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Longitudinal shear resistance of PVA-ECC composite slabs
Structures 5 (2016) 247–257
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Regular article
Longitudinal shear resistance of PVA-ECC composite slabs Bashar S. Mohammed ⁎,1, Muhammad Aswin 1, Walden Harry Beatty 1, Muhammad Hafiz 1 Department of Civil and Environmental Engineering, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak Darul Ridzuan, Malaysia
a r t i c l e
i n f o
Article history: Received 5 September 2015 Received in revised form 27 November 2015 Accepted 23 December 2015 Available online 4 January 2016 Keywords: Engineered cementitious composite (ECC) Composite slab Profiled steel sheeting Longitudinal shear bond strength m–k values
a b s t r a c t Including polyvinyl alcohol (PVA) fiber to develop a ductile engineered cementitious composite (ECC) is an effective way to overcome the brittle behavior of conventional concrete and to increase the efficiency of composite slab construction. Two groups of full-scale composite slabs were prepared, cast and tested until failure. Each group consists of three PVA-ECC composite slabs and one control specimen made with normal concrete topping. The first group has been tested with long shear span of 900 mm while for the second group, the shear span was 450 mm. Test results have been analyzed and the longitudinal shear bond strength has been computed using both methods of m–k and partial shear connection (PSC). In addition, comparison with other types of composite slabs using the same profiled steel sheeting was also carried out. The longitudinal shear bond strength was found to be 0.46 MPa and 0.387 MPa using m–k and PSC, respectively, with difference of 15.87%. The results showed that the failure modes of the PVA-ECC composite slabs are ductile with enhanced load carrying capacity. Therefore the use of PVA-ECC composite slabs in the construction industry would consider a better economic solution. © 2015 The Institution of Structural Engineers. Published by Elsevier Ltd. All rights reserved.
1. Introduction The composite beam is one of general types of slab in steel constructions. The composite beam usually consists of steel beams and concrete slab where steel beams support the concrete slab and shear connectors are used to produce the integrity of the system. These shear connectors are welded from one side to the upper flange of the steel beam, and are extended into the concrete slab. Therefore, the formwork for concrete slab is commonly designed to carry the weight of fresh concrete before it can withstand the hardened self-weight. The formwork of concrete slabs usually involves sheathings, joists, stringers and shores [1]. This system can add to problems in construction weight, construction speed, etc. Mullett [2] stated that steel skeleton-framed buildings incorporating the composite slabs are quite popular and economical in steel construction due to their lightweight nature and speed of erection. Composite slabs are the main system of floor construction in steelframed buildings [3]. They are constructed from cold formed profiled steel sheeting with concrete topping. The profiled steel sheeting has two main advantages. Firstly, during construction process, profiled steel sheeting will act as a permanent formwork which can safely carry the weight of fresh concrete without supporting false-work. Secondly, after concrete is hardened, the profiled steel sheeting will act as tensile reinforcement to resist tensile stresses at the soffit of the composite slab via the composite action between the two materials [4]. The composite action is achieved by mechanical and physical means. ⁎ Corresponding author. E-mail address: [email protected] (B.S. Mohammed). 1 Tel.: +605 3687305.
The mechanical interlocking is provided by the embossment on the profiled steel sheeting and the physical is obtained by the frictional interlocking between the re-entrant of the profiled steel sheeting and concrete topping [3]. Other advantages of the composite slab system are quick installation, working platform and reduced floor to floor height which leads to reduction in the total dead load of the building [3,5]. The common failure mode of composite slabs is the longitudinal shear failure in which the slip would occur between the profiled steel sheeting and concrete topping which mainly depends on the composite action between the two materials. However, it is worth noting that the failure load of composite slabs is far from the ultimate bending moment [6]. There are two methods to design the composite slabs which are shear bond method, also known as m–k method and partial shear connection method (τ). Both methods are required experimental data which can be obtained from testing two groups of identical full-scale composite slabs; each group consists of 3 slabs. The first group is to be tested with long shear span of 900 mm, while for the second group; the shear span is 450 mm. These constants (m–k and τ) are used by manufacturers to provide designers with tables containing the range of load-span values for specific profiled steel sheeting. The load carrying capacity of composite slabs depends on the shear-bond resistance at the interface between the profiled steel sheeting and concrete [7–9]. The performance of composite action also depends on several factors involving geometry and thickness of steel deck, concrete compressive strength and loading arrangement [10]. Moreover, other composite actions can be achieved by using the shear studs or similar shear devices [11]. Ability to resist the vertical separation is also obtained by the amount and type of embossments, as well as by adopting a proper shape of the steel deck profile. Furthermore, the shear-bond resistance is affected
http://dx.doi.org/10.1016/j.istruc.2015.12.003 2352-0124/© 2015 The Institution of Structural Engineers. Published by Elsevier Ltd. All rights reserved.
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Table 1 ECC and normal concrete proportions. Ingredients
ECC
Normal concrete
Cement (kg/m3) Coarse aggregate (kg/m3) Fine aggregate (kg/m3) Water cement ratio Fly ash (kg/m3) Superplasticizer (kg/m3) Polyvinyl alcohol (%)
583 – 467 0.32 700 9.5 2
500 520 1105 0.44 – – –
avoiding fiber clogging. The mixing lasted for at least 8 min until the fibers were fully dispersed. The ECC mixture then poured into the prepared profiled steel sheeting with side formwork. The formwork was de-molded after 24 h and soaked in water jute bags which covered the slabs to allow for proper curing until the testing day. The composite slabs made with normal concrete topping were also prepared under the same environment and curing condition. The hardened properties (average of 3 samples) of the ECC are reported in Table 2. 2.2. Properties of the profiled steel sheeting
by compatibility of deformation properties between concrete and profiled steel sheeting. Profiled steel sheeting is ductile and normal concrete is brittle, if the two materials provide the poor compatibility so they will contribute the adverse effect for the efficiency of the composite slabs [12]. Therefore, enhancing the ductility of concrete will lead to improve the resistance of longitudinal shear stresses and subsequently increasing the load-carrying capacity of the theses composite slabs. Engineered cementitious composites (ECCs) were developed by Li [13] based on the micromechanical principles. The ECC mixtures exhibit wide strain hardening with up to 6% strain capacity. Due to the ductile behavior of ECC beams subjected to flexural and shear loadings, Li [13] has suggested to use the ECC in structural applications that require ductility and high energy absorption. Further details on ECC properties and behavior have been reported by other researchers [14–19]. Therefore, the main objective of the research work presented in this paper is to investigate the structural behavior of composite slabs made with ECC and also to evaluate the m–k and the partial shear connection values. 2. Experimental program 2.1. ECC materials, mix proportions and mechanical properties Mix proportions of ECC and normal concrete is shown in Table 1. For preparation of the ECC mixture, OPC, fly ash and sand were dry mixed in a concrete pan mixer for 3 min with half part of the PVA fiber added to the mix. Half portion of the water mixed with superplasticizer and added to the dry mixture, the mixing process continued for another 3–5 min. At this stage, the other portions of PVA and water were added slowly into the mixture to allow for thorough distribution and
Table 2 Mechanical properties of ECC. Mechanical properties
Average of 3 samples
Testing standard
Compressive strength (MPa) Splitting tensile strength (MPa) Modulus of rupture (MPa) Elastic modulus (GPa)
86.5 6.18 6.35 22
ASTM C39/C39M-04 A ASTM C496/C496M ∓04 ASTM C293-02 ASTM C469∓02
As shown in Fig. 1, the profiled steel sheeting used in this study has depth (hp) of 54 mm, thickness of 1 mm, and cross sectional area (Ap) of 980 mm2. The centroid axis of the profiled steel sheeting is 109.5 mm (measured from the outer top fiber of concrete), and 550 MPa tensile yield strength. Dimensions of the profiled steel sheeting are 2700 mm length and 590 mm width. 2.3. Composite slabs testing setup and instrumentations To achieve the objective of this study, eight full-scale composite slabs were prepared, cast and tested. Six slabs were made with engineered cementitious composite (ECC) topping and the other two with a normal concrete topping for comparison purposes. All slabs were prepared, cast and tested in accordance with the requirements of part 1.1 of Eurocode4 [20]: The composite slabs were divided into two groups, each group comprises of three ECC composite slabs and one control specimen with normal concrete topping. The first group was tested for long shear span of 900 mm and the second group for short shear span of 450 mm. Each composite slab had dimensions of 2700 mm length, 590 mm width and 125 mm overall depth. The concrete thickness above the ribs (hc) and steel profiled sheeting of depth (hp) are 71 mm and 54 mm, respectively. The schematic diagram of the experimental setup is shown in Fig. 2, whereas SG is the strain gauge, T is transducer and Ls is the shear span. 3. Results and discussion 3.1. General observation The experimental setup of ECC composite slabs with long and short shear spans is shown in Figs. 3 and 4, respectively. The load carrying capacity of composite slabs depends on the longitudinal shear bond strength at the interface between the profiled steel sheeting and concrete topping. In its turn, the longitudinal shear bond resistance depends on the mechanical and physical bonds of the concrete topping with the re-entrant and embossment of the steel deck, respectively. In order for the composite slab system to work effectively under vertical loading, the profiled steel decks and normal concrete or ECC toppings
Fig. 1. Profiled steel sheeting.
