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INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING
Volume 1, No 1, 2010
Copyright 2010 All rights reserved Integrated Publishing services
Research article ISSN 0976 4399
101
Shear Resistance of High Strength Concrete Beams Without Shear
ReinforcementSudheer Reddy.L
1, Ramana Rao .N.V
2, Gunneswara Rao T.D
3
1. Assistant Professor, Faculty of Civil Engineering, Kakatiya Institute of Technology andScience, Warangal
2. Professor and Principal, Faculty of Civil Engineering, Jawaharlal Nehru Technological
University, Hyderabad3. Assistant Professor, Faculty of Civil Engineering, National Institute of Technology (NITW),
ABSTRACT
The use of high strength concrete in major constructions has become obligatory, whosemechanical properties are still at a research phase. This paper deals with the review of available
data base and shear models to predict the shear strength of reinforced concrete beams withoutweb reinforcement. An attempt has been made to study shear strength of high strength concrete
beams (70 Mpa) with different shear span to depth ratios (a/d = 1, 2, 3 & 4) without webreinforcement and compare the test results with the available shear models. Five shear models
for comparison are considered namely, ACI 318, Canadian Standard, CEP-FIP Model, ZsuttyEquation and Bazant Equation. The results revealed that the most excellent fit for the test data is
provided by Zsuttys Equation and a simplified equation is proposed to predict the shear capacityhigh strength concrete beams without shear reinforcement.
Keywords:High-strength Concrete, Shear, Shear span to Depth Ratio (a/d).
1. IntroductionUse of high strength concrete in construction sector, has increased due to its improved
mechanical properties compared to ordinary concrete. One such mechanical property, shearresistance of concrete beams is an intensive area of research. To Estimate the shear resistance of
beams, standard codes and researchers all over world have specified different formulaeconsidering different parameters into consideration. The parameters considered are varying for
different codes and researchers leading to disagreement between researchers, making it difficultto choose an appropriate model or code for predicting shear resistance of reinforced concrete.
Therefore an extensive research work on shear behavior of normal and high strength concrete is
being carried out all over the world. The major researchers include Bazant Z.P. [1], Zsutty T.C,[2] Piotr Paczkowski [3], Jin-Keun Kim [4], and Imran A. Bukhari [5] and many more.Estimation of shear resistance of high strength concretes is still controversial therefore its a
thrust area for research.The shear failure of reinforced concrete beams without web reinforcement is a distinctive
case of failure which depends on various parameters such as shear span to effective depth ratio(a/d), longitudinal tension steel ratio (), aggregate type, strength of concrete, type of loading,
and support conditions, etc. In this research, shear span-to-effective depth ratio is taken as main
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variable keeping all other parameters constant.Most of the researchers concluded that failure mode is strongly dependent on the shear
span to depth ratios (a/d). Berg [6], Ferguson [7], Taylor [8], Gunneswara Rao [9], found thatshear capacity of reinforced concrete beams varied with a/dratio.
2. Objectives To study the shear response of concrete beams with out shear reinforcement varying
shear span to depth ratio (a/d) from 1 to 4 (1,2,3 & 4). To compare the shear formulae formulated by eminent codes with the experimental test
data. To propose a simplified formula to predict shear strength of HSC beams with out shear
reinforcement.
3. Research Significance
This paper provides the test data related to behaviour of HSC beams in shear. The data is usefulfor developing constitutive models for shear response of structural elements where high strengthconcrete is used.
4. Experimental ProgrammeEight reinforced high strength concrete beams were cast and tested, under two point loadingvarying the shear span to effective depth ratio (a/d). The test specimens are divided into four
series. Each series consisted of two high strength concrete beams with out shear reinforcementwith a/d ratio 1, 2, 3 & 4. For all the series, the parameters viz., concrete proportions and
percentage of longitudinal steel were kept constant. The details are listed in the Table 1 below:
Table 1:Reinforced beams without shear reinforcement
Serial
No
Beam
Designation
Length of
beam (m)
a/d
Ratio
No. of
Beams
1 R01 0.7 1 2
2 R02 1.0 2 2
3 R03 1.3 3 2
4 R04 1.6 4 2
4.1 Test Materials
4.1.1 CementOrdinary Portland cement whose 28- day compressive strength was 53Mpa was used.
4.1.2 Fine AggregateNatural River sand confirming with specific gravity is 2.65 and fineness modulus 2.33 was used.
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4.1.3 Coarse AggregateCrushed Coarse aggregate of 20mm and 10 mm procured from local crusher grading withspecific gravity is 2.63 was used.
