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Heterogeneity of recycled concrete aggregates, an intrinsic variability
Construction and Building Materials 175 (2018) 705–713
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Heterogeneity of recycled concrete aggregates, an intrinsic variability Eliane Khoury a,b,⇑, Weslei Ambrós a,c, Bogdan Cazacliu a, Carlos Hoffmann Sampaio c, Sébastien Remond b a
IFSTTAR, Aggregates and Materials Processing Laboratory, Route de Bouaye-CS4, 44344 Bouguenais Cedex, Nantes, France IMT Lille Douai, Univ. Lille, EA 4515 – LGCgE – Laboratoire de Génie Civil et géoEnvironnement, département Génie Civil & Environnemental, F-59000 Lille, France c Mineral Processing Laboratory, Federal University of Rio Grande do Sul, 9500 Bento Gonçalves Avenue, 91501-970 Porto Alegre, Brazil b
h i g h l i g h t s Heterogeneity of Recycled concrete aggregates (RCA) is one of their critical properties. RCA sorted by density were characterized. A given granular class of RCA can exhibit very large physical heterogeneities. Heterogeneity of RCA mainly depends on their cement paste content. Water jig is a very efficient method to sort RCA by density.
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Article history: Received 23 January 2018 Received in revised form 17 April 2018 Accepted 20 April 2018
Keywords: Recycled concrete aggregates Water jig Sorting Heterogeneity Cement paste content Water absorption Density
a b s t r a c t Recycled concrete aggregates (RCA) are composed of two different materials: natural aggregate and attached cement paste. It is generally admitted that finer the RCA greater is the quantity of adhered cement paste. In this study it is shown that for a given granular class, very large disparities may be present in the adhered cement paste content, which could generate dispersion of the results of characterization tests of RCA. The heterogeneity of a narrow granular class of coarse RCA (6.3/10 mm) was investigated by sorting samples according to their densities using a water jig. The water jig sorting was essentially a separation by density and not by size. It was shown that the average water absorption after an immersion in water during 24 h was about 5% for a representative sample of the feed RCA, before sorting tests, while for the homogenous specimens separated by density, the water absorption ranged from 2% to 9%. The distribution of the water absorption in a sample of 120 kg of RCA was well estimated by a log-normal distribution. This intrinsic heterogeneity of RCA implies being able to have representative samples with sufficient accuracy, not only concerning the size but also the mineral composition of grains in the stockpiles of RCA. So, after discussing the results of this study, water jig appeared to be an efficient tool to separate RCA and obtain representative samples for their characterization. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction Many countries have made great efforts to deal with large quantities of construction and demolition wastes (CDW) in order to reintroduce them into the construction life cycle. However, the reuse of these wastes to produce new concrete has not been widely adopted yet, and landfill is still the main solution for removing these materials [1,2]. On the other hand, different studies have been conducted to develop concrete production with recycled concrete aggregates (RCA) that comply with applicable codes and standards [3–19]. Until now, only a small percentage of natural ⇑ Corresponding author at: IFSTTAR, Aggregates and Materials Processing Laboratory, Route de Bouaye-CS4, 44344 Bouguenais Cedex, Nantes, France. E-mail address: [email protected] (E. Khoury). https://doi.org/10.1016/j.conbuildmat.2018.04.163 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
aggregates were replaced by these materials to formulate concrete. The difficulty in the use of RCA in new concrete is related to their high water absorption and its temporal heterogeneity. Due to their high porosity [5,11], RCA absorb part of the mixing water which may reduce concrete workability unless the absorbed water is taken in account in the mix design. A poor estimation of the water absorption coefficient (WA) of RCA leads to inadequate effective water for the recycled concrete, the latter induces poor mechanical properties (excess in the mixing water) or poor workability of recycled concretes (lack of water). Pre-saturation of RCA before mixing could appear as a potential technical solution. However, presaturation before mixing is a complex task that is most generally not applied in recycled concrete manufacturing. Therefore, better knowledge of in-situ RCA water absorption capacity is necessary.
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More generally, sophisticated characterization (mineral composition, particle size distribution, particle shape, abrasion, density, water absorption and cement mortar content) must be carried out to get a better understanding of the effects of RCA on recycled concrete production and performance [20–24]. Thus, the first step of characterization is to extract a representative sample and determine various properties according to the intended recycling paths [25]. Natural aggregates may present heterogeneities, but the procedures described in European standards for sampling and characterization make it possible to produce materials that are sufficiently consistent for the purposes of construction. On the other hand, RCA are composed of natural aggregates but also of hardened cement paste and other impurities that are not homogeneously distributed in the different granular classes. Hence RCA have a much significant heterogeneity compared to natural aggregates. Therefore, the heterogeneity of recycled concrete aggregates limits their use due to the uncertainty in their expected behavior [13,14]. Indeed, a wide dispersion of results can be observed in the literature, mainly caused by the different sources and quality of original concretes that are recycled [15–18]. These results highlighted the heterogeneity and the inaccurate characterization measurement of RCA and confirmed the necessity to control their properties for different applications [13,19]. It was already demonstrated that the density increases and the adhered cement paste content decreases with the size fraction of the crushed concrete [26–28]. It was also shown that, in situ, crushed concrete obtained from CDW is always mixed with other crushed materials like brick, asphalt, gypsum, glass and so on. In the literature, jigs were proposed to concentrate the concrete particles and/or to diminish the impurities of a mixed recycled aggregate as a result of a density sorting [29–34]. The objective of this paper is to show that even in a given granular class of well controlled industrially manufactured coarse RCA, large disparities may be present in the adhered cement paste content, generating dispersion in the properties of the RCA. A laboratory water jig was used to sort crushed concrete aggregates with different densities in three generations. Homogeneous density samples of RCA were characterized in order to determine water absorption, granular size distribution, densities, porosity and cement paste content, to assess their statistical distribution in the original material.
mately 20 kg per batch (Fig. 1). The total mass studied was about 120 kg. The complete jigging device consists of a separating chamber supported on a perforated plate (Ø = 1 mm), which is allocated on a tank containing water, which in turn is pulsed into the jig container by means of an air flow controlled by a rotary piston valve and a knife gate valve. Thus, the jigging stroke and the frequency of pulsation can be regulated by adjusting the valves. During the tests, the particle bed within the jig container is submitted to numerous cycles of expansion and contraction by means of the pulse of water, which creates the conditions for particles of different densities to move with different vertical velocities. The net result is the bed stratification according to the particles density and/or particle size distribution with the concentration of the lighter fraction in the upper layers and the denser fraction in the lower layers of the bed. The separating chamber was assembled by overlying 6 layers of PlexiGlass (400 400 25 mm), so that after each test, the bed can be vertically sliced in 6 different fractions. For all test cases, the pulse frequency, bed expansion and jigging time were kept constant at 90 RPM (rotations per minute), 5 cm and 120 s, respectively. 2.3. Characterization of RCA 2.3.1. Water absorption WA The water absorption coefficient determined after 24 h of immersion in water, noted WA, was measured by the pycnometer method according to the European NF EN 1097-6 standard. It involves immersing a dry sample in a pycnometer filled with water to measure the sample mass increase due to the penetration of water in the pores. The WA was determined using Eq. (1)
WA ¼ ðM SSD M OD Þ=M OD
where MSSD is the Saturated Surface Dry (SSD) mass after 24 h of saturation in water, and MOD is 48 h-oven-dry mass (at 75 ± 5 °C) of the sample after the test. The drying temperature was fixed to 75 °C instead of 110 °C in order to avoid a loss of chemically bound water from adhered cement paste [35].
