Behaviour of simply supported steel reinforced SIFCON two way slabs in punching shear
H Sudarsana Raoa*, K Gnaneswara & N V Ramanab
aCivil Engineering, Jawaharlal Nehru Technological University College of Engineering, Anantapur 515 002, India
bRayalaseema Thermal Power Project, Aandhrapradesh Power Generation Corporation, V V Reddy Nagar, Kadapa, 516 312, India
Received 6 September 2007; accepted 11 June 2008
This paper reports the behaviour of slurry infiltrated fibrous concrete (SIFCON) two-way slabs in punching shear.
SIFCON slabs are cast with 8, 10 and 12% fibre volume fraction and for comparison, fibre reinforced concrete (FRC) with 2% fibre volume fraction and reinforced cement concrete (RCC) slabs are cast and tested. The results of the experimentation show that the SIFCON slabs with 12% fibre volume fraction exhibits excellent performance in punching shear among other slabs. The experimental results have been compared with the provisions of ACI and IS codes. A regression model has been proposed for estimating the punching shear strength of reinforced SIFCON slabs.
Slurry infiltrated fibrous concrete (SIFCON) is one of the new addition to the high performance concrete family. SIFCON is the extension for conventional FRC that differs in terms of fabrication and composition. In FRC the fibre content varies from 1 to 3% by volume whereas, in SIFCON, the fibre content may vary from 6 to 20%. SIFCON is prepared by infiltrating cement slurry into a bed of preplaced fibres. Even though, SIFCON is a recent construction material, it has found applications in the areas of pavements repairs, repair of bridge structures, safe vaults and defense structures due to its excellent energy absorption capacities1-3. Due to its extraordinary ductility characteristics, it has a lot of potential for applications in structures subjected to impact and dynamic loading. Lankard1 presented the basic properties of SIFCON (prepared with 12.5% of fibres) such as load-deflection curve, ultimate compressive and flexural strengths, impact and abrasion resistance. Bhupinder Singh et al.4 presented the method of production, structural properties and applications of SIFCON, compact reinforced composites (CRC) and densified small particles (DSP). Naman and Baccouche5 presented the shear response of dowel reinforced SIFCON and observed that the shear strength of SIFCON is 10 times higher than that of the plain matrix. Yan et al.6 studied mechanical properties and digital image analysis of SIFCON with 4%, 6%, 8% and 10% fibre volume
content. Fractal dimension is used as a parameter to characterize the crack pattern on the surface of SIFCON and concluded that there exists a good correlation between mechanical properties and fractal dimension. Sudarsana Rao and Ramana7 tested the SIFCON slab elements under flexure and compared the results with FRC and plain cement concrete (PCC) slabs and concluded that SIFCON slabs exhibit superior performance in flexure when compared to FRC and PCC slabs. Yazici et al.8 studied the effect of incorporating high volume of class C fly ash on mechanical properties of autoclaved SIFCON and concluded that by increase in the fibre volume remarkably increases flexural strength and toughness of SIFCON. This behaviour was more pronounced at 10% fibre volume. Mansur et al.9 reported the punching shear test on square ferro cement slabs.
This paper presents experimental results that describe the behaviour of steel reinforced SIFCON simply supported slabs in punching shear. In this experimental study, SIFCON slabs are produced by using black steel wire fibres. The investigation envisages studying the strength and deflection behaviour of steel reinforced SIFCON slabs under punching shear. The results are compared with slabs made of reinforced cement concrete (RCC) and fibre reinforced concrete (FRC).
Experimental Procedure
The experimental program comprise of casting and testing of nine reinforced SIFCON slabs (8, 10 and
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12% fibre volume), three fibre reinforced concrete slabs (2% fibre) and three RCC slabs (M20 grade concrete and Fe 415 steel) simply supported on all four edges. The mix proportions of the various slabs are presented in Table 1. All the slabs are square and are of size 600 × 600 × 50 mm. The slabs are white washed for easy identification of crack patterns and placed over the platform for testing. To simulate the simply supported edge condition, 16 mm mild steel rods are placed in between the slab and platform.
Materials
Material properties of cement, fine aggregate and water are presented in Table 2.
Cement
Ordinary Portland cement of 53 grade manufactured by Birla Company confirming to IS 12269 was used. The specific gravity of the cement was 3.01. The initial and final setting times were found as 40 min and 340 min respectively.
