Monotonic and cyclic behaviour of lightweight concrete beams with and without steel fiber reinforcement

Construction and Building Materials 122 (2016) 23–35

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Monotonic and cyclic behaviour of lightweight concrete beams with and without steel fiber reinforcement Angelo Caratelli, Alberto Meda, Zila Rinaldi ⇑ University of Rome Tor Vergata, via del Politecnico 1, 00133 Rome, Italy

h i g h l i g h t s
Experimental tests on lightweight concrete beams with and without steel fiber reinforcement are developed.
The cement was partially substituted by ashes coming from the combustion of Municipal Solid Waste.
Both monotonic and cyclic loads are applied.
The results show the effectiveness of fiber reinforcement in improving the element performance.

a r t i c l e

i n f o

Article history: Received 18 February 2016 Received in revised form 23 May 2016 Accepted 14 June 2016

Keywords: Fiber reinforced beams Cyclic loads Municipal Solid Waste Experimental tests

a b s t r a c t Experimental tests on four lightweight concrete beams with and without steel fiber reinforcement, subjected to monotonic and reverse cyclic loads, were set-up in order to investigate the influence of the fiber reinforcement on the strength, ductility and energy dissipation. The tests were performed on beams with 200  300 mm cross section and clear span of 3000 mm. Furthermore, in the framework of the definition of sustainable materials, the cement of the concrete matrix was partially substituted by ashes coming from the combustion of Municipal Solid Waste (MSW), with pozzolanic reaction. The obtained results are presented, discussed and compared. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction The use of Fiber Reinforced Concrete (FRC) is continuously growing, particularly in pavements, shotcrete and precast industry [5,22,17]. In some applications, such as precast tunnel segments, the possibility of totally or partially replace the traditional reinforcement with FRC allows several advantages not only in terms of cost reduction but also related to an increase of the quality and structural performance [32,26,9,15,10,29]. With reference to the structural aspects, the fiber reinforcement improves the performance of the material under tensile actions, remarkably increasing the toughness and enhancing the cracking control [34,36]. Furthermore, the presence of fibers in the concrete matrix has important effects in increasing the fatigue and the impact resistance [16,7]. Guidelines and codes for FRC structures have been developed in different Countries [13,11] and the fib Model Code 2010 includes specific indications on FRC structural design [25]. Nevertheless, there is a clear gap in the literature on ⇑ Corresponding author. E-mail address: [email protected] (Z. Rinaldi). http://dx.doi.org/10.1016/j.conbuildmat.2016.06.045 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.

the application of steel fiber reinforced concrete (SFRC) to enhance the seismic -and thus cyclic – response of a structure and to assess the potential ductility and energy absorption capacity of such composites. Indeed under reverse cyclic loading, concrete is subjected to more severe damage and the presence of fibers can reduce the strain magnitude and control the crack openings [14,33]. Very few papers are nowadays available on the cyclic behaviour of fiber reinforced elements, and mainly beam. Monotonic and cyclic experimental tests on small steel FRC beams (150  150  500 mm) were made by Campione and Mangiavillano [8]. Several advantages given by the fibers under cyclic action were observed, such as a reduced cover spalling process, a significant increase in shear strength due to the bridging actions of the fibers across the principal cracks, fewer pinching effects. Twelve two-span beam specimens (150  200  1000 mm) were tested by Kotsovos et al. [27] using different reinforcement diameters and concrete grades, with or without steel fibers in the mix, with the aim of studying the effect of fibers on the behaviour of reinforced-concrete (RC) structures designed in accordance with Eurocode 8 [19]. Both monotonic and cyclic loading were considered. Based on the experimental results, the authors concluded

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Table 1 Concretes mix design and density (amount in kg/m3).

Cement MSW ashes (40% in slurry) Sand (0–4 mm) Coarse Aggregate (2–16 mm) Expanded clay (0–15 mm) Water (total) Plasticizers Shrinkage reducer agent Steel Fibers Measured density

Table 2 Concretes compressive mechanical properties.

