Mechanical Behaviour of Concrete with Recycled Aggregate

Transcription

1 Mechanical Behaviour of Concrete with Recycled Aggregate A. Caggiano 1, C. Faella 2, C. Lima 3, E. Martinelli 4, M. Mele 5, A. Pasqualini 6, R. Realfonzo 7, M. Valente 8 ABSTRACT: This study is aimed at evaluating the mechanical behaviour of recycled aggregate concretes (RAC). An extended experimental investigation has been carried out on RAC prepared by replacing variable amounts of virgin aggregates with recycled concrete aggregates (RCA) deriving from building demolitions. The mechanical properties measured on RAC specimens are compared to the corresponding ones obtained for a conventional concrete with 1% of natural aggregates. The watercement ratio (W/C) has been kept constant in all specimens. Some key concrete properties (i.e. strength and permeability) as well as some durability-related parameters have been measured with the aim of assessing feasibility and suitability of a RAC for structural use. Furthermore, the possible replacement of fine sand aggregates with Fly Ash has been also considered and compared. Stress-strain curves and the compressive strength values of both RACs and conventional concretes having different curing ages, have been evaluated and compared. 1 INTRODUCTION Nowadays, the problem of tacking care of the environment is getting more and more relevant and is conditioning all fields of the human activities. The industrial processes generally produce a huge quantity of wastes which require complex disposing procedures thus representing a great environmental issue. Also the construction industry is obviously involved in these problems. In particular, the processes of maintenance or demolition of existing structures (as buildings, bridges, etc) - as well as those of constructing new structures - cause the production of large quantities of hazardous wastes (well known as construction and demolition wastes C&DWs). For this reason, it is very appealing the hypothesis to recycle a large portion of these wastes by employing them as raw material for constructing new structures. The employ of wastes coming from demolished concrete structures or from industrial production of precasted concrete members, is a primary choice for obtaining recycled aggregates - generally indicated as RCA (acronym of "recycled concrete aggregate") - useful in producing new concrete products. Concretes produced with recycled aggregates are the subject of several papers recently published in the technical literature. Generally, performances of recycled aggregate concrete (RAC) are compared with those of concrete made of virgin aggregates. Several investigations point out as the physical and mechanical properties of a RAC are 1 PhD student, University of Salerno, Italy, ancaggiano@unisa.it 2 Full Professor, University of Salerno, Italy, c.faella@unisa.it 3 Research Assistant, University of Salerno, Italy, clima@unisa.it 4 Assistant Professor, University of Salerno, Italy, e.martinelli@unisa.it 5 Quality control manager, Calcestruzzi Irpini SpA, Avellino, Italy, m.mele@irpiniacalcestruzzi.it 6 Engineer, General Admixtures SpA, Ponzano Veneto (TV), Italy, apasqualini@gageneral.com 7 Associate Professor, University of Salerno, Italy, rrealfonzo@unisa.it 8 President of General Admixtures SpA, Ponzano Veneto (TV), Italy, mvalente@gageneral.com

