Structural shear behaviour of recycled concrete with silica fume

Construction and Building Materials 23 (2009) 3406–3410

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Structural shear behaviour of recycled concrete with silica fume González-Fonteboa Belén *, Martínez-Abella Fernando 1, Martínez-Lage Isabel 2, Eiras-López Javier 3 The School of Civil Engineering, Department of Construction Engineering of the University of A Coruña, E.T.S.I. Caminos, Canales, Puertos, Campus Elviña s/n, 15071 La Coruña, Spain

a r t i c l e

i n f o

Article history: Received 28 July 2008 Received in revised form 9 June 2009 Accepted 18 June 2009 Available online 15 July 2009 Keywords: Recycled aggregates Recycled concrete Silica fume Structural behaviour Shear behaviour

a b s t r a c t This paper is the outcome of the second stage of studies carried out on the structural behaviour of recycled concrete. The first stage determined, using beams specimens, the shear behaviour of recycled concrete with which 50% of the coarse aggregate was replaced by recycled coarse aggregate (obtained from concrete demolition waste). This stage revealed minor differences (recycled concrete – conventional concrete) in ultimate loads and these differences were found to increase when cracking was taken into account. In this study, it was determined that the addition of 8% silica fume to recycled concrete (recycled concrete with silica fume) improved this behaviour. Moreover, the use of this material in recycled concrete produced changes in its structural behaviour similar to those induced in conventional concrete (conventional concrete with silica fume). Finally, experimental results were compared using current codes and it had been seen that all of them were conservative so t can be used for the shear design of recycled concrete beams with silica fume. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction

2. Interest in the proposed research and objectives

In the first stage of this research work [1] two concrete mixes were designed, a conventional one and a concrete mix containing 50% recycled coarse aggregate, both having similar compressive strengths and traction resistance. The first conclusion reached in this stage was that with the recycled aggregates used, it was possible to obtain recycled concrete with a mechanical behaviour similar to that of conventional concrete, as has been reported in other studies [2,3]. Next, for every concrete, four reinforced beams with a different amount of transverse reinforcement were made and were tested to failure. The results of the study of shear structural behaviour showed that there were only minor differences in behaviour in terms of ultimate load or deflections. However, the cracking analysis detected the formation of premature cracking and notable splitting cracks along the tension reinforcement. Therefore the introduction of stricter limits on minimum stirrup spacing was proposed as a possible solution. Lastly, in this stage, experimental results were compared using current codes, leading to the conclusion that all of them were conservative and could be used subsequently for the shear design of recycled concrete beams.

In keeping with the line of investigation described above, this work set out to determine the possibility of improving the behaviour of recycled concrete by the inclusion of a puzzolanic addition, silica fume. It was initially decided to examine the effect of this addition on the basic properties of recycled concrete [4]. This study showed that recycled concrete underwent changes similar to conventional concrete when this type of addition was used. The next step was to carry out a structural study that aimed to shed light on the structural behaviour of recycled concrete with silica fume, thus opening up a field that has received very little attention [5–7]. Another objective was to determine whether or not the addition of this percentage of silica fume to the recycled concrete would succeed in controlling the premature cracking detected in previous studies, thus avoiding the need to impose stricter limits on minimum stirrup spacing, which would result in reduced costs. Therefore, if this solution is found to be effective, the new recycled concretes with silica fume will open the way to a new field that will lead to a more sustainable construction. 3. Experimental program 3.1. Materials

* Corresponding author. Tel.: +34 981167000x1442; fax: +34 981167170. E-mail addresses: [email protected] (B. González-Fonteboa), [email protected] (F. Martínez-Abella), [email protected] (I. Martínez-Lage), [email protected] (J. EirasLópez). 1 Tel.: +34 981167000x1443; fax: +34 981167170. 2 Tel.: +34 981167000x1441; fax: +34 981167170. 3 Tel.: +34 981167000x5433; fax: +34 981167170. 0950-0618/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2009.06.035

This stage used the same materials as in the first stage with the addition of silica fume. Cement (C): Portland CEM I – 42,5R (RC-08 [8]) cement. An analysis of the physical, chemical and mechanical characteristics of the cement was carried out. The results were considered to be satisfactory taking the standards into account (RC-08).