2. Experimental method 2.1. Materials 6.3/10 mm RCA sourced from the ‘‘Gonesse Recycling Platform” located in France and composed of 99% recycled concrete and 1% of other inert materials were used in this study. A narrow granular fraction was chosen in order to emphasize the differences in density rather than the differences in particle size. Main physical properties of the RCA employed were determined according to the NF EN 1097-6 standard. The average of the results of 4 tests on 4 samples of 1–2 kg each and the corresponding standard deviation were the following: - Water absorption coefficient (WA) 4.88 ± 0.11% - Real dried density in water (qrd) (NF EN 1097-6) 2.31 ± 0.01 g/ cm3 - Absolute density in water (qa) (NF EN 1097-6) 2.60 ± 0.01 g/ cm3. 2.2. Water jig The sorting of RCA was conducted in a batch scale water jig, model AlljigÒ S 400 from AllMineral with a capacity of approxi-
ð1Þ
Fig. 1. Water jig, AllMineral Company, model Alljig S 400/600X400Ò.
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2.3.2. Grading curves with video grader (VDG) The particle size distribution of RCA was characterized by an optoelectronic video grader consisting of a single camera video imaging [36]. This device determines the two main dimensions of the shadow of each particle, and then extrapolates the preceding two-dimensional characterization to a three dimensional particle shape with size expressed as an equivalent sieve diameter. This method is faster than the sieve analysis. 2.3.3. Densities measured in water According to the European standard NF EN 1097-6, two densities in water can be identified as following, - The real dried density (qrd), ratio obtained by dividing the mass of a sample of the oven-dried aggregate (drying after immersion) by the volume it occupies in water, including the volume of any closed pore and the volume of any pore accessible to water. This density is called also effective particle density in British Standards Institute (BSI) and envelope density in American Society for Testing and Materials (ASTM) [37] - The absolute density (qa), ratio obtained by dividing the mass of a dried aggregate sample in an oven by the volume it occupies in water, including the volume of any pore inaccessible to water (closed pore), but excluding the volume of water present in all pores accessible to water. This density is also called apparent particle density in British Standards Institute (BSI) and Skeletal density in American Society for Testing and Materials (ASTM) [37]. 2.3.4. Mercury Intrusion porosimetry The porosity and pore size distribution of the recycled aggregates were determined using mercury intrusion porosimetry (MIP). A porosimeter (Micrometrics Autopore IV) with low and high pressure positions was used for accessing pore radius between 1.8 nm and 60 mm. The MIP samples were small, about 10 g of material, representing only a few grains. In order to diminish the errors of sampling, this test was used here only for the less heterogeneous aggregates obtained after jigging. In addition, skeletal and envelope densities were determined in Mercury. These densities have the same definition as in water but the difference is simply due to the fact that the volume of accessible pores is not identical, pores less easily accessible with mercury. 2.3.5. Helium pycnometer For each sorted RCA, representative samples of 8 g ± 0.5 g were dried in the oven at a temperature of 75 °C and crushed (d 200 mm), then the specific density qS(gas) was measured by using a helium pycnometer (Micromeritics AccuPyc 1330). The samples were crushed to powder. Assuming that grinding allows the opening of all the non-accessible porosity, it can be considered that density measured in Helium pycnometer corresponds to the absolute powder density after exclusion of all the spaces (pores and voids) [37]. 2.3.6. Mass loss of RCA between 75 °C and 475 °C The chemically bonded water was estimated by determining the mass loss of RCA between 75 °C and 475 °C. This method is used to estimate the adherent cement paste content [38,39]. 100 g of each sample were dried at 75 °C for 24 h and then weighed to obtain the mass M75°C. Then they were heated at 475 °C for five hours and weighed again, to obtain the mass M475°C. A maximum temperature of 475 °C was chosen to avoid the decarbonation of carbonated hydrates [38,39] and to determine the water mass loss that corresponds to the hydrated phases. The temperature of 75 °C has been chosen instead of 110 °C to avoid losing of chemically bound water from adhered cement paste [35]. The
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mass loss proportion noted DML75°C–475°C can then be calculated according to the Eq. (2). This method does not allow for the determination of adherent cement paste content but to a quantity that is proportional to it. It allowed therefore comparing the cement paste content that is present in the different fractions separated with the water JIG.
DML75 C475 C ¼ 100 ðM75 C M 475 C Þ=M 75 C
ð2Þ
where, DML75°C–475°C is the proportion of the oven dried mass loss between 75 °C and 475 °C, M75°C is the oven dried mass at 75 °C and M475°C is the oven dried mass at 475 °C. 2.4. Jigging procedure At first, the 120 kg feed RCA was sorted in six batches of jigging, noted JIG 1, JIG 2, JIG 3, JIG 4, JIG 5 and JIG 6, with a mass of 20 kg of RCA each filling the box up to about 15 cm. Each batch was separated in six layers of 2.5 cm called L1 (bottom) to L6 (top) to obtain the ‘‘Generation 1” sorted RCA: L1, L2, L3, L4, L5 and L6 like schematized in Fig. 2. To finish the first step of jigging, the water absorption (WA) and densities (qrd and qa) of the ‘‘Generation 1” sorted RCA from the JIG 1, JIG 2, JIG 3, and JIG 4 were determined with the standard test NF EN 1097-6. In the second step of jigging, the layers corresponding to the same level in the six jigs were gathered, mixed, and submitted again to separation. The new six runs of jigging: JIG L1 – JIG L2 – JIG L3 – JIG L4 – JIG L5 and JIG L6, like schematized in Fig. 2, produced 36 ‘‘Generation 2” sorted RCA: LiLj with i = j=1 to 6. For all the ‘‘Generation 2” sorted RCA the water absorption (WA) and densities (qrd and qa) were determined according to the standard test NF EN 1097-6. In the final step, the most homogenous samples have been chosen from Generation 2 and then placed in new JIG to confirm the heterogeneity of the RCA. Six ‘‘Generation 2” sorted RCA (L3L3, L3L4, L3L5, L5L3, L5L4 and L5L5) having closed WA values were therefore mixed together and separated a third time to obtain 6 layers ‘‘Generation 3” sorted RCA. The standard test NF EN 1097-6 was used to measure the water absorption and density of these ‘‘Generation 3” sorted RCA. Complementary characterization of the feed RCA and some sorted RCA was made: porosity, mass loss proportion between 75 °C and 475 °C (proportional to cement paste content), particle size distribution and pore size distribution (Mercury Intrusion Porosimetry). Table 1 summarizes all the characterization tests performed for feed and sorted RCA.
