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INFLUENCE OF THE INCORPORATION OF RECYCLED CONCRETE
COARSE AGGREGATES ON THE PUNCHING STRENGTH OF
CONCRETE SLABS
Nuno Carvalho Martins dos Reis
Extended Abstract
Master’s Degree in Civil Engineering
Supervisors
Prof. Dr. Jorge Manuel Caliço Lopes de Brito
Prof. Dr. João Pedro Ramôa Ribeiro Correia
Jury
President: Prof. Dr. José Manuel Matos Noronha da Câmara
Supervisor: Prof. Dr. Jorge Manuel Caliço Lopes de Brito
Member: Prof. Dr. António Manuel Pinho Ramos
September 2014
1
1. Introduction
The construction sector is responsible for 40% of the consumption of extracted natural re-
sources and the major waste producer worldwide. Therefore, proper management of waste and
resources consumption is essential to achieve sustainable construction. With this purpose,
based on a study carried by Ortiz et al. (2010), 80% of the total construction and demolition
waste (CDW) is capable of being recycled and reused. However, according to a Bio Intelligence
Service report, only about 46% of all waste produced in the European Union (EU) are reused /
recycled. The use of recycled aggregates (particularly originating from crushing concrete) in
concrete is a recent approach in the reuse and recycling of CDW.
In order to contribute to the knowledge about the incorporation of recycled concrete aggregates
(RCA) in concrete, a research line has been developed in Instituto Superior Técnico (IST). Sev-
eral studies on this subject (investigating the processing of demolished products, mix proportion
design, mechanical properties and durability aspects have provided a better understanding
about the performance of concrete made with RCA (CRCA) and indicated positive prospects
concerning its use. Recently, the structural performance and economic aspects of using recy-
cled concrete coarse aggregate concrete (RCCAC) have also been analyzed and they pointed
out the potential of applying RCCAC in civil engineering structures.
The mechanical and durability properties of RCCAC have been studied in depth in the last
years. However the structural performance of this concrete type is still underdeveloped. Thus,
this work aims to contribute to the knowledge of the structural behavior of RCCAC. Particularly,
it evaluates the effect of the incorporation of recycled concrete coarse aggregates (RCCA) on
the punching strength of concrete slabs. According to the best of the author’s knowledge, this
topic has been addressed only by Sudarsana Rao et al. (2012).
The first step of this research consisted of a literature review, in order to collect information on
RCCA properties and the effect of their incorporation in terms of concrete mechanical properties
(such as compressive strength, tensile strength and modulus of elasticity) and structural perfor-
mance. The comparison between RCCAC and natural aggregate concrete (NAC) demonstrates
a generalized worsening of the mechanical properties with the increase of RCCA content (Brito,
2005). The punching tests on concrete slabs performed by Sudarsana Rao et al. (2012) indicat-
ed a decreasing trend in the punching strength of concrete slabs when the RCCA content in-
creases, as well as lower stiffness and cracking load.
The next stage was to prepare and execute the experimental campaign. First, the RCCA and
natural aggregates (NA) properties were analyzed, aiming at designing the four concrete mixes
produced. Their workability was determined to guarantee compliance with the target slump (125
± 10 mm). Concrete cubes and cylinder test specimens (to characterize the mechanical proper-
ties) and eight slabs (two for each type of concrete, with 1100 x 1100 x 90 mm, all submitted to
the punching test) were prepared. Finally, the hardened concrete and punching tests were per-
2
formed and their results were analyzed and discussed in detail.
Alongside the experimental campaign, a three-dimensional non-linear finite element model
(FEM) was also developed using ATENA 3D program to simulate the punching test of the slabs.
Numerical results were compared with experimental data, allowing for a better understanding
about the structural response of the tested slabs and also to validate the experiments.
2. Experimental programme
2.1. Materials
In this research, two main materials were used: concrete and steel.
Two types of river sand and three different grades of crushed limestone were used as NA. The
RCCA were obtained by crushing concrete beams (cast in situ at the Construction Laboratory of
IST) with 28 days of age, using a concrete jaw crusher. The other constituents used were tap
water and CEM II 42.5 Portland cement.
To design the concrete mixes, the measurement of some aggregate properties was essential.