2 strongly dependent on the quality (nature and sizing) of the recycled aggregate (Topcu, 1997). A RAC requires more water than a conventional natural aggregate concrete (NAC), in order to obtain the same workability (Casuccio et al., 28). Furthermore, material density, compressive strength and modulus of elasticity of a RAC are generally lower than those of a NAC (Corinaldesi and Moriconi, 29). Many international (RILEM Recommendation, 1994; ACI Committee 555, 22) and national codes (in Italy the Decree of the Ministry of Public Work, January 28) point out the importance and feasibility of the used of RACs as gravel aggregates. A further eco-friendly way to produce concrete consists in using of fly ash (FA) in addition or in partial substitution of Portland cement. Fly ash is a fine, glass-like powder recovered from gases created by coal-fired electric power generation. The use of FA in concrete production provides a twofold advantage: reduce the amount of industrial waste disposal; decrease the Portland cement demand and consequently the emission of CO 2 in the atmosphere. The use of FA combined with RCA in the concrete production allows improving the mechanical properties of a RAC making them comparable with those of a concrete made with virgin aggregate (Sani et al., 25). This paper presents the preliminary results from an experimental study on the performances of green concretes, i.e. of concrete made with RCA in partial or total substitution of virgin aggregates. Several NAC and RAC specimens were tested at Labs of the University of Salerno and of the General Admixtures SpA, in Ponzano Veneto. The behaviour of the considered concretes has been investigated at both fresh and hardened state. Some concrete specimens, made with aggregates coming from C&DWs of concrete structures, contained also fly ash, as addition or substitution to Portland cement. Since some properties of RCAs i.e. their surface shape, sizing, water absorption capacity - are rather different respect to those of the virgin ones, the mix-design approaches, aimed at achieving target properties at the fresh-state or at the hardenedstate, has been carefully revised and adapted. The durability-related issues and the mechanical properties of a RAC, deriving by the physical and chemical nature of the recycled aggregates have been also investigated. 2 EXPERIMENTAL PROGRAMME: MATERIALS AND MIXTURES Accurate screening and mechanical characterization of aggregates deriving from recycling processes represent the preliminary phases for obtaining a high quality RAC. In this work, several RAC specimens have been obtained by using aggregates coming from the demolition of a concrete building (located in the Emilia Romagna region, Italy); the debris have been selected, cleaned and sieved in laboratory. The preliminary sieve process has been carried out with the objective of establishing the granular distribution of the particles to be used as aggregates in RAC specimens. Figure 1 shows the screening and sieving phases. Figure1. sieve operations and sieved aggregates.

3 The aggregate fractions (sand, fine and coarse size of both natural and recycled) were mixed according to the Bolomey curve (Bolomey, 1975), with the aim of reproducing an optimal grain size distribution. Both natural (common crushed limestone) and recycled aggregates, with maximum diameter of 31.5 mm, were subdivided in four size-classes (see Figure 1b): N3, with nominal diameters ranging between 2 and 31.5 mm; N2, with nominal diameters ranging between 1 and 2 mm; N1, with nominal diameters ranging between 2 and 1 mm; sand, with nominal diameters smaller than 2 mm; The water absorption capacity (at 24h) of both natural and recycled aggregates was evaluated according to the procedures proposed by ASTM Standards C127 (for coarse aggregates) and C128 (for fine aggregates). Results from tests - shown in Table 1 - confirmed that the recycled aggregates absorb much more water than natural ones. Table 1. Water absorption (%) at 24h of both natural and recycled aggregates. Type Sand (-2 mm) N1 (2-1 mm) N2 (1-2 mm) N3 ( mm) Natural Recycled Eleven different types of concrete have been manufactured: a reference concrete made with 1% natural aggregates (labeled as NAC ); two concretes obtained by partially replacing the natural aggregates with recycled ones (i.e., RAC3 obtained by substituting the 3% in volume of virgin aggregates with RCAs; RAC6, with 6% of RCA and 4% of virgin one); a concrete made of 1% recycled concrete aggregates ( RAC1 ); three concretes having fly ash as substitute for the sand fraction, ( RAC3+FA, RAC6+FA, etc) keeping constant the amount of cement; four concretes made of recycled aggregates and fly ash used in partial substitution of the Portland cement amount ( NAC+FA sub, RAC3+FA sub, etc). The mix design of these concretes is outlined in Table 2. It has to be noted that a W/C equal to.56 has been considered; however, this ratio has been effectively used only in NAC production. In fact, in order to keep unchanged the water amount needed for the cement reaction, a slightly greater water-cement ratio has been used in producing RACs. The water surpluses needed in RAC mix design account for the larger water absorptions evidenced by recycled aggregates (Table 1). Table 2. Mixture proportions [in kn/m 3 ] and slumps [in mm]. NAC RAC3 RAC6 RAC1 RAC3 RAC6 RAC1 NAC RAC3 RAC6 RAC1 + FA + FA + FA +FA sub + FA sub + FA sub + FA sub Sand 8,99 9,4 7,56-8,24 7,48-7,5 8,91 7,4 - RCA (Sand) - - 1,16 7, ,24 - -,82 6,1 N1 1,34 1, , ,65 1, RCA (N1) - - 1,16 1,15-1,18 1, ,14 1,14 N2 4,21 3, , ,7 3,3 - - RCA (N2) - 1,1 3,66 3,62 1,1 3,72 3,72-1, 3,59 3,59 N3 4, , RCA (N3) - 4,6 3,99 3,94 4,5 4,6 4,6-4,1 3,95 3,91 Cement 3,9 3,11 3,5 3,2 3,1 3,1 3,1 2,5 2,78 2,78 2,78 FA ,6 1,2 2,4,8,6 1,2 2,4 Slump