B.S. Mohammed et al. / Structures 5 (2016) 247–257
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Fig. 2. The experimental setup for the composite slab.
must have the ability to transfer the induced longitudinal shear forces. With sufficient load carrying capacity, under loading, the composite slab behaves as a single structural component. Slight slippage has been occurred between the ECC or concrete toppings and the steel deck at initial stage of loading. However, at comparison loading stage for long shear span (LS) and short shear span (SS) of composite slabs, shear cracks have been initiated in normal concrete much earlier than ECC slabs due to early loss of composite action between normal concrete and profiled steel sheeting, which also indicated that ECC topping provides better bonding to the steel deck. For the SS of composite slab, the shear force at the support is larger than that of LS. It has been observed that cracks initiated in the shear span area at soffit of the ECC or concrete topping and propagated upward the loading point. The failure mode has been classified as a shear bond failure in which transverse tension cracks in topping materials (ECC or normal concrete) at or near
one of the loading point accompanied with longitudinal separation between topping material and profiled steel sheeting which lead to complete loss of composite action as shown in Fig. 5. The failure mode of SS concrete composite slabs was brittle mode, while ECC composite slab failed in ductile mode. Unlike concrete composite slab, after reaching the peak condition, ECC slabs didn't rupture suddenly and also had not been accompanied by crushing of ECC at the top surface (compression zone). Whereas for LS composite slabs, the shear force at the support is lower than SS. It has been noticed that, for ECC composite slabs, cracks were approximately occurred in vertical direction in between loading points. However, when plastic bending capacity of the composite slab is reached, longitudinal shear failure would occur characterized by slip between the profiled steel sheeting and ECC topping. In addition, for all LS of ECC composite slabs, final deflection at rupture was higher
Fig. 3. Testing setup of ECC composite slab with long shear span.
Fig. 4. Testing setup of ECC composite slab with short shear span.
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The direct tensile test has been carried out on 3 test specimens of each ECC and normal strength concrete (NSC) as shown in Fig. 6. As shown in Fig. 7 and Table 3, in comparison to NSC, the direct tensile strength and strain capacity of ECC specimens are increased by 103.6% and 562.8%, respectively. As shown in Fig. 7, ECC specimens can develop strain hardening until the peak load, then tension softening occurred after post-peak. It has also been noticed that ECC specimens do not rupture suddenly after peak condition but it can still perform plastic deformation until rupture.
3.2. Strains of composite slab at ECC and profiled steel sheeting
Fig. 5. Transverse separation of ECC and longitudinal separation between ECC and profiled steel sheeting.
than SS of ECC composite slabs and also fails in ductile mode; while the failure mode of LS concrete composite slab was brittle. This is due to loss of composite action (separation between concrete topping and profiled steel sheeting) at the shear span area.
a) test setup
The relationship between ECC strains versus loading at short shear span (SS) and long shear span (LS) is shown in Fig. 8. The ECC strains and at ultimate load have exhibited more superiority in comparison to normal concrete (NC), in which the mean value of ECC strain for SS is 350 μm/m or 48.94% higher than NC, whereas for LS is 850 μm/m or 466.67% larger than NC. While from Fig. 9, the values of steel stains of composite slabs with ECC for SS of 1040 μm/m and LS of 1575 μm/m are greater than those produced from composite slab with normal concrete by 28.40% and 43.18%, respectively. Although the maximum strains capacity of the ECC (1300 μm/m) and profiled steel sheeting (2750 μm/m) at failure load haven't been reached, nevertheless the loading capacity of the composite slab with ECC is much higher that those of normal concrete. Therefore, it can be suggested that using
b) test specimens
Fig. 6. Experimental setup of direct tensile test and test specimens.
a) ECC composite slabs
b) normal concrete composite slabs
Fig. 7. Stress–strain relationships from direct tensile tests of ECC & NSC.
B.S. Mohammed et al. / Structures 5 (2016) 247–257 Table 3 Direct tensile test results of ECC & NSC.
ECC Max strain (εmax: μm/m) Strain at rupture (εr: μm/m) Tensile strength (ftE: MPa)
Specimen 1 Specimen 2 Specimen 3
Average
55.4 82.6
64.5 87.8
59.5 81.4
59.8 83.9
3.3
3.3
3.7
3.4
11.9 12.1
14.9 14.9
12.5 12.7
NSC Max strain (εmax: μm/m) 10.8 Strain at rupture 11.0 (εr: μm/m) 1.8 Tensile strength (ftN: MPa) Comparison: ECC to NSC
1.7
1.6 Max strain Strain at rupture Tensile strength
1.7 4.8 6.6 2
251
Eurocode4, composite slab behavior is considered ductile if the maximum load exceeds the load causing the first 0.5 mm of the end slip by more than 10%. The loads that cause first slippage of 0.5 mm for LS and SS of composite slab are almost equal to the load that occurs at the first crack, as shown in Table 4. The relationship between loads and slippage at left and right ends of the short shear span (SS) and long shear span (LS) of the composite slabs is shown in Figs. 10 and 11, respectively. All of ECC composite slabs that have the load that causes the first 0.5 mm of end slippage were higher than 10% of maximum load. Unlike the normal concrete composite slab, all ECC composite slabs failed in ductile manner. This is mainly attributed to the ductility of ECC which forms better compatibility with profiled steel sheeting so that ECC composite slabs have better longitudinal shear resistance.
3.4. Mid-span deflection of composite slabs shear transfer devices might lead to utilizing the full capacity of ECC composite slabs.
3.3. Slippage at end of composite slabs Slight slippage has been occurred between the ECC or concrete topping and the steel deck at initial stage of loading. According to
Eurocode4 (EC4) has specified that failure load of a composite slab is considered as the load causes the mid-span deflects of span/50 (i.e. 2500/50 = 50 mm) except if the failure has already occurred. Most of the ECC composite slabs fail with high deflection (less than 50 mm) which achieves the serviceability limit to the utmost capacity. As shown in Fig. 12, the deflection at the mid-span of the composite slabs increases as the applied loading increases. However, at comparable
a) short span
b) long span Fig. 8. Load versus strain of ECC.
a) short span
b) long span Fig. 9. Load versus strain of profiled steel sheeting.
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Table 4 The load at first crack and load at slippage of 0.5 mm. Span
Specimen type
Shear Force, Vt (kN)
Failure load, P (kN)
Load at first crack, Pfc (kN)
Load at first slippage of 0.5 mm, Pfs (kN)
Long span (LS)
E1 E2 E3 NSC1 E1 E2 E3 NSC2
34.235 35.985 32.485 26.710 48.755 48.180 49.330 31.575
68.47 71.97 64.97 53.42 97.51 96.36 98.66 63.15
18.25 21.43 17.11 5.18 29.27 26.68 29.74 5.98
17.66 21.02 16.42 4.98 28.46 25.72 29.11 5.24
Short span (SS)
loads, composite slabs with long shear spans deflect more than those of short shear spans. This is due to the fact that at same loading stage, flexural moment acting on long shear spans is higher. However, the appearance of cracks is usually accompanied by cease of the linear trend of deflections. Therefore, cracks initiated in long shear span slabs at loading stages lower than that of short shear span slabs as shown in Table 4. It also has been noticed that deflection of ECC composite slabs is greater than normal concrete slabs and this is due to the more ductility and less rigidity of the ECC in comparison to the brittle and high ri-
a) left end slippage
gidity behavior of the normal concrete. Wong [21] indicates that composite slabs without shear transfer devices fail in brittle manner whereas the plastic deformation is small. While in composite slabs provided with shear transfer devices, large plastic deformation going together with substantial rise in the load carrying capacity is identified. Although the ECC composite slabs have not been provided with shear transfer devices, they can be categorized as ductile composite slabs with large plastic deformation and high failure load as shown in Fig. 12. It can be indicated that ECC composite slab system will provide
b) right end slippage
Fig. 10. Load versus end slippage in short shear span composite slabs.
a) left end slippage
b) right end slippage
Fig. 11. Load versus end slippage in long shear span composite slabs.