Thus, the following properties were determined: particle density and water absorption at 24
hours (according to NP EN 1097-6 (2003)); bulk density (NP EN 1097-3 (2002)); Los Angeles
coefficient (that measures the abrasion resistance, according to NP EN 1097-2 (2010)); shape
index (NP EN 933-4 (2008)); and water content of RCCA (NP EN 1097-5 (2011)). The experi-
mental results of these tests are presented in Table 1.
Table 1 - Experimental results of the aggregates’ tests
Property Fine sand
Coarse sand
Fine gravel
Intermediate gravel
Coarse gravel RCCA
Particle density (kg/dm3) 2.55 2.51 2.53 2.54 2.56 2.28
Water absorption (%) 0.3 0.6 1.4 1.3 1.2 6.2
Bulk density (kg/dm3) 1.55 1.59 1.43 1.44 1.45 1.23
Los Angeles coefficient (%) - - 23.2 26.5 29.2 41.9
Shape index (%) - - 17.2 17.6 14.9 23.1
Water content (%) - - - - - 3.05
As expected, the cement paste in the structure of the RCCA decreases their quality compared to
the NA, mainly due to their high porosity and lightness. Thus, these aggregates present lower
particle and bulk density and higher water absorption and content, shape index and Los Angeles
coefficient than NA. As a consequence of the higher water absorption of RCCA, the mixing water
must be compensated to maintain the workability of the RCCAC.
As top and bottom reinforcement of the slabs, 6 and 8 mm diameter hot-rolled steel (S500) bars
were used. These present yielding and ultimate stresses of 600 MPa and 640 MPa, respective-
ly, and elasticity modulus of 200 GPa.
3
2.2. Concrete mixes design
Four different concrete compositions (Table 2) were designed according to the Faury’s method
with various replacement rates of NCA by RCCA: 0% - reference concrete (RC); 20% (C20);
50% (C50); and 100% (C100). The RC mix was designed to have a strength class C30/37,
equivalent to the concrete that was crushed (the RCCA source), with C20, C50 and C100 hav-
ing the same target class, and an Abrams cone slump of 125 ± 10 mm (given by an effective
water/cement (w/c) ratio of 0.54). In order to maintain constant the workability in all the mixes
and due to the higher RCCA’s water absorption, additional water was added to the mix (equal to
the absorbed water in 15 minutes by the RCCA minus their initial water content times the vol-
ume of RCCA). This explains the difference between the apparent and effective w/c ratio in
RCCAC mixes. As mentioned, Portland cement (CEM II 42.5 R) was used and the maximum
particle size was 22.4 mm.
Table 2 - Composition of the mixes
Component Volume (m
3/m
3)
RC C20 C50 C100
Natural coarse
aggregates
22.4 - 16 mm 0.121 0.097 0.061 0.000
16 - 11.2 mm 0.120 0.096 0.060 0.000
11.2 - 8 mm 0.047 0.038 0.024 0.000
8 - 5.6 mm 0.047 0.037 0.023 0.000
5.6 - 4 mm 0.041 0.033 0.020 0.000
Recycled concrete
coarse ag-gregates
22.4 - 16 mm 0.000 0.024 0.061 0.121
16 - 11.2 mm 0.000 0.024 0.060 0.120
11.2 - 8 mm 0.000 0.009 0.024 0.047
8 - 5.6 mm 0.000 0.009 0.023 0.047
5.6 - 4 mm 0.000 0.008 0.020 0.041
Coarse sand 0.250 0.250 0.250 0.250
Fine sand 0.054 0.054 0.054 0.054
Cement 0.115 0.115 0.115 0.115
Water 0.188 0.193 0.199 0.210
Apparent w/c ratio 0.54 0.55 0.57 0.60
Effective w/c ratio 0.54 0.54 0.54 0.54
The slump of the mixes was measured based on the Abrams slump cone test (in accordance
with NP EN 12350-2 (2009)). The bulk density of the fresh concrete was calculated according to
NP EN 12350-6 (2009). The results of both tests are, respectively, presented in Table 3 and
Figure 1. As observed, the slump results are in the defined range and the bulk density decreas-
es as the content of RCCA increases, which is explained by the lower density of the RCCA.
