REINFORCED CONCRETE BEAMS WITH RECYCLED AGGREGATES FROM DEMOLISHED CONCRETE OF A STADIUM José Roberto dos SANTOS Research Assistant 2 Fernando BRANCO Full Professor 3 Jorge de BRITO Associated Professor 4 Nuno ALMEIDA Research Assistant
1
1 Civil Engineering Department, Technical University of Lisbon, Av. Rovisco Pais,
[email protected] 2 Civil Engineering Department, Technical University of Lisbon, Av. Rovisco Pais,
[email protected] 3 Civil Engineering Department, Technical University of Lisbon, Av. Rovisco Pais,
[email protected] 4 Civil Engineering Department, Technical University of Lisbon, Av. Rovisco Pais,
[email protected]
Lisbon 1049-001, Portugal Lisbon 1049-001, Portugal Lisbon 1049-001, Portugal Lisbon 1049-001, Portugal
Keywords: demolished concrete, recycling, recycled aggregates, mechanical properties, reinforced concrete
Abstract The demolition of José de Alvalade Stadium, located in Lisbon, was decided within the stadia renovation program for the 2004 UEFA European Championship. In the composition of demolition waste of the stadium, concrete was paramount (about 95%). Recycling of the demolition waste (CDW) was considered as a way of reducing the waste harmful effect in the urban environment through the use of CDW recycled aggregates in road pavement bases. Nevertheless their use in concretes and mortars was also envisaged. In order to introduce this type of aggregates in the concrete market, it was necessary to investigate its contamination risks and find its potential applications, taking into account its quality, limitations and the technical-economical aspects that characterise its background. In this paper results are presented related to the analysis of the mechanical properties that can be achieved with concrete made with the recycled aggregates. The sorting, crushing and screening of waste was done in the demolition site using a mobile plant. The study considered two types of recycled concrete and a conventional concrete (with natural aggregates). For the analysis, the water/cement ratio of the conventional concrete was maintained in one recycled concrete, while in the other the slump was maintained. This led to a characterization of the main mechanical properties of the recycled concrete and to a set of recommendations for the use of recycled aggregates in concrete.
1.
Introduction
The European construction industry is responsible for huge amounts of waste/rubble, generally referred to as Construction and Demolition Waste (CDW). Up to very recently, in most of UE countries, this waste was generally unheard of and no regulations existed concerning its production and disposal. This happened mostly due to the fact that other much less environmentally friendly types of waste drew all the media attention, with a strong emphasis on the so-called toxic waste. Nevertheless, the sheer volume of CDW (180 Mtons in 1998) is depleting available space in Solid Urban Waste landfills and causing widespread unlawful dumping (Hendriks et al-2000). The recent need to demolish three major stadiums for UEFA Euro 2004 has provided authorities with a strong motivation to promote the process of recycling CDW. The results of these efforts are exemplified here by the Lisbon José de Alvalade Stadium (Sporting Club of Portugal) whose debris was processed mainly into aggregates for road pavements. This paper shows the results of an experimental program in which the aggregates from the stadium were used to prepare concrete mixes and concrete beams whose behaviour was compared with the one of conventional concrete made with natural aggregates, aiming at analysing the viability of concrete produced with recycled concrete aggregates (RCA) as a structural material. The results are discussed and conclusions are drawn from them.
2.
Demolition of Stadium
2.1
Demolition of Structure
Part of the structure of the José de Alvalade Stadium was erected in the late 50’s, comprising the South, North and West sectors. The main structure comprised 86 reinforced concrete cross-frames approximately 6,0 m apart in which the stand seats structure and the floor slabs, also in reinforced concrete, were supported (IST/ICIST report-2001). The East sector was built in the middle 80’s (Figure 1). In this case there were 27 prestressed concrete cross frames approximately 5.5 m apart in which also the stand seats structure and the floor slabs were supported (IST/ICIST report-2001). The demolition was planned aiming at maximizing reuse of demolition debris. To begin with, before the concrete structure was cut, reusable materials and components, such as glass frames, metals, doors, WC equipment, central heating boilers and window frames, were dismantled and sorted. The demolition contractor was free to sell these components.
Figure 1
Demolition of the first modules between frames with a mobile crane positioned outside the stadium
The demolition of the structure followed, using mechanical equipment such as hydraulic hammers, shears and breakers, connected to a mobile crane. The first modules between frames were cut using mobile cranemounted hydraulic shears positioned outside the stadium (Figure 1). 2.2
Recycling of Demolition Waste
The composition of demolition waste was mostly concrete rubble that was processed in order to produce recycled concrete aggregates (RCA). Processing consisted of crushing and screening the demolition material, which was performed at the demolition site using a mobile plant (Figure 2). Concrete waste was fed into a mobile jaw-crusher, with a vibrating feeder and a discharge conveyor. After the rubble had been crushed, it was dropped on a mobile screen and three size fractions of aggregates were obtained.