B.S. Mohammed et al. / Structures 5 (2016) 247–257
a) short span
253
b) long span
Fig. 12. Mid-span deflection versus applied loads of composite slabs.
economic solution by saving on the cost of the transfer shear devices and the time associated with their installation. In addition, constants (m–k and τ) can be used by manufacturers to provide designers with tables of load-span values for specific profiled steel sheeting used in the ECC composite slab system as these constants are limited to composite slabs without shear transfer devices. 3.5. Longitudinal shear bond strength of ECC composite slabs 3.5.1. Evaluation of shear bond strength using m–k method The evaluation of m–k values has been carried out according to the requirements of Eurocode4. As shown in Fig. 13, linear regression was obtained using Eqs. 1 & 2 to determine the two empirical constants, m and k. The parameters m and k stand for mechanical interlocking and the friction between concrete and profiled steel sheet, respectively. The m and k values have been computed from the reduced regression line. The reduced regression line has been obtained by applying 10% reduction to the actual regression line. The reduction is used to account for test variations and also to assure that line approaches a lower bound for experimental values. The obtained m and k values are 106.52 MPa and 0.27 MPa, respectively. Based on these values, the longitudinal shear bond strength (τu , Rd) of composite slab can be obtained using Eq. 3. The shear bond strength (τu , Rd) for each ECC
composite slab is shown in Table 5. The average value of τu, Rd is found to be 0.46. V t ¼ bdp
τu;Rd ¼
m AP þk b Lv
ð1Þ
AP mþk b Lv
ð2Þ
AP m þ k =γ vs b Ls
ð3Þ
Vt ¼ bdp
τu;Rd ¼
where, Vt is experimental shear force, b is width of composite slab, dp is depth measured between extreme fiber of the concrete slab in compression and the centroid of the profiled steel sheet, Ap is cross-sectional area of profiled steel sheet, γvs is the partial safety factor for the ultimate limit state and Lv is shear span. 3.5.2. Evaluation of shear bond strength using partial shear connection (PSC) The PSC method can also be used to calculate the longitudinal shear bond strength (τu,Rd) of the composite slab as detailed in Annex E of the Eurocode4 by using Eq. 4. Where Lo is length of overhang part of composite slab, η ¼ NNcfc is degree of shear connection, Ncf is compressive force in the concrete at full shear connection (η =1), and Nc is compressive force in the concrete related to the testing moment. Before calculation of τu , Rd, values of η (degree of shear connection) have to be determined. The procedure detailed in EC4, as explained in Fig. 14. τu;Rd ¼
η Ncf 1 b ðLs þ Lo Þ γvs
ð4Þ
The plastic moment resistance of composite slab at full shear connection (η = 1) and the profile moment of composite slab at zero shear connection (η = 0) have to be found first, as shown in Figs. 15 and 16 by using Eqs. 5–15.
Fig. 13. Linear regression for m–k method.
f yp;d ¼ 0:87f yp
ð5Þ
f cm ¼ 0:85 f cd ¼ 0:567 f ck
ð6Þ
Ncf ¼ 0:567f ck bx
ð7Þ
Np ¼ f yp;d Asp
ð8Þ
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Table 5 m–k values and τu,Rd. Span type
LS
SS
Slab ID.
E1 E2 E3 E1 E2 E3
b
dp
Lv
Ap
Vt
m
(mm)
(mm)
(mm)
(mm2)
(N)
(N/mm2)
0.38 0.38 0.38 0.54 0.54 0.54
109.65 109.65 109.65 109.65 109.65 109.65
900.00 900.00 900.00 450.00 450.00 450.00
1007.00 1007.00 1007.00 1007.00 1007.00 1007.00
34,235.00 35,985.00 32,485.00 48,755.50 48,180.50 49,330.50
106.52 106.52 106.52 106.52 106.52 106.52
Mean of τu,Rd
k
ϒvs
0.2743 0.2743 0.2743 0.2743 0.2743 0.2743
1.25 1.25 1.25 1.25 1.25 1.25
τu,Rd (N/mm2) 0.38 0.38 0.38 0.54 0.54 0.54 0.46
Fig. 14. Degree of shear connection.
Mpl;Rd ¼ Np z
ð9Þ
z ¼ dp −0:5x
ð10Þ
Mt ¼ V t Ls
ð11Þ
where Np is tensile force of steel deck, fyp is yield stress of steel deck, fck is compressive strength of concrete (ECC), z is lever arm of moment, Mpl,Rd is internal moment capacity of composite slab, and Mt is applied moment. The calculation result can be seen in Table 6. Np1 ¼ f yp;d Asp1
ð12Þ
Np2 ¼ f yp;d Asp2
ð13Þ
e ¼ D−dp
ð14Þ
Mprofile ¼ Np1 z1
ð15Þ
where, Np1 is tensile force of steel deck in tension area, Np2 is compressive force of steel deck in compression region, D is total height of composite slab, Asp1 is section area of steel deck in tension area, Asp2 is section area of steel deck in compression area, hcs is thickness of concrete slab above steel deck, hsd is height of steel deck, z1 is lever arm of moment, and Mprofile is internal moment capacity of steel deck at η = 0 (no shear connection between concrete and steel deck), and by using Eqs. 12–15: Mprofile is 7.396 kNm and Mprofile/Mpl,Rd ratio is 0.149. For the maximum condition in producing Fig. 17 of shear connection degree, the maximum test moment was taken equal to the plastic moment resistance of composite slab; so that the moment ratio is one. The value of ηtest in Fig. 17 is an example of η graph for the maximum test moment in which Mt is 32.38 kNm, Mt/Mpl,Rd is 0.654, and ηtest is 0.594. The overall values of ηtest can be seen in Table 7, whereas Eq. 4 can be used to calculate τu,Rd for each specimen.
Fig. 15. Force equilibrium at full shear connection.
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Fig. 16. Profile moment of composite slab at η = 0.
For each type of composite slabs either long shear span (LS) or short shear span (SS), it can be noted that the ability of composite slab to withstand the applied moment is indicated by the value of η. Therefore, the largest value of η is showed that the test moment of composite slab is maximum. In comparison to SS, LS has larger η which means that the ability of LS has higher moment capacity. In addition, if the test moment and η is maximum, so the value of τu,Rd is maximum. Therefore, it can be agreed that higher longitudinal shear bond strength of composite slab (specified by higher η) can be achieved by higher composite action and higher compatibility between the profiled steel sheeting and ECC topping. To evaluate accuracy of the longitudinal shear bond strength obtained by m–k and PSC methods, experimental direct bonding test (DBT) has been carried out as shown in Fig. 18. Three small scales of test specimens were prepared, cast and tested up to failure. The test results are shown in Table 8.
Table 6 Plastic moment resistance of composite slab. Slab No.