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B. González-Fonteboa et al. / Construction and Building Materials 23 (2009) 3406–3410 Water-reducing admixture: a superplastizer (S) was used. to obtain a workability defined by slump values between 5 and 10 cm. Recycled aggregates: a size fraction 0–40 mm (0–40R) of Spanish material obtained from real demolition debris was used. This material, composed principally of concrete (72%) and stone (20%), was washed and screened into two size fractions (4–12 mm, 4–12R, and 10–25 mm, 10–25R) which were used as recycled coarse aggregates (RCA). The remaining size fractions were rejected (0–4R and 25–40R). Natural aggregates: to size fractions of crushed quartz (4–12 mm, 4–12C and 10–25 mm, 10–25C) were used as natural coarse aggregates (NCA). Crushed limestone with a maximum size of 4 mm (0–4C) was used as natural fine aggregate (FA). These natural aggregates were washed and screened in the plant. Reinforcement: The reinforcing bars were Spanish high ductility B 500 SD (EHE [9]), with a yield point stress of 500 MPa (tested 571 MPa) and a tensile strength of 690 MPa. Silica fume: The characteristics of this new material, silica fume (SF), are summarized in Table 1.

Table 3 and Fig. 1 show the details of the eight new beams tested until shear failure.

4. Experimental results The crack patterns that appeared during the test were marked and crack widths measured according to the procedures used in the first stage (Fig. 2). Beam load, deflections, reinforced bar strains and concrete strains were monitored in the same way as in the first stage. Deflections at the middle of each span (S6 and S8) and at the middle of the beam were recorded (Fig. 3 and 4), as were the shear strains (Fig. 5) at the middle of each span (measured by a pair of linear variable displacement transducers inclined at 45°). Continuous readings of the longitudinal reinforcement on the tension and compression sides were taken. And if the beam contained stirrups, the strains produced on them were also measured (Fig. 6). The strain–load curves of these graphs allow to obtaining the shear force at cracking and the shear force at stirrup yield. Concrete strains were measured with both unidirectional gauges (fitted at the same place as the gauges of the longitudinal reinforcement on the flexural compression side) and rosette gauges. Figs. 3 and 4 show that the recycled concrete beams always present greater deflections, in middle of the shear span and in the center of the beam, than those beams that have been made with conventional concrete. The deformability of the mortar adhered to the aggregate produces this phenomenon.

3.2. Mix proportions The proportions designed in stage 1 were maintained for the purpose of obtaining a reinforced concrete with a workability defined by slump values between 5 and 10 cm and subjected to exposure levels IIa and IIb (according to Spanish concrete code EHE [9]). Lastly, in keeping with the objectives of this research work, the amount of silica fume to be added to both the conventional and the recycled concrete was set at 8% of the weight of the cement. This resulted in the design of the conventional concrete with silica (CCS) and the recycled concrete with silica (RCS). Table 2 presents a summary of the proportion parameters. 3.3. Test specimens Eight new beams were made (four using CCS and four with RCS) to test until shear failure occurred. The same type of geometry and test procedures designed in the first stage were used.

Table 1 Physical and chemical characteristics of SF.

Limits Results

Density (t/m3)

Apparent density (t/m3)

Specific surface (m2/g)

Activity index (%) (28 days)

Loss to fire %

SO3 (%)

SiO2 (%)

Cl (%)

Álkali (%)

Free CaO (%)

– 2.35

– 0.58

>15 and <35 26.86

>100

<4 5.6

<2 0.3

P85 94

<0.1 0.03

– 0.63

<1 0.37

Table 2 Mix proportions.

CCS RCS a

0–4C%

4–12C%

4–12R%

10–25C%

10–25R%

SF/C kg/kg (%)

C (kg/m3)

W/C (kg/kg)

S/C kg/kg (%)

FA/CAa (kg/kg)

(CA + FA + SF)/C (kg/kg)

100 100

100 50

0 50

100 50

0 50

8.0 8.0

325 345

0.55 0.55

1.19 1.24

0.87 0.86

5.44 4.84

CA = NCA + RCA.