Fine aggregate
Locally available river sand passing through 4.75 mm IS sieve was used. The specific gravity of the sand is found to be 2.62.
Coarse-aggregate
Crushed granite aggregate available from local sources has been used. To obtain a reasonably good grading, 50% of the aggregate passing through 20 mm IS sieve and retained on 12.5 mm IS sieve and 50% of the aggregate passing through 12.5 mm IS sieve and retained on 10 mm IS sieve was used in preparation of FRC and RCC slab specimens.
Reinforcement
All the slabs are reinforced with 8 mm diameter Fe 415 grade steel rods, placed at 150 mm spacing in both directions. The yield strength of steel bars was 250 N/mm2.
Fibres
The present investigation aims at producing SIFCON with locally available fibres. Accordingly, black annealed steel wire fibres of 1.0 mm diameter were used. The fibres were cut to the required length of 50 mm by using shear cutting equipment giving an aspect ratio of 50. The ultimate tensile strength of fibre was 395 MPa. These black steel wires are commercially available and are generally used for binding the steel reinforcement in RCC works.
Table 1 — Mix proportions S.No Type of slab Mix proportion Number of
slab specimens
Volume fraction of
fibre
W/C ratio Dosage of super plasticizer
Mode of vibration
1 SIFCON (8%) Cement and sand (1:1 by wt)
3 8% 0.45 1.5% Hand tamping
2 SIFCON (10%) Cement and sand (1:1 by wt)
3 10% 0.45 1.5% Hand tamping
3 SIFCON (12%) Cement and sand (1:1 by wt)
3 12% 0.45 1.5% Hand tamping
4 FRC (2%) Cement, sand and coarse aggregate (1:1.54:3.17)
3 2% 0.50 Nil Table vibration
5 RCC Cement, sand and
coarse aggregate (1:1.54:3.17)
3 No fibres 0.50 Nil Table vibration
Table 2 — Material properties
Cement Fine aggregate Water
Fineness of cement(Blains specific surface area) – 320 m2/kg Specific gravity - 2.62 pH value – 7.0
Specific gravity – 3.01 Fineness modulus – 2.74 Turbidity (NT unit) – 3.0
Initial setting time – 40 minutes Final setting time –340 minutes
Density
Loose state – 14.67 kN/m3 Dense state – 16.04 kN/m3
Hardness(mg/L) – 400
Compressive strengths 3 days – 28.2 N/mm2 7 days – 38.7 N/mm2 28 days – 54.3 N/mm2
Grading – zone II Sulphates (mg/L) – 300 Chlorides (mg/L) – 280
Water
Potable fresh water available from local sources was used for mixing and curing of SIFCON, FRC and RCC slabs.
Super plasticizer
To improve the workability in slurry and concrete, CONPLAST-300 confirming to IS 9103-1999 and ASTM C 494 type F high range water-reducing agent has been used.
Casting of test specimens
Steel moulds were used to cast the slab specimens of required size. Two L-shaped frames with a depth of 50 mm were connected to a flat plate at the bottom using nuts and bolts. Cross-stiffeners were provided to the flat plate at the bottom to prevent any possible deflection while casting the specimens. The gaps were effectively sealed by using thin card-boards and wax to prevent any leakage of cement-sand slurry in SIFCON slab specimens. The moulds are shown in Fig. 1. Initially, the steel mould was coated with waste oil so that the slab specimens can be removed easily from the moulds. Then the mat of 8 mm steel rods @ 150 mm c/c was kept, at the bottom of mould over 10 mm thick layer of sprinkled fibres. Then the steel fibres are placed randomly in the mould such that they occupy the entire volume of the mould. In the mean time, cement-sand slurry was prepared using CONPLAST – 300 which was later poured into the mould uniformly over the pre-placed mat and fibres.
The details of casting are shown in Fig 2. In case of fibre reinforced concrete slabs, fibres are first mixed in the dry mixture of cement and sand and then spread over the heap of coarse aggregate. Hand mixing was done after adding required quantity of water to achieve uniform dispersion of fibres and to prevent the segregation or balling of fibres during mixing. For
both FRC and RCC specimens, table vibration was adopted. The test specimens were de-moulded after 24 h and were cured for 28 days in curing water ponds. After removing the slab specimens from the curing pond, they were allowed to dry under shade for a while and then they were coated with white paint on both sides, to achieve clear visibility of cracks during testing. The loading position on the top and the dial gauge position at the bottom of the slab were marked with black paint.