3. Results and discussion 3.1. Densities Figs. 3 and 4 show the results of real dried density qrd determined for ‘‘Generation 1” sorted RCA and ‘‘Generation 2” sorted RCA respectively. As can be seen, qrd decreased with the height of the layers in the jig. For ‘‘Generation1” sorted RCA, as shown in Fig. 3, qrd of layer 1 of all jigs varied from 2.41 g/cm3 to 2.44 g/cm3 and varied from 2.08 g/cm3 to 2.15 g/cm3 for layer 6. For ‘‘Generation 2” sorted RCA, as shown in Fig. 4, for JIG L1, qrd differed from layer 1 (qrd = 2.56 g/cm3) to layer 6 (qrd = 2.30 g/ cm3). For JIG L6, qrd differed from layer 1 (qrd = 2.30 g/cm3) to layer 6 (qrd = 2.02 g/cm3). Thus, the additional cycle of jig allowed a better separation of the RCA. This interpretation of the result holds for all the layers after Generation 2.
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Fig. 2. Experimental procedure of water jigging.
As seen in the literature, the real dried density qrd of most natural aggregates lies between 2.40 and 2.60 g/cm3. It ranges between 1.60 and 2.00 g/cm3 for harden cement paste [1,15,37]. Moreover, literature shows that cement paste content of RCA can vary depending on the original concrete composition and particle size. The real dried densities qrd of ‘‘Generation 2” sorted RCA in layer 1 spreading over the range 2.30 g/cm3 to 2.56 g/cm3 were close to those of natural aggregates. For layer 6 of ‘‘Generation 2” sorted RCA, the real dried densities spread over the range of 2.02 g/cm3 to 2.30 g/cm3; these values show that the ‘‘Generation 2” sorted RCA in layer 6 had significantly larger amounts of cement paste than the ‘‘Generation 2” sorted RCA in layer 1. 3.2. Particle size distribution Fig. 5 presents the particle size distribution taken from the results of VDG of ‘‘Generation 2” sorted RCA. Only five grading curves amongst 22 results are presented for clarity.
Little differences were observed between the grading curves. However, they seem arranged according to the height of the layer and proportional to real dried density, suggesting that separation in the JIG was due both to density and particle size. To confirm this observation, the D50 was determined for each generation of jig tests. Fig. 6 shows the variation of D50 (size for which the cumulative function is equal to 50%) as a function of real dried density of heterogeneous RCA (before any jig) and for the RCA products of generation 1 and 2 of jig tests. As it can be seen in the graph, the D50 was smaller for the less dense aggregates. D50 varied linearly with qrd with R2 = 0.99 and 0.94 for ‘‘Generation 1” sorted RCA and ‘‘Generation 2” sorted RCA respectively. However, differences in size were small; D50 varied between 8.4 mm and 9.2 mm, showing that the separation may be considered essentially by density. The influence of particle size is negligible compared to that of density owing to the fact that a narrow granular class has been chosen for the study.
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E. Khoury et al. / Construction and Building Materials 175 (2018) 705–713 Table 1 Summary of characterization tests of feed and sorted RCA. Characterization of sorted RCA
Feed RCA ‘‘Generation 1” sorted RCA from
Water Absorption/ Densities (NF EN 1097-6) WA
Grading Curves with Video grader (VDG)
JIG JIG JIG JIG
1 2 3 4
x x x x x
x x x x x
‘‘Generation 2” sorted RCA from
JIG JIG JIG JIG JIG JIG
L1 L2 L3 L4 L5 L6
x x x x x x
x x x x x x
‘‘Generation 3” sorted RCA
JIG L3L5
x
x
Mercury Intrusion Porosimetry
Helium pycnometer
Mass Loss between 75 °C and 475 °C
8 sorted RCA having different WA
8 sorted RCA having different WA
x x x x x x
Fig. 3. Real dried densities (qrd) for ‘‘Generation 1” sorted RCA.
Fig. 5. Particle size distributions for five distinguished ‘‘Generation 2” sorted RCA.
Fig. 4. Real dried densities (qrd) for ‘‘Generation 2” sorted RCA.
3.3. Relationship between cement paste content and real dried density Fig. 7 shows the correlation between the proportion of chemically bonded water (DML75°C–475°C) and the real dried density (g/ cm3) (standard method EN 1097-6) of the ‘‘Generation 2” sorted RCA. It can be seen that, when DML75°C–475°C decreased, real dried density increased. In addition, the real dried density varied linearly with DML75°C-475°C with a determination factor equal to R2 = 0.87. The same results were also observed in [38,39]. This mass loss is proportional to the cement paste content in RCA. The measurement of the chemically bound water is therefore an indirect measure of the cement paste content. These results confirmed that the density variation in a given granular class RCA was mainly due to different paste contents.
Fig. 6. Variation of the diameter D50 of the ‘‘Generation 1” and ‘‘Generation 2” sorted RCA as a function of their real dried density (g/cm3).
3.4. Water absorption Table 2 summarizes the results of WA, the weighted average of WA for each level of all jigs and the corresponding standard deviation of WA of ‘‘Generation 1” sorted RCA. In addition, Fig. 8 shows the variation of WA of ‘‘Generation 1” sorted RCA produced from the six layers of four tests of jigging. And, Fig. 9 shows the variation of WA (24h) of ‘‘Generation 2” sorted RCA. The WA of ‘‘Generation 1” sorted RCA varied from 3.1% to 8.3%. And, the WA of ‘‘Generation 2” sorted RCA varied from 1.8% to 9.1%. As seen in the results of densities as well, the additional cycle of jig
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Fig. 7. DML75°C–475°C as function of real dried density (g/cm3) of ‘‘Generation 2” sorted RCA.