Table 3 Beam specimens. Beam Specimen

Material

V0CCS V0RCS V24CCS V24RCS V17CCS V17RCS V13CCS V13RCS

CCS RCS CCS RCS CCS RCS CCS RCS

a b c d e f

fc (MPa)

fct (MPa)

Asa

Ascb

ul = A s / (b  d) (%)

Transverse reinforcement (span S6)

ut6 = Atc/ (b  st) (%)

Transverse reinforcement (span S8)

ut8 = At/ (b  st) (%)

46.77 41.45 43.66 43.25 45.16 44.49 42.75 41.45

3.06 3.08 3.09 3.33 3.42 3.61 3.66 3.36

232 + 116

216

2.98

without web reinforcement

0.00

2.98

stirrups 26 spaced 240 mm

stirrups 28 spaced 100 mm

0.50

e

stirrups 28 spaced 240 mm

0.21

d

0.12

2.98

stirrups 26 spaced 170 mm

0.17

stirrups 28 spaced 170 mm

0.30

2.98

stirrups 26 spaced 130 mm

0.22f

stirrups 28 spaced 130 mm

0.39

As = area of longitudinal reinforcement on flexural tension side. Asc = area of longitudinal reinforcement on flexural compression side. At = area of the shear reinforcement perpendicular to flexural tension reinforcement. Minimum amount of shear reinforcement according to Spanish concrete code ðAt  fyd P 0:02  fcd  bÞ. Less than the minimum amount of shear reinforcement. More than the minimum amount of shear reinforcement.

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Fig. 1. Test geometry.

Fig. 2. Cracking at failure of beams.

500

SPA N S6

SPA N S6

SPA N S8

400 X=800

300

400

200

0 -10

-8

-6

-4

-2

0

2

4

6

8

10

Load (kN)

Load (kN)

CCS RCS

100

-100

CCS RCS

300

200

-12

SPA N S8

500

100 0 -20

-200

-100

-300

-200

-400

-300

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

2

4

6

8

10

12

14

-400

-500 Deflection at span S6 (mm)

Fig. 3. Deflections at the middle of the span S6.

-500 Deflection at the middle of the beam (mm)

Fig. 4. Deflections at the middle of the beam.

On the other hand, the shear deformation of recycled concrete beams with high quantity of reinforcement (V17 and V13) are lower than those obtained with conventional concrete being, however, higher in the case of beams with low transverse reinforcement (Fig. 5). In this case, comparing the results obtained with beams V0 and the different concretes, CCS, RCS, CC and RC, some of the first phase, it can be deduced that the shear deformations grow

up in the following order: V0CCS, V0RCS, V0CC, V0RC. This confirms the positive effect of the silica fume to control the deformability of the materials. All the beams suffered shear failure owing to diagonal traction. It was found that the splitting phenomena in the new beams designed with silica fume were of little significance. It was thus

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SPAN S6

240

V13

200

2.0

V0

X=800

1.9

160 120 Shear (kN)

Vy/Vcr

1.8

80

CCS RCS

40

1.6

0

-40 -6

-5

-4

-3

-2

-1

1

2

3

4

1.4

-120 -160 -240

V17

V24

Shear strain γ xy (mm/m)

VANO S6

350

X=885

300 250 200 150

1 2 3 L 1

100

SPAN S8

SPAN S6

50 0

0.15

0.18

0.21

Amount of transverse reinforcement (%) (SPAN S6)

it possible to determine the strength reserve (from the standpoint of computation) of the beams fabricated with the different types of concrete. Low values (Table 4) are generally observed when recycled aggregates are used, while higher values are obtained when silica fume is added. This confirms that, after cracking, concrete with recycled aggregate reached Vyield before those that do not incorporate these aggregates. Furthermore, it can be seen that the inclusion of silica fume (8% in the RCS) partially corrects this effect; thus, RCS beams present larger ratios (Vyield/Vcrack) than the RC beams, although without reaching those offered by the CC beams.

VANO S8

X=885

1.3 0.21

Fig. 7. Relationship between shear force at stirrup yield–shear force at cracking in terms of the amount of transverse reinforcement (span S6) and type of material.

