensation increases for higher RCCA incorporation rates. The capillary ascension height results represented in Figure 13 are coherent with water absorption by capillarity results. Pre‐saturated RCCA mixes present higher capillary ascension levels than those with water compensation. The almost constant values between incorporation rates of 50 and 100% are due to a pressure balance between exterior water surface and water surface inside pores. The capillarity water absorption results, complemented with absorption by immersion results, allow concluding that mixes with pre‐saturated RCCA have worse durability than those with water compensation.
Figure 12 – Water absorption by capillarity after 72 hours versus RCCA incorporation rate
Figure 13 – Capillary ascension height after 72 hours versus RCCA incorporation rate
5.8. Observation with magnifying glass The observation with a magnifying glass of mixes’ surface show a homogeneous and blurred interfacial transition zone between cement paste and RCCA, both in reference concrete and mixes with water compensation (Figure 14). On the other hand, pre‐saturated RCCA mixes showed a slightly more marked and visible interfacial transition zone (Figure 15). This may be a result of the lower “nailing effect” in these mixes.
Figure 14 – B100 photograph with 20x zoom
Figure 15 – B100PS photograph with 20x zoom
6. Conclusions It is possible to conclude that RCCA have different proprieties from NA. These differences are evident in particle and loose bulk density (lower in RCCA) and water absorption (higher in RCCA). No significant differences were found in aggregates shape by flakiness index. The differences between RAC and NA are a consequence of the low density and high porosity of the adhered mortar in RCCA. The higher adhered mortar quantity in RCCA compared to other investigations contrasted with matching results of other properties may indicate a good original concrete quality. It is also concluded that water absorption in RCCA is very intense in the first instants of submerging into water, leading RCCA to rapidly achieve a high humidity level. This test is very important for determining mixing and pre‐saturation time periods and the absorbed water by RCCA. Concerning concrete properties, fresh concrete density and hardened concrete dry density are lower in mixes with pre‐saturated RCCA. This may indicate that mixes with pre‐saturated RCCA have a
higher porosity. In fresh concrete density the difference between mixes with pre‐saturated RAC and those with water compensation show a growing trend for increasing incorporation rates. However, in hardened concrete dry density this trend is not seen. Compressive strength and elasticity modulus are also lower in mixes with pre‐saturated RCCA. This may be due to a weaker interfacial transition zone caused by a lower “nailing effect”. The difference in compressive strength for mixes with water compensation shows a diminishing trend for increasing RCCA incorporation rates which is caused by a higher number of weak zones. Drying shrinkage is higher in mixes with pre‐saturated RCCA. This is possibly due to the lower “nailing effect”, higher porosity and lower elasticity modulus of these mixes. Autogeneous shrinkage results are not conclusive. Considering this, it is concluded that RCCA pre‐saturation leads to a worse mechanical behaviour of concrete. Water absorption by immersion is higher in mixes with pre‐saturated RCCA. Water absorption by capillarity and capillary ascension height show the same trend even though more evidently. The difference in water absorption by capillarity, between mixes with pre‐saturated RCCA and those with water compensation, increases for higher RCCA incorporation rates. It is then concluded that mixes with pre‐saturated RCCA have worse durability. Moreover, it is concluded that durability is more affected by pre‐saturation of RCCA than mechanical behaviour. The observation with magnifying glass allows concluding that a better, more homogeneous, interfacial transition zone is obtained in mixes with water compensation. All in all, it is concluded that, in order to prevent RCCA water absorption from affecting concrete’s properties, it is preferable to compensate mixing water rather than pre‐saturating the RCCA.
7. Bibliography
• ALAEJOS, P., SANCHEZ, M., ALEZA, F., BARRA, M., BURÓN, M., CASTILLA, J., DAPENA, E., ETXEBERRIA, M., FRANCISCO, G., GONZÁLEZ, B., MARTÍNEZ, F., MARTÍNEZ, I., PARRA, J., POLANCO, J., SANABRIA, M., VAZQUEZ, E., “Utilización de árido reciclado para la fabricación de hormigón estructural”, Comisión 2, Grupo de Trabajo 2/5 “Hormigón reciclado", Monograph M‐11 ACHE, Madrid, 2006.
• BARRA, M., VASQUEZ, E., “The influence of retained moisture in aggregates from recycling on the properties of new hardened concrete”, Waste Management, Vol. 16, pp. 113‐117, 1996.
• ETXEBERRIA, M., VÁZQUEZ, E., MARÍ, A., BARRA, M., “Influence of amount of recycled coarse aggregates and production process on properties of recycled aggregate concrete”, Cement and Concrete Research nº 37, pp. 735‐742, 2007.
• GOMES, M., “Betões estruturais com incorporação de agregados reciclados de betão e cerâmicos com reboco”, MSc. thesis in Construction, Instituto Superior Técnico, Technical University of Lisbon, Lisbon, 2007.
• LEITE, M. B., “Avaliação de propriedades mecânicas de concretos produzidos com agregados reciclados de resíduos de construção e demolição”, PhD. Thesis in Civil Engineering, Engineering School, Federal University of Rio Grande do Sul, Porto Alegre, 2001
• LIMA, J., “Proposição de diretrizes para produção e normalização de resíduo de construção reciclado e de suas aplicações em argamassas e concretos”, MSc. Thesis in Architecture and Urbanism, São Carlos Engineering School of University of São Paulo, São Carlos, 1999.
• POON, C.S., SHUI, Z.H., LAM, L., FOK, H., KOU, S.C., “Influence of moisture states of natural and recycled aggregates on the slump and compressive strength of concrete”, Cement and Concrete Research nº 34, pp. 31‐36, 2004.
• SANCHEZ, M., “Estudio sobre la utilización de árido reciclado para la fabricación de hormigón estructural”, PhD. thesis, Polytechnic University of Madrid, Madrid, 2004.
• SANTOS, J., BRANCO, F. A., BRITO, J. de, “Utilização de agregados grossos reciclados de betão na produção de novos betões”, Estruturas 2002, pp. 227‐236, LNEC, Lisbon, 2002.
• EN 932‐2:1998, Tests for general properties of aggregates – Part 2: Methods for reducing laboratory samples.
• EN 933‐3:1997, Tests for geometrical properties of aggregates – Part 3: Determination of particle shape – Flakiness index.
• EN 1097‐3:1998, Tests for mechanical and physical properties of aggregates – Part 3: Determination of loose bulk density and voids.
• EN 1097‐5:1999, Tests for mechanical and physical properties of aggregates – Part 5: Determination of the water content by drying in a ventilated oven.
• EN 1097‐6:2000, Tests for mechanical and physical properties of aggregates – Part 6: Determination of particle density and water absorption.
• EN 12350‐2:1999, Testing fresh concrete – Part 2: Slump test. • EN 12350‐6:1999, Testing fresh concrete – Part 6: Density. • EN 12390‐3:2001, Testing hardened concrete – Part 3: Compressive strength of test
specimens. • EN 12390‐7:2000 Testing hardened concrete – Part 7: Density of hardened concrete. • LNEC E393, Concrete. Determination of absorption of water through capillarity, 1993, LNEC. • LNEC E394, Concrete. Determination of absorption of water by immersion, 1993, LNEC. • LNEC E397, Hardened concrete. Determination of the modulus of elasticity of concrete in
compression, 1993, LNEC. • LNEC E398, Hardened concrete. Determination of the shrinkage and of the swelling, 1993,
LNEC. • UNE 83‐318‐94, Concrete tests. Determination of the modulus of elasticity in compression.