Ls (mm)
Vt (kN)
Mt (kNm)
Ncf (kN)
Np (kN)
z (mm)
Mpl,Rd (kNm)
Mt/Mpl,Rd
LS-1 LS-2 LS-3 SS-1 SS-2 SS-3
900 900 900 450 450 450
34.23 35.98 32.48 48.75 48.18 49.33
30.81 32.38 29.23 21.93 21.68 22.19
481.85 481.85 481.85 481.85 481.85 481.85
481.85 481.85 481.85 481.85 481.85 481.85
102.70 102.70 102.70 102.70 102.70 102.70
49.49 49.49 49.49 49.49 49.49 49.49
0.623 0.654 0.591 0.443 0.438 0.449
Referring to the result test of full scale model, the difference in calculating the longitudinal shear bond strength (τu , Rd) using the m–k and PSC methods can be obtained as follows: Difference ¼
ðm‐k valueÞ−ðPSC valueÞ 100 ¼ 15:87%: ðm‐k valueÞ
Based on the direct bonding test results, the average values of the longitudinal shear bond strength (τu , Rd) is 0.35; this value is lesser than m–k and PSC values. Many parameters may contribute to the differences of these values such as: number of embossment, type of test (type of applied load), and contact surface area. Comparing the direct bonding test results of small scale model to these values, the difference in calculating the longitudinal shear bond strength (τu , Rd) can be obtained as follows: Difference ¼
ðm‐k valueÞ−ðDBT valueÞ 100 ¼ 23:69% ðm‐k valueÞ
Difference ¼
ðPSC valueÞ−ðDBT valueÞ 100 ¼ 9:30% ðPSC valueÞ
The longitudinal shear bond strength not only can be evaluated by using the full scale model as suggested by EC4, whereas full scale model testing is costly. But it can also be evaluated by another method using small scale model as has been reported by other researchers [22]. Therefore, the longitudinal shear bond strength can be evaluated using small scale models with good accuracy. 4. Comparison with previous studies using the same type of profiled steel sheeting and different concrete toppings Two studies have been carried out earlier to determine the shear bond strength of composite slabs with rubbercrete topping [12] and palm oil clinker concrete (POCC) topping [8]. The composite slabs geometry, lab preparation, testing conditions and type of the profiled
Table 7 Values of ηtest and τu,Rd for each Mtest in LS and SS type. Type LS
Fig. 17. Value of ηtest for maximum Mtest.
Slab No.
Ls (mm)
Vt (kN)
LS-1 900 34.24 LS-2 900 35.99 LS-3 900 32.49 Mean value for LS SS SS-1 450 48.76 SS-2 450 48.18 SS-3 450 49.33 Mean value for SS Mean value for LS and SS
Mpl,Rd (kNm)
Mtest (kNm)
Mtest/Mpl,Rd
η
τu,Rd (MPa)
49.49 49.49 49.49
30.81 32.39 29.24
49.49 49.49 49.49
21.94 21.68 22.20
0.623 0.654 0.591 0.623 0.443 0.438 0.449 0.443 0.533
0.556 0.594 0.519 0.556 0.346 0.339 0.352 0.346 0.451
0.363 0.388 0.339 0.363 0.410 0.403 0.418 0.410 0.387
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a) test setup
b) test specimen
Fig. 18. Experimental direct bonding strength setup & test specimens.
steel sheeting are the same. All these composite slabs have showed a ductile failure mode, whereas the ultimate applied load was higher by more than 10% than the load causing 0.5 mm end-slippage. The longitudinal shear bond strength (τu,Rd) values based on m–k method and PSC method for the three types of composite slab topping are shown in Table 9. The composite slabs with PVA-ECC topping have showed higher longitudinal shear bond strength than the other two types of concrete. However, the results proved that the higher ductility of concrete topping would result in higher longitudinal shear bond strength, where the PVA-ECC has more ductile than the rubbercrete which in turn showed higher ductility than palm oil clinker concrete. In addition, by examining the average ultimate loads of theses slabs: PVA-ECC, rubbercrete and POCC for short shear span are 84 kN, 63.5 kN and 59 kN and also for long shear span are 68.5 kN, 50.3 kN and 45 kN, respectively. This provides evidence that the load carrying capacity of composite slabs increases as ductility of concrete topping increases which leads to better economic solution on using composite slabs in the construction industry. ECC is one of the materials that recommended to be used as concrete topping of composite slab, many benefits of using ECC [13–19] such as:
Table 8 Test results of direct bonding strength. Spec. No
Tensile load, P (kN)
1 22.66 2 21.82 3 24.53 Average of τu
Contact surface area, As (mm2)
Direct bonding strength, τu (MPa)
65,455 65,455 65,455
0.35 0.33 0.37 0.35
high long term strength, high early strength, high durability, lightweight, high tensile strength, high tensile strain, and high ductility. ECC is also better material as concrete topping of composite slab, compared to normal concrete. In addition, ECC has good bonding with steel deck due to the mechanical interaction between fiber, matrix and interface so that ECC still can resist stresses and strains even the crack width is growing wider as shown in Fig. 19, so that ECC has higher longitudinal shear bond strength compared to normal concrete. In addition, EC4 states that the weight of concrete contributes to the deflection of steel deck which termed as ponding effect. Therefore, to avoid ponding effect, ECC is one solution that can be used as a topping material in composite slab. 5. Observations and conclusions A series of short-term static load tests on large-scale ECC composite slabs has been described and the results have been presented. Two groups of identical composite slabs were prepared, cast and tested with two shear spans of long-span (900 mm) and short-span (450 mm) until failure. Each group comprises of three PVA-ECC composite slabs and one slab made with normal concrete as control slab. The main objective of the research work reported in this paper is to investigate the enhanced performance of PVA-ECC composite slabs in comparison to the normal concrete and other types of concrete composite slabs. Based on the experimental and analytical data presented in this study, the following conclusions can be made: 1- The failure mode of PVA-ECC composite slab is ductile, therefore it would be safe to be used in the construction industry. In addition, PVA-ECC composite slab fulfill the serviceability requirements as the deflection at the failure is within the limit.
Table 9 The longitudinal shear bond strength (τu,Rd) values for different types of composite slabs. Type of composite slab topping
τu,Rd (m–k method) (N/mm2)
τu,Rd (PSC method) (N/mm2)
Percentage of differences (m–k and PSC methods) (%)
Palm oil clinker concrete Rubbercrete PVA-ECC
0.248 0.342 0.46
0.215 0.242 0.387
13.3 29.2 15.87
B.S. Mohammed et al. / Structures 5 (2016) 247–257
Fig. 19. The stress–strain-crack width relationship of ECC (Lepech & Li, 2009).
2- PVA-ECC composite slabs would provide better economic solution as their load carrying capacity is greater than other types of concrete topping. 3- Due to the ductile failure mode of the PVA-ECC composite slabs, longitudinal shear bond strength can be evaluated using m–k method and partial shear connection method. 4- The strains on top fiber of PVA-ECC and soffit of the profiled steel sheeting didn't reach the ultimate strain values which indicates that the load carrying capacity of these slabs can be improved further by providing shear transfer devices. 5- The PVA-ECC composite slabs are lighter than the normal weight concrete slabs by about 8.2%. Therefore, using these slabs will lead to lower self-weight of the structure. Acknowledgment The authors would like to thank the Ministry of Education (MOE) of Malaysia and Universiti Teknologi PETRONAS for granting the project under codes FRGS 2013-2 and STIRF 38/2012, respectively. References [1] Kaveh A, Behnamb AF. Cost optimization of a composite floor system, one-way waffle slab, and concrete slab formwork using a charged system search algorithm. Sci Iran A 2012;19(3):410–6.
