using fine recycled aggregates from con- struction

USING FINE RECYCLED AGGREGATES FROM CONSTRUCTION AND DEMOLITION WASTE IN CONCRETE PRODUCTION: A STATE-OF-THE-ART REVIEW Jorge de Brito1, Luís Evangelista2 1. Instituto Superior Técnico/ICIST, Technical University of Lisbon, Portugal 2. Lisbon’s Polytechnic Engineering Institute (ISEL-IPL), Portugal

ABSTRACT. As the use of coarse recycled aggregates (CRA) from Construction and Demolition Waste (CDW) is becoming a common practice, researches now focus on different directions. One the most important lines of research currently being developed is related to the possibility of using fine recycled aggregates (FRA) from CDW as replacement (either partially or totally) of natural sands in concrete. This goal serves a greater environmental purpose, as it fights abiotic resources depletion, namely by reducing river banks and coastal sand extraction. Investigations on this matter have been set aside mostly because some early attempts on the use of these materials proved to be excessively detrimental to concrete’s performance. In this paper, an extensive review of the current level of knowledge on this subject is given, considering the latest information and results published worldwide regarding the behaviour of concrete made with fine recycled aggregates, as well as the most recent investigations carried out by the authors. It can now be stated with a significant degree of certainty that concrete made with fine recycled aggregates is feasible as structural concrete, considering the effect of the inherent properties of these materials on its design, production and application. Keywords: Construction and demolition waste, fine recycled aggregates, state-of-the-art. Jorge de Brito, MSc and PhD in civil engineering from IST. Full professor at IST and head of ICIST research centre. Member of IABSE, FIB, CIB and IABMAS. Research work: deterioration, rehabilitation and management of concrete structures; sustainable construction. Coauthor of over one hundred papers in referenced international journals. Luís Evangelista, member of ACI, graduated in Civil Engineering and MSc from IST, based on his research on the use of fine recycled concrete aggregate in concrete production. He is currently continuing his research on this subject for his PhD. Author of several papers related to the use of recycled aggregates in concrete production. Lecturer at Lisbon’s Polytechnic Institute of Engineering and also a structural designer.

INTRODUCTION As the world population grows to levels never reached before, the environmental concerns linked to that growth also increase. Furthermore there is a mass exodus of rural populations to urban centres, especially in emerging countries, with high demographic growth rates and concentration of inhabitants per km2 [1]. In order to accommodate the growth of cities that shelter these people there is a frantic growth of the construction industry, which becomes one of the motors of local economies. However, an unregulated and poorly directed urban development creates environmental issues greater than the solutions it provides [2] [3]. Knowing that the construction industry is an activity that greatly contributes to the present environmental unbalances, it is consensual that it is necessary to deeply change the existing paradigm, in order to: i) drastically reduce the consumption of non-renewable natural resources and ii) limit the dumping of waste from construction and demolition (CDW) that takes up ever more limited space. Since concrete is one of the materials most often used in the construction of new urban and transport infrastructures [4-6], huge amounts of aggregates are needed for its production. In order to understand the impact that the use of aggregates causes the worldwide consumption of concrete is quantified. Since the onset of the industrial revolution the consumption of nonrenewable natural resources has not stopped growing and unsustainable values are expected soon. In 2010 the yearly consumption of aggregates was 37400 millions of tonnes and this will increase to 48000 millions of tonnes until 2015 [7]. Since 90% of them are obtained from quarries and pits (41% come from sand and gravel extraction) and 45% of these are used in concrete [8], it is concluded that the yearly aggregates production (coarse and fine) used in concrete reached 152000 millions of tonnes. The extraction of sand and gravel reached around 7000 millions of tonnes. The uncontrolled consumption of natural sands has led to situations of exhaustion of availability of these aggregates, with various warnings that it is going to happen in several regions [9]. Resorting to crushed sands has shown to be unviable given the cost (fundamentally energyrelated) associated to its production as well as the fact that the shape of these particles is not the most suitable to achieve the best workability for concrete. On the other hand, the extraction of sans in coastal areas brings along deterioration of the ecosystems with unpredictable long-term repercussions, such as the elimination of local species and consequent unbalances [10]. Studies have been developed to colonize again the areas affected [11] that implied costs and increased the final cost of the sands. Furthermore, it is not guaranteed that the tides are able to renovate the topography of the sea bottom, which may lead to adverse effects in the adjoining coastal areas [12]. Besides the environmental problems associated to the extraction of natural resources the construction industry is also confronted with the environmental impacts of its waste, in the construction of infrastructures, in their maintenance and at the end of their life cycle. The management of CDW is a theme very present in the societal consciences of most civilized nations [1318]. Even though it is not easy to estimate the world production of CDW, due to lack of data from some of the greatest producers, the values that surfaced are worrying: in the USA it is estimated that CDW production nears 130 millions of tonnes per year, making up 35% of the waste generated [19]; in the European Union the international recycling federation (FIR) estimates that 380 millions of tonnes per year (2007 data) are produced, in contrast with the