Loading arrangement and testing
The set-up for loading the slab consists of a solid plate of 100 × 100 × 20 mm placed at the center of the top face of slab specimen. Over this solid plate, solid circular rod of 50 mm diameter was kept to distribute the load from hydraulic jack to the slab specimen. The whole arrangement has been made to obtain the punching shear effect on the slab specimen, as shown in Fig. 3. The loading platform consists of four welded steel beams of ISMB 200@254 N/m in square shape. These steel beams were stiffened using small size steel I-Sections (ISMB100@50N/m). This loading platform has been supported by brick walls on two sides and the other two sides are supported with two steel rods. The load was applied through hydraulic jack and was measured with a calibrated proving ring of 500 kN capacity. The vertical deflections were measured by using dial gauge with a least count of 0.01 mm. The vertical deflections were measured at the centre of the slab specimens.
The load has been applied incrementally. The load increment was selected such that there will be as many number of readings as possible. The load was applied in increments of 833.3 N which corresponds to one unit of proving ring. Deflections have been recorded for each load increment. The load at the first crack and the corresponding deflection at the bottom
Fig. 1 — Slab mould filled with fibre and steel reinforcement Fig. 2 — Casting of steel reinforced SIFCON slabs
centre of the slab were recorded. The ultimate punching shear load and corresponding deflection at the centre were also observed and recorded.
The determine cube compressive strength of RCC and FRC, six numbers of cubes (3-RCC, 3-FRC) of 150 × 150 × 150 mm size have been cast and tested, and the average compressive strength of three cubes for RCC and FRC is 20.1 N/mm2 and 32 N/mm2 respectively. Similarly to determine cube compressive strength of SIFCON, thirty number of cubes (10 – SIFCON 8%, 10 – SIFCON 10% and 10 – SIFCON 12%) were cast and tested and the compressive strengths are given in Table 3.
Results and Discussion
The results of the experimental investigation are summarized in Table 4. The values presented here represent the average of punching shear strengths, load and deflection obtained for three slab specimens in each series. The effect of percentage of fibres on the ultimate punching shear load of the steel reinforced SIFCON slabs is shown in Table 4. From Table 4, it is observed that there is an increase in first
crack strength with the increase in volume fraction of fibres in punching shear. The slab specimens reinforced with higher volume fraction of fibres behaved better than those containing lower volume fractions of fibres. The maximum first crack load of 61.82 kN has been achieved for slabs reinforced with 12% volume fraction of fibres. FRC slab specimens and RCC slab specimens have failed at significantly lower loads. The percentage increase in first crack load in steel reinforced SIFCON slab specimen when compared to RCC and FRC slab specimens is in the range of 1121-1755% and 876-1384% respectively for different volume fractions of fibres. This confirms the superior performance of steel reinforced SIFCON specimens in punching shear.
From Table 4, it is observed that ultimate punching shear strength increases with increase of volume fraction of fibre in steel reinforced SIFCON slab specimens. The maximum ultimate punching shear load of 124 kN has been obtained for steel reinforced SIFCON slabs with 12% volume fraction of fibres which is 21% higher than that of 8% volume fraction slab specimens. The ultimate punching shear strength of steel reinforced SIFCON slab specimens is about 665-830% when compared to FRC slab specimens with maximum values corresponding to 12% volume fraction of fibres. The increment when compared to RCC specimens is about 841-1045% .
The maximum central deflection values of various slab specimens are presented in Table 4. From this table, it is also observed that the maximum central deflections and the ultimate punching shear load of
Fig. 3 — Testing of slabs for punching shear
Table 3 — 28 day cube compressive strength of SIFCON Cube compressive strength (N/mm2) Nomenclature
1 2 3 4 5 6 7 8 9 10
SIFCON-8% 40.00 45.00 41.00 47.11 39.07 47.95 44.44 51.55 48.84 43.51
SIFCON-10% 51.06 51.50 53.28 55.05 48.84 51.04 51.06 53.28 48.84 51.84
SIFCON-12% 52.80 57.72 57.72 52.39 52.39 55.50 55.50 54.56 52.80 56.32
Table 4 — Details of test results Nomenclature First crack
load (kN)
Ultimate punching shear load
(kN)
Maximum Central deflection in
punching shear (mm)
RCC 3.332 10.829 10.25
FRC-2% 4.165 13.328 15.50
SIFCON-8% 40.690 102.000 20.50
SIFCON-10% 51.900 114.000 21.50
SIFCON-12% 61.820 124.000 22.50
reinforced SIFCON slab specimens are considerably higher than RCC and FRC specimens indicating higher energy absorption capacities of reinforced SIFCON slab specimens. The steel reinforced SIFCON slabs have not only carried higher loads, but also sustained greater deflections till ultimate stage.