L-MSW

L-FRC-MSW

472 140 922 165 344 170 38 32 – 2064

460 133 1113 – 295 162 48 32 30 2156

that for the specimens without fibers, under cyclic loading, designing to current code provisions does not safeguard against a premature brittle failure. On the contrary, specimens with fibers satisfied the performance requirements of Eurocode 8 for strength and ductility, for concrete strength up to 60 MPa. For higher values of concrete strength, in spite of the significant improvement in performance, the code requirements were not fulfilled in terms of ductility. Schumach [35] found that in some circumstances the introduction of fibers for the enhancement of a particular structural behaviour (e.g., shear, impact resistance, behaviour at service load, crack control, etc.) can limit the ductility under flexure. This situation occurred in particular conditions, mainly when both steel rebars with a low hardening ratio and a FRC having a high toughness are adopted. The combination of these two aspects was found to be particularly detrimental with regard to the overall ductility. In those circumstances, an increase in the rebar bond leads to a localization of deformations at a crack. Similar conclusions were found by Meda et al. [30]. The lack of works dealing with the influence of the fiber reinforcement on the cyclic behaviour of full-scale beams, and the emerging contrasting views, clearly show the need to provide more experimental data and models. Recently, a comprehensive research on the adoption of fiber reinforced concretes, with different mix designs, both in beam and slab elements [31] has been developed at University of Rome Tor Vergata. The use of innovative concrete in RC structures is receiving widespread attention, with particular reference to the idea of reducing the environmental impact of new concrete structures [24]. In this context, the possibility of partially replacing the cement with the product of incineration of the Municipal Solid Waste (MSW) can be of great interest [23,6,12,28,1,3]. A research on innovative lightweight concrete with cement partially replaced with ashes coming from the burning of MSW, having pozzolanic properties, has been developed by the authors, and its possible application in combination with steel fiber reinforcement is here proposed for beams subjected to cyclic loads. At this aim,

L-MSW

Mean concrete cubic strength Rc [MPa] Time (days)

L-MSW

L-FRC-MSW

3 7 14 28 160

33.55 47.50 51.14 58.56 65.60

34.33 42.50 47.27 52.77 65.00

70

60

Rcm [MPa]

24

L-MSW 50

L-FRC-MSW 40

30 0

14

28

42

56

70

84

98

112 126 140 154 168

time [days] Fig. 2. Mean compressive strength at different curing times.

experimental tests have been carried out on beams with two concrete mixes, also in presence of lightweight aggregates, with and without fiber reinforcement. The full-scale specimens are subjected to cyclic loads and the effectiveness of the FRC material solutions is remarked. 2. Material properties and characterization Four beams were cast with a lightweight concrete (with clay aggregate) and with a partial replacement of the cement with ashes coming from the burning of Municipal Solid Waste. In two specimens, steel fiber reinforcement was added also. The specimens were subjected to monotonic and cyclic loads. 2.1. Concrete mixes Two concrete mixes were cast and adopted in the present research: – Lightweight concrete with ash from Municipal Solid Waste (L-MSW); – Lightweight fiber-reinforced concrete with ashes from Municipal Solid Waste (L-FRC-MSW). The fiber reinforced concrete is characterized by the addition of 30 kg/m3 of hooked steel fibers with length (l) equal to 30 mm and diameter (d) of 0.30 mm (aspect ratio l/d equal to 100). The steel wire strength is higher than 2000 MPa. The mix design for the concretes used in the experimental program is shown in Table 1.

L-FRC-MSW

Fig. 1. Slump flow test results.

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Section A-A

A

hsp=125

150

150

A

b=150

150

250

250

125

σ

250

L=500

500

(a)

(b)

Fig. 3. Flexural test; a) specimen geometry; b) clip cage for CMOD measure.

Fig. 4. FRC tensile characterization; a) flexural test set-up (UNI EN 14651 [20]); b) reference strength values (MC2010).

4

fR3

fR2

5

Strength

fR4

fR1

fL [MPa] fR1 [MPa] fR2 [MPa] fR3 [MPa] fR4 [MPa]

3

0 0.0

0.5

1.0

1.5

CMOD4

CMOD3

1

CMOD2

2 CMOD1

Nominal stress [MPa]

6

2.0

2.5

3.0

3.5

Mean Characteristic Value Value 2.62 1.92 3.40 2.78 4.14 3.07 4.21 3.06 3.93 3.02

4.0

CMOD [mm] Fig. 5. Results of the flexural response of the fiber reinforced concrete, mean and characteristic values of residual flexural strength.