Fig. 5. Shear force–shear strain cxy at the span S6.

Load (kN)

CC CCS RC RCS

1.5

-80

-200

1.7

5. Predictions of shear strength -4

-2

0

2

4

6

8

10

12

14

16

Strains (%o)

Fig. 6. Load–stirrups and longitudinal tensile reinforcement strains V24RCS.

detected that this phenomenon, which was especially evident in the beams containing recycled aggregates of the first phase, could be mitigated by including silica fume in the RCS beams of this second stage. Table 4 summarizes the deflections measured at midspan S6 when failure occurs and shear force at cracking (Vcrack), stirrup yield (Vyield) and failure (Vu). It also shows the ratio of the shear force at stirrup yield to the shear force at cracking (Vyield/Vcrack). It was also noted that only V17RCS achieve the Vyield in the spans S8. Fig. 7 also presents the experimental results of stage 1 which allows for comparisons to be made between the four types of concrete (CC, CCS, RC and RCS). It also shows the ratios between the shear force at stirrup yield and shear force at cracking, which make

The predicted capacities of the beams specimens using the expressions of different codes (without safety factors) and the RESPONSE (MCFT) [10] are given in Table 5. The calculations were performed at a section located at a distance ‘‘d” from the face of the loading plate. It can be observed (Table 5) that results generated by the codes studied are all conservative. It means that can be used for the shear design of any beam of any material (included recycled concrete with silica fume). In the case of beams without transverse reinforcement, the codes studied are closer to experimental results than in beams with stirrups. If the predictions of different theories and codes are compared, it can be observed that MCFT theory and CSA code produce values that are much closer than any other to those of current tests, and in some cases (in the absence of safety coefficients) MCFT values are above those of the tests. In the case of beams without transverse reinforcement, the standards are much closer to experimental results than when transverse reinforcement is used. The EHE [9] code is the most conservative of all those used.

Table 4 Test results. Specimens

V13CCS V13RCS V17CCS V17RCS V24CCS V24RCS V0CCS V0RCS a

Vcrack (kN)

Vyield (kN)

Vyield/Vcrack

Vu (kN)

Vu/Vcrack

Deflectionsa

S6

S8

S6

S8

S6

S6

S6

at Vfail (mm)

104.00 95.00 104.00 100.00 92.00 89.50 90.00 74.00

95.20 76.00 80.00 93.70 81.00 104.00 – –

197.00 175.00 187.00 176.00 140.00 130.50 – –

Not reached Not reached Not reached 175.50 Not reached Not reached Not reached Not reached

1.89 1.84 1.80 1.76 1.52 1.46 – –

220.08 202.36 199.79 192.92 150.07 147.33 100.53 83.88

2.12 2.13 1.92 1.93 1.63 1.65 1.12 1.13

15.62 17.54 14.54 17.08 10.62 11.52 5.72 4.77

Deflection measured at center of beam.

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Table 5 Test shear (kN) and predicted shear (kN).

V13CCS V13RSC V17CCS V17RSC V24CCS V24RSC V0CCS V0RSC

Exp.

EHE

220.08 202.36 199.79 192.92 150.07 147.33 100.53 83.88

130.44 128.76 116.97 114.69 98.93 99.78 78.88 74.83

MCFT (1.69) (1.57) (1.71) (1.68) (1.52) (1.48) (1.27) (1.12)

207.90 203.10 180.70 176.20 140.40 142.30 97.29 101.30

(1.06) (1.00) (1.11) (1.09) (1.07) (1.04) (1.03) (0.83)

CSA A23.3-AASTHO

ACI318

176.90 174.07 160.93 156.75 135.20 137.01 85.76 80.64

153.89 151.19 140.22 136.21 118.00 119.82 77.31 75.71

(1.24) (1.16) (1.24) (1.23) (1.11) (1.08) (1.17) (1.04)

(1.43) (1.34) (1.42) (1.42) (1.27) (1.23) (1.30) (1.11)

AS3600

NZS3101

168.39 166.15 153.98 151.05 133.36 134.36 97.38 92.37

154.93 152.00 141.62 137.56 119.33 120.93 84.78 78.27

(1.31) (1.22) (1.30) (1.28) (1.13) (1.10) (1.03) (0.91)

(1.42) (1.33) (1.41) (1.40) (1.26) (1.22) (1.19) (1.07)

Note: In brackets (experimental shear/code shear).