allowed a better separation of the RCA. The lower limit corresponded approximately to the water absorption capacity of natural aggregates and the upper limit to the WA of RCA with large amount of adhered cement paste. These later values of WA were comparable to those of some recycled sand with water absorption capacity varying between 8 and 10% [38–40]. The WA of ‘‘Generation 2” sorted RCA produced from L3, L4, and L5 of JIG L3 and JIG L5 varies between 6.2% and 6.8% with an average around 6.5%. These RCA were put together in another JIG to form the Generation 3. The WA of ‘‘Generation 3” sorted RCA varied from 4.5% to 8.2% with an average of WA for all layers about 6.42%. So, after 2 stages of s, the RCA still showed a large heterogeneity. The results of WA after all the three cycles of jigging showed a lot of variability in a sample of 120 kg of 6.3/10 mm RCA. It can be concluded that the water jig made the separation of these very heterogeneous aggregates possible and thus offered a real possibility to characterize them very precisely (even if in routine control it cannot be envisaged). 3.5. Porosity According to the literature, RCA are much more porous than natural ones, mainly as a consequence of the amount of old cement paste in the RCA. Fig. 10 presents the pore size distribution of ‘‘Generation 2” sorted RCA. It shows that the porous network of studied RCA was composed of two classes of pores [0.01–0.1] and [0.1–10] mm. As expected, RCA with high WA and lower density has the highest volume of pores, but the distribution of the size of pores remained mostly similar. Also, the amount of pores of the sample with low WA in this range was very low, which seems conforming that this aggregate was mostly composed from natural aggregate and had little amount of attached mortar. 3.6. Sample alteration by jigging The packing densities in Table 3 were determined for the three generations of jigging. The packing density was determined after
Fig. 8. Variation of water absorption of ‘‘Generation 1” sorted RCA as a function of height of the layer in the JIG.
weighting known volumes and real dried density of each generation sorted RCA. The packing densities presented are the average of packing densities of products of all the layers of each jig generation. The standard deviation of these measures is also indicated. One can statistically conclude that packing density was constant before and after jigging. So, it can be supposed that the shape of RCA was not significantly by jigging in all generations (1, 2, and 3). It was also noted that jigging did not influenced (or very slightly) the weighted average WA before and after the second jigging (5.35% and 5.26% respectively) or the third jigging (6.46% and 6.42% respectively) – see the results in Section 3.4. It is true that a significant deviation was observed before and after the first jigging. Indeed, the WA determined according to NF EN1097-6 and using representative samples of heterogeneous RCA before jigging was, in mean of 4 samples, 4.88%. This error could be explain by the high level of density heterogeneity in the feed material. The standard sampling method, defined for natural aggregates, should probably be better adapted to this particularity of the RCA. 4. Discussion 4.1. Accessible and non-accessible porosity Fig. 11 shows the densities of ‘‘Generation 2” sorted RCA determined by different protocols as a function of their WA. It can be observed linear variations. All the curves intersect approximately for WA = 0% at an average value around 2.68 g/cm3. This value could reasonably be considered having the magnitude of the density of original natural aggregates from the RCA. Using these densities, accessible and non-accessible porosities can be calculated for the different fluids used during the porosity tests, in the corresponding protocol used to measure the density. For example, the difference in skeletal volumes determined in mercury and in water is due to the fact that water and mercury will not
Table 2 Water absorption of « Generation 1 » sorted RCA. Level (from bottom to top)
L1 L2 L3 L4 L5 L6 Weighted Average of WA/jig (%) Total weighted Average of all tests (%)
WA of « Generation 1 » sorted RCA (%) JIG1
JIG2
JIG3
JIG4
3.3 4 5.7 6 6.1 7.4 5.35 5.35
3.4 3.9 5.2 5.5 5.9 7.9 5.29
3.1 4 5.6 6.2 6 7.1 5.3
3.2 4.2 5.7 5.6 6.1 8.3 5.46
Weighted Average of WA/level (%)
Standard deviation of WA/level (%)
3.25 4.03 5.55 5.83 6.02 7.67
0.13 0.13 0.24 0.33 0.1 0.53
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Fig. 9. Variation of water absorption coefficient WA of ‘‘Generation 2” sorted RCA.
Table 3 Packing densities of RCA produced after each jig generation. Jig generation
Compacity
1 2 3
0.387 ± 0.021 0.382 ± 0.024 0.378 ± 0.005
fill the same volume of pores during the experiment (water being allowed to enter smaller pores than mercury). The proportion of non-accessible porosity in water to the total porosity of the sample is equal to 10.3%, whereas it is 26.4% in Mercury. Fig. 10. Pore size distribution of eight samples between ‘‘Generation 2” sorted RCA.
Fig. 11. Different densities for ‘‘Generation 2” sorted RCA as function of WA with the equations and coefficient of determination R2 for each curve of each density.
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Fig. 12. Volume distribution (the axis in the left side) and cumulative volume distribution (the axis in the right side) of WA of 120 kg of 6.3/10 RCA.
4.2. WA distribution in the feed RCA Fig. 12 shows the WA probability distribution function in volume and the cumulative volume distribution of the 120 kg of 6.3/10 RCA. The graphs were dressed according to the results of WA of ‘‘Generation 2” sorted RCA, by giving to the material of each jig box the normalized standard deviation observed after the ‘‘Generation 3” jigging. It can be seen in the figure that WA follows a log-normal distribution with an average of 5.2% and a standard deviation of 1.6% showing a large heterogeneity of the material. 5. Conclusions Previous studies have investigated the variability of recycled concrete aggregates and showed the influence of the size fraction on the density and mortar content of the crushed concrete particles. It is generally admitted that the finer the recycled concrete aggregates the higher is the quantity of adhered cement paste present in its composition. The results of this study highlight that even a narrow granular class (here 6.3/10 mm) RCA can exhibit very large heterogeneity in terms of density and water absorption coefficient. It is proved that very large disparities (aggregates with almost 100% natural aggregates or grains with large amounts of adhered cement paste) may be present. The distribution of WA of a given RCA granular class is shown to follow a log-normal distribution with a high standard deviation. These disparities are at the origin of the dispersion of the results of characterization tests with RCA. This study also shows that a water jig is very efficient to sort a given granular class RCA. It helps to a better characterization of the RCA (useful for mercury porosimetry for example). The jig could be an excellent tool for separating materials in the laboratory and improving their characterization, but its industrial use would be a source of significant additional cost. Given the results presented here, it is suggested that the standard sampling method, defined for natural aggregates, should be better adapted to the particularity of the recycled concrete aggregates. Conflict of interest The authors declare that they have no conflict of interest. References [1] R. Cardoso, R.V. Silva, J. de Brito, R. Dhir, Use of recycled aggregates from construction and demolition waste in geotechnical applications: a literature review, Waste Manag. 49 (2016) 131–145. [2] A. Coelho, J. De Brito, Handbook of Recycled Concrete and Demolition Waste, Elsevier, 2013.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Heterogeneity of recycled concrete aggregates, an intrinsic variability Eliane Khoury a,b,⇑, Weslei Ambrós a,c, Bogdan Cazacliu a, Carlos Hoffmann Sampaio c, Sébastien Remond b a
IFSTTAR, Aggregates and Materials Processing Laboratory, Route de Bouaye-CS4, 44344 Bouguenais Cedex, Nantes, France IMT Lille Douai, Univ. Lille, EA 4515 – LGCgE – Laboratoire de Génie Civil et géoEnvironnement, département Génie Civil & Environnemental, F-59000 Lille, France c Mineral Processing Laboratory, Federal University of Rio Grande do Sul, 9500 Bento Gonçalves Avenue, 91501-970 Porto Alegre, Brazil b
h i g h l i g h t s Heterogeneity of Recycled concrete aggregates (RCA) is one of their critical properties. RCA sorted by density were characterized. A given granular class of RCA can exhibit very large physical heterogeneities. Heterogeneity of RCA mainly depends on their cement paste content. Water jig is a very efficient method to sort RCA by density.