4 Furthermore, in RAC+FA production an amount of fly ash increasing with the recycled aggregate percentages has been used. Portland cement, type CEMII/A-L42.5R (European Standards EN-197/1, 2), has been always used; its weight per unit volume was 3,3 kn/m 3. An acrylic-based superplasticizer ( PRiMIUM RM28 by General Admixtures SpA) has been also used to achieve a good workability of the fresh concrete. The Table also reports the mean value of the measured slumps (in mm). The materials utilized in the concrete mixtures are characterized by the values of bulk specific gravity reported in Table 3. Figure 2 shows the slump tests performed for the conventional concrete + fly ash and the RAC 1 fresh mixture. Table 3. Densities [kn/m 3 ] Natural Aggregate Recycled Aggregate Cement Fly Ash 26,9 23,69 3,3 21, Figure 2. Slump test: NAC+FA sub and RAC1 3 EXPERIMENTAL PROGRAMME AND TEST RESULTS For each type of concrete fourteen 15x15x15 mm cube specimens were cast in polyurethane forms (EN , 23) and cured at 22 C with about 1% humidity. Table 4 shows the test matrix of the whole proposed experimental programme; the Table reports indications about tests completely or partially performed. The mechanical properties of concretes were evaluated by means of compression tests at 2, 7, 28 and 6 days of curing. Table 4. Experimental program. Mixtures # Compression tests Permeability 6 days tests () NAC RAC RAC RAC RAC3+FA* RAC6+FA* RAC1+FA* NAC+FA sub RAC3+FA sub ** RAC6+FA sub ** RAC1+FA sub ** * tests partially done; ** tests not done

5 According to the EN UNI (22), a measure of the concrete durability was obtained by performing permeability tests at (see Figure 3). The permeability was determined by injection of water at a constant pressure P i = 5 bar. Figure 3. Some concrete specimens during and after the permeability tests. The preliminary experimental results are presented in Table 5 which reports the mean values of the compressive strength and the coefficients of permeability (the experimental campaign is in progress and the remaining tests will be performed in the near future). Mixtures R cm Table 5. Test results. Compression Strength [MPa] R cm R cm R cm 6 days Permeability [mm] NAC * RAC * 2.2 RAC * RAC * RAC 3 + FA * * RAC 6 + FA * * RAC 1 + FA * * NAC + FA sub * * RAC3 + FA sub * * * * * RAC6 + FA sub * * * * * RAC1 + FA sub * * * * * * to be performed Figure 4 shows the results in terms of compressive strengths for different curing times. The available experimental results demonstrate that the substitution of virgin aggregate with RCA significantly affects the compressive strength (for each curing age); larger the RCA amount is, lower the compressive strength is R cm [MPa] 2 1 NAC NAC + FAsub RAC3 RAC6 RAC1 RAC 3 + FA RAC 6 + FA RAC 1 + FA Figure 4. Compressive strengths at 2, 7 and.

