Experimental Evaluation on Components of Shear resistance of Reinforced Concrete Beams with Shear Reinforcement Provided


 The contribution of aggregate interlocking and dowel force in shear strength of reinforced concrete beams was topic of research for many years. The precise forecasts of shear behavior were challenging to determine due to complication involved. The existing theories had focused on aggregate interlocking force and shear resistance arising due to concrete compression zone, neglecting the contribution of dowel force despite considering as significant constituent in shear transfer mechanism. The present investigation focuses on cogitating all components in shear transfer mechanism by providing shear reinforcement and keeping clear cover and effective span to depth ratio constant. Sixteen specimens were considered for parametric study by employing suitable variables such as increase in strength of concrete and variation in flexural reinforcement. Eight specimens were conventional beams and the remaining eight specimens were provided with preformed cracks. Moment vs. displacement curvature and strain vs. moment curvature were plotted to evaluate shear at uncracked compression zone and accordingly aggregate interlocking force and dowel force were determined based on the empirical formulas proposed. From the result it was confirmed that contribution of aggregate interlocking force and dowel force were insignificant and shear resistance due to uncracked compression zone is the sole contributor in shear transfer mechanism. Structural behavior of concrete beams was also studied and it was confirmed that beams with preformed cracks exhibited better structural behavior when related to conventional beams.


INTRODUCTION
Shear failure is often sudden with little or no advanced warning and the design for shear must ensure that shear strength for every member exceeds its flexural strength. Shear resistance of reinforced concrete beams is provided by shear transfer in uncracked compression zone, aggregate interlocking across the crack surface, stirrups crossing through the shear crack and dowel action of longitudinal reinforcing bars crossing the crack in the concrete.
"ACI Committee 318 (2019)", had suggested that in reinforced concrete beams the shear resistance is determined by the amount of the influence of concrete V c and the influence of shear reinforcement V s. Failure prediction and occurrence of shear cracks is tough to measure due to inconvenience involved in shear transfer mechanism. The initial study on shear failure was done by " Kani (1964)" and considered the arch action and ignored resistance of shear rising from aggregate interlock and dowel force and later several theories were proposed on the shear resistance of Reinforced concrete beams and their behavior. Several empirical formulas were proposed by considering varied design variables like bar size, flexural reinforcement ratio, characteristic strength of concrete in relation to effective span and shear depth ratio. " Taylor (1970)" had conducted experiments on reinforced concrete beams by providing shear reinforcement and concluded that shear due to concrete compression zone is the major contributor. Further "Paulay and Loeber (1974)" considered arch action of the concrete and neglected bond action caused by dowel force of longitudinal reinforcement and concluded that aggregate interlocking force holds major contribution in shear resistance but majorly depends on flexural reinforcement provided. Comparable works was carried out by "Walraven (1987)", "Thomas (1988)", and " Reineck (1991)" and concluded that with increase in strength of concrete, the contribution of aggregate interlocking force decreases. This led to modelling the relationship between bond action of longitudinal reinforcement and further intensified to develop empirical formulas in predicting various components in shear transfer mechanism. " Sarkar et al. (1999)" conducted tests on high strength concrete beam as represented in Fig.1.  Further " Kim et al. (2018)", had concluded that aggregate interlock effect was 15%-25% and dowel force was 20%-27% by keeping shear span to depth ratio constant and proposed an empirical formula to evaluate various components in shear strength.
From the above literature, it was evident that flexural reinforcement, bar diameter and concrete compressive strength are significant in determining the dowel force and purely depends on the type of test setup.

RESEARCH SIGNIFICANCE
It is a common agreement that structural behaviour of reinforced concrete members in bending is well understood. This is primarily due to various procedures mentioned for design strength in the codes are reasonably consistent.
However, shear behaviour was not fully explained as there is a great variation between code-to-code provisions in determining the shear strength which is instigating the research for the last two decades. Understanding the shear behaviour is becoming a major challenge due to complexity involved and varying influence parameters are being corrected throughout the years through testing. It was also observed that magnitude of dowel force was given minimum attention due to wide-ranging nature of experimental observations and it was also observed that impact of aggregate interlocking force was overlooked which affords stages to calculate shear at uncracked compression which makes to determine dowel force accordingly. Limited efforts were done to establish the components in shear transfer with suitable design variables such as strength of concrete, flexural reinforcement effective span to depth ratio and clear cover.
The current experimental investigation emphasizes on establishing the dowel force by employing the suitable design variables as mentioned above.