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[2] Mullett DL. Composite floor systems. The Steel Construction Institute, Blackwell Science Publishing; 2000. [3] VV D. Strength of composite slabs with end anchorages. Part II: parametric studies. J Constr Steel Res 2014;94:163–75. [4] A. G, RI. G, MA. B, ZT. C. Longitudinal shear stress and bond–slip relationships in composite concrete slabs. Eng Struct 2014;69:37–48. [5] SAL. A, PJGDS. V, JGSDS. S, TH. T. Standardized composite slab systems for building constructions. J Constr Steel Res 2004;60:493–524. [6] H. C, F. M. Experimental study on shear behavior of composite slabs according to Eurocode4. J Constr Steel Res 2013;82:99–110. [7] M. C, P. E. New method for the design of composite slabs. Proceedings of composite construction in steel and concrete. South Africa: American Society of Civil Engineers; 2004. [8] BS. M, MA. A-G, M. A. Analytical and experimental studies on composite slabs utilizing palm oil clinker concrete. Construct Build Mater 2011;25(8):3550–60. [9] V. M, S. S, SA. J, A. C, TK. B, D. D. Experimental studies on composite deck slabs to determine the shear-bond characteristics (m–k) values of the embossed profiled sheet. J Constr Steel Res 2007;63:791–803. [10] M P, R B. Partial shear connection design of composite slabs. Eng Struct J 1994;V16: 348–62. [11] H K, Y J. Ultimate strength of a steel–concrete composite bridge deck slab with profiled sheeting. Eng Struct J 2010;V32:534–46. [12] BS M. Structural behavior and m–k value of composite slab utilizing concrete containing crumb rubber. Construct Build Mater 2010;24:1214–21. [13] VC L. Engineered cementitious composites through micromechanical modeling. Fiber reinforced concrete: present and the future. Canadian Society of Civil Engineers; 1997. p. 1–38. [14] VC. L, MD. L, M. L. Field demonstration of durable link slabs for jointless bridge decks based on strain-hardening cementitious composites. Research report R-1471; 2005 (Machigan). [15] MD. L, VC. L. Application of engineered cementitious for bridge deck link slabs. Mater Struct 2009;42:1185–95. [16] - Fukuyama H., Sato Y., Li VC., Matsuzaki Y., and Milhashi H. Ductile engineered cementitious composite elements for structural application 2000; in 12 WWCEE, paper 1672, (Auckland, New Zealand) [17] - Li VC. On engineered cementitious composite (ECC). Adv Concr Technol 1 2003; 3: 215–230. [18] VC L. From micromechanics to structural engineering — the design of cementitious composite for civil engineering applications. JSCE J Struct Mech Earthq Eng 1993; 10(2):37–48. [19] K. T-F, B. F, R. R, BY L, VC L. The role of flaw size and fiber distribution on tensile ductility of PVA-ECC. Compos Part B 2014;56:536–45. [20] EN 1994-1-1: Eurocode4; design of composite steel concrete structures, Part 1.1: general rules and rules for buildings; 2004. [21] CK W. Feasibility studies of two-way composite steel-deck slab. Texas Tech University; 1987(December. Master thesis). [22] - Holomek J, Bajer M, Kala J. Comparison of design methods for composite slabs using small-scale shear tests. Advances in mathematics and statistical sciences 2015.
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Structures journal homepage: www.elsevier.com/locate/structures
Regular article
Longitudinal shear resistance of PVA-ECC composite slabs Bashar S. Mohammed ⁎,1, Muhammad Aswin 1, Walden Harry Beatty 1, Muhammad Hafiz 1 Department of Civil and Environmental Engineering, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak Darul Ridzuan, Malaysia
a r t i c l e
i n f o
Article history: Received 5 September 2015 Received in revised form 27 November 2015 Accepted 23 December 2015 Available online 4 January 2016 Keywords: Engineered cementitious composite (ECC) Composite slab Profiled steel sheeting Longitudinal shear bond strength m–k values
a b s t r a c t Including polyvinyl alcohol (PVA) fiber to develop a ductile engineered cementitious composite (ECC) is an effective way to overcome the brittle behavior of conventional concrete and to increase the efficiency of composite slab construction. Two groups of full-scale composite slabs were prepared, cast and tested until failure. Each group consists of three PVA-ECC composite slabs and one control specimen made with normal concrete topping. The first group has been tested with long shear span of 900 mm while for the second group, the shear span was 450 mm. Test results have been analyzed and the longitudinal shear bond strength has been computed using both methods of m–k and partial shear connection (PSC). In addition, comparison with other types of composite slabs using the same profiled steel sheeting was also carried out. The longitudinal shear bond strength was found to be 0.46 MPa and 0.387 MPa using m–k and PSC, respectively, with difference of 15.87%. The results showed that the failure modes of the PVA-ECC composite slabs are ductile with enhanced load carrying capacity. Therefore the use of PVA-ECC composite slabs in the construction industry would consider a better economic solution. © 2015 The Institution of Structural Engineers. Published by Elsevier Ltd. All rights reserved.
1. Introduction The composite beam is one of general types of slab in steel constructions. The composite beam usually consists of steel beams and concrete slab where steel beams support the concrete slab and shear connectors are used to produce the integrity of the system. These shear connectors are welded from one side to the upper flange of the steel beam, and are extended into the concrete slab. Therefore, the formwork for concrete slab is commonly designed to carry the weight of fresh concrete before it can withstand the hardened self-weight. The formwork of concrete slabs usually involves sheathings, joists, stringers and shores [1]. This system can add to problems in construction weight, construction speed, etc. Mullett [2] stated that steel skeleton-framed buildings incorporating the composite slabs are quite popular and economical in steel construction due to their lightweight nature and speed of erection. Composite slabs are the main system of floor construction in steelframed buildings [3]. They are constructed from cold formed profiled steel sheeting with concrete topping. The profiled steel sheeting has two main advantages. Firstly, during construction process, profiled steel sheeting will act as a permanent formwork which can safely carry the weight of fresh concrete without supporting false-work. Secondly, after concrete is hardened, the profiled steel sheeting will act as tensile reinforcement to resist tensile stresses at the soffit of the composite slab via the composite action between the two materials [4]. The composite action is achieved by mechanical and physical means. ⁎ Corresponding author. E-mail address: [email protected] (B.S. Mohammed). 1 Tel.: +605 3687305.
The mechanical interlocking is provided by the embossment on the profiled steel sheeting and the physical is obtained by the frictional interlocking between the re-entrant of the profiled steel sheeting and concrete topping [3]. Other advantages of the composite slab system are quick installation, working platform and reduced floor to floor height which leads to reduction in the total dead load of the building [3,5]. The common failure mode of composite slabs is the longitudinal shear failure in which the slip would occur between the profiled steel sheeting and concrete topping which mainly depends on the composite action between the two materials. However, it is worth noting that the failure load of composite slabs is far from the ultimate bending moment [6]. There are two methods to design the composite slabs which are shear bond method, also known as m–k method and partial shear connection method (τ). Both methods are required experimental data which can be obtained from testing two groups of identical full-scale composite slabs; each group consists of 3 slabs. The first group is to be tested with long shear span of 900 mm, while for the second group; the shear span is 450 mm. These constants (m–k and τ) are used by manufacturers to provide designers with tables containing the range of load-span values for specific profiled steel sheeting. The load carrying capacity of composite slabs depends on the shear-bond resistance at the interface between the profiled steel sheeting and concrete [7–9]. The performance of composite action also depends on several factors involving geometry and thickness of steel deck, concrete compressive strength and loading arrangement [10]. Moreover, other composite actions can be achieved by using the shear studs or similar shear devices [11]. Ability to resist the vertical separation is also obtained by the amount and type of embossments, as well as by adopting a proper shape of the steel deck profile. Furthermore, the shear-bond resistance is affected
http://dx.doi.org/10.1016/j.istruc.2015.12.003 2352-0124/© 2015 The Institution of Structural Engineers. Published by Elsevier Ltd. All rights reserved.