4
Table 3 - Abrams cone slump test results and w/c ratios
Concrete Slump (mm)
Apparent w/c
Effective w/c
RC 129 0.54 0.54
C20 124 0.55 0.54
C50 131 0.57 0.54
C100 121 0.60 0.54
Figure 1 - Density of fresh concrete
2.3. Test specimens
The test specimens included eight reinforced concrete slabs (two per type of concrete analyzed)
and their size was established in order to simulate the area around a column, in which the radial
moments at the contour are approximately null. As seen in Figure 2, the total size of the slab
corresponds to 44% of the span length (Lv) that it intends to simulate, considering that the mo-
ment is zero at a distance of 0.22Lv from the column axis. Thus, the tests were performed in
square slabs with 1100 x 1100 x 90 mm. The distance between (radial) supports was 1000 mm,
in order to simulate a 2.30 m spam between columns. The ratio Lv/h of the slabs is 25, which is
within the range of values recommended by EC2 for this type of structural elements.
In all specimens, as mentioned hot-rolled bars of steel (S500) were used as reinforcement. The
amount of the top flexural reinforcement is 0.93% (in a 8 mm//75 mm grid). The bottom rein-
forcement consists of 6 mm diameter bars spaced by 150 mm. A cover of 10 mm was used.
Thus, the mean depth (d) is about 72 mm.
Plywood moulds were used to cast the specimens. After 3 days, the specimens were de-moulded
and subjected to curing (watering): once a day for the first 7 days and thereafter every 2 days up
to the test date (28 days).
Figure 2 – Approximate radial moments distribution in slabs
In order to evaluate the mechanical properties of each type of concrete used to cast the slabs,
the following tests were performed: compressive strength at 28 and 56 days (according to NP
EN 12390-3 (2009)); splitting tensile strength at 28 days (NP EN 12390-6 (2009)); and modulus
of elasticity at 28 days (LNEC E-397 (1993)). The specimens used in these tests (150 mm cu-
2385 2357
2325
2267
2250
2300
2350
2400
0 10 20 30 40 50 60 70 80 90 100
De
nsi
ty (
kg/m
3 )
Replacement rate of NCA by RCCA (%)
0.22L 0.44L
L
5
bes for compression strength and cylinders with 300x150 (d) mm for the other properties) were
de-moulded 24 hours after casting and placed in a wet curing chamber during 28 days.
2.4. Punching tests setup, instrumentation and procedure
The setup used in the punching tests is presented in Figure 3. The slabs were simply supported
on 8 supports uniformly distributed in a circular pattern, placed on top of four concrete blocks.
Each of the metal supports comprised three thick steel plates, the upper two enclosing 20 mm
steel rods that allowed free rotation in the radial direction. To avoid restricting the horizontal
movement of the slabs, 1–mm thick sheets of PTFE were placed between the bottom two steel
plates of the supports. Figure 4 illustrates the supports described.
Figure 3 - Punching test setup Figure 4 - Support devices
A vertical (descending) point load was applied with an Enerpac hydraulic jack (load capacity of
600 kN, mounted on a steel reaction frame) placed on the center of the slab and contacting with
it through a 150 x 150 x 30 mm steel plate that aimed at simulating a column. A spherical hinge
was placed between this plate and the hydraulic jack to correct possible geometrical imperfec-
tions. The slab specimens were tested upside down relative to the normal orientation of a slab-
column connection. As a result, the tensile flexural reinforcement was located on the bottom
side and the column (in this case, the steel plate) was located above the slab. The overall ge-
ometry and dimensions of the slabs, as well as the arrangement of the longitudinal reinforce-
ment and position of the supports are presented in Figure 5.
During the preparation of the test system, fine layers of plaster were used to level the contact
surfaces between: the laboratory strong floor and the concrete blocks; the concrete blocks and
the supports; the supports and the slab; and the slab and the column (steel plate). In these op-
erations, a level was used to guarantee that the upper face of the elements of the test system
was horizontal.
To monitor the structural behavior of the slab specimens, various measurements were continu-
ously recorded. The applied load was measured with a Novatech load cell (capacity of 400 kN)
6
placed between the hydraulic jack and the steel plate. Two displacement transducers (TML
brand) were used on the top and bottom surfaces of the slab to measure the deflection of the
center point. The top one has a range of 500 mm and the bottom one is more accurate and has
a range of 25 mm. The rotation of the slab was measured by two TML clinometers located
above diametrically opposite supports. Therefore, the correct evolution of the test (i.e. perfectly
symmetric punching) was guaranteed by checking similar slopes in the two clinometers.