6 Nevertheless, the concrete strength of a RAC mixture can be significantly improved by partially substituting the fine fraction of natural aggregate with fly ash; in the case of a RAC1, the improvement of the concrete compressive strength due to the fly ash contribute is considerable (compare RAC1 with RAC1+FA ). Natural aggregate concrete specimens made with fly ashes in substitution of a certain amount of Portland cement ( NAC+FA sub ) have lower strengths than the reference ones ( NAC ) at 2 and of curing, but they reach similar strength values at 28 days, probably due to the different times of reaction of the fly ash with respect to the cement. Figure 5 shows the densities of the considered concretes. It can be observed that the RAC density is always lower than the NAC one; this is in agreement with the observation that recycled aggregates are characterized by high percentages of internal pores. For the same reason the permeability index typically increases in the case of recycled concretes density [kn/m 3 ] RAC 3 + RAC 6 + RAC 1 + NAC NAC + FA RAC 3 RAC 6 RAC 1 FA FA FA avg. 24,1 23,29 23,22 22,8 2,19 22,99 22,7 2,25 Figure 5. Mean concrete densities. Figures 6 and 7 respectively shown the NAC and RAC specimens after compression tests, while the stress strain experimental curves are presented in Figures 8 and 9. Figure 6. Compression tests on NAC and on NAC+FA sub at -curing age. Figure 7. Compression tests at on RAC 1 and RAC 1 + FA.

7 NAC ,% NAC + FAsub ,% 2,%.% 3,% 1.% 2.% 3.% Figure 8. Compression tests on NAC and con NAC+FAsub : stress strain curves. RAC 3 RAC 3 + FA ,% ,% 2,%,% 3,% RAC 6 1,% 2,% RAC 6 + FA ,% ,% 2,%,% 3,% 1,% 2,% ,% RAC 1 + FA RAC ,% 3,% 1,% 2,% 3,%.% 1.% 2.% 3.% Figure 9. Compression tests on RAC and RAC+FA specimens: stress strain laws

8 Comparing Figures 8 and 9, a more ductile behaviour is observed for the RAC specimens. This observation confirms what reported in the literature, i.e. that concrete ductility is strongly dependent on the concrete quality (Van Mier, 1997). In particular, higher the concrete strength is, lower its ductility is. 4 CONCLUSIONS In this paper a study has been conducted in order to further develop the knowledge of using recycled aggregate concretes (RAC) for structural purposes. In particular, preliminary results from a wide experimental campaign on RACs have been presented and commented. The experimental results have confirmed that the use of aggregates coming from C&DWs of concrete structures in substitution of virgin aggregates leads to concretes having lower strengths and higher permeability. Nevertheless, the use of fly ash as additive in recycled aggregate concrete enhances its physical and mechanical properties thus mitigating the worsening effect of the recycled aggregates. REFERENCES ACI Committee 555 (22). "Removal and reuse of hardened concrete", ACI Material Journal, 99(3), ASTM C (21). "Standard Test Method for Density, Relative Density (Specific Gravity), and Absorption of Coarse Aggregate". ASTM C (21). "Standard Test Method for Density, Relative Density (Specific Gravity), and Absorption of Fine Aggregate". EN (2). "Cement part 1: composition, specifications and conformity criteria for common cements". EN (23). Testing hardened concrete. Part 3: compressive strength of test specimens. EN (22). Testing hardened concrete - Depth of penetration of water under pressure. Italian Ministry of Public Work (28), Decree of January 14 th : New Italian Code for Constructions (in Italian), Ordinary Supplement n. 3 to the Italian Official Journal of 4 February 28. Rilem recommendation (1994). "Specification for concrete with recycled aggregates", Mater. Struct., 27(173), Bolomey, J. (1975). "Revue Matèr Constr.", Trav. Publ. Edition C, p Corinaldesi, V. and Moriconi, G. (29). "Influence of mineral additions on the performance of 1% recycled aggregate concrete", Construction and Building Materials, 23, Casuccio, M., Torrijos, M.C., Giaccio, G. and Zerbino, R. (28). "Failure mechanism of recycled aggregate concrete", Construction and Building Materials, 22, Sani, D., Moriconi, G., Fava, G. and Corinaldesi, V. (25). "Leaching and mechanical behaviour of concrete manufactured with recycled aggregates", Waste Management, Topcu, I.B. (1997). "Physical and mechanical properties of concrete produced with waste concrete". Cem. Concr. Res., 27(12), Van Mier, J.G. (1997). Fracture Processes of Concrete, CRC Press.