a r t i c l e
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Article history: Received 23 January 2018 Received in revised form 17 April 2018 Accepted 20 April 2018
Keywords: Recycled concrete aggregates Water jig Sorting Heterogeneity Cement paste content Water absorption Density
a b s t r a c t Recycled concrete aggregates (RCA) are composed of two different materials: natural aggregate and attached cement paste. It is generally admitted that finer the RCA greater is the quantity of adhered cement paste. In this study it is shown that for a given granular class, very large disparities may be present in the adhered cement paste content, which could generate dispersion of the results of characterization tests of RCA. The heterogeneity of a narrow granular class of coarse RCA (6.3/10 mm) was investigated by sorting samples according to their densities using a water jig. The water jig sorting was essentially a separation by density and not by size. It was shown that the average water absorption after an immersion in water during 24 h was about 5% for a representative sample of the feed RCA, before sorting tests, while for the homogenous specimens separated by density, the water absorption ranged from 2% to 9%. The distribution of the water absorption in a sample of 120 kg of RCA was well estimated by a log-normal distribution. This intrinsic heterogeneity of RCA implies being able to have representative samples with sufficient accuracy, not only concerning the size but also the mineral composition of grains in the stockpiles of RCA. So, after discussing the results of this study, water jig appeared to be an efficient tool to separate RCA and obtain representative samples for their characterization. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction Many countries have made great efforts to deal with large quantities of construction and demolition wastes (CDW) in order to reintroduce them into the construction life cycle. However, the reuse of these wastes to produce new concrete has not been widely adopted yet, and landfill is still the main solution for removing these materials [1,2]. On the other hand, different studies have been conducted to develop concrete production with recycled concrete aggregates (RCA) that comply with applicable codes and standards [3–19]. Until now, only a small percentage of natural ⇑ Corresponding author at: IFSTTAR, Aggregates and Materials Processing Laboratory, Route de Bouaye-CS4, 44344 Bouguenais Cedex, Nantes, France. E-mail address: [email protected] (E. Khoury). https://doi.org/10.1016/j.conbuildmat.2018.04.163 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
aggregates were replaced by these materials to formulate concrete. The difficulty in the use of RCA in new concrete is related to their high water absorption and its temporal heterogeneity. Due to their high porosity [5,11], RCA absorb part of the mixing water which may reduce concrete workability unless the absorbed water is taken in account in the mix design. A poor estimation of the water absorption coefficient (WA) of RCA leads to inadequate effective water for the recycled concrete, the latter induces poor mechanical properties (excess in the mixing water) or poor workability of recycled concretes (lack of water). Pre-saturation of RCA before mixing could appear as a potential technical solution. However, presaturation before mixing is a complex task that is most generally not applied in recycled concrete manufacturing. Therefore, better knowledge of in-situ RCA water absorption capacity is necessary.
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More generally, sophisticated characterization (mineral composition, particle size distribution, particle shape, abrasion, density, water absorption and cement mortar content) must be carried out to get a better understanding of the effects of RCA on recycled concrete production and performance [20–24]. Thus, the first step of characterization is to extract a representative sample and determine various properties according to the intended recycling paths [25]. Natural aggregates may present heterogeneities, but the procedures described in European standards for sampling and characterization make it possible to produce materials that are sufficiently consistent for the purposes of construction. On the other hand, RCA are composed of natural aggregates but also of hardened cement paste and other impurities that are not homogeneously distributed in the different granular classes. Hence RCA have a much significant heterogeneity compared to natural aggregates. Therefore, the heterogeneity of recycled concrete aggregates limits their use due to the uncertainty in their expected behavior [13,14]. Indeed, a wide dispersion of results can be observed in the literature, mainly caused by the different sources and quality of original concretes that are recycled [15–18]. These results highlighted the heterogeneity and the inaccurate characterization measurement of RCA and confirmed the necessity to control their properties for different applications [13,19]. It was already demonstrated that the density increases and the adhered cement paste content decreases with the size fraction of the crushed concrete [26–28]. It was also shown that, in situ, crushed concrete obtained from CDW is always mixed with other crushed materials like brick, asphalt, gypsum, glass and so on. In the literature, jigs were proposed to concentrate the concrete particles and/or to diminish the impurities of a mixed recycled aggregate as a result of a density sorting [29–34]. The objective of this paper is to show that even in a given granular class of well controlled industrially manufactured coarse RCA, large disparities may be present in the adhered cement paste content, generating dispersion in the properties of the RCA. A laboratory water jig was used to sort crushed concrete aggregates with different densities in three generations. Homogeneous density samples of RCA were characterized in order to determine water absorption, granular size distribution, densities, porosity and cement paste content, to assess their statistical distribution in the original material.
mately 20 kg per batch (Fig. 1). The total mass studied was about 120 kg. The complete jigging device consists of a separating chamber supported on a perforated plate (Ø = 1 mm), which is allocated on a tank containing water, which in turn is pulsed into the jig container by means of an air flow controlled by a rotary piston valve and a knife gate valve. Thus, the jigging stroke and the frequency of pulsation can be regulated by adjusting the valves. During the tests, the particle bed within the jig container is submitted to numerous cycles of expansion and contraction by means of the pulse of water, which creates the conditions for particles of different densities to move with different vertical velocities. The net result is the bed stratification according to the particles density and/or particle size distribution with the concentration of the lighter fraction in the upper layers and the denser fraction in the lower layers of the bed. The separating chamber was assembled by overlying 6 layers of PlexiGlass (400 400 25 mm), so that after each test, the bed can be vertically sliced in 6 different fractions. For all test cases, the pulse frequency, bed expansion and jigging time were kept constant at 90 RPM (rotations per minute), 5 cm and 120 s, respectively. 2.3. Characterization of RCA 2.3.1. Water absorption WA The water absorption coefficient determined after 24 h of immersion in water, noted WA, was measured by the pycnometer method according to the European NF EN 1097-6 standard. It involves immersing a dry sample in a pycnometer filled with water to measure the sample mass increase due to the penetration of water in the pores. The WA was determined using Eq. (1)
WA ¼ ðM SSD M OD Þ=M OD
where MSSD is the Saturated Surface Dry (SSD) mass after 24 h of saturation in water, and MOD is 48 h-oven-dry mass (at 75 ± 5 °C) of the sample after the test. The drying temperature was fixed to 75 °C instead of 110 °C in order to avoid a loss of chemically bound water from adhered cement paste [35].