Figure 5 - Overall geometry of the slab specimens: plan view (top) and section view (bottom)
After placing the slabs and the elements described in the planned position, the following proce-
dure was carried out:
checking the instrumentation;
application of a load up to 20 kN (for the settlement of the test system);
complete unloading of the slab;
reset the instruments;
loading up to 20 kN;
register of the crack pattern (by unaided visual observation);
loading up to 120 kN (in 20 kN steps in order to register changes in the crack pattern);
loading up to the failure;
recording the failure mode and the crack pattern.
7
For safety reasons, it was decided not to register the crack pattern beyond 120 kN, since this
procedure was conducted under the test specimen.
The readings obtained from the different sensors were gathered in a PC using an HBM data
logger. The slabs were tested under load control. An average load speed of 0.5 kN/s was used.
3. Experimental results
3.1. Concrete properties
Table 4 shows the compressive strength (fcm,28), the splitting tensile strength (fctm,28) and the
modulus of elasticity (Ecm,28), all at 28 days of age. The relative performance reduction between
RCCAC and RC mechanical properties (RC,28) is also presented.
As expected, concrete’s mechanical properties were not significantly affected by the incorpora-
tion of RCCA. All the mixes present a high compressive strength (of the strength class expected
- C30/37) and similar values between the various mixes, demonstrating the RCCA’s good quali-
ty. The splitting tensile strength experienced a slight reduction (regardless of the RCCA incorpo-
ration rate) with the incorporation of RCCA. Finally, the modulus of elasticity was reduced. Such
reduction was lower than that reported in most studies concerning RCCAC, possibly due to the
increasing of effective w/c ratio that reduces the effect of a weak aggregates’ quality.
Table 4 - Mechanical properties for each type of concrete
Concrete fcm,28 (MPa) RC,28 (%) fctm,28 (MPa) RC,28 (%) Ecm,28 (GPa) RC,28 (%)
RC 46.8 ± 2.0 - 3.17 ± 0.42 - 33.7 ± 0.6 -
C20 44.3 ± 2.3 -5.4 2.90 ± 0.24 -8.3 32.8 ± 0.7 -2.7
C50 46.6 ± 1.0 -0.5 2.94 ± 0.21 -7.0 32.7 ± 0.7 -3.1
C100 45.6 ± 1.2 -2.5 2.86 ± 0.32 -9.5 31.5 ± 0.6 -6.7
3.2. Punching tests
3.2.1. Load-deflection curves
Figure 6 shows the load-displacement curves from the tests of the various specimens.
The load-displacement curves of the various specimens show a qualitatively similar progress.
The following four behavioral brunches are highlighted: (i) initial linear progress (corresponding
to the uncracked state behaviour); (ii) cracking development (between loads of 20 kN and
40 kN) that causes a pronounced stiffness reduction; (iii) linear progress once again (cracked
state behaviour); (iv) slight stiffness reduction before failure due to the yielding of the steel rein-
forcement (around 140 kN).
8
Figure 6 - Load-displacement curves of the test specimens
3.2.2. Stiffness
Table 5 shows the stiffness of the test specimens at two stages: before (uncracked state) and
after (cracked state) pronounced cracking development.
Table 5 - Uncracked (KI) and cracked (KII) state stiffness of the test specimens
Test specimens
KI,EXP (kN/mm)
KI,EXP,avg
(kN/mm) KI,EXP,RCCAC
/ KI,EXP,RC Ec,RCCAC
/ Ec,RC KII,EXP
(kN/mm) KII,EXP,avg
(kN/mm) KII,EXP,RCCAC
/ KII,EXP,RC
RC-1 138 134.0 1.00 1.00
17.5 17.4 1.00
RC-2 130 17.3
C20-1 136 131.7 0.98 0.97
17.0 17.0 0.98
C20-2 127 17.0
C50-1 108 113.0 0.84 0.97
16.8 17.2 0.99
C50-2 118 17.6
C100-1 111 103.9 0.78 0.93
17.7 17.3 1.00
C100-2 97 16.9
As seen in Figure 6, the overall stiffness of the test specimens decreases as the RCCA’s con-
tent increases. This reduction occurs mainly due to the cracked state stiffness drop (2% for
C20, 16% for C50 and 22% for C100) that should be related to the modulus of elasticity reduc-
tion experienced by those concrete mixes. However, Table 5 demonstrates that the uncracked
state stiffness reduction is unexpectedly not proportional to the modulus of elasticity reduction.