The crack patterns of different slab specimens are depicted in Figs 4-8. From Figs 4-6, it is observed that the crack pattern is almost similar in all SIFCON slabs. In all slabs, the first crack originated at the centre and propagates radially towards the corners. At higher loads, already formed cracks get widened with formation of new cracks. The new formations of cracks are mainly concentrated at the point of application of punching load. Cracks are mainly localized up to a particular distance from the loading point in circular area. This circular area increases with increase in volume fraction of fibres in steel reinforced SIFCON slab specimens. A few cracks are identified on the top surface steel reinforced SIFCON slab specimens. Similar type of failure pattern was reported earlier9.
From Fig. 7, it is observed that the FRC slab specimens show more number of cracks underneath
the slab. A few hair cracks are noticed on the top surface. From Fig. 8, it is observed that the RCC slab specimens show very much pealing of concrete on bottom face. Regarding strength, load deflection and crack pattern the steel reinforced SIFCON slab specimens exhibit superior performance compared to FRC and RCC slab specimens. This may be due to the incorporation of higher volume fraction of fibres which lead to crack arresting and crack bridge mechanism in the matrix.
Comparisons of experimental results with codal provisions
There is hardly any code for SIFCON material in punching shear is reported so far. However, in the present analysis, the two major building codes ACI 318-2005 and IS 456-2000 have been considered for comparison. In the strict sense, the above two building code methods may not be applicable to SIFCON material.
As per the ACI 318-2005 code, the ultimate punching shear strength Pu is taken as the smallest value given by the following
Pu = (0.166+(0.332/Bc))√fc u d ...(1)
Fig. 4 — Tested steel reinforced SIFCON – 8% slab
Fig. 5 — Tested steel reinforced SIFCON – 10% slab
Fig. 6 — Tested steel reinforced SIFCON – 12% slab
Fig. 7 — Tested FRC – 2% slab
Pu = (0.166+(0.083 α d/u))√fc u d …(2)
Pu = 0.332√fc u d … (3)
According to the Indian standard code IS: 456- 2000, the expression for calculating the punching shears strength Pu by considering partial safety factor for material as unity is given as
Pu=Ks
τ
cu d …(4)Ks = (0.5+Bo) ≤ 1
τ
c=0.25 √fckThe above two specified code provisions are used to calculate the ultimate punching shear load and the predicted values are given in Table 5. From this table, it can be seen that the experimentally observed values are higher than those calculated as per ACI and I.S code procedures. The experimental loads for SIFCON slabs are higher by 52-67% and 102-122% when compared with ACI code and IS code respectively.
From the results it can be concluded that the IS code is more conservative than the ACI code and there is a need to define specific procedures for computation of punching shear of SIFCON. Accordingly a regression model is prepared for estimating the punching shear strength of SIFCON.
Regression model for punching shear strength of steel reinforced SIFCON slabs
A simple regression model has been developed from the results of present investigation for predicting the punching shear strength of steel reinforced SIFCON slabs. To develop the punching shear strength model, linear regression technique has been adopted.
The proposed model for punching shear is given as:
Pu =τ us d ...(5)
A simple linear regression equation has been developed between compressive strength (fc) and shear stress (τ) of SIFCON by using Tables 3 and 6 and presented below.