2. Experimental method 2.1. Materials 6.3/10 mm RCA sourced from the ‘‘Gonesse Recycling Platform” located in France and composed of 99% recycled concrete and 1% of other inert materials were used in this study. A narrow granular fraction was chosen in order to emphasize the differences in density rather than the differences in particle size. Main physical properties of the RCA employed were determined according to the NF EN 1097-6 standard. The average of the results of 4 tests on 4 samples of 1–2 kg each and the corresponding standard deviation were the following: - Water absorption coefficient (WA) 4.88 ± 0.11% - Real dried density in water (qrd) (NF EN 1097-6) 2.31 ± 0.01 g/ cm3 - Absolute density in water (qa) (NF EN 1097-6) 2.60 ± 0.01 g/ cm3. 2.2. Water jig The sorting of RCA was conducted in a batch scale water jig, model AlljigÒ S 400 from AllMineral with a capacity of approxi-
ð1Þ
Fig. 1. Water jig, AllMineral Company, model Alljig S 400/600X400Ò.
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2.3.2. Grading curves with video grader (VDG) The particle size distribution of RCA was characterized by an optoelectronic video grader consisting of a single camera video imaging [36]. This device determines the two main dimensions of the shadow of each particle, and then extrapolates the preceding two-dimensional characterization to a three dimensional particle shape with size expressed as an equivalent sieve diameter. This method is faster than the sieve analysis. 2.3.3. Densities measured in water According to the European standard NF EN 1097-6, two densities in water can be identified as following, - The real dried density (qrd), ratio obtained by dividing the mass of a sample of the oven-dried aggregate (drying after immersion) by the volume it occupies in water, including the volume of any closed pore and the volume of any pore accessible to water. This density is called also effective particle density in British Standards Institute (BSI) and envelope density in American Society for Testing and Materials (ASTM) [37] - The absolute density (qa), ratio obtained by dividing the mass of a dried aggregate sample in an oven by the volume it occupies in water, including the volume of any pore inaccessible to water (closed pore), but excluding the volume of water present in all pores accessible to water. This density is also called apparent particle density in British Standards Institute (BSI) and Skeletal density in American Society for Testing and Materials (ASTM) [37]. 2.3.4. Mercury Intrusion porosimetry The porosity and pore size distribution of the recycled aggregates were determined using mercury intrusion porosimetry (MIP). A porosimeter (Micrometrics Autopore IV) with low and high pressure positions was used for accessing pore radius between 1.8 nm and 60 mm. The MIP samples were small, about 10 g of material, representing only a few grains. In order to diminish the errors of sampling, this test was used here only for the less heterogeneous aggregates obtained after jigging. In addition, skeletal and envelope densities were determined in Mercury. These densities have the same definition as in water but the difference is simply due to the fact that the volume of accessible pores is not identical, pores less easily accessible with mercury. 2.3.5. Helium pycnometer For each sorted RCA, representative samples of 8 g ± 0.5 g were dried in the oven at a temperature of 75 °C and crushed (d 200 mm), then the specific density qS(gas) was measured by using a helium pycnometer (Micromeritics AccuPyc 1330). The samples were crushed to powder. Assuming that grinding allows the opening of all the non-accessible porosity, it can be considered that density measured in Helium pycnometer corresponds to the absolute powder density after exclusion of all the spaces (pores and voids) [37]. 2.3.6. Mass loss of RCA between 75 °C and 475 °C The chemically bonded water was estimated by determining the mass loss of RCA between 75 °C and 475 °C. This method is used to estimate the adherent cement paste content [38,39]. 100 g of each sample were dried at 75 °C for 24 h and then weighed to obtain the mass M75°C. Then they were heated at 475 °C for five hours and weighed again, to obtain the mass M475°C. A maximum temperature of 475 °C was chosen to avoid the decarbonation of carbonated hydrates [38,39] and to determine the water mass loss that corresponds to the hydrated phases. The temperature of 75 °C has been chosen instead of 110 °C to avoid losing of chemically bound water from adhered cement paste [35]. The
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mass loss proportion noted DML75°C–475°C can then be calculated according to the Eq. (2). This method does not allow for the determination of adherent cement paste content but to a quantity that is proportional to it. It allowed therefore comparing the cement paste content that is present in the different fractions separated with the water JIG.
DML75 C475 C ¼ 100 ðM75 C M 475 C Þ=M 75 C
ð2Þ
where, DML75°C–475°C is the proportion of the oven dried mass loss between 75 °C and 475 °C, M75°C is the oven dried mass at 75 °C and M475°C is the oven dried mass at 475 °C. 2.4. Jigging procedure At first, the 120 kg feed RCA was sorted in six batches of jigging, noted JIG 1, JIG 2, JIG 3, JIG 4, JIG 5 and JIG 6, with a mass of 20 kg of RCA each filling the box up to about 15 cm. Each batch was separated in six layers of 2.5 cm called L1 (bottom) to L6 (top) to obtain the ‘‘Generation 1” sorted RCA: L1, L2, L3, L4, L5 and L6 like schematized in Fig. 2. To finish the first step of jigging, the water absorption (WA) and densities (qrd and qa) of the ‘‘Generation 1” sorted RCA from the JIG 1, JIG 2, JIG 3, and JIG 4 were determined with the standard test NF EN 1097-6. In the second step of jigging, the layers corresponding to the same level in the six jigs were gathered, mixed, and submitted again to separation. The new six runs of jigging: JIG L1 – JIG L2 – JIG L3 – JIG L4 – JIG L5 and JIG L6, like schematized in Fig. 2, produced 36 ‘‘Generation 2” sorted RCA: LiLj with i = j=1 to 6. For all the ‘‘Generation 2” sorted RCA the water absorption (WA) and densities (qrd and qa) were determined according to the standard test NF EN 1097-6. In the final step, the most homogenous samples have been chosen from Generation 2 and then placed in new JIG to confirm the heterogeneity of the RCA. Six ‘‘Generation 2” sorted RCA (L3L3, L3L4, L3L5, L5L3, L5L4 and L5L5) having closed WA values were therefore mixed together and separated a third time to obtain 6 layers ‘‘Generation 3” sorted RCA. The standard test NF EN 1097-6 was used to measure the water absorption and density of these ‘‘Generation 3” sorted RCA. Complementary characterization of the feed RCA and some sorted RCA was made: porosity, mass loss proportion between 75 °C and 475 °C (proportional to cement paste content), particle size distribution and pore size distribution (Mercury Intrusion Porosimetry). Table 1 summarizes all the characterization tests performed for feed and sorted RCA.