850 millions of tonnes predicted by the European Commission [20]; in Chine, a country booming economically, data are unknown, but it is estimated that 325-345 millions of tonnes of CDW are produced per year [21]. Considering these values, it is an obligation to find solutions that allow reducing the consumption of natural resources and the dumping of waste. One of the ways of improving the environmental efficiency of the construction industry is to promote CDW recycling. The use of these materials as replacement of natural aggregates is an appealing alternative, considering the benefits that it brings to the two issues mentioned above. The use of recycled aggregates (RA) as replacement of natural aggregates in concrete production has been the object of research since the middle of the XX century [22] [23], but it has been intensified in the last two decades. The development of case studies [24] [25] and the growing commercial interest for these materials [26] [27] led to a growth of the scientific knowledge on the subject in what concerns the use of coarse recycled concrete aggregates (CRA) [28-33]. Even though advances reached in the study of concrete with coarse recycled aggregates have been enough to reliably say that it is possible to make concrete with CRA with perfectly acceptable characteristics for current use, the use of the fine fraction of these RA is still limited or banned. In the early reports the fine recycled aggregates (FRA) showed such detrimental characteristics that their use would lead to non-acceptable performance losses. The main causes for this weak performance of FRA were the low particles’ density, associated to a high water absorption capacity, the high level of contaminants, as well as the irregular shape of the particles, which hinders the mixing. RILEM’ 1994 recommendation [34] clearly states that the use of FRA is limited due to insufficient knowledge on the performance of concrete incorporating them. Consequently, most standards and recommendations available on the use of RA in concrete limit or even forbid the use of FRA. In the Gonçalves compilation [35], out of the national specifications and standards known, only the Swiss [36], Japanese [37] and Russian [38] ones allow the use up to 100% of FRA, if in unreinforced concrete (accepting up to 20% of FRA for classes up to C30/37 in Switzerland and strength glasses up to 18 MPa and 15 MPa in Japan and Russia respectively. In Brazil [39] it is possible to use up to 100% of FRA as long as concrete is non-structural, while in Denmark [40] 20% are allowed with a limitation of maximum strength of 20 MPa and 40 MPa, depending on whether the FRA result from CDW or concrete, respectively. In Holland [41-43] it is possible to use up to 100% of FRA if the coarse aggregates are natural (limited to non-aggressive environments and maximum strength classes of C20/25 and C40/50 for FRA from CDW and concrete, respectively). The remaining standards analysed (Germany [44], Hong-Kong [45], UK [46], Portugal [47] and Spain [48]) strictly forbid the use of FRA, whatever their nature or the concrete final application. More recent studies, such as those of, Leite [49], Khatib [50], Evangelista and de Brito [51, 52] and Kou and Poon [53], have shown that it is possible to make concrete with FRA with no significant performance loss. Therefore, in order to reverse the trend of restricting the use of FRA, it is necessary to keep on developing work on the analysis and characterization of FRA and the concrete made with them. In this paper the intent is to present a state-of-the-art review on the processing and characterization of FRA, as well as on the production and performance of concrete made with them.