On the other hand, the cracked state stiffness is not affected by the incorporation of RCCA. This
was expected since the role of the steel reinforcement (identical for all specimens) is more im-
portant in this state, thus decreasing the concrete influence.
3.2.3. Cracking load and pattern
Figure 7 shows the crack pattern of the bottom face of test specimens (a) BR-1 and (b) B100-2
before the failure of the slabs.
0
20
40
60
80
100
120
140
160
180
0 5 10 15 20
Load
(kN
)
Displacement (mm)
RCC20C50C100
9
During and after the tests, the crack pattern was similar in all the specimens, as depicted in
Figure 7. The evolution of the crack pattern on the bottom face was expected and showed the
following steps: first, tangential and radial cracks from the column developed; afterwards, the
typical shear-crack appeared, forming the circular pattern around the column.
Some data on the cracking load are presented in Table 6. This parameter tends to decrease
with the increase of the RCCA content. For C20 and C50 mixes, there is a slight reduction
(about 3% for both) when compared to RC, while C100 shows a reduction of 21%. Comparing
these reductions with the tensile strength variation, it can be stated that they are not directly
related. In particular, for C100 mix, the tensile strength reduction was 10% while the cracking
load dropped approximately twice. This fact could be related with the relatively high scatter ex-
hibited by the tensile strength, much higher than that of other properties for all concrete mixes
(coefficients of variation: RC - 13.2%; C20 - 8.4%; C50 - 7.0%; and C100 - 11.2%).
Table 6 - Cracking load of the test specimens
Test specimens
Pcr,EXP (kN)
Pcr,EXP,avg (kN)
Pcr,EXP,RCCAC / Pcr,EXP,RC
fctm (MPa) fctm,RCCAC / fctm,RC
RC-1 24.3 24.0 1.00 3.24 ± 0.42 1.00
RC-2 23.7
C20-1 23.9 23.4 0.97 2.90 ± 0.24 0.92
C20-2 22.8
C50-1 22.8 23.2 0.97 2.94 ± 0.21 0.93
C50-2 23.6
C100-1 19.6 19.1 0.79 2.86 ± 0.32 0.90
C100-2 18.5
(a)
(b)
Figure 7 - Bottom face of test specimens (a) BR-1 and (b) B100-2 before failure
10
3.2.4. Failure mode
As expected, the test specimens collapse was due to the punching failure mechanism and the
failure surfaces were very similar for all specimens. Figures 8 and 9 show the test specimen
BR-1 after failure.
Figure 8 shows the failure surface around the load application area (bottom surface), formed by
the pitched shear-crack. The top face of the specimen (Figure 9) presents an indent under the
load application area caused by the formation of the truncated cone (typical of the punching
phenomenon).
Figure 8 - Bottom face of test specimen BR-1 after failure
Figure 9 - Top face test specimen BR-1 after fail-ure
3.2.5. Punching strength
Table 7 presents the experimental results of the test specimens punching strength as well as its
prediction based on EC2 and ACI 318-11 design guidelines. Figure 10 shows the influence of
the RCCA introduction on the punching strength of the concrete slabs.
As shown in Table 7, the maximum variation of the RCCAC slabs punching strength is 4.8% (in
the C20 specimens) compared to RC slabs. Figure 10 illustrates that the incorporation of RCCA
in concrete does not significantly affect the punching strength of slabs, even for a total replace-
ment rate of NA by RCCA (average value of 163.3 kN for RC slabs compared to 160.1 kN for
C100 slabs).