Fig. 6. Tensile test on steel rebars; a) set-up; b) measured stress-strain relationships.

25

26

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Fig. 7. Beam geometry.

1000

1000

300

50

1000

Fig. 8. Static schemes for monotonic and cyclic tests.

(a) Fy 2/3 Fy 1/3 Fy Fcr

(b)

Fig. 9. Loading history: a) pre-yielding cycles; b) post-yielding cycles.

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27

load, the opening of the notch at the bottom face of the beam, named Crack Mouth Opening Displacement (CMOD). This last quantity has been directly measured with a clip gauge, as shown in Fig. 3b. The Limit of Proportionality strength fL and the reference strength values fR1, fR2, fR3 and fR4, defined by EN14651 and used by MC2010 for defining the constitutive relationship or the material, are related to CMOD equal to 0.5, 1.5, 2.5 and 3.5. These quantities are evaluated from the F-CMOD relationship and shown in Fig. 4b. In particular, in the hypothesis of linear stress distribution (r, Fig. 3a), and being l the span length (500 mm) and b the specimen width (150 mm):

Right side

Left side

fL ¼

3F L l 2

2bhsb

f R;j ¼

3F j l 2

2bhsb

ð1Þ

where: fL is the limit of proportionality as defined in EN 14651; fRj is the residual flexural tensile strength corresponding to CMOD = CMODj; Fj is the measured load corresponding to CMOD = CMODj; hsp the distance between the notch tip and the top of the specimen (125 mm).

Fig. 10. Testing system.

The concrete has been prepared in a truck mixer. For every batch a quantity of concrete equal to 2.5 m3 has been mixed and the mixer plant was 30–40 min from the laboratory, where the specimens where prepared. The concrete rheology was measured before starting the casting procedure through a slump flow test, carried out according to EN 12350 [21]. The results are shown in Fig. 1. The combination of lightweight aggregate and fiber reinforcement allows keeping the material density lower than that of the ordinary normal weight concrete (usually in the range 2300–2400 kg/m3). The adoption of lightweight concrete in the beam L-MSW gives a dead load reduction of about 10–14%, with respect to the ordinary normal weight concrete (see Table 1).

2.2. Concrete strength characterization The average concrete strength measured on four cubes having a side of 150 mm, at different curing times is reported in Table 2 and plotted in Fig. 2. The last date corresponds to the test age. It can be noted that after 28 days a lower value of compressive strength is measured for the fiber reinforced mix (about 10% with respect to L-MSW), but this difference becomes absolutely negligible with the time. In particular after 160 days from the cast (date of the test), the compressive strengths of the L-MSW and L-FRC-MSW mixes are almost the same (65.60 MPa and 65.00 MPa, respectively). Finally, the tensile behaviour of the fiber reinforced materials was characterized through flexural test on beams (150  150  600 mm3), according to the UNI EN 14651 [20] (Figs. 3a and 4a). In particular, the nominal values of the material properties are defined by performing a three point bending test on a notched beam (notch width and height equal to 5 mm and 25 mm, respectively, Fig. 3a), and by measuring, besides the

The nominal stress–CMOD diagrams related to the five specimens are shown in Fig. 5; the mean and characteristic residual strengths are remarked in the same figure. The characteristic values are evaluated in agreement with the EN 1990 [18]. In particular, for each strength (fi), the average value (fm), the standard deviation (s) and the coefficient of variation (Vx) have been evaluated:

fm

n X ¼ fi i¼1

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pn 2 s i¼1 ðfi  fmÞ Vx ¼ s¼ n1 fm

ð2Þ

being (n) the number of specimens (five in the analysed case). In the hypothesis of normal distribution, the characteristic values can be calculated as:

f k ¼ f m  kn s

ð3Þ

The value of kn is suggested by [18] as a function of the specimens, for unknown and known coefficient of variation (Vx). In particular, for fiber reinforced concrete, it can be considered (Vx) known from prior knowledge, and the coefficient kn can be assumed equal to 1.8. Finally, the possibility of adopting the cast FRC as a structural material is checked according to the ModelCode 2010 prescriptions, and then it is verified that both the code requirements are widely fulfilled:

f R1k =f LK ¼ 1:45 > 0:4 f R3k =f R1k ¼ 1:10 > 0:5

2.3. Steel characterization The beams where reinforced with 16 mm diameter steel rebars. All the bars came from the same production and the tensile behaviour (Fig. 6) was characterized through tensile tests on three specimen. The average yielding and ultimate strengths were equal to 520 MPa and 650 MPa, respectively.

