There is a common phenomenon of shear failure in RCC beams, especially in old buildings and bridges. Any possible strengthening of such beams is needed to be explored that could strengthen and make them fit for serviceable conditions. The present research has been made to determine the performance of predamaged beams strengthened with three-layered wire mesh polymer-modified ferrocement (PMF) with 15% styrene-butadiene-rubber latex (SBR) polymer. Forty-eight shear-designed and shear-deficient real-size beams were used in this experimental work. Ultimate shear load-carrying capacity of control beams was found at two different shear-span (
With the passage of time, many of the existing RCC structures deteriorate due to increases in service loads, corrosion of reinforcement, and poor ductile detailing, which results in loss of strength, cracking, and spalling of the structural components. Such structural elements need special attention and must be retrofitted using suitable strengthening techniques to restore strength and the design life. Many researchers have worked on the development of various materials and techniques for repairing, retrofitting, and strengthening of such structural elements. The selection of a particular strengthening material and technique depends on the type, cause, and nature of distress to be addressed.
All RCC elements are designed to fail in a ductile manner by making suitable detailing of reinforcement. During an earthquake, a sudden catastrophic failure can occur due to increased shear loads [
In the recent years, the usage of different advanced materials such as ferrocement, glass fibre-reinforced polymer (GFRP), fibre-reinforced polymer (FRP), carbon fibre-reinforced polymer (CFRP), and steel plate jacketing has increased for retrofitting and strengthening of concrete structures. These materials have excellent properties such as high strength, light weight, and corrosion resistance abilities. Some researchers have explored the effect of various advanced composite bonding materials as well as their orientation on the flexure and shear strength properties of retrofitted beams [
Conventional ferrocement made with cement mortar matrix showed some deficiencies and got cracks under loads even much smaller than the ultimate loads, leading to reduced life [
In the past, a few experimental studies had been carried out to gauge the effect of the high-performance ferrocement strengthening technique on RCC beams. Kumar and Vidivelli [
It is concluded that the application of polymer-modified ferrocement as an outer strengthening material is a viable technology for improving the structural performance of RCC beams in flexure. At present, there is no such research work recorded to study the effects of polymer-modified ferrocement to strengthen the beams in shear. It is the need of the hour to explore the utility of PMF as a strengthening material in the specified domain of shear. Many factors impart shear strength of RCC beams like
A preliminary study has been done to determine the properties of ingredients required for this experimental work.
Portland pozzolana cement (PPC) with a 28-day compressive strength of 34.2 N/mm2, specific gravity 2.9, fineness 2.1%, consistency 34%, initial setting time 98 minutes, final setting time 240 minutes, and soundness of 1 mm conforming to IS 1489-Part 1 [
Properties of fine and coarse aggregates.
Material description | Fineness modulus | Specific gravity | Water absorption (%) | Moisture content (%) | Grading zone |
---|---|---|---|---|---|
Fine aggregate (FA-1) | 2.24 | 2.67 | 1.9 | 0.22 | 3 |
Fine aggregate (FA-2) | 2.65 | 2.675 | 1.35 | 0.16 | 2 |
Coarse aggregate (CA-1) 20 mm | 6.69 | 2.69 | 1.18 | Nil | All-in-aggregate |
Coarse aggregate (CA-2) 12.5 mm | 6.11 | 2.685 | 1.11 | Nil | All-in-aggregate |
The thermomechanical-treated (TMT) 12 mm diameter bars with an ultimate tensile strength of 710 N/mm2 were used as tensile reinforcement, and 8 mm diameter TMT bars which have an ultimate tensile strength of 697.5 N/mm2 were used as compressive reinforcement. Plain mild steel (MS) 6 mm diameter reinforcement bars with an ultimate tensile strength of 491.5 N/mm2 were used as shear reinforcement. Galvanised square woven wire mesh of 0.49 mm diameter with centre-to-centre spacing of 8 mm and having an ultimate tensile strength of 950 N/mm2 was used in polymer-modified ferrocement as per ACI 549.1R guidelines [
Properties of reinforcement steel bars.
Diameter (mm) | Yield stress (MPa) | Ultimate stress (MPa) | Elongation (%) |
---|---|---|---|
12 | 556.5 | 710.0 | 22.0 |
8 | 548.5 | 679.5 | 18.4 |
6 | 465.0 | 491.5 | 5.0 |
0.49 (square woven wire mesh) | 665.0 | 950.0 | 18.2 |
Commercially available Sika® Latex Power [
Concrete of grade M20 with C : FA : CA in a ratio of 1 : 2.1 : 3.4 was designed as per IS 10262 [
Detail of concrete mix design.
Cement (C) | Fine aggregate (FA-2) | Coarse aggregate (CA-1 : CA-2) (60 : 40) | Water (W) |
---|---|---|---|
340 | 714.0 | 1156 | 187 |
All quantities are in Kg.