PRODUCTION, TREATMENT AND PROPERTIES OF FRA Production and treatment Usually FRA are a product from the crushing of CDW and are an unwanted sub-product, containing high contaminants ratios [54, 55]. Bianchini et al. [56] refer that FRA may be used if these contaminants are separated at the onset of the CDW. Angulo [57] and Rodrigues et al. [58] refer that the CDW’s source and their crushing process significantly affect the FRA’s composition. Knowing that the quality of RA depends on their process of production and treatment [59] [60], some authors have developed methods to improve their performance to facilitate their use in concrete. Some studies on CRA’s improvement have been successfully developed, namely by application of ultra-sounds and immersion in silica fume solutions [61], surface agents [62] and nanosilica [63], but no similar studies are known using FRA. Ulsen [64] produced FRA using techniques from the separation and crushing of minerals, in order to improve them and obtained characteristics very close to those of fine natural aggregates (FNA). They used a tertiary crushing, consisting of an initial crushing at the recycling plant, followed by crushing with a jaw crusher and finally crushing using a vertical shaft impactor. The resulting product was benefited using attrition techniques to remove the cement paste and size grading studies of the FRA were performed both by density and magnetism. The results from chemical analysis, SEM, DRX and FRX showed that the FRA thus obtained showed rounder shapes and had lower contents of adhered mortar. Properties of the AFR The properties of FRA have been the object of various studies, even though seldom treated separately from those of the coarse fraction. This option often prevents the establishment of the differences in the influence of FRA and CRA on concrete. It is consensual that one of the most distinguishing characteristics between FNA and FRA is their density. The difference arises from the porous material adhered to the stone material particles (mortar, ceramics, plaster, among others) that not only decreases the density but it also increases the water retention capacity. The results from various authors range significantly, denoting a great scatter of this entity, directly linked with the nature of the recycled materials. The density values show variations between 1.89 g/cm3 [65] and 2.7 g/cm3 [66]. The water absorption has values between 4.3% [66] and 13.1% [51], depending on the FRA’s nature (from concrete (FRCA), masonry (FRMA) or undifferentiated (FRUA)). Table 1 shows the values from the literature review for the particle densities (oven-dry [ρs], saturated surface-dry [ρsss] and apparent [ρa]) and the water absorption (W). Fumoto and Yamada [67] obtained very similar characteristics of FRA from concrete structures of various ages (0, 45 and 70 years), seemingly indicating that this factor is not relevant. The microscopic analysis of FRA shows that the particles have more angular shapes than FNA (Figures 1 and 2 [69]). This characteristic is one of the main reasons for the loss of workability, for the same effective water/cement (w/c) ratio [51]. This greater angularity, coupled with the existence of open pores on the FRA’s surface, increases their specific surface, as demonstrated by Fumoto and Yamada [67]. These authors obtained values determined by BET (Brunauer, Emmett and Teller) up to 400% higher than those of FNA. This increase in specific surface is also probably the cause for the loss of efficacy of superplasticizers. Pereira et al. [72] noticed a loss of performance of the superplasticizers they used in concrete with fine recycled aggregates (CFRA), leading to believe that the polymeric chains have larger contact areas with the FRA than with the FNA. Kou [73] analysed the microstructure of FRA using MEV and found that it shows cracking linked to their production procedure (Figure 3).

Table 1 Density and water absorption of FRA from various studies Authors (s)

Type of FRA

ρs (g/cm3)

Hansen and Narud [68] Müeller and Winkler* [66] Kikushi et al. [69] Fumoto and Yamada [67] Katz [28] Khatib [50] Lyn et al. [70] Solyman [71] Evangelista and de Brito [51] Levy [65] Levy [65] Kou and Poon [53] Yaprak et al.[66] Pereira et al. [72]

FRCA FRUA FRCA FRCA FRCA FRCA FRCA FRCA FRCA FRCA FRMA FRCA FRCA FRCA

2.28 2.06-2.23 1.99-2.18 2.23 2.34 2.25 2.36 1.91 2.34 2.31 2.01

ρsss (g/cm3)

ρa (g/cm3)

2.31 -

2.66 -

2.48 2.17 2.32 1.89 2.23

2.56 2.56 2.57

W (%) 9.8 5.5 7.3-10.0 8.1-11.4 12.7 6.2 11.3 8.0 13.1 10.3 13.0 11.9 4.28 10.9

* Average value for 13 specimens of CDW; values for FRA+CRA

Figure 1 MEV of FNA [71]

Figure 2 MEV of FRA [71] Figure 3 Cracking in FRA [73]