6. Conclusions  It was found that the addition of silica fume to recycled concrete causes changes in its behaviour similar to the changes that occur when it is added to conventional concrete. The proportioning method used in RCS is valid and increased mechanical resistance is obtained.  With the exception of V17, the beams with silica fume did not attain yielding in the spans that did not reach failure (spans S8).  The notable splitting cracks along the tension reinforcement observed in recycled concrete beams of the first phase [1] were mitigated by the addition of silica fume. Hence, the RCS beams showed practically no evidence of this phenomenon.  All the codes studied were conservative and subsequently can be used for the shear design of recycled concrete beams with silica fume. In this study the instantaneous shear behaviour of recycled concrete to not very aggressive exposure levels (IIa and IIb according to EHE08 [9]) was determined. Compared with the CC beams, in the RC beams was detected [1] premature shear cracking (in relation to the failure load). From these results the spanish recommendations about recycled concrete (EHE08 [9]) proposed a maximum stirrups separation of 200 mm. This investigation has shown that the inclusion of silica fume significantly mitigates this phenomenon. It would be interesting to consider the phenomenon of shear cracking under service loads in order to limit the influence of aggressive environments (chloride, carbonation, etc.) on the longterm behaviour of the recycled concrete. Acknowledgments This study is part of the project entitled ‘‘3.2-358/2005/3-B: an experimental study on the use of RCDs in recycled concrete used in

structural applications (RECNHOR)”, financed by the Spanish Environmental Ministry. In order to go on with this line of research a new project has been given to the group. It bellows to the program CENIT (spanish Ministry of Industry) and it is named ‘‘Clean, efficient and nice construction along its life cycle (CLEAM)”. References [1] González-Fonteboa B, Martínez-Abella F. Shear strength of recycled concrete beams. Constr Build Mater 2007;21:887–93. doi:10.1016/j.conbuildmat. 2005.12.01. [2] Eguchia K, Teranishib K, Nakagomea A, Kishimotoa H, Shinozakia K, Narikawac M. Application of recycled coarse aggregate by mixture to concrete construction. Constr Build Mater 2007;21:1542–51. doi:10.1016/j. conbuildmat.2005.12.02. [3] Rahal K. Mechanical properties of concrete with recycled coarse aggregate. Build Environ 2007;42:407–15. doi:10.1016/j.buildenv.2005.07.03. [4] González-Fonteboa B, Martínez-Abella F. Concretes with aggregates from demolition waste and silica fume. Materials and mechanical properties. Build Environ 2008;43:429–37. doi:10.1016/j.buildenv.2007.01.00. [5] Mukai T, Kikuchi M. Properties of reinforced concrete beams containing recycled aggregate. In: Kasai Y, editor. Demolition and reuse of concrete and masonry vol. 2: reuse of demolition waste. Proceedings of the second international RILEM symposium; November 1988. p. 670–9. ISBN 0-41232110-6. [6] Yagishita M, Sano M, Yamada M. Behaviour of reinforced concrete beams containing recycled aggregate. In: Erik K. Lauritzen, editor. Demolition and reuse of concrete and masonry. Proceedings of the third international RILEM symposium; 1993. p. 331–43. ISBN 0-412-32110-6. [7] Sogo. Shear behaviour of reinforced recycled concrete beams. In: Vázquez E, Hendriks ChF, Janssen GMT, editors. International RILEM conference on the use of recycled materials in building and structures; 2004. p. 610–8. ISBN 2912143-52-7. doi:10.1617/2912143756.067. [8] RC-08. Instrucción para la recepción de cementos. Ministerio de Fomento; 2008. [9] EHE. Instrucción de Hormigón Estructural. Publicaciones del Ministerio de Fomento. Secretaría General Técnica; 2008. [10] Bentz, Evan C, Collins, Michael P. Response – 2000. Reinforced concrete sectional analysis using the modified compression field theory. Toronto; 2000.