Polymer-modified mortar (PMM) with a cement (C) to sand (FA-1) ratio of 1 : 2 and having 15% of SBR was used to develop the polymer-modified ferrocement (PMF). The water-cement ratio of mortar was found as 0.56 for a flow value of 105 ± 5% [
This experimental study was conducted on 48 full-size (127 mm × 229 mm × 2700 mm) RCC beams. Out of these 48 beams, 24 were designed for shear-designed beams (DBs), and 18 stirrups of 6 mm diameter were provided at a spacing of 150 mm c/c. The rest 24 beams were designed as shear-deficient beams (SDBs), and 7 stirrups of 6 mm diameter were provided at a spacing of 450 mm c/c. All the beams were confined with 2–12 mm diameter bars on tensile face and 2–8 mm diameter bars on compression face. Beam section and reinforcement details of both types of beams are shown in Figure
Beam section and reinforcement details: (a) shear-designed beams (DBs); (b) shear-deficient beams (SDBs).
Shear-designed and shear-deficient RCC beam designation detail.
Designation of beams | Loading description | Shear-span ratio ( |
No. of samples |
---|---|---|---|
DB-ad-1 | Shear-designed controlled beams | 1.0 | 3 |
RDB45-ad-1 | 45% predamage + strengthening | 1.0 | 3 |
RDB75-ad-1 | 75% predamage + strengthening | 1.0 | 3 |
RDB95-ad-1 | 95% predamage + strengthening | 1.0 | 3 |
DB-ad-3 | Shear-designed controlled beams | 3.0 | 3 |
RDB45-ad-3 | 45% predamage + strengthening | 3.0 | 3 |
RDB75-ad-3 | 75% predamage + strengthening | 3.0 | 3 |
RDB95-ad-3 | 95% predamage + strengthening | 3.0 | 3 |
SDB-ad-1 | Shear-deficient controlled beams | 1.0 | 3 |
RSDB45-ad-1 | 45% predamage + strengthening | 1.0 | 3 |
RSDB75-ad-1 | 75% predamage + strengthening | 1.0 | 3 |
RSDB95-ad-1 | 95% predamage + strengthening | 1.0 | 3 |
SDB-ad-3 | Shear-deficient controlled beams | 3.0 | 3 |
RSDB45-ad-3 | 45% predamage + strengthening | 3.0 | 3 |
RSDB75-ad-3 | 75% predamage + strengthening | 3.0 | 3 |
RSDB95-ad-3 | 95% predamage + strengthening | 3.0 | 3 |
DB: shear-designed beam; RDB: strengthened shear-designed beam; SDB: shear-deficient beam; RSDB: strengthened shear-deficient beam.
After 28 days of curing, the shear-designed and shear-deficient controlled beams were tested over a loading frame fixed with a hydraulic jack. The load test on all the beams was performed for two different shear-span (
Schematic diagram of the test setup: (a) beams tested at
Test results of controlled DB and SDB.
Designation of beams |
|
Avg. ultimate load value (kN) | Avg. deflection at ultimate load (mm) | Calculated load value for different stress levels (kN) | ||
---|---|---|---|---|---|---|
45% | 75% | 95% | ||||
DB-ad-1 | 1 | 145.68 | 5.899 | 65.56 | 109.26 | 138.40 |
DB-ad-3 | 3 | 64.53 | 11.674 | 29.04 | 48.40 | 61.30 |
SDB-ad-1 | 1 | 144.28 | 6.857 | 64.93 | 108.21 | 137.07 |
SDB-ad-3 | 3 | 59.55 | 10.371 | 26.80 | 44.66 | 56.57 |
According to the test results of controlled beams, the other sets of beams were loaded on the same setup and predamaged for three different levels of initial stresses corresponding to 45%, 75%, and 95% of the ultimate load. The DB45 and SDB45 beams were initially loaded for 45% level of damage. Similarly, DB75 and SDB75 and DB95 and SDB95 were loaded for 75% and 95% levels of damage, respectively. These damaged beams were then unloaded and strengthened with 20 mm thick U-shaped polymer-modified ferrocement jacketing which contained 15% SBR latex and three layers of square woven steel wire mesh. The detail of strengthening is given in the subsequent section.
The behaviour of strengthened beams is highly dependent upon the surface preparation and application of the strengthening material. The repaired surface of the beams should be free from dirt, oil, dust, existing matter, and curing compounds. An improper preparation of the surface can result in debonding of PMF jacketing. Before the strengthening procedure, the beams were turned upside down to expose their soffit. The particular portion of all the predamaged beams was cleaned with a wire brush at their soffit and side faces where the jacketing is supposed to be applied. The surface was cleaned with such a way to expose the aggregates and to make the surface sufficiently rough for application of repairing mortar. Water was sprayed on the prepared surface to make it wet.
The another study done by the authors resulted that the three-layered square woven steel wire mesh PMF having an optimum percentage of 15% SBR showed better strength properties and hence adopted in this present investigation to strengthen the predamaged RCC beams. The polymer-modified repairing mortar was constituted with cement, sand, SBR, and water in the ratio of 1 : 2 : 0.15 : 0.35 [
Detail of PMF strengthening: (a) beams tested at
Application of PMF jacketing on the soffit and side faces of the beams (by turning the beams upside down).