The mineralogical analysis of FRA, both with XRD and XRF, shows a scatter inherent to the recycled materials’ nature. Solyman [71] obtained SiO2 contents between 60.1% and 81.1%, where the highest values are associated to the FRCA that went through two crushing phases. The samples showed CaO contents between 4.3% and 12.4% and the remaining oxides had residual values. Given the nature of FRA, the SO3 contents are insignificant (between 0.01% and 0.37%), thus demonstrating the absence of sulphates. In the case of FRUA the contents of the mineral present varies significantly. Rodrigues et al. [58] obtained gypsum contents in FRUA from Portuguese recycling plants between 0 and 1.2%, while the remaining phases (quartz, calcite, muscovite, K feldspar and Na feldspar) had high scatter. Ulsen et al. [74] determined the contents of the main oxides within FRUA subjected to improvement and obtained values between 65% and 75% for SiO2, between 7% and 11% for Al2O3, and around 2.5% for Fe2O3, proving the efficacy of FRA processing. New methods to determine the water absorption of FRA One of the main problems in the characterization of fine aggregates, especially FRA, is the determination of water absorption and consequently of the particles densities. The current test techniques devised for FNA are difficult to execute, particularly due to the presence of highly porous phases in the particles, sometimes cohesive. Tam et al. [75] report the main difficulties of current tests as: drying at 105 ± 5 ºC decomposes the products, by removing the con-

stituent water; the saturation time is very long and varies with the type and nature of the RA. Evangelista and de Brito [76] noticed that FRCA tested formed a weak mortar during saturation, making the test procedures very difficult. In order to solve these problems various researches have been developed. Gagnon [77] developed a method that allows quickly determining on site the particles density and water absorption of RA, using to that effect a densimeter and a micro-wave oven, obtaining reliable results except for RA containing bitumen. Leite [49] developed a method where the water absorption is obtained as the average of the absorptions of the oven-dry and the immersed material, using a hydrostatic scale with non-stop measurements. The method proposed provides only an approximation of the real absorption, since it is not possible to know the hydrostatic mass of the dried material. Pi [78] presented an automatic method to determine the particles density and water absorption of FRA and CRA using a device, called SSDetect, which detects the moment when the particles reach the saturated surface-dry condition, through the reflection of infrared rays. The author’s results show that the scatter of the parameters determined is substantially reduced and the repeatability of the test is significantly improved. Rodrigues et al. [79] proposed a method to determine the water absorption where the aggregates’ tests are performed in a solution of sodium hexametaphosphate (Na6P6O18), in order to prevent the agglomeration of particles. The results show that the use of this product prevents the formation of agglomerates, improving the reliability of the test. Kasemchairisi and Tangtermsirikul [80] propose that the parameters considered in the mixes is not water absorption but water retained, consisting on the sum of the water absorbed with that adsorbed on the FRA’s surface. In order to determine that variable they developed a method where the FRA are subjected to centrifugal forces that release the excess water in between particles. The results seem to be reliable with low standard deviations, thus seemingly indicating that the method is valid.

TECHNIQUES FOR MIXING CONCRETE WITH FRA A fundamental problem in designing and producing CFRA is the need to mind their high porosity, which predictably leads to adding supplementary water to the mix. Skipping this water demand will affect negatively the performance of concrete [67] [81], since part of the mixing water, needed to hydrate the cement and guarantee paste fluidity, will be absorbed by the FRA. If the FRA are saturated, the dynamics of the mixing may lead to a release of the water to the paste, indirectly increasing the w/c ratio. Ferreira et al. [82] concluded that concrete made with VRA subjected to pre-wetting in order to become only partially saturated contained more homogeneous paste-aggregate interfaces than those with saturated CRA. Leite [28] and Evangelista and de Brito [51] [52] made concrete with FRA pre-wetted with part of the mixing water, before introducing the remaining materials in the mixer. Leite analysed the interface between the FRA and the cement paste (Figures 4 and 5) and found out that it is more cohesive than with the FNA. Tamura et al. [83] proposed the improvement of the interface between the cement paste and the RA using their porosity and a vacuum decompression during mixing, followed by a sudden release. The authors managed to obtain substantially better interface bonding.

Figure 4 Interface between FNA and cement paste [28]

Figure 5 Interface between FRCA and cement paste [28]

Tam et al. [84] proposed a method to make concrete with RA called “Two stage mixing approach”. It is similar to that used by other authors [28] [51] [81] except for the fact that the RA are pre-wetted with part of the mixing water and cement, after which the remaining components are added to the mix. The results show that the method performs well [84-86].