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Table 1 ECC and normal concrete proportions. Ingredients
ECC
Normal concrete
Cement (kg/m3) Coarse aggregate (kg/m3) Fine aggregate (kg/m3) Water cement ratio Fly ash (kg/m3) Superplasticizer (kg/m3) Polyvinyl alcohol (%)
583 – 467 0.32 700 9.5 2
500 520 1105 0.44 – – –
avoiding fiber clogging. The mixing lasted for at least 8 min until the fibers were fully dispersed. The ECC mixture then poured into the prepared profiled steel sheeting with side formwork. The formwork was de-molded after 24 h and soaked in water jute bags which covered the slabs to allow for proper curing until the testing day. The composite slabs made with normal concrete topping were also prepared under the same environment and curing condition. The hardened properties (average of 3 samples) of the ECC are reported in Table 2. 2.2. Properties of the profiled steel sheeting
by compatibility of deformation properties between concrete and profiled steel sheeting. Profiled steel sheeting is ductile and normal concrete is brittle, if the two materials provide the poor compatibility so they will contribute the adverse effect for the efficiency of the composite slabs [12]. Therefore, enhancing the ductility of concrete will lead to improve the resistance of longitudinal shear stresses and subsequently increasing the load-carrying capacity of the theses composite slabs. Engineered cementitious composites (ECCs) were developed by Li [13] based on the micromechanical principles. The ECC mixtures exhibit wide strain hardening with up to 6% strain capacity. Due to the ductile behavior of ECC beams subjected to flexural and shear loadings, Li [13] has suggested to use the ECC in structural applications that require ductility and high energy absorption. Further details on ECC properties and behavior have been reported by other researchers [14–19]. Therefore, the main objective of the research work presented in this paper is to investigate the structural behavior of composite slabs made with ECC and also to evaluate the m–k and the partial shear connection values. 2. Experimental program 2.1. ECC materials, mix proportions and mechanical properties Mix proportions of ECC and normal concrete is shown in Table 1. For preparation of the ECC mixture, OPC, fly ash and sand were dry mixed in a concrete pan mixer for 3 min with half part of the PVA fiber added to the mix. Half portion of the water mixed with superplasticizer and added to the dry mixture, the mixing process continued for another 3–5 min. At this stage, the other portions of PVA and water were added slowly into the mixture to allow for thorough distribution and
Table 2 Mechanical properties of ECC. Mechanical properties
Average of 3 samples
Testing standard
Compressive strength (MPa) Splitting tensile strength (MPa) Modulus of rupture (MPa) Elastic modulus (GPa)
86.5 6.18 6.35 22
ASTM C39/C39M-04 A ASTM C496/C496M ∓04 ASTM C293-02 ASTM C469∓02
As shown in Fig. 1, the profiled steel sheeting used in this study has depth (hp) of 54 mm, thickness of 1 mm, and cross sectional area (Ap) of 980 mm2. The centroid axis of the profiled steel sheeting is 109.5 mm (measured from the outer top fiber of concrete), and 550 MPa tensile yield strength. Dimensions of the profiled steel sheeting are 2700 mm length and 590 mm width. 2.3. Composite slabs testing setup and instrumentations To achieve the objective of this study, eight full-scale composite slabs were prepared, cast and tested. Six slabs were made with engineered cementitious composite (ECC) topping and the other two with a normal concrete topping for comparison purposes. All slabs were prepared, cast and tested in accordance with the requirements of part 1.1 of Eurocode4 [20]: The composite slabs were divided into two groups, each group comprises of three ECC composite slabs and one control specimen with normal concrete topping. The first group was tested for long shear span of 900 mm and the second group for short shear span of 450 mm. Each composite slab had dimensions of 2700 mm length, 590 mm width and 125 mm overall depth. The concrete thickness above the ribs (hc) and steel profiled sheeting of depth (hp) are 71 mm and 54 mm, respectively. The schematic diagram of the experimental setup is shown in Fig. 2, whereas SG is the strain gauge, T is transducer and Ls is the shear span. 3. Results and discussion 3.1. General observation The experimental setup of ECC composite slabs with long and short shear spans is shown in Figs. 3 and 4, respectively. The load carrying capacity of composite slabs depends on the longitudinal shear bond strength at the interface between the profiled steel sheeting and concrete topping. In its turn, the longitudinal shear bond resistance depends on the mechanical and physical bonds of the concrete topping with the re-entrant and embossment of the steel deck, respectively. In order for the composite slab system to work effectively under vertical loading, the profiled steel decks and normal concrete or ECC toppings
Fig. 1. Profiled steel sheeting.
B.S. Mohammed et al. / Structures 5 (2016) 247–257
249
Fig. 2. The experimental setup for the composite slab.
must have the ability to transfer the induced longitudinal shear forces. With sufficient load carrying capacity, under loading, the composite slab behaves as a single structural component. Slight slippage has been occurred between the ECC or concrete toppings and the steel deck at initial stage of loading. However, at comparison loading stage for long shear span (LS) and short shear span (SS) of composite slabs, shear cracks have been initiated in normal concrete much earlier than ECC slabs due to early loss of composite action between normal concrete and profiled steel sheeting, which also indicated that ECC topping provides better bonding to the steel deck. For the SS of composite slab, the shear force at the support is larger than that of LS. It has been observed that cracks initiated in the shear span area at soffit of the ECC or concrete topping and propagated upward the loading point. The failure mode has been classified as a shear bond failure in which transverse tension cracks in topping materials (ECC or normal concrete) at or near
one of the loading point accompanied with longitudinal separation between topping material and profiled steel sheeting which lead to complete loss of composite action as shown in Fig. 5. The failure mode of SS concrete composite slabs was brittle mode, while ECC composite slab failed in ductile mode. Unlike concrete composite slab, after reaching the peak condition, ECC slabs didn't rupture suddenly and also had not been accompanied by crushing of ECC at the top surface (compression zone). Whereas for LS composite slabs, the shear force at the support is lower than SS. It has been noticed that, for ECC composite slabs, cracks were approximately occurred in vertical direction in between loading points. However, when plastic bending capacity of the composite slab is reached, longitudinal shear failure would occur characterized by slip between the profiled steel sheeting and ECC topping. In addition, for all LS of ECC composite slabs, final deflection at rupture was higher
Fig. 3. Testing setup of ECC composite slab with long shear span.
Fig. 4. Testing setup of ECC composite slab with short shear span.
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The direct tensile test has been carried out on 3 test specimens of each ECC and normal strength concrete (NSC) as shown in Fig. 6. As shown in Fig. 7 and Table 3, in comparison to NSC, the direct tensile strength and strain capacity of ECC specimens are increased by 103.6% and 562.8%, respectively. As shown in Fig. 7, ECC specimens can develop strain hardening until the peak load, then tension softening occurred after post-peak. It has also been noticed that ECC specimens do not rupture suddenly after peak condition but it can still perform plastic deformation until rupture.
3.2. Strains of composite slab at ECC and profiled steel sheeting
Fig. 5. Transverse separation of ECC and longitudinal separation between ECC and profiled steel sheeting.
than SS of ECC composite slabs and also fails in ductile mode; while the failure mode of LS concrete composite slab was brittle. This is due to loss of composite action (separation between concrete topping and profiled steel sheeting) at the shear span area.
a) test setup
The relationship between ECC strains versus loading at short shear span (SS) and long shear span (LS) is shown in Fig. 8. The ECC strains and at ultimate load have exhibited more superiority in comparison to normal concrete (NC), in which the mean value of ECC strain for SS is 350 μm/m or 48.94% higher than NC, whereas for LS is 850 μm/m or 466.67% larger than NC. While from Fig. 9, the values of steel stains of composite slabs with ECC for SS of 1040 μm/m and LS of 1575 μm/m are greater than those produced from composite slab with normal concrete by 28.40% and 43.18%, respectively. Although the maximum strains capacity of the ECC (1300 μm/m) and profiled steel sheeting (2750 μm/m) at failure load haven't been reached, nevertheless the loading capacity of the composite slab with ECC is much higher that those of normal concrete. Therefore, it can be suggested that using
b) test specimens
Fig. 6. Experimental setup of direct tensile test and test specimens.
a) ECC composite slabs
b) normal concrete composite slabs
Fig. 7. Stress–strain relationships from direct tensile tests of ECC & NSC.
B.S. Mohammed et al. / Structures 5 (2016) 247–257 Table 3 Direct tensile test results of ECC & NSC.
ECC Max strain (εmax: μm/m) Strain at rupture (εr: μm/m) Tensile strength (ftE: MPa)
Specimen 1 Specimen 2 Specimen 3
Average
55.4 82.6
64.5 87.8
59.5 81.4
59.8 83.9
3.3
3.3
3.7
3.4
11.9 12.1
14.9 14.9
12.5 12.7
NSC Max strain (εmax: μm/m) 10.8 Strain at rupture 11.0 (εr: μm/m) 1.8 Tensile strength (ftN: MPa) Comparison: ECC to NSC
1.7
1.6 Max strain Strain at rupture Tensile strength
1.7 4.8 6.6 2
251
Eurocode4, composite slab behavior is considered ductile if the maximum load exceeds the load causing the first 0.5 mm of the end slip by more than 10%. The loads that cause first slippage of 0.5 mm for LS and SS of composite slab are almost equal to the load that occurs at the first crack, as shown in Table 4. The relationship between loads and slippage at left and right ends of the short shear span (SS) and long shear span (LS) of the composite slabs is shown in Figs. 10 and 11, respectively. All of ECC composite slabs that have the load that causes the first 0.5 mm of end slippage were higher than 10% of maximum load. Unlike the normal concrete composite slab, all ECC composite slabs failed in ductile manner. This is mainly attributed to the ductility of ECC which forms better compatibility with profiled steel sheeting so that ECC composite slabs have better longitudinal shear resistance.
3.4. Mid-span deflection of composite slabs shear transfer devices might lead to utilizing the full capacity of ECC composite slabs.