Fig. 11. Test set-up for cyclic loads: load transmission system a); b) detail.

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(a)

(b)

Steel pad Neoprene layer

Fig. 12. Details of the test set-up; a) point load; b) support.

(a) W1 W5 W2R

W3R W4

(b)

W3L

W2L

Fig. 13. Instrumentation; a) right surface; b) left surface (Fig. 10).

Load [kN]

3. Experimental tests

200 180 160 140 120 100 80 60 40 20 0

FRCM OCM

0

20

40

60

80

100

120

Displacement [mm] Fig. 14. Load-midspan displacement diagram.

140

160

In the Laboratory of the University of Rome Tor Vergata two beams were cast with the concrete matrix L-MSW, named OC elements (ordinary concrete), and two beams were made with L-FRC-MSW concrete material and named FRC elements. The subscripts M and C will point out the nature of load, monotonic and cyclic, respectively. The beams have a length of 3.50 m (net span 3.00 m) and a 200  300 mm cross-section. The elements are reinforced with 3£16 symmetrical longitudinal reinforcement (Fig. 7). The transversal reinforcement is made with £8 stirrups spaced 100 mm. A spacing equal to 50 mm is given close to the supports and to the point load position (Fig. 7). Both monotonic and cyclic tests are carried out, according to the schemes of Fig. 8. A grid (50  50 mm, see Fig. 8) was drawn on the lateral surfaces in order to simplify the crack pattern detection.

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Right side

50

0

100

150

25

200

250

300

300

25

Left side

50

0 25

100

150

200

250

300

300

25

Fig. 15. Beam OCM, Monotonic test, crack pattern.

Fig. 16. Beam OCM, Monotonic test: a) crack pattern at yielding (Fy = 140 kN); b) crack width at yielding; c) failure stage for concrete crush.

The loading history, related to the cyclic loads, is represented in Fig. 9. In particular, starting from the cracking load (Fcr), four triplets of cycles are given up to the yielding load (Fy), as shown in Fig. 9a. Then, (Fig. 9b) the triplet of cycles are given in terms of displacements (d), starting from the yielding one (dy) up to the beam failure (du).

3.1. Test set-up and instrumentation The test set-up is made with structural steel HEB300 (Fig. 10). The lower part includes two main beams that can move on steel wheels and slide on suitable guides, allowing a simple positioning of the specimen under the load cell. A transversal beam connects the main ones, and acts as stiffening element. The intermediate

part consists in two steel columns for reaching the fixed height, while the upper part, constituted by secondary beams bolted to the columns, allows the lodging of the support system. The described set-up has been improved for the cyclic tests. In particular for avoiding the lifting during the inverse loading, the testing system has been connected to the floor slab with £20 mm steel threaded rebars. The load is transmitted through a system shown in Fig. 11 composed of a main beam, two small beams and four ties, able to grant the same behaviour during the up-loading and un-loading phases. All the tests have been performed under a contrast frame of 4000 kN and the load is given by a 1000 kN electromechanical jack, in displacement control.A steel pad is inserted between the cylindrical point load and the beam surface. Furthermore a layer of neoprene is placed in correspondence of the point load, under

30

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Right side

50

0

100

150

25

200

250

300

300

25

Left side

50

0 25

100

150

200

250

300

300

25

Fig. 17. Beam FRCM, Monotonic test, crack pattern.