PROPRETIES OF CONCRETE WITH FRA Mechanical properties The evaluation of the mechanical properties of CFRA is fundamental to characterize and rank these materials for structural purposes. Due to the presence of more porous and fragile particles, it is expected that CFRA have lower performances than conventional concrete. Initial works by Merlet and Pimienta [87] showed compressive strength losses between 19% and 38% for concrete with total incorporation of FRA and CRA. Leite [28] tested several concrete families with various replacement ratios of FNA by FRUA and w/c ratios. The author surprisingly obtained gains of around 10% in compressive strength for total replacement FNA-FRUA. Similar performances were obtained for tensile strength and modulus of elasticity (in both cases only for w/c ratios above 0.47), with gains of up to 32% and around 15%, respectively. The author justified this improvement with a better bond between the FRA and the new cement paste. For lower w/c ratios where results were the opposite, the author believes the mixes had hydration problems because the FRA absorbed too much mixing water. Khatib [50] tested concrete with replacement of FNA by FRCA and brick FRA (FRBA) up to 100%. The author obtained maximum losses of compressive strength of around 30% for concrete with FRBA and steady values for concrete with FRBA. The author justifies the second trend with the presence of reactive silica and alumina released when the bricks were crushed. In terms of the dynamic modulus of elasticity, the author obtained a similar loss for both types of concrete (losses up to 20%) for the mixes with total replacement. Solyman [71] studied various families of CFRA with FRCA and FRUA from German recycling plants. The results show that the mixes with FRCA had a similar compressive strength to those with FNA, with maximum losses of 6.5%, for full replacement of FNA. For the mixes with FRUA there were losses up to 29%, mostly due to the presence of non-mineral materials, such as asphalt. When analysing the mixes with various cement contents and w/c ratios, the author saw no distinct trends, i.e. the variations were similar, independently of the range of strength analysed. The tensile strength had losses up to around 20%, regardless of the FRA

used. The modulus of elasticity decreased for total replacement between 17% and 25%, depending on the type of FRA used. Evangelista and de Brito [51] studied concrete with FRCA and got results similar to those of Solyman [71]. The compressive strength remained practically constant (maximum losses of 8%), near 55 MPa, which was justified by the better bond between the new paste and the FRA and the eventual presence of non-hydrated cement. The tensile strength and the modulus of elasticity showed decreases of 30.5% and 18.5%, respectively. Kou and Poon [53] evaluated the performance of self-compacting concrete (SCC) with incorporation of FRCA and found that it was not affected for ratios between 25% and 40% of FRA. Furthermore, they obtained SCC with compressive strength around 64 MPa in mixes with integral replacement of FNA by FRA. Pereira et al. [72] [88] analysed the effect of superplasticizers in CFRA, with a regular performance plasticiser and a high-performance superplasticizer. The results showed there was an insignificant loss of compressive strength with the incorporation of FRA for current concrete and with the superplasticizer (less than 5%) and maximum losses of around 15% for the mixes with the lower performance plasticiser. The authors justified this difference with the lower robustness of the latter product, loosing efficacy with the increase of the specific surface of the FRA. For splitting tensile strength and modulus of elasticity, the authors had losses of performance between 15.6% and 24.3% and between 20.7% and 33%, respectively. In every situation the authors predicted that the use of small (extra) contents of plasticiser would be enough to offset the loss of performance due to the presence of FRA. Table 2 presents an overall analysis of the mechanical properties determined by various authors, providing a relative view of the performance of CFRA, compared to a reference concrete, made with FNA. The type of FRA used, as well as the maximum replacement rate (MRR) used in each of the investigations is also given. The vast majority of the researches yield general losses of mechanical performance, although the extent of these is uncertain. The latest researches tend to have better results than the earlier ones, most probably because of the new approaches to the mixes’ design and production currently used, which seem to better address FRA particularities. Table 2 Mechanical properties of CFRA from various studies

Authors

Type of FRA

Merlet and Pimienta [87] CRA + FRCA Leite [28] FRUA Khatib* [50] FRCA * Khatib [50] FRBA Solyman [71] FRCA Solyman [71] FRUA Evangelista and Brito [51] FRCA Kou and Poon† [53] FRCA Pereira et al [72] [88] FRCA

Variation of performance relative to the reference concrete (%)