EXPERIMENTAL INVESTIGATION
The present experimental investigation emphasizes on evaluating the dowel force of flexural reinforcement by increasing the percentage of flexural reinforcement and strength of concrete and keeping effective span to depth ratio and clear cover constant. One set of beams was conventional beams and in the second set of beams, diagonal tension cracks was initiated. Maximum shear load was recorded and structural behaviour of the beams were noticed and stress occurring at a depth 'y' from the neutral axis were calculated. M30 and M50 were considered with suitable mix proportions as represented in Table 1 and Table 2 respectively.   Table 3 and Table 4 respectively, by employing ratio of flexural reinforcement in the proportion of 0.30%, 0.60%, 0.90%. Clear span of the beam was 2200 mm with cross section 150 mm × 300 mm with a/d ratio 1.26.   Sarkar et al. (1999)" was marked at 380 mm from supports and 60 mm away from the bottom reinforcement with iron plates 5mm thickness at an angle of 45 0 as presented in Fig. 3. Iron plates were taken away after four hours, and kept for curing for twenty-eight days as presented in Fig.4.

Procedure for Testing
The test was conducted on loading frame of 200-ton capacity and was similar to the set up by "Jelic et al.
Two support conditions were placed at a distance of 100mm from both the ends. Hinged support was placed at the left end and roller support was placed at right end. Failure of the beam was observed under four-point bending load and ultimate load carrying capacity was recorded with LVDT placed at mid span. Specimen failure represented in Fig. 5. and pictorially represented in Fig. 6., was analysed from moment vs. displacement curvature, moment vs. strain curvature and shear components was determined accordingly.
The V cz , was calculated from strain vs. moment curvature as represented in Figs. 9 and 10 and slope represents . E c was evaluated as 5000√ as specified by IS 456-2000.
'V d ' was calculated from formulas as presented.
For conventional beams, formula suggested by "Kim et al. (2018)" reprseneted in Eqs. (2) and (3). was taken into consideration to calculate V a and accordingly V d was determined.
For preformed cracks, empirical formula suggested by "Panda and Apparao (2017)" as represented in Eq.

RESULTS AND DISCUSSIONS
The results obtained is represented in the method of moment vs. displacement curvature and moment vs. strain curvature and V and V cz are determined consequently.

Displacement vs. Moment Curvature
From Figs. 8 and 9, the moment vs. lateral displacement at each level is evaluated and slope at individual level represent V as proposed in Eq. (1).    From the Fig. 11., it was witnessed all the beams underwent a gradual drop due to ductility with increase in lateral strain. It was concluded that, shear at compression zone is the main contributor for shear resistance with increase in characteristic strength of concrete and increase in percentage of flexural reinforcement.
Results obtained from Figs. 8-11, was used to determine 'V'and 'V cz ' as represented in Tables 5 and 6.

Evaluation of Vd
After evaluating V and V cz , V d was derived based on the equations mentioned in Eqs. (2), (3) and (4) as denoted below.

a) Conventional Beams
'V a ' was determined based on the Eqs. (2) and (3) and accordingly 'V d' was determined as represented in Table 7. From the Table 7. for conventional beams, with the presence of 'V a ', contribution of 'V d' had reduced with increase in strength of concrete and percentage of flexural reinforcement and is maximum for the beam with preformed cracks without any shear and tensile reinforcement provided.

b) Beams with preformed cracks
For the beams with preformed cracks, the contribution of 'V a ' was removed and 'V d' was determined and the results obtained are presented Table 8.  Eq.(4) was applied to determine V d as represented in Table 9. From the Table 9, huge variation was observed between numerical value and experimental value

Degradation Curvature
After shear components were determined, structural behaviour was examined by employing stiffness degradation.
As the stiffness is trivial under the load, degradation was observed with respect to the crack propagation by calculating with ratio of specific shear force and the ultimate shear force as represented in Figs.14. and 15. From the Fig. 14, it was understood that, plain concrete beams varied linearly and with provision of flexural reinforcement, linear curve was observed initially and later "S" shaped had taken place indicating stiffness before the failure and found to be maximum for beam H1. In the Fig. 15 it was understood that, plain concrete beams varied linearly and with provision of flexural reinforcement, linear curve was observed initially and later "S" shaped had taken place indicating stiffness before the failure and found to be maximum for beam H2.  i. From moment vs. displacement curvature, it can be concluded that design variables as discussed above does not contribute significantly as shear resistance was found be minimum at supports.
ii. It is also concluded that, by eliminating aggregate interlocking force, shear at uncracked compression zone is the major contributor for shear resistance and was in agreement with as concluded by "Taylor (1970)".
Hence it can be concluded that empirical formula holds good in determining the shear strength of concrete beams with shear reinforcement provided.
iv. From the discussions above, huge variation was noticed between numerical values and experimental values.
As such, formula proposed by "Panda SS and Apparao G (2017)" is not applicable.
v. From stiffness degradation curvature, it was observed that beams with preformed cracks had displayed better stiffness compared to conventional beams and there was decrease of cracks in shear. It can be decided that contribution of V a holds minimum contribution to shear strength of concrete beams with shear reinforcement provided.
vi. Finally, it can be decided that V cz is major contributor in shear resistance of concrete beams which was in agreement with "Zararis and Papadakis (2001)" with shear reinforcement provided.