Grading curves of RCA are given in Figure 3, showing that two types of coarse RCA (RCA1 and RCA2) and a fine RCA (RCA3) were obtained. Tests on some samples of RCA2 showed that the concrete percentage in mass was about 95%.
Figure 2
Processing of concrete rubble at the demolition site using a mobile jaw crusher 100 90
% passing the sieve
80 70 60 50
RCA 1 RCA 2 RCA 3
40 30 20 10 0 0.01
0.1
1
10
100
sieve opening (mm)
Figure 3
Grading curves of recycled concrete aggregates
3.
Experimental Program
3.1
General Remarks
To study the viability of using RCA in recycled concrete, a sample of RCA2 from the South sector was selected, to perform the research program. Tests aimed at assessing the effect of coarse RCA on concrete mechanical properties, namely the compressive strength and modulus of elasticity, and analysing the flexural behaviour of reinforced concrete beams produced with RCA. Three concrete mixes were considered: one (NAC) only with natural aggregates, a second one (RCA-WC) with RCA2 replacing the entire amount of coarse natural aggregates and with the same water/cement ratio as NAC concrete and a third one (RCA-SL) also with RCA2 only but with the same slump as NAC concrete. To limit the number of variables in test results, it was decided to use RCA2 with a similar grading as the coarse natural aggregates (NA). 3.2
Concrete Mix Proportions
The compressive strength of the original concrete (OC) from the stadium was initially assessed through drilled cores tests. The mean value obtained was 37 MPa. The natural aggregates (NA) used for NAC were limestone 4.76/12.7 mm and river sand 0.297/2.38 mm. The Portland cement used was type II, grade 32.5 MPa according to NP 2064 (1991).
Standard tests were performed to characterize the coarse natural and recycled aggregates. RCA2 water absorption amounted to 6% that compares with 0.8% for coarse NA, as a result of the higher porosity of RCA2 associated with the mortar adherent3to RCA surfaces. For RCA2, the value of apparent specific 3 density (saturated dry surface) (2460 kg/m ) was lower than NA’s (2660 kg/m ). NAC and RCA concrete mixes were proportioned following the mix design method described by Helene et al (1993). NAC concrete mix proportions were: NAC concrete 3 cement = 358 kg/m 3 coarse aggregate = 1053 kg/m 3 fine aggregate = 737 kg/m water/cement ratio = 0.55 In both RCA-WC and RCA-SL mixes, the coarse aggregate contents were adjusted to obtain the same volume as in NAC. RCA-WC and RCA-SL mix proportions were: RCA-WC concrete 3 cement = 366 kg/m 3 coarse aggregate = 963 kg/m 3 fine aggregate = 754 kg/m water/cement ratio = 0.55 RCA-SL concrete cement = 360 kg/m3 coarse aggregate = 947 kg/m3 fine aggregate = 742 kg/m3 water/cement ratio = 0.63 NAC and RCA-SL mixes yielded a slump of 62 mm. For RCA-WC an 18 mm slump was obtained, lower than for NAC, due to the higher porosity of RCA and because of its use in air-dry condition, thus decreasing the amount of free water in the mix. When RCA are not saturated with water before mixing, they absorb part of the water added during mixing (De Pauw et al-1998). NAC mixing water was considered free due to the limestone’s low water absorption that led to a water/cement ratio of RCA-SL higher than NAC’s. It was then necessary to increase the amount of water in order to maintain the slump (Hansen-1992). 3.3
Flexural Tests in Reinforced Concrete Beams
Reinforced concrete beams and test specimens were cast for each concrete type. The beams had a rectangular cross section with a 0.15 m width and a 0.20 m height, and a length of 2.20 m. The beams were designed in order to produce flexural failure in the concrete compression flange. All the beams were reinforced with 2 φ16 mm bars at the bottom, 2 φ8 mm bars at the top and φ8 mm/10 cm as vertical stirrups. Samples of reinforcement steel were tested and the following mean values were obtained: fy = 550 MPa and Es = 210 GPa. The reinforced concrete beams were tested in bending by applying two point loads with increasing loading cycles, until collapse. The beams had 1.80 m span and each load (P) was applied at 0.63 m from the end bearings. The five cycles considered were: 0-12 kN, 12-0-25 kN, 25-0-41 kN, 41-51 kN and 51 kN until failure. The test set-up adopted for all the three beams is shown in Figure 4.
Figure 4
Test set-up for flexural tests in all three reinforced concrete beams
4.