Fig. 18. Beam FRCM; Beam OCM, monotonic test – collapse state a) crack pattern; b) concrete crush; c) rebar failure.

the steel slab, for assuring a uniform contact (Fig. 12a). The support system has been designed and made in order to allow the horizontal displacements and the rotations with respect to the support axis (Fig. 12b). The displacements are measured through seven wire transducers, placed as shown in Fig. 13 and in particular one close to the midspan (W1); four close to the point load (W2R, W2L, W3R, W3L) and two located at 50 cm from the support (W4, W5). The total load F is measured with a 1000 kN (±1%) cell.

4. Results of monotonic tests The experimental results of the monotonic tests developed for both the ordinary (OCM) and fiber reinforced (FRCM) beams are here summarised and discussed. It is worth remarking that both monotonic and cyclic tests were developed on beams with the same geometry and support conditions. The load-midspan displacement diagrams measured during both the tests are compared in Fig. 14.

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A. Caratelli et al. / Construction and Building Materials 122 (2016) 23–35

0

displacement [mm]

-20

0

500

1000

1500

2000

2500

3000

-40 -60

OCM

-80 -100 -120

FRCM

-140 -160

x [mm] Fig. 19. Displacement pattern for yielding and ultimate stages.

200

OCM 150

Load [kN]

OCC

100 50 0

-100

-80

-60

-40

-20

0

20

40

60

80

100

-50 -100 -150 -200

Displacement [mm] Fig. 20. Load displacement diagram; reinforced concrete beams OCM and OCC: Comparison between monotonic and cyclic bejaviour.

Right side

50

0

100

150

25

200

250

300

300

25

Left side

50

0 25

100

150

200 300

250

300 25

Fig. 21. Beam OCC. Cyclic test; crack pattern evolution.

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A. Caratelli et al. / Construction and Building Materials 122 (2016) 23–35

Fig. 22. Beam OCC. Cyclic test: crack pattern for the first cycle (4 dy).

Fig. 23. Beam OCC. Cyclic load: Crack pattern for the second cycle (5 dy).

200 150

Load [kN]

100 50 0 -120 -10 0

-80

-60

-40

-20

0

20

40

60

80

100

12 0

140

16 0

-50 -100 -150 -200

Displacement [mm] Fig. 24. Load displacement diagram; Fiber Reinforced beams FRCM and FRCC: Comparison between monotonic and cyclic bejaviour.

The loads related to the first cracking (Fcr) and first yielding (Fy) of the beam OCM were equal to 19 kN and 140 kN. The related midspan displacements were about 0.6 mm and 15.5 mm, respectively. The crack pattern evolution measured during the test is summarised in Fig. 15. More in detail, the crack pattern at yielding stage is shown in Fig. 16a and the maximum crack width at the same load level, equal to about 0.2 mm, is shown in Fig. 16b. The maximum load, almost coincident with the failure one, was about 165 kN, and the ultimate displacement was about 82 mm. The failure was due to the concrete crush (Fig. 16c). With reference to the fiber reinforced beam FRCM, the load related to the first cracking (Fcr) and first yielding (Fy) were equal to about 20 kN and 170 kN. The related midspan displacements

were about 1.12 mm and 16 mm, respectively. The maximum load was about 196 kN, related to a midspan displacement of about 91 mm. The ultimate displacement was about 140 mm. The complete crack pattern detected during the test is summarised in Fig. 17. The failure was due to the contemporaneous effects of concrete crush and rebar failure. (Fig. 18a–c). In can be clearly noted the variation of failure mode due to the fiber presence. In particular in the FRCM beam the fibers avoid the sudden and brittle concrete crushing, due to their capacity of improving the concrete toughness in compression [30]. The fiber presence leads to an increase of the yielding and ultimate load of about 21% and 19%, respectively, in agreement with other experimental tests [4,2]. The increase of the ductility defined as the ratio between the

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Right side

50

0

100

150

25

200

250

300

300

25

Left side

50

0 25

100

150

200 300

250

300 25

Fig. 25. Beam FRCC. Cyclic test; crack pattern evolution.

Fig. 26. Beam FRCC. Cyclic test. Crack pattern: a) second cycle (5 dy); b) third cycle 5 dy; c) rebar failure.

ultimate and yielding displacements is equal to about 65%. Finally, the displacements, measured along both the beams, at yielding and close to the failure stages are plotted and compared in Fig. 19. Negligible differences are found up to the yielding force, while a significant increase of displacements in the fiber reinforced beam is measured for load values close to the ultimate one.