3.3. Slippage at end of composite slabs Slight slippage has been occurred between the ECC or concrete topping and the steel deck at initial stage of loading. According to
Eurocode4 (EC4) has specified that failure load of a composite slab is considered as the load causes the mid-span deflects of span/50 (i.e. 2500/50 = 50 mm) except if the failure has already occurred. Most of the ECC composite slabs fail with high deflection (less than 50 mm) which achieves the serviceability limit to the utmost capacity. As shown in Fig. 12, the deflection at the mid-span of the composite slabs increases as the applied loading increases. However, at comparable
a) short span
b) long span Fig. 8. Load versus strain of ECC.
a) short span
b) long span Fig. 9. Load versus strain of profiled steel sheeting.
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Table 4 The load at first crack and load at slippage of 0.5 mm. Span
Specimen type
Shear Force, Vt (kN)
Failure load, P (kN)
Load at first crack, Pfc (kN)
Load at first slippage of 0.5 mm, Pfs (kN)
Long span (LS)
E1 E2 E3 NSC1 E1 E2 E3 NSC2
34.235 35.985 32.485 26.710 48.755 48.180 49.330 31.575
68.47 71.97 64.97 53.42 97.51 96.36 98.66 63.15
18.25 21.43 17.11 5.18 29.27 26.68 29.74 5.98
17.66 21.02 16.42 4.98 28.46 25.72 29.11 5.24
Short span (SS)
loads, composite slabs with long shear spans deflect more than those of short shear spans. This is due to the fact that at same loading stage, flexural moment acting on long shear spans is higher. However, the appearance of cracks is usually accompanied by cease of the linear trend of deflections. Therefore, cracks initiated in long shear span slabs at loading stages lower than that of short shear span slabs as shown in Table 4. It also has been noticed that deflection of ECC composite slabs is greater than normal concrete slabs and this is due to the more ductility and less rigidity of the ECC in comparison to the brittle and high ri-
a) left end slippage
gidity behavior of the normal concrete. Wong [21] indicates that composite slabs without shear transfer devices fail in brittle manner whereas the plastic deformation is small. While in composite slabs provided with shear transfer devices, large plastic deformation going together with substantial rise in the load carrying capacity is identified. Although the ECC composite slabs have not been provided with shear transfer devices, they can be categorized as ductile composite slabs with large plastic deformation and high failure load as shown in Fig. 12. It can be indicated that ECC composite slab system will provide
b) right end slippage
Fig. 10. Load versus end slippage in short shear span composite slabs.
a) left end slippage
b) right end slippage
Fig. 11. Load versus end slippage in long shear span composite slabs.
B.S. Mohammed et al. / Structures 5 (2016) 247–257
a) short span
253
b) long span
Fig. 12. Mid-span deflection versus applied loads of composite slabs.
economic solution by saving on the cost of the transfer shear devices and the time associated with their installation. In addition, constants (m–k and τ) can be used by manufacturers to provide designers with tables of load-span values for specific profiled steel sheeting used in the ECC composite slab system as these constants are limited to composite slabs without shear transfer devices. 3.5. Longitudinal shear bond strength of ECC composite slabs 3.5.1. Evaluation of shear bond strength using m–k method The evaluation of m–k values has been carried out according to the requirements of Eurocode4. As shown in Fig. 13, linear regression was obtained using Eqs. 1 & 2 to determine the two empirical constants, m and k. The parameters m and k stand for mechanical interlocking and the friction between concrete and profiled steel sheet, respectively. The m and k values have been computed from the reduced regression line. The reduced regression line has been obtained by applying 10% reduction to the actual regression line. The reduction is used to account for test variations and also to assure that line approaches a lower bound for experimental values. The obtained m and k values are 106.52 MPa and 0.27 MPa, respectively. Based on these values, the longitudinal shear bond strength (τu , Rd) of composite slab can be obtained using Eq. 3. The shear bond strength (τu , Rd) for each ECC
composite slab is shown in Table 5. The average value of τu, Rd is found to be 0.46. V t ¼ bdp
τu;Rd ¼
m AP þk b Lv
ð1Þ
AP mþk b Lv
ð2Þ
AP m þ k =γ vs b Ls
ð3Þ
Vt ¼ bdp
τu;Rd ¼
where, Vt is experimental shear force, b is width of composite slab, dp is depth measured between extreme fiber of the concrete slab in compression and the centroid of the profiled steel sheet, Ap is cross-sectional area of profiled steel sheet, γvs is the partial safety factor for the ultimate limit state and Lv is shear span. 3.5.2. Evaluation of shear bond strength using partial shear connection (PSC) The PSC method can also be used to calculate the longitudinal shear bond strength (τu,Rd) of the composite slab as detailed in Annex E of the Eurocode4 by using Eq. 4. Where Lo is length of overhang part of composite slab, η ¼ NNcfc is degree of shear connection, Ncf is compressive force in the concrete at full shear connection (η =1), and Nc is compressive force in the concrete related to the testing moment. Before calculation of τu , Rd, values of η (degree of shear connection) have to be determined. The procedure detailed in EC4, as explained in Fig. 14. τu;Rd ¼
η Ncf 1 b ðLs þ Lo Þ γvs
ð4Þ
The plastic moment resistance of composite slab at full shear connection (η = 1) and the profile moment of composite slab at zero shear connection (η = 0) have to be found first, as shown in Figs. 15 and 16 by using Eqs. 5–15.
Fig. 13. Linear regression for m–k method.
f yp;d ¼ 0:87f yp
ð5Þ
f cm ¼ 0:85 f cd ¼ 0:567 f ck
ð6Þ
Ncf ¼ 0:567f ck bx
ð7Þ
Np ¼ f yp;d Asp
ð8Þ
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Table 5 m–k values and τu,Rd. Span type
LS
SS
Slab ID.
E1 E2 E3 E1 E2 E3
b
dp
Lv
Ap
Vt
m
(mm)
(mm)
(mm)
(mm2)
(N)
(N/mm2)
0.38 0.38 0.38 0.54 0.54 0.54
109.65 109.65 109.65 109.65 109.65 109.65
900.00 900.00 900.00 450.00 450.00 450.00
1007.00 1007.00 1007.00 1007.00 1007.00 1007.00
34,235.00 35,985.00 32,485.00 48,755.50 48,180.50 49,330.50
106.52 106.52 106.52 106.52 106.52 106.52
Mean of τu,Rd
k
ϒvs
0.2743 0.2743 0.2743 0.2743 0.2743 0.2743
1.25 1.25 1.25 1.25 1.25 1.25
τu,Rd (N/mm2) 0.38 0.38 0.38 0.54 0.54 0.54 0.46
Fig. 14. Degree of shear connection.
Mpl;Rd ¼ Np z
ð9Þ
z ¼ dp −0:5x
ð10Þ
Mt ¼ V t Ls
ð11Þ
where Np is tensile force of steel deck, fyp is yield stress of steel deck, fck is compressive strength of concrete (ECC), z is lever arm of moment, Mpl,Rd is internal moment capacity of composite slab, and Mt is applied moment. The calculation result can be seen in Table 6. Np1 ¼ f yp;d Asp1
ð12Þ
Np2 ¼ f yp;d Asp2
ð13Þ
e ¼ D−dp
ð14Þ
Mprofile ¼ Np1 z1
ð15Þ
where, Np1 is tensile force of steel deck in tension area, Np2 is compressive force of steel deck in compression region, D is total height of composite slab, Asp1 is section area of steel deck in tension area, Asp2 is section area of steel deck in compression area, hcs is thickness of concrete slab above steel deck, hsd is height of steel deck, z1 is lever arm of moment, and Mprofile is internal moment capacity of steel deck at η = 0 (no shear connection between concrete and steel deck), and by using Eqs. 12–15: Mprofile is 7.396 kNm and Mprofile/Mpl,Rd ratio is 0.149. For the maximum condition in producing Fig. 17 of shear connection degree, the maximum test moment was taken equal to the plastic moment resistance of composite slab; so that the moment ratio is one. The value of ηtest in Fig. 17 is an example of η graph for the maximum test moment in which Mt is 32.38 kNm, Mt/Mpl,Rd is 0.654, and ηtest is 0.594. The overall values of ηtest can be seen in Table 7, whereas Eq. 4 can be used to calculate τu,Rd for each specimen.
Fig. 15. Force equilibrium at full shear connection.
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Fig. 16. Profile moment of composite slab at η = 0.