4.1.4 WaterPortable water free from any harmful amounts of oils, alkalis, sugars, salts and organic materialswas used for proportioning and curing of concrete.
4.1.5 Super plasticizerIn the present experimental investigations naphthalene based superplasticizer conplast337 wasused for enhancing workability.
4.1.6 Fly Ash:Class F fly ash was used acquired from KTPS, Kothagudam, Andhra Pradesh, India.
4.1.7 Ground Granulated Blast Furnace SlagThe slag was procured from Vizag. The physical requirements were confirming to BS: 6699.
4.1.8 Tension Reinforcement16 mm diameter bars were used as tension reinforcement whose yield strength was 475Mpa.
4.2 Mix DesignThe high strength concrete mix design was done using Erntroy and Shacklock method. By
conducting trial mixes and with suitable laboratory adjustments for good slump and strength thefollowing mix proportion was arrived as shown in Table 2
Table 2:Mix Proportion of Concrete
Cement
(Kg)
Fine
Aggregate
(Kg)
Coarse
Aggregate
(Kg)
Water
(lit)
Fly Ash
(By Wt.
of
Cement)
GGBS
(By Wt.
of
Cement)
Super
Plasticizer
(By wt. of
Cement)
520 572 1144 130 5% 15% 1.5%
.
4.3 Specimen detailsTests were carried out on sixteen beams, simply supported under two point loading. All the
beams had constant cross section of 100mm x 150mm illustrated in Fig 1. The length of beam
was worked out to be 0.7m, 1.0m, 1.2m and 1.6m for corresponding a/d ratio = 1, 2 , 3 & 4respectively. All the four series of beams were provided with 3 16 mm diameter HYSD bars as
longitudinal reinforcement to avoid any possible failure by flexure and the grade of concrete waskept constant.
4.4 Test Procedure
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The beams were tested under two point loading on 100 Ton loading frame. The test specimenwas simply supported on rigid supports. Two point loads were applied through a rigid spread
beam. The specimen was loaded using a 100 ton jack which has a load cell to monitor the load.Based on the shear span to depth ratio, the support of the spread beam was adjusted. Two
LVDTs were provided, one at the centre of the span and other at the centre of the shear span tomeasure deflections
Figure 1:Details of test beams with arrangement of loads and supports
. The load and deflections were monitored for every 5 seconds. The load that produced the
diagonal crack and the ultimate shear crack were recorded. Crack patterns were marked on the
beam. The average response of two beams tested in a series, was taken as the representativeresponse of the corresponding series. The test set up is presented in figure 2.
Figure 2:Beam and LVDT arrangement in 100 ton loading Frame
0.15m
Span of the beam (L)
(0.7m, 1.0m, 1.3m, 1.6m)
0.1m
P
0.1 a 0.2 a 0.1
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5. Different Models to predict shear capacityComparative analysis is made for well known shear models which are used to calculate shear
resistance of beams without web reinforcement. The following equations are used:
ACI code Equation.
Canadian Equation. CEP-FIP Model. Zsutty Equation. Bazant Equation.
5.1 Shear Design by ACI code EquationAccording to ACI Building Code 318 [9], the shear strength of concrete members withouttransverse reinforcement subjected to shear and flexure is given by following equation
(1)
(1a)
- Compressive strength of concrete at 28 days in MPa.
bwd - Width and depth of Effective cross section in mm.
MuVu Factored moment and Factored shear force at Cross section. Longitudinal Reinforcement Ratio.
Many researchers [10] have expressed certain imperfections in the Eq (1), as it
underestimates the effect of shear span to depth ratio on shear resistance.
5.2 Shear Design by Canadian EquationAccording to Canadian Standard [11], the shear strength of concrete members is given by
following equation
(N). (2)
- Compressive strength of concrete at 28 days in MPa.
bwd - Width and depth of Effective cross section in mm.
The Canadian standard in Eq (2) has not considered the effect of shear span to depth ratio
and longitudinal tension reinforcement effect on shear strength of concrete.