5. Results of the cyclic tests Some of the main results obtained from the cyclic tests developed on both the ordinary reinforced concrete beam (OCC) and fiber reinforced one (FRCC) are here summarised and discussed. The applied loading history has been discussed in Section 3.

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FRCC

200 150

Load [kN]

100 50 0 -120

-100

-80

-60

-40

-20

OCC

0

20

40

60

80

100

120

-50 -100 -150 -200

Displacement [mm] Fig. 27. Beams OCC and FRCC: comparison of the cyclic behaviour.

10 9

FRCC

8

E [kN m]

7 6

OCC

5 4 3 2 1 0 1

2

3

4

5

6

7

8

9

10

11

12

13

14

cycle number Fig. 28. Beams OCC and FRCC: energy dissipated.

5.1. Reinforced concrete beam OCC The load midspan displacement diagram measured for the beam OCC, is plotted in Fig. 20. In the same figure the monotonic response, discussed in the previous section, is superimposed. The maximum cyclic load was about 151 kN. If compared to the monotonic behaviour a decrease of this property of about 9.3% is measured. On the contrary, an increase of the ultimate cyclic displacement is observed, of about 20%. The crack pattern evolution, detected during the test, is shown in Fig. 21. The response of the beam related to the third cycle (4 dy) and to the second cycle (5 dy) is shown in Fig. 22. The complete failure took place at the second cycle (5 dy), as highlighted in Fig. 23. A complete crush of the compressive concrete occurred (Fig. 23). 5.2. Fiber reinforced concrete beam FRCC The load midspan displacement diagram measured for the beam FRCC, is plotted in Fig. 24. In the same figure the monotonic response, discussed in Section 4, is superimposed. The beam collapsed in correspondence of the cycle (4 dy). The maximum load was about 182 kN. If compared to the monotonic behaviour a slight decrease of this property of about 7% is measured. The ultimate displacement also appears reduced of about 28%. The crack pattern evolution, during the test, is shown in Fig. 25. The beam behaviour close to the failure is shown in

Fig. 26a and b, with reference to the second cycle (5 dy) and the last cycle (third 5 dy), respectively. The collapse took place at the third cycle (5 dy), as highlighted in Fig. 26c, with a failure of the rebars at the bottom side. 5.3. Comparisons The cyclic behaviour of the beams OCC and FRCC, expressed in terms of load-midspan displacement, are finally superimposed in Fig. 27. In agreement with the monotonic behaviour an increase of about 14% of the maximum load was noted. The energy dissipated in each cycle is shown in Fig. 28. It can be noted the very stable cyclic behaviour, since the three cycles of each step dissipate approximately the same energy. In both the beams the energy increases with the cycle number, but a higher dissipation takes place in the FRC element, mainly for higher cycle number (energy dissipation increase of about 20%). 6. Conclusions In the present paper, an experimental research on the monotonic and cyclic behaviour of beams cast with a lightweight aggregate concrete with and Municipal Solid Waste, with and without steel fiber reinforcement, is presented. On the basis of the obtained results the following remarks can be addressed:

A. Caratelli et al. / Construction and Building Materials 122 (2016) 23–35

– in the monotonic test, the addition of about 30 kg/m3 of hooked steel fibers steel fibers leads to an increase of about 20% of the ultimate load: the increase of ductility (defined as the ratio between the ultimate and yielding displacements) is equal to about 65%. – in the cyclic tests, the fiber reinforced beam exhibits an increase of the ultimate load of about 14%, with respect to the reference one without fibers. Furthermore, the fibers addition causes an increase of dissipated energy (of about 20%) and more stable behaviour for higher cycle number, – the adoption of lightweight aggregates allows reducing the selfweight of the composite material of about 10–14%, with respect to the traditional concrete. The combination of lightweight aggregate and fiber reinforcement allows keeping the material density equal or lower than the ordinary concrete; – the adoption of Municipal Solid Waste does not provide significant differences in the compressive behaviour of the material and in the final global behaviour of the beam elements.

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