τ = 0.311√fc …(6)
However, the compressive strength (fc) of SIFCON is dependent on fibre volume fraction. In the present work three fibre volume fraction, viz., 8,10 and 12%
have been used. Ten cubes of 150 × 150 × 150 mm size for each percentage of fibre were cast and tested for compressive strength. The various 28 day compressive strength (fc) values of SIFCON obtained from the experimentation are given in Table 3. A regression model has been developed by using the results of Table 3 for cube compressive (fc) strengths of SIFCON in terms of fibre volume fraction (Fv), as it will be convenient to estimate fc with fibre volume fraction (Fv). The regression model is given as:
fc = 25.32 + 2.505 Fv ...(7)
Substituting the Eq. (7) in Eq. (6)
τ = 0.311√(25.32 + 2.505 Fv) …(8) The critical perimeter us can be defined with reference to Fig. 9 as:
Table 5 — Comparison of Experimental punching shear values with standard codes of practice
Nomenclature Ultimate Punching Shear loads (kN) EXP values/ EXP values/
Exp values ACI 318-2005 IS: 456-2000 ACI 318-2005 IS 456-2000
RCC 10.83 21.00 15.81 0.516 0.685
FRC-2% 13.33 28.17 21.22 0.473 0.628
SIFCON-8% 102.00 67.08 50.40 1.520 2.023
SIFCON-10% 114.00 70.69 53.10 1.612 2.146
SIFCON-12% 124.00 74.12 55.80 1.672 2.223
Fig. 8 — Tested RCC slab
us = 4[ b+ 2d tanθ] …(9) From the present experimental work, it is observed that the dispersion angle θ depends on fibre volume fraction FV. Accordingly a simple linear regression equation is developed to connect θ with FV from the results of the present work and is presented as Eq.(10).
Θ = 44.055+1.312(Fv) …(10)
Substituting Eq.(10) in Eq.(9)
us = 4[ b+ 2d tan(44.055+1.312(Fv))] …(11) Substituting Eqs (8) and (11) in Eq.(5), a model for predicting punching shear strength is obtained as:
Pu ={(0.311√(25.32 + 2.505 Fv))( 4[ b
+2d tan(44.055+1.312(Fv))]) d} …(12) A comparison of the ultimate punching shear loads predicted by the regression model (Eq.(12)) with experimental values is given in Table 6. From Table 6, it can be observed that the proposed model compared well with the experimental ultimate loads.
The ratio of experimental punching shear load to that predicted by regression model is given in column 4 of Table 6. It can be observed that the ratio varies from 1.012 to 0.985. Thus, the proposed regression equation is able to predict the ultimate punching shear loads of SIFCON slabs quite satisfactorily.
Conclusions
Analyzing the results obtained from this investigation, the following conclusions are drawn:
(i) The punching shear carrying capacity of the steel reinforced SIFCON slabs is much higher than the fibre reinforced concrete and reinforced cement concrete slab specimens.
(ii) With increase of fibre volume, the punching strength carrying capacity increases in steel reinforced SIFCON slabs.
(iii) The zone of influence area (dispersion angle) increases with increase of volume fraction of fibres.
(iv) The steel reinforced SIFCON slabs are intact even after ultimate failure but this is not so in RCC slab specimens.
(v) The steel reinforced SIFCON slabs with higher volume fraction of fibre sustain greater deflection with high ultimate load.
(vi) In steel reinforced SIFCON slabs the stiffness increases with increase of fibre volume and the stiffness of steel reinforced SIFCON slabs specimens is very much higher than the FRC and RCC slab specimens.
(vii) Existing codal provisions for punching shear are not suitable for steel reinforced SIFCON slabs.
There is need to develop specific codal provisions for punching shear of steel reinforced SIFCON slabs.
Nomenclature
b = breadth of the patch load, mm
Bc = the ratio of long side to short side of the loaded area Bo = ratio of short side to long side of column
d = effective depth of the slab, mm.
fc = specified compressive strength of concrete, N/mm2 fck = characteristic cube compressive strength of concret, N/mm2 Fv = fibre volume fraction, %
Pu = ultimate punching shear strength, N.
u = length of the critical perimeter (mm), taken at a distance of d/2 from the column/pedestal (for ACI and IS codal provision)
us = length of the critical perimeter (depending on angle of dispersion for SIFCON slabs), mm
α = 40 for symmetric punching.
τc = shear stress in concrete, N/mm2 θ = dispersion angle
Table 6 — Performance of regression model Nomenclature Exp Ultimate
load, (kN)
Ultimate Load predicted by
Regression model, (kN)
Exp value/
Predicted value
SIFCON-8% 102.000 100.752 1.012
SIFCON-10% 114.000 112.566 1.012
SIFCON-12% 124.000 125.793 0.985
Fig. 9 — Dispersion angle for slabs under punching shear
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