3. Results and discussion 3.1. Densities Figs. 3 and 4 show the results of real dried density qrd determined for ‘‘Generation 1” sorted RCA and ‘‘Generation 2” sorted RCA respectively. As can be seen, qrd decreased with the height of the layers in the jig. For ‘‘Generation1” sorted RCA, as shown in Fig. 3, qrd of layer 1 of all jigs varied from 2.41 g/cm3 to 2.44 g/cm3 and varied from 2.08 g/cm3 to 2.15 g/cm3 for layer 6. For ‘‘Generation 2” sorted RCA, as shown in Fig. 4, for JIG L1, qrd differed from layer 1 (qrd = 2.56 g/cm3) to layer 6 (qrd = 2.30 g/ cm3). For JIG L6, qrd differed from layer 1 (qrd = 2.30 g/cm3) to layer 6 (qrd = 2.02 g/cm3). Thus, the additional cycle of jig allowed a better separation of the RCA. This interpretation of the result holds for all the layers after Generation 2.
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Fig. 2. Experimental procedure of water jigging.
As seen in the literature, the real dried density qrd of most natural aggregates lies between 2.40 and 2.60 g/cm3. It ranges between 1.60 and 2.00 g/cm3 for harden cement paste [1,15,37]. Moreover, literature shows that cement paste content of RCA can vary depending on the original concrete composition and particle size. The real dried densities qrd of ‘‘Generation 2” sorted RCA in layer 1 spreading over the range 2.30 g/cm3 to 2.56 g/cm3 were close to those of natural aggregates. For layer 6 of ‘‘Generation 2” sorted RCA, the real dried densities spread over the range of 2.02 g/cm3 to 2.30 g/cm3; these values show that the ‘‘Generation 2” sorted RCA in layer 6 had significantly larger amounts of cement paste than the ‘‘Generation 2” sorted RCA in layer 1. 3.2. Particle size distribution Fig. 5 presents the particle size distribution taken from the results of VDG of ‘‘Generation 2” sorted RCA. Only five grading curves amongst 22 results are presented for clarity.
Little differences were observed between the grading curves. However, they seem arranged according to the height of the layer and proportional to real dried density, suggesting that separation in the JIG was due both to density and particle size. To confirm this observation, the D50 was determined for each generation of jig tests. Fig. 6 shows the variation of D50 (size for which the cumulative function is equal to 50%) as a function of real dried density of heterogeneous RCA (before any jig) and for the RCA products of generation 1 and 2 of jig tests. As it can be seen in the graph, the D50 was smaller for the less dense aggregates. D50 varied linearly with qrd with R2 = 0.99 and 0.94 for ‘‘Generation 1” sorted RCA and ‘‘Generation 2” sorted RCA respectively. However, differences in size were small; D50 varied between 8.4 mm and 9.2 mm, showing that the separation may be considered essentially by density. The influence of particle size is negligible compared to that of density owing to the fact that a narrow granular class has been chosen for the study.
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E. Khoury et al. / Construction and Building Materials 175 (2018) 705–713 Table 1 Summary of characterization tests of feed and sorted RCA. Characterization of sorted RCA
Feed RCA ‘‘Generation 1” sorted RCA from
Water Absorption/ Densities (NF EN 1097-6) WA
Grading Curves with Video grader (VDG)
JIG JIG JIG JIG
1 2 3 4
x x x x x
x x x x x
‘‘Generation 2” sorted RCA from
JIG JIG JIG JIG JIG JIG
L1 L2 L3 L4 L5 L6
x x x x x x
x x x x x x
‘‘Generation 3” sorted RCA
JIG L3L5
x
x
Mercury Intrusion Porosimetry
Helium pycnometer
Mass Loss between 75 °C and 475 °C
8 sorted RCA having different WA
8 sorted RCA having different WA
x x x x x x
Fig. 3. Real dried densities (qrd) for ‘‘Generation 1” sorted RCA.
Fig. 5. Particle size distributions for five distinguished ‘‘Generation 2” sorted RCA.
Fig. 4. Real dried densities (qrd) for ‘‘Generation 2” sorted RCA.
3.3. Relationship between cement paste content and real dried density Fig. 7 shows the correlation between the proportion of chemically bonded water (DML75°C–475°C) and the real dried density (g/ cm3) (standard method EN 1097-6) of the ‘‘Generation 2” sorted RCA. It can be seen that, when DML75°C–475°C decreased, real dried density increased. In addition, the real dried density varied linearly with DML75°C-475°C with a determination factor equal to R2 = 0.87. The same results were also observed in [38,39]. This mass loss is proportional to the cement paste content in RCA. The measurement of the chemically bound water is therefore an indirect measure of the cement paste content. These results confirmed that the density variation in a given granular class RCA was mainly due to different paste contents.
Fig. 6. Variation of the diameter D50 of the ‘‘Generation 1” and ‘‘Generation 2” sorted RCA as a function of their real dried density (g/cm3).
3.4. Water absorption Table 2 summarizes the results of WA, the weighted average of WA for each level of all jigs and the corresponding standard deviation of WA of ‘‘Generation 1” sorted RCA. In addition, Fig. 8 shows the variation of WA of ‘‘Generation 1” sorted RCA produced from the six layers of four tests of jigging. And, Fig. 9 shows the variation of WA (24h) of ‘‘Generation 2” sorted RCA. The WA of ‘‘Generation 1” sorted RCA varied from 3.1% to 8.3%. And, the WA of ‘‘Generation 2” sorted RCA varied from 1.8% to 9.1%. As seen in the results of densities as well, the additional cycle of jig
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Fig. 7. DML75°C–475°C as function of real dried density (g/cm3) of ‘‘Generation 2” sorted RCA.