5.3 Shear Design by CEP FIP ModelAccording to CEP FIP Model [12], the shear strength of concrete members is given byfollowing equation
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(N) (3)
- Compressive strength of concrete at 28 days in Mpa.
bwd - Width and depth of Effective cross section in mm.a/d Shear span to Depth ratio.
Longitudinal Reinforcement Ratio.
The CEP FIP model as formulated in Eq (3) takes into formula, the size effect andlongitudinal steel effect, but still underestimates shear strength of short beams.
5.4 Shear Design by Zsutty EquationZsutty (1968) [2] has formulated the following equation for shear strength of concrete members
.. (4)
.. (4a)
- Compressive strength of concrete at 28 days in MPa.
bwd - Width and depth of Effective cross section in mm.
a/d Shear span to Depth ratio. Longitudinal Reinforcement Ratio.
Most of researchers suggested that Zsutty equation is more appropriate and simple to
predict the shear strength of both shorter and long beams as it takes into account size effect andlongitudinal steel effect.
5.5 Shear Design by Bazant EquationBazant (1987)[1] has formulated the following equation for shear strength of concrete members
d (N). (6)
- Compressive strength of concrete at 28 days in MPa.
bwd - Width and depth of Effective cross section in mm.
a/d Shear span to Depth ratio. Longitudinal Reinforcement Ratio.
The Eq (6) stated by Bazant (1987) to predict shear strength of concrete members lookscomplicated, but takes into account all the parameters involved in predicting the shear strength of
concrete members.
6. Discussion on Test Results
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Comparison of the experimental results (Table 3 with ACI code [Eq 1] , Canadian Code[Eq 2],CEP-FIP model[Eq 3], Zsutty Equation[Eq 4] and Bazant equation[Eq 5] ) reveals that a/d ratio
significantly effects the shear capacity of the high strength concrete beams. Most of theequations are under estimating the shear capacity at lower a/d ratios. When the a/d ratio is less
than 2.0, strut action prevails and the shear resistance is very high. For a/d ratios up to 2 theexperimental values showed remarkable increase in shear strength compared to various design
approaches. Only predicted shear capacity using Zsutty Equation for a/d ratio up to 2 were closerto experimental values. For a/d ratios 2 to 4 almost all the models predicted values of shear force
were fair and were closer to experimental values where arch action prevails.
The results tabulated in Table 3 and comparison illustrated in Figure 3, the discussion may beconcluded as follows:
ACI code underestimates the shear capacity of high strength concrete beams withoutweb reinforcement.
Canadian code has not taken into account the effect of shear span to depth ratio. Theshear resistance of HSC member predicted based on Canadian code, under estimates
the actual shear capacity of member at all a/d ratios. Shear capacity of the HSC members predicted based on CEP- FIP model, showed
lower values at all a/d ratios. Shear resistance of HSC members using Zsutty Equation closely predict the shear
capacity of high strength concrete beams without web reinforcement. The shear capacity calculated using Bazant equation indicate that the equation
moderately under estimates the shear capacity of HSC beams.
Table 3:Predicted and Experimental ResultsVpredicted
(kN)
Beam
ID MPa
a/d Vexp(kN)
ACI
CODE
CAN
CODE
CEP-
FIP
MODEL
ZSUTTY
EQ
BAZANT
EQ
R01 70 1 129 35.36 24.19 51.75 131.79 62.90
R02 70 2 78.5 27.36 24.19 41.08 52.31 32.51
R03 70 3 55.5 24.69 24.19 35.89 36.56 28.35
R04 70 4 42.5 23.35 24.19 32.61 33.22 27.14
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0
20
40
60
80
100
120
140
0.0 1.0 2.0 3.0 4.0 5.0
a/d Ratio
ShearForce(kN)
ACI CAND CEB-FIP ZSUTTY BAZANT EX
Figure 3:Influence of a/d on shear resistance
The variation of deflection with load of HSC beams without shear reinforcement for a/d =1, 2, 3, and 4 are shown in Fig 4, which indicate the increase in a/d ratio has shown reduction in
shear capacity of the beam. At lower a/d ratios the ultimate load was observed to be more thantwice at diagonal cracking. The deflections increased with a/d ratio, which signify that at lower
a/d ratios i.e. up to 2 the strut behavior and above 2 the arch behaviour of the beams. At lowera/d ratios (up to 2), the failure was observed to be sudden compared to failure pattern observed
for higher a/d ratio (a/d 2 to 4).