The PMF-strengthened beams were again placed on the same loading frame setups (as specified in Figure
Load-deflection plot of shear-designed beams tested at
Load-deflection plot of shear-designed beams tested at
Load-deflection plot of shear-deficient beams tested at
Load-deflection plot of shear-deficient beams tested at
Reloading test results of predamaged strengthened beams.
Beam description | Ultimate load (kN) | Percentage increase in ultimate load | Deflection at ultimate load (mm) | Failure mode |
---|---|---|---|---|
DB-ad-1 | 145.68 | — | 5.899 | Shear |
RDB45-ad-1 | 149.14 | 2.38% | 11.891 | Shear |
RDB75-ad-1 | 153.45 | 5.33% | 12.494 | Shear flexural |
RDB95-ad-1 | 163.20 | 12.03% | 14.882 | Shear flexural |
DB-ad-3 | 64.53 | — | 11.674 | Shear flexural |
RDB45-ad-3 | 71.75 | 11.19% | 14.564 | Shear-flexural |
RDB75-ad-3 | 70.22 | 8.82% | 17.534 | Shear |
RDB95-ad-3 | 70.24 | 8.85% | 17.887 | Shear flexural |
SDB-ad-1 | 144.28 | — | 6.857 | Shear |
RSDB45-ad-1 | 154.60 | 4.60% | 14.048 | Shear flexural |
RSDB75-ad-1 | 161.87 | 7.57% | 10.450 | Shear flexural |
RSDB95-ad-1 | 155.96 | 6.29% | 10.412 | Shear flexural |
SDB-ad-3 | 59.55 | — | 10.371 | Shear |
RSDB45-ad-3 | 67.20 | 12.85% | 14.146 | Shear |
RSDB75-ad-3 | 64.05 | 7.56% | 13.763 | Shear |
RSDB95-ad-3 | 62.24 | 4.52% | 12.523 | Shear |
In this current experimental program, a total of 48 beams were tested. The testing was aimed to achieve many objectives by comparing the behaviours of these beams. The controlled and strengthened beams were loaded up to ultimate failure. Most of the beams showed diagonal cracking patterns and the shear mode of failure. The effects of different levels of initial stresses,
All the strengthened beams which were initially damaged with 45%, 75%, and 95% of the ultimate load had showed a complete restoration and further enhancement of the original strength up to 12.03% after strengthening. The deflection behaviour of these beams was also changed after strengthening, and the beams exhibited more ductility as compared to controlled beams. Hence, higher deflection values were observed for all the strengthened beams at the ultimate failure level.
The strengthened shear-designed beams when tested at
The strengthened shear-deficient beams having initial damages of 45%, 75%, and 95% were also tested for different
Test results showed that the performance of PMF jacketing is very consistent for both shear-deficient and shear-designed beams at
The controlled as well as strengthened shear-designed (DB) and shear-deficient (SDB) beams were tested for two different
The controlled shear-designed beams tested at
Comparison of ultimate loads of shear-designed beams tested at
A similar trend was recorded for controlled shear-deficient SDB-ad-1 beams, and 142.28% higher load-carrying capacity was observed in comparison with SDB-ad-3 beams. The strengthened shear-deficient beams also behaved in a similar manner, and the ultimate load values of RSDB95-ad-1 beams (tested at
Comparison of ultimate loads of shear-deficient beams tested at
At an
It is concluded that the PMF jacketing technique contributes more towards improving the shear resistance of beams and further enhances the ultimate load values when the beams were tested at
The beams having stirrups at a spacing of 150 mm c/c were designated as shear-designed beams (DBs), and the beams have stirrups at a spacing of 450 mm c/c were designated as shear-deficient beams (SDBs). Both the controlled and strengthened beams were tested for two different
This is attributable to the arch action of the concrete element and the efficiency of PMF jacketing. The strengthening technique is observed to be more efficient in case of shear-deficient beams when tested at
Comparison of ultimate loads of shear-designed and shear-deficient beams tested at
The shear-deficient strengthened beams (RSDB45-ad-1 and RSDB75-ad-1) with initial stresses of 45% and 75% showed a higher strength of 1.19% and 1.14% over the shear-designed strengthened beams (RDB45-ad-1 and RDB75-ad-1), respectively, when tested at
Comparison of ultimate loads of shear-designed and shear-deficient beams tested at
The PMF jacketing is fully effective to restore and enhance the original strength of both types of initially damaged beams. Furthermore, in the case of shear-deficient beams, it is observed that the cracks had developed at an angle of 30° to 45°. As the stirrups were at spaced as far as 450 mm c/c, many of these cracks were not interfered by the stirrups and therefore caused failure of beams in shear. The use of PMF tends to enhance the inertia and ductility of the beam section in such cases and is