The comparison of the concrete’s mechanical properties (splitting tensile strength and compres-
sion strength) and the slabs’ punching strength (presented in Table 8) shows a close correla-
tion. The slight reduction of the compressive strength seems to explain (better than the reduc-
tion of the tensile strength) the similar punching strength of test specimens. This relationship is
also accounted for in the EC2 and ACI 318-11 design guidelines. These results are consistent
with those presented by Sudarsana Rao et al. (2012), where the compressive strength reduc-
tion was consistent with the strength decrease of slab specimens submitted to the punching
test.
11
Table 7 - Experimental and estimate of the test specimens’ punching strength
Test specimens
fcm,cylinder,28
(MPa)
Punching strength
PEXP (kN)
PEXP,avg (kN)
PEXP,avg,RC (%)
PEC2 (*) (kN)
PACI (kN)
PEXP / PEC2 PEXP / PACI
RC-1 37.1 ± 1.6
157.7 163.3 - 169.3 120.8
0.93 1.31
RC-2 168.9 1.00 1.40
C20-1 35.1 ± 1.8
158.6 155.4 -4.8 166.3 117.5
0.95 1.35
C20-2 152.2 0.92 1.30
C50-1 36.9 ± 0.8
163.6 169.2 +3.6 169.1 120.5
0.97 1.36
C50-2 174.8 1.03 1.45
C100-1 36.2 ± 1.0
161.8 160.1 -2.0 167.9 119.2
0.96 1.36
C100-2 158.3 0.94 1.33
Avg. value 0.96 1.36
Variation coeff. 0.04 0.05
(*) Note: PEC2 value were obtained ignoring k’s upper limit (≤ 2).
Figure 10 - Punching strength of the test specimens
Table 8 - Relation between the punching strength and concrete mechanical properties (fcm and fctm)
Concrete PEXP,RCCAC / PEXP,RC fcm,RCCAC / fcm,RC fctm,RCCAC / fctm,RC
RC 1.00 1.00 1.00
C20 0.95 0.95 0.92
C50 1.04 1.00 0.93
C100 0.98 0.98 0.90
The slight influence of the incorporation of RCCA on the punching strength of concrete slabs is
possibly explained by two reasons: the lower RCCA’s mechanical strength; and the better ce-
ment paste/aggregates connection due to the RCCA’s higher cement content, porosity and
roughness. Since punching strength is also influenced by these properties of the aggregates
along the shear-crack plan, they offset each another, making the punching strength similar in all
the concrete slabs.
The ratio between the experimental data and the analytical predictions of the punching strength
(presented in Table 7) proves that EC2 (with an average value of 0.96 ± 0.04) is more accurate
than ACI 318-11 (1.36 ± 0.05). The similarity between the experimental results and the EC2
estimates also validates the punching tests setup and procedure used in the present study.
130
140
150
160
170
180
190
200
0 10 20 30 40 50 60 70 80 90 100
Pu
nch
ing
stre
ngt
h (k
N)
Replacement rate of NCA by RCCA (%)
12
4. Numerical modelling
4.1. Description of the FE models
The numerical study included the development of a three-dimensional non-linear FEM using
ATENA 3D software (developed by Cervenka Consulting) with the aim of simulating the punch-
ing tests. This software has been successfully used in the simulation of punching in concrete
slabs (Ericsson et al., 2010; Öman and Blomkvist, 2006).
4.1.1. Geometry, type of elements and boundary conditions
In order to reduce the computation time and simplify the non-linear analysis, only a quarter of
the slab (550 x 550 x 90 mm) was simulated, making use of the slab’s symmetry. Concrete
characteristics (whose behaviour is explained in a next subchapter) was assigned to this com-
ponent. Brick elements (hexahedral with 8 integration nodes) were used as finite elements.
Figure 11 present the mesh refinement, whose elements maximum size is 2.5 cm. Near the
column, at a horizontal distance from the border of 3 times the depth, the mesh was refined -
1.25 cm elements were used. Note that the nodes in the “refinement border” are not connected;
this is solved based on the Master-Slave method (Cervenka et al., 2010), that makes the adjoin-
ing elements cinematically compatible. The quarter column steel plate and the two steel sup-
ports (included in the quarter slab) were defined using ATENA’s pre-defined steel characteris-
tics. Since the analysis of these elements is not relevant to this investigation, tetrahedral ele-
ments with 4 integration nodes were used. All these elements were built using isoparametric
formulations with bilinear interpolation functions and integrated by Gauss integration.