MRR (%) 100 100 100 100 100 100 100 40 100

* The author determined the dynamic modulus of elasticity

†

Compressive strength

Tensile strength

Modulus of elasticity

-38 +10 -30 0 -6.5 -29 -8 0 -5 to -15

+32 -20 -20 -30.5 0 -16 to -24

+15 -20 -20 -17 -25 -18.5 0 -21 to -33

Self compacting concrete

Durability Merlet and Pimienta [87] determined the shrinkage of concrete with FRCA and found that the use of superplasticizers, as well as the pre-wetting of the FRCA, improved the concrete’s performance. The CFRA’s shrinkage was between 20% and 50% higher than that of concrete with FNA. Solyman [71] evaluated the shrinkage of CFRA and found that the average value after one year was 15% higher in mixes with FRCA than in those with FNA. This increase rose to 40% for mixes with FRUA. The carbonation of CFRA with FRUA was up to 17% higher after one year. Again FRCA had more interesting results with increases of around half of that. The freeze/thaw tests showed losses of performance between 60% and 143% for a maximum replacement ratio of 70% of FNA by FRA. For this requirement, the author suggests a maximum replacement ratio of 30%. Khatib [50] determined the shrinkage of concrete with FRCA and FRBA and found that the presence of these recycled fines deteriorates the concrete performance, since all mixes showed higher shrinkage than the control mix. For mixes with FRCA the increase was around 52% at 90 days for 100% replacement ratio. The mixes with FRBA showed random results and no conclusions could be drawn. Evangelista and de Brito [52] tested the water absorption by immersion and by capillarity of concrete with FRCA. They obtained excellent correlations between the replacement ratio and the absorption values, with increases of water absorption of 46% and 70.3% for immersion and capillarity respectively, for integral replacement of FNA by FRA. In the test of chloride ion penetration an increase of around 33% was found for mixes made with FRA only, while the 21-day carbonation increased around 110% for the same mixes. Levy and Helene [65] studied the effect of FRCA and FRMA in concrete with various ranges of strength. In each range various replacement ratios were tested in terms of water absorption, pores volume, carbonation and electrical resistivity. The results showed that for a given strength range those properties were similar, and in some cases they were slightly better in the CFRA than in the concrete with FNA. The authors justified this phenomenon with the need to add higher cement contents in the CFRA in order to reach the same strength. This increase of the cement content improves the matrix, leading to the results obtained. Zega and Di Maio [89] analysed concrete with FRCA in terms of waster absorption under pressure, by immersion and by capillarity. Complementarily, they determined the shrinkage of the same mixes. They found that the shrinkage of the various mixes (with 0%, 20% and 30% replacement of FNA by FRA) was similar, because the FRA absorbed mixing water and therefore the w/c dropped. The various water absorptions measured yielded similar results in the various families leading the authors to conclude that it is possible to produce concrete with durability demands with replacement ratios up to 30%. A summary of the results is provided in Table 3, where the values for each of the collected investigations are presented. A comparative analysis of the various investigations can become difficult, considering that there are several types of tests that can be made at different ages. However, it is consensual for all researchers that durability deteriorates with the presence of FRA.

Table 3 Durability performance of CFRA from various studies

Authors

Type of FRA

Merlet and Pimienta [87] CRA + FRCA Solyman [71] FRCA Solyman [71] FRUA Khatib [50] FRCA Evangelista and Brito [52] FRCA Evangelista and Brito [52] FRCA Zega and Di Maio [89] FRCA * 1 year

†

Variation of performance relative to the reference concrete (%) MRR (%) Water Shrinkage Carbonation absorption 100 100 100 100 100 100 30

+20 to +50 +15* +40* +52† 0

+46¥ +70§ 0

+8.5* +17* +110× +110× -

after 90 days × after 21 days ¥ by immersion § by capillarity

CONCLUSIONS Even though the use of coarse recycled aggregates in concrete production is an increasingly common practice, encouraged or imposed by the national authorities, the use of FRA, whatever their nature, is still very restricted or even barred, due to the lack of consolidated knowledge in this area. Nonetheless, the growing search for natural resources, namely stone, is creating an unsustainable environmental pressure and therefore it is urgent to change the existing paradigm of not allowing the use of FRA in concrete production. Present knowledge on this subject allows establishing that, if the specific characteristics of this material are considered, it is possible to produce concrete with acceptable quality using significant replacement ratios. In terms of RA processing (collection and production), there are treatment and beneficiation operations to improve the quality of FRA. The typical characteristics of this material are that it is lighter and more porous than FNA. Their mineralogical nature is directly linked with the original material and it is possible that FRA comply with the standards requirements. The characterization of FRA must bear in mind their greater porosity and there must a unified procedure to determine their water absorption, given the scatter of existing works/proposals. The production of concrete with FRA cannot be done the same way as for conventional concrete. The presence of more porous aggregates than the traditional ones imposes that the water absorption of the FRA must be taken into account to calibrate the w/c ratios.

ACKNOWLEDGMENTS The authors acknowledge the support of the Foundation for Science and Technology (FCT), namely for the scholarship of the second author, and of the ICIST research centre from IST.

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