0
50
100
150
200
250
300
0 5 10 15 20 25
Deflection (mm)
Load(kN)
R01 R02 R03 R04
R01 - a/d=1
R02 - a/d=2
R03 - a/d=3
R04 - a/d=4
Figure 4:Load - Deflection illustration for R01 (a/d=1), R02 (a/d=2), R03 (a/d=3) & R04
(a/d=4)The failure pattern of the beams shown in Fig. 5 clearly indicate that for a/d 1 and 2
crack initiated approximately at 45degrees to the longitudinal axis of the beam. A compressionfailure finally occurred adjacent to the load which may be designated as a shear compression
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failure. For a/d 3 and 4 the diagonal crack started from the last flexural crack and turnedgradually into a crack more and more inclined under the shear loading. The crack did not
proceed immediately to failure, the diagonal crack moved up into the zone of compressionbecame flatter and crack extended gradually at a very flat slope until finally sudden failure
occurred up to the load point. The failure may be designated as diagonal tension failure.
Figure 5: Crack patterns and load points of the failed specimens- R01, R02, R03 and R04
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As the present research work focuses on enhancement of shear capacity of HSC beamswithout web reinforcement, the tensile strength of concrete plays a vital role. The shear equations
proposed by different codes cited in shear resistance models clearly disclose that shear resistanceis factor of tensile strength of concrete, shear span to depth ratio (a/d) and tensile reinforcement
ratio. To estimate the shear capacity of HSC beams the parameters viz.., tensile strength ofconcrete, shear span to depth ratio (a/d) and tensile reinforcement ratio were taken into account
in form of Shear Influencing Parameter (SIP).
( )tfSIP ad r =
(7)
tf - Tensile strength of concrete in Mpa.
a/d Shear Span to Depth Ratio. Tensile Reinforcement Ratio.
y = 31.988x 0.7962
R2 = 0.9964
0
2
4
6
8
10
0.00 0.05 0.10 0.15 0.20 0.25
SIP
ShearStress(MPa)
Figure 6: Variation of shear strength with Shear Influencing Parameter (SIP)
The influence of shear resistance, which is taken as average value of the two specimens
tested with SIPs calculated using Eq.7 is illustrated in Fig. 6. To estimate the shear resistance(Vc) a linear regression equation was set in power series.
( )0.8
32 tc w
fV b d
ad
r =
(N) . (8)
where
bw and d - Width and depth of Effective cross section in mm.
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The empirical shear stress values calculated from the Eq.8 and the shear stress valuesobtained by testing the beams for shear span to depth (a/d) ratio = 1, 2, 3 & 4 are listed in Table 6.
The values clearly signify that the experimental and empirical values fall within +5% and -5%variation. Thus the proposed equation can fairly estimate the shear resistance of HSC beams
without stirrup reinforcement, under shear loading.
Table 4:Experimental and Empirical shear stress
S. No a/d
Ratio
Experimental
Shear Stress
(MPa)
Empirical
Shear Stress
(MPa)
1 1 8.60 8.68
2 2 5.23 5.01
3 3 3.70 3.62
4 4 2.83 2.88
7. Conclusions
With the discussion on shear models and the experimental studies conducted on HSC beamswithout shear reinforcement the following conclusions can be drawn:
The prediction of shear capacity of High strength concrete beams without shearreinforcement using the shear equations (Eqs 1 to 6) listed in this paper with a/d ratio is
less than 2.0, a separate equation has to be used as there is remarkable difference between
experimental values and predicted values and for a/d ratio more than 2.0, the availableequations satisfactorily predict the shear capacity of the beams. The equation (Eq. 8) stated above includes almost all the parameters required to predict
the shear capacity beams without shear reinforcement. Therefore a single simplified
equation can be used to predict the shear capacity of HSC beams with a/d = 1, 2, 3 & 4.
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without Web Reinforcement.ACI Materials Journal, V 93, No. 3, May- Jun 1996. pp. 213-221.
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17. Elzanaty, A.H., Nilson, A.H., and Slate. F.O., Shear Capacity of Reinforced ConcreteBeams Using High-strength Concrete,ACI Journal, Proceedings,83(2)(1986), pp. 290296.
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