For each type of composite slabs either long shear span (LS) or short shear span (SS), it can be noted that the ability of composite slab to withstand the applied moment is indicated by the value of η. Therefore, the largest value of η is showed that the test moment of composite slab is maximum. In comparison to SS, LS has larger η which means that the ability of LS has higher moment capacity. In addition, if the test moment and η is maximum, so the value of τu,Rd is maximum. Therefore, it can be agreed that higher longitudinal shear bond strength of composite slab (specified by higher η) can be achieved by higher composite action and higher compatibility between the profiled steel sheeting and ECC topping. To evaluate accuracy of the longitudinal shear bond strength obtained by m–k and PSC methods, experimental direct bonding test (DBT) has been carried out as shown in Fig. 18. Three small scales of test specimens were prepared, cast and tested up to failure. The test results are shown in Table 8.
Table 6 Plastic moment resistance of composite slab. Slab No.
Ls (mm)
Vt (kN)
Mt (kNm)
Ncf (kN)
Np (kN)
z (mm)
Mpl,Rd (kNm)
Mt/Mpl,Rd
LS-1 LS-2 LS-3 SS-1 SS-2 SS-3
900 900 900 450 450 450
34.23 35.98 32.48 48.75 48.18 49.33
30.81 32.38 29.23 21.93 21.68 22.19
481.85 481.85 481.85 481.85 481.85 481.85
481.85 481.85 481.85 481.85 481.85 481.85
102.70 102.70 102.70 102.70 102.70 102.70
49.49 49.49 49.49 49.49 49.49 49.49
0.623 0.654 0.591 0.443 0.438 0.449
Referring to the result test of full scale model, the difference in calculating the longitudinal shear bond strength (τu , Rd) using the m–k and PSC methods can be obtained as follows: Difference ¼
ðm‐k valueÞ−ðPSC valueÞ 100 ¼ 15:87%: ðm‐k valueÞ
Based on the direct bonding test results, the average values of the longitudinal shear bond strength (τu , Rd) is 0.35; this value is lesser than m–k and PSC values. Many parameters may contribute to the differences of these values such as: number of embossment, type of test (type of applied load), and contact surface area. Comparing the direct bonding test results of small scale model to these values, the difference in calculating the longitudinal shear bond strength (τu , Rd) can be obtained as follows: Difference ¼
ðm‐k valueÞ−ðDBT valueÞ 100 ¼ 23:69% ðm‐k valueÞ
Difference ¼
ðPSC valueÞ−ðDBT valueÞ 100 ¼ 9:30% ðPSC valueÞ
The longitudinal shear bond strength not only can be evaluated by using the full scale model as suggested by EC4, whereas full scale model testing is costly. But it can also be evaluated by another method using small scale model as has been reported by other researchers [22]. Therefore, the longitudinal shear bond strength can be evaluated using small scale models with good accuracy. 4. Comparison with previous studies using the same type of profiled steel sheeting and different concrete toppings Two studies have been carried out earlier to determine the shear bond strength of composite slabs with rubbercrete topping [12] and palm oil clinker concrete (POCC) topping [8]. The composite slabs geometry, lab preparation, testing conditions and type of the profiled
Table 7 Values of ηtest and τu,Rd for each Mtest in LS and SS type. Type LS
Fig. 17. Value of ηtest for maximum Mtest.
Slab No.
Ls (mm)
Vt (kN)
LS-1 900 34.24 LS-2 900 35.99 LS-3 900 32.49 Mean value for LS SS SS-1 450 48.76 SS-2 450 48.18 SS-3 450 49.33 Mean value for SS Mean value for LS and SS
Mpl,Rd (kNm)
Mtest (kNm)
Mtest/Mpl,Rd
η
τu,Rd (MPa)
49.49 49.49 49.49
30.81 32.39 29.24
49.49 49.49 49.49
21.94 21.68 22.20
0.623 0.654 0.591 0.623 0.443 0.438 0.449 0.443 0.533
0.556 0.594 0.519 0.556 0.346 0.339 0.352 0.346 0.451
0.363 0.388 0.339 0.363 0.410 0.403 0.418 0.410 0.387
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a) test setup
b) test specimen
Fig. 18. Experimental direct bonding strength setup & test specimens.
steel sheeting are the same. All these composite slabs have showed a ductile failure mode, whereas the ultimate applied load was higher by more than 10% than the load causing 0.5 mm end-slippage. The longitudinal shear bond strength (τu,Rd) values based on m–k method and PSC method for the three types of composite slab topping are shown in Table 9. The composite slabs with PVA-ECC topping have showed higher longitudinal shear bond strength than the other two types of concrete. However, the results proved that the higher ductility of concrete topping would result in higher longitudinal shear bond strength, where the PVA-ECC has more ductile than the rubbercrete which in turn showed higher ductility than palm oil clinker concrete. In addition, by examining the average ultimate loads of theses slabs: PVA-ECC, rubbercrete and POCC for short shear span are 84 kN, 63.5 kN and 59 kN and also for long shear span are 68.5 kN, 50.3 kN and 45 kN, respectively. This provides evidence that the load carrying capacity of composite slabs increases as ductility of concrete topping increases which leads to better economic solution on using composite slabs in the construction industry. ECC is one of the materials that recommended to be used as concrete topping of composite slab, many benefits of using ECC [13–19] such as:
Table 8 Test results of direct bonding strength. Spec. No
Tensile load, P (kN)
1 22.66 2 21.82 3 24.53 Average of τu
Contact surface area, As (mm2)
Direct bonding strength, τu (MPa)
65,455 65,455 65,455
0.35 0.33 0.37 0.35
high long term strength, high early strength, high durability, lightweight, high tensile strength, high tensile strain, and high ductility. ECC is also better material as concrete topping of composite slab, compared to normal concrete. In addition, ECC has good bonding with steel deck due to the mechanical interaction between fiber, matrix and interface so that ECC still can resist stresses and strains even the crack width is growing wider as shown in Fig. 19, so that ECC has higher longitudinal shear bond strength compared to normal concrete. In addition, EC4 states that the weight of concrete contributes to the deflection of steel deck which termed as ponding effect. Therefore, to avoid ponding effect, ECC is one solution that can be used as a topping material in composite slab. 5. Observations and conclusions A series of short-term static load tests on large-scale ECC composite slabs has been described and the results have been presented. Two groups of identical composite slabs were prepared, cast and tested with two shear spans of long-span (900 mm) and short-span (450 mm) until failure. Each group comprises of three PVA-ECC composite slabs and one slab made with normal concrete as control slab. The main objective of the research work reported in this paper is to investigate the enhanced performance of PVA-ECC composite slabs in comparison to the normal concrete and other types of concrete composite slabs. Based on the experimental and analytical data presented in this study, the following conclusions can be made: 1- The failure mode of PVA-ECC composite slab is ductile, therefore it would be safe to be used in the construction industry. In addition, PVA-ECC composite slab fulfill the serviceability requirements as the deflection at the failure is within the limit.
Table 9 The longitudinal shear bond strength (τu,Rd) values for different types of composite slabs. Type of composite slab topping
τu,Rd (m–k method) (N/mm2)
τu,Rd (PSC method) (N/mm2)
Percentage of differences (m–k and PSC methods) (%)
Palm oil clinker concrete Rubbercrete PVA-ECC
0.248 0.342 0.46
0.215 0.242 0.387
13.3 29.2 15.87
B.S. Mohammed et al. / Structures 5 (2016) 247–257
Fig. 19. The stress–strain-crack width relationship of ECC (Lepech & Li, 2009).
2- PVA-ECC composite slabs would provide better economic solution as their load carrying capacity is greater than other types of concrete topping. 3- Due to the ductile failure mode of the PVA-ECC composite slabs, longitudinal shear bond strength can be evaluated using m–k method and partial shear connection method. 4- The strains on top fiber of PVA-ECC and soffit of the profiled steel sheeting didn't reach the ultimate strain values which indicates that the load carrying capacity of these slabs can be improved further by providing shear transfer devices. 5- The PVA-ECC composite slabs are lighter than the normal weight concrete slabs by about 8.2%. Therefore, using these slabs will lead to lower self-weight of the structure. Acknowledgment The authors would like to thank the Ministry of Education (MOE) of Malaysia and Universiti Teknologi PETRONAS for granting the project under codes FRGS 2013-2 and STIRF 38/2012, respectively. References [1] Kaveh A, Behnamb AF. Cost optimization of a composite floor system, one-way waffle slab, and concrete slab formwork using a charged system search algorithm. Sci Iran A 2012;19(3):410–6.
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