Figure 11 - Finite elements mesh and monitoring points
The steel reinforcement bars were modelled with frame elements (pre-defined as reinforcement
bars). The top reinforcement bars, with a diameter of 6 mm, were placed at depths of 1.3 cm
and 1.9 cm relative to the top face of the slabs. The bottom bars, with a diameter of 8 mm, were
positioned in two different levels at distances of 1.4 cm and 2.2 cm from the bottom face of the
slabs. These elements were modelled using a perfect bond with concrete.
13
Under the steel supports’ middle point the vertical displacement was restricted, simulating the
pinned support. In the symmetrical plans of the entire slab, corresponding to the borders of the
modeled slab, the horizontal displacements and the rotation throughout these plans were restrict-
ed. The same simplification was adopted for the column’s steel plate, where the load is applied.
4.1.2. Material properties
To simulate concrete’s behaviour, ATENA’s pre-defined concrete material with the reference
CC3DNonLinCementitious2 was used. This model describes the (plastic) tensile and compressive
behaviour of concrete. A concrete biaxial failure criterion is used when tension and compression
exist simultaneously in the perpendicular principal directions. Therefore, the compressive strength
decreases based on the Menétrey-William theory (Cervenka, 2009). In this model, the cracks
can close, leading to crushing of concrete. The mechanical properties adopted in the different
mixes were the ones measured in the experimental campaign (Table 4) and = 0.2.
The cracking and shear behaviours are based on a classical smeared crack model, particularly in
a fixed crack model, that provides an accurate description of the cracking behaviour in concrete
structures (Ericsson et al., 2010; Öman and Blomkvist, 2006). In this model (as well as in the
rotated crack model), cracks occur when the principal tensile stress exceeds the tensile strength
of the concrete. The crack direction is defined by the direction of the principal stress. In the fixed
crack model, the cracks direction remains the same under continued loading. The shear modu-
lus is reduced to represent the reduction in shear stiffness due to crack opening. To model
crack propagation, the constitutive law is used in combination with the crack band theory based
on a crack-opening law and fracture energy.
For concrete class C30/37, FIB (2013) indicates an average value of the fracture energy of Gf =
80 N/m for a maximum aggregate diameter of 22.4 mm. According to Mendes (2011), this pa-
rameter has a high scatter, being affected by several factors, particularly the size effect. Since
the slab thickness is smaller than half of that of a current slab, the size effect is particularly rele-
vant. So, the fracture energy was used to calibrate the FEM. According to studies reported in
the literature (e.g. Li, 2008), RCCAC has lower fracture energy than RC. However, in the pre-
sent study a similar value for all mixes (25 N/m) best described the various concrete slabs me-
chanical behaviour.
For the steel bars a classical elasto-plastic bilinear model was adopted with the following mate-
rial parameters: Esy = 200 GPa and fsy = 600 MPa (yield stress). This behavior is valid for ten-
sion and compression stresses.
4.1.3. Type of analysis
The analysis was performed using an applied displacement (with a 0.2 mm step) at the center
of the column steel plate. The slabs were loaded up to failure under displacement control. In
14
order to obtain a load vs. displacement curve similar to that of the experimental campaign, the
displacement was measured at the bottom corner of the model, corresponding to the center of
the entire slab’s bottom face.
4.2. Results and discussion
Figure 12 shows the numerical (FEM) and experimental results, in terms of load vs. displace-
ment curves.
(a)
(b)
(c)
(d)
Figure 12 - Load vs. displacement curves of the numerical and experimental results: (a) RC; (b) C20; (c) C50; (d) C100
The load vs. displacement curves resulting from the numerical modeling show a qualitatively
similar progress for all concrete types, compared to the experimental counterparts. The numeri-
cal and experimental curves show a slight quantitative difference in the results, the former con-
sistently presenting higher stiffness; several factors can cause this difference, such as not con-
sidering the supports’ deformability and the fact that FEM generally present higher stiffness
(Zienkiewicz et al., 2005).
Table 9 summarizes the results of the cracking load, stiffness and punching strength of the nu-
merical models. In this table, the difference between RC and RCCAC’s numerical results ()
and its difference to the experimental results (Diff) are also presented.