allowed a better separation of the RCA. The lower limit corresponded approximately to the water absorption capacity of natural aggregates and the upper limit to the WA of RCA with large amount of adhered cement paste. These later values of WA were comparable to those of some recycled sand with water absorption capacity varying between 8 and 10% [38–40]. The WA of ‘‘Generation 2” sorted RCA produced from L3, L4, and L5 of JIG L3 and JIG L5 varies between 6.2% and 6.8% with an average around 6.5%. These RCA were put together in another JIG to form the Generation 3. The WA of ‘‘Generation 3” sorted RCA varied from 4.5% to 8.2% with an average of WA for all layers about 6.42%. So, after 2 stages of s, the RCA still showed a large heterogeneity. The results of WA after all the three cycles of jigging showed a lot of variability in a sample of 120 kg of 6.3/10 mm RCA. It can be concluded that the water jig made the separation of these very heterogeneous aggregates possible and thus offered a real possibility to characterize them very precisely (even if in routine control it cannot be envisaged). 3.5. Porosity According to the literature, RCA are much more porous than natural ones, mainly as a consequence of the amount of old cement paste in the RCA. Fig. 10 presents the pore size distribution of ‘‘Generation 2” sorted RCA. It shows that the porous network of studied RCA was composed of two classes of pores [0.01–0.1] and [0.1–10] mm. As expected, RCA with high WA and lower density has the highest volume of pores, but the distribution of the size of pores remained mostly similar. Also, the amount of pores of the sample with low WA in this range was very low, which seems conforming that this aggregate was mostly composed from natural aggregate and had little amount of attached mortar. 3.6. Sample alteration by jigging The packing densities in Table 3 were determined for the three generations of jigging. The packing density was determined after
Fig. 8. Variation of water absorption of ‘‘Generation 1” sorted RCA as a function of height of the layer in the JIG.
weighting known volumes and real dried density of each generation sorted RCA. The packing densities presented are the average of packing densities of products of all the layers of each jig generation. The standard deviation of these measures is also indicated. One can statistically conclude that packing density was constant before and after jigging. So, it can be supposed that the shape of RCA was not significantly by jigging in all generations (1, 2, and 3). It was also noted that jigging did not influenced (or very slightly) the weighted average WA before and after the second jigging (5.35% and 5.26% respectively) or the third jigging (6.46% and 6.42% respectively) – see the results in Section 3.4. It is true that a significant deviation was observed before and after the first jigging. Indeed, the WA determined according to NF EN1097-6 and using representative samples of heterogeneous RCA before jigging was, in mean of 4 samples, 4.88%. This error could be explain by the high level of density heterogeneity in the feed material. The standard sampling method, defined for natural aggregates, should probably be better adapted to this particularity of the RCA. 4. Discussion 4.1. Accessible and non-accessible porosity Fig. 11 shows the densities of ‘‘Generation 2” sorted RCA determined by different protocols as a function of their WA. It can be observed linear variations. All the curves intersect approximately for WA = 0% at an average value around 2.68 g/cm3. This value could reasonably be considered having the magnitude of the density of original natural aggregates from the RCA. Using these densities, accessible and non-accessible porosities can be calculated for the different fluids used during the porosity tests, in the corresponding protocol used to measure the density. For example, the difference in skeletal volumes determined in mercury and in water is due to the fact that water and mercury will not
Table 2 Water absorption of « Generation 1 » sorted RCA. Level (from bottom to top)
L1 L2 L3 L4 L5 L6 Weighted Average of WA/jig (%) Total weighted Average of all tests (%)
WA of « Generation 1 » sorted RCA (%) JIG1
JIG2
JIG3
JIG4
3.3 4 5.7 6 6.1 7.4 5.35 5.35
3.4 3.9 5.2 5.5 5.9 7.9 5.29
3.1 4 5.6 6.2 6 7.1 5.3
3.2 4.2 5.7 5.6 6.1 8.3 5.46
Weighted Average of WA/level (%)
Standard deviation of WA/level (%)
3.25 4.03 5.55 5.83 6.02 7.67
0.13 0.13 0.24 0.33 0.1 0.53
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Fig. 9. Variation of water absorption coefficient WA of ‘‘Generation 2” sorted RCA.
Table 3 Packing densities of RCA produced after each jig generation. Jig generation
Compacity
1 2 3
0.387 ± 0.021 0.382 ± 0.024 0.378 ± 0.005
fill the same volume of pores during the experiment (water being allowed to enter smaller pores than mercury). The proportion of non-accessible porosity in water to the total porosity of the sample is equal to 10.3%, whereas it is 26.4% in Mercury. Fig. 10. Pore size distribution of eight samples between ‘‘Generation 2” sorted RCA.
Fig. 11. Different densities for ‘‘Generation 2” sorted RCA as function of WA with the equations and coefficient of determination R2 for each curve of each density.
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Fig. 12. Volume distribution (the axis in the left side) and cumulative volume distribution (the axis in the right side) of WA of 120 kg of 6.3/10 RCA.
4.2. WA distribution in the feed RCA Fig. 12 shows the WA probability distribution function in volume and the cumulative volume distribution of the 120 kg of 6.3/10 RCA. The graphs were dressed according to the results of WA of ‘‘Generation 2” sorted RCA, by giving to the material of each jig box the normalized standard deviation observed after the ‘‘Generation 3” jigging. It can be seen in the figure that WA follows a log-normal distribution with an average of 5.2% and a standard deviation of 1.6% showing a large heterogeneity of the material. 5. Conclusions Previous studies have investigated the variability of recycled concrete aggregates and showed the influence of the size fraction on the density and mortar content of the crushed concrete particles. It is generally admitted that the finer the recycled concrete aggregates the higher is the quantity of adhered cement paste present in its composition. The results of this study highlight that even a narrow granular class (here 6.3/10 mm) RCA can exhibit very large heterogeneity in terms of density and water absorption coefficient. It is proved that very large disparities (aggregates with almost 100% natural aggregates or grains with large amounts of adhered cement paste) may be present. The distribution of WA of a given RCA granular class is shown to follow a log-normal distribution with a high standard deviation. These disparities are at the origin of the dispersion of the results of characterization tests with RCA. This study also shows that a water jig is very efficient to sort a given granular class RCA. It helps to a better characterization of the RCA (useful for mercury porosimetry for example). The jig could be an excellent tool for separating materials in the laboratory and improving their characterization, but its industrial use would be a source of significant additional cost. Given the results presented here, it is suggested that the standard sampling method, defined for natural aggregates, should be better adapted to the particularity of the recycled concrete aggregates. Conflict of interest The authors declare that they have no conflict of interest. References [1] R. Cardoso, R.V. Silva, J. de Brito, R. Dhir, Use of recycled aggregates from construction and demolition waste in geotechnical applications: a literature review, Waste Manag. 49 (2016) 131–145. [2] A. Coelho, J. De Brito, Handbook of Recycled Concrete and Demolition Waste, Elsevier, 2013.
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