Results and Discussion
4.1
Compressive Strength and Modulus of Elasticity
Compressive strength (fc) was assessed according to LNEC E 226 (1968) and the modulus of elasticity (Ec) was evaluated according to LNEC E 397 (1993) (Table 1). Regarding compressive strength, a lower value was expected for RCA-WC as compared with NAC’s, for this level of concrete strength (Barra et al-1998) (Limbachiya et al-1998). The similitude of the values can be explained by the lower amount of free water in RCA-WC due to the coarse RCA higher absorption, which decreased its effective water/cement ratio and enabled an increase in compressive strength. It must be also referred that in the RCA concretes, values of compressive strength similar to the original concrete were obtained. Table 1 Compressive strength and modulus of elasticity of original concrete and concrete mixes Concrete OC NAC RCA-WC RCA-SL
fc (MPa) 37 38.4 38.4 32.7
Ec (GPa) 32.4 28 24.5
Concerning the modulus of elasticity, lower values were obtained for both RCA concrete (14% for RCA-WC and 24% for RCA-SL). The higher porosity and consequently lower specific density of RCA increases the aggregate stiffness, which also increases the deformability of RCA concrete (Santos et al-2002). The lowest value of modulus of elasticity for RCA-SL can be explained by the higher water/cement ratio. 4.2
Crack Pattern
The crack pattern in all the beams, during bending, was similar. The first cracks were very short flexural cracks at the mid-span. As load increased, the existing cracks extended and new cracks occurred. When the recorded load was 51 kN, the maximum crack widths were: 0.15 mm (NAC beam), 0.24 mm (RCA-WC beam) and 0.25 mm (RCA-SL beam). The crack pattern in the mid-span area of the RCA-SL beam, for 51 kN, is shown in Figure 5.
Figure 5
Crack pattern in the mid-span area of RCA-SL beam for 51 kN
The cracking load (Pcr) was obtained directly from the load versus bottom reinforcement strain diagram of the beam tests (Figure 6). The diagram shows for the first cycle of loading the following values of Pcr: 5.3 kN (NAC beam), 2.8 kN (RCA-WC beam) and 3.8 kN (RCA-SL beam). Using the Pcr values from the diagram, the cracking moment and the tensile strength of concrete (ft) for the three beams were evaluated, considering the members in state I (un-cracked) (Table 2). Lower values for RCA beams were obtained as compared with NAC (Tavakoli et al-1996). The inverse relation between the tensile strength of the RCA-WC beam and the RCA-SL beam (Figure 6) may arise from a local defect leading to an early cracking.
7 6
P (kN)
5 4
3 NAC RCA-SL RCA-WC
2 1 0 0
25
50
75
100
125
150
-6
strain (x10 )
Figure 6
Load bottom reinforcement strain diagram
Table 2 Compressive strength and modulus of elasticity of original concrete and concrete mixes Beam NAC RCA-WC RCA-SL 4.3
ft (MPa) 3.3 1.8 2.4
ft/fc 1/11.6 1/21.5 1/13.9
Bending Strength and Deflection
As initially designed, collapse of all beams was originated by compression failure in the concrete top flange. Figure 7 presents the load/mid-span deflection diagram. Experimental failure loads (Pu), as well as the analytical values (Put) assessed using a parabola-rectangle stress-strain diagram, are given in Table 3. 60
50
P (kN)
40
30
RCA-WC NAC
20
RCA-SL 10
0 0
5
10
15
deflection (mm)
Figure 7
Load/mid-span deflection diagram
20
25
Table 3 Experimental and theoretical failure loads Beam NAC RCA-WC RCA-SL
Pu (kN) 57 53 52
Put (kN) 54.4 52.7 51.9
As seen in Figure 7, there were no pre-failure qualitative differences in the behaviour of the beams. The stiffness of the RCA beams was lower than the NAC beam’s, as expected due to their lower modulus of elasticity. The differences in terms of stiffness of the beams are registered in Figure 8. With a load of 35 kN (about 65% of failure load), the following mid-span deflections were obtained: 7.1 mm (NAC beam), 7.86 mm (10% higher than NAC’s), for the RAC-WC beam, and 8.83 mm (24% higher than NAC’s), for the RCA-SL beam. These results are coherent with the ones in Table 1. 14 12
P (kN)
10 8 6 RCA-WC NAC RCA-SL
4 2 0 0
0.5
1
1.5
2
2.5
deflection (mm)
Figure 8
5.
Load/mid-span deflection for the first load cycle
General Conclusions
This paper presents a set of test results concerning concrete mixes and concrete beams made with coarse recycled concrete aggregates (RCA) obtained from the demolition of José Alvalade Stadium. They present a generally good behaviour for this type of concrete as compared with equivalent conventional concrete made with coarse natural aggregates (NAC), showing the viability of this material for structural elements. Nevertheless, the higher water absorption of the mortar adherent to recycled aggregates, decreases the workability of RCA concrete if the same water content is used (RCA-WC). This implies the use of more water to maintain the slump (RCA-SL). Both procedures were tested and the main results were: • RCA-WC shows no decrease in compressive strength, a slight decrease in the modulus of elasticity and a slightly reduced flexural failure load and stiffness; • RCA-SL shows generally the same behaviour as RCA-WC but with higher differences when compared with NAC.
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