It can be seen that the global stiffness of the slabs obtained from the numerical model decreas-
es as the RCCA’s content increases. This reduction is less pronounced than in the experimental
0
50
100
150
200
0 5 10 15 20
Load
(kN
)
Displacement (mm)
RC-1RC-2RC-FEM
0
50
100
150
200
0 5 10 15 20
Load
(kN
)
Displacement (mm)
C20-1C20-2C20-FEM
0
50
100
150
200
0 5 10 15 20
Load
(kN
)
Displacement (mm)
C50-1C50-2C50-FEM
0
50
100
150
200
0 5 10 15 20
Load
(kN
)
Displacement (mm)
C100-1C100-2C100-FEM
15
results, as the uncracked stiffness loss in the numerical models seems to be directly related to
the reduction of the modulus of elasticity. In opposition, similarly to the experiments, the cracked
state stiffness remained approximately constant.
Table 9 - Summary of numerical results
Concrete
Cracked stiffness Cracking load Uncracked stiffness Punching strength
KI,FEM (kN/mm)
(%)
Diff
(%) Pcr,FEM (kN)
(%)
Diff
(%) KII,FEM
(kN/mm)
(%) Diff
(%) PFEM (kN)
(%)
Diff
(%)
RC 181.1 - +26 36.1 - +33 21.6 - +19 162.5 - -0.5
C20 174.2 -3.8 +24 34.7 -3.9 +33 21.3 -1.4 +20 161.0 -0.9 +3.5
C50 174.1 -3.9 +35 34.7 -3.9 +33 21.5 -0.5 +20 162.1 -0.2 -4.4
C100 169.5 -6.4 +39 34.8 -3.6 +45 21.7 +0.5 +20 161.6 -0.6 +0.9
Regarding the cracking process, no relevant conclusions can be drawn, due to the high de-
pendence of the cracking load on the displacement step. This should be improved in a future
work, decreasing the displacement step in order to discretize the initial part (elastic behaviour)
of the load vs. displacement curves. Despite this fact, there is a good correlation between the
variation of the tensile strength and the cracking load, unlike in the experimental results. This
was expected, since the tensile strength of the experimental results (unlike the numerical re-
sults) is associated to a high scatter (standard deviation).
The failure mode (due to the punching phenomenon) obtained in the numerical modeling (pre-
sented in Figure 13) was similar in the various concrete models and the experimental tests.
Figure 13 - Principal strains (m/m) plotted in the deformed configuration of the models after failure
The punching strength is very similar in the various concrete types (maximum variation of
0.9%), as in the experimental results. For the different mixes, there was also a very good
agreement between experimental and numerical punching strength, which reinforces the result
that RCCA’s incorporation (with similar properties to those used in this experimental campaign)
has reduced influence in the punching strength, even for total replacement of the coarse frac-
tion.
16
Finally, based on the results presented, it can be stated that the model described is appropriate
to simulate the mechanical behaviour of the experimental slabs, particularly in terms of the
punching strength (the main goal of this study). However, due to the reasons already pointed
out, the model provided a worse agreement in terms of stiffness and crack parameters.
5. Conclusions
This document presents an experimental and numerical study in order to evaluate the effect of
the incorporation of RCCA on the mechanical behaviour of concrete slabs submitted to punch-
ing tests. In these tests, it has been concluded that RCCA (with similar properties to those used
in this experimental campaign) can be incorporated in concrete mixes without affecting signifi-
cantly the structural properties. The results obtained in this research allow drawing the following
general conclusions:
1) The load vs. displacements curves (behaviour pattern) is qualitatively similar in all concrete
slabs;
2) As the RCCAC’s content increases, the uncracked state stiffness decreases; somehow,
such reduction is higher than that experienced by the elasticity modulus;
3) The cracked state stiffness is identical in all the mixes;
4) RCCAC slabs have a similar punching strength to RC slabs (less than 2% and 0.6% for the
concrete slabs with 100% of NCA replaced by RCCA compared to RC slabs, respectively in
the experimental and numerical results);
5) The punching strength of all RCCAC slabs was predicted with very good accuracy using
EC2 recommendations; less accurate predictions were obtained using ACI 318;
6) The results of numerical modelling showed a good agreement with the experimental results,
particularly in terms of punching strength, adopting a similar fracture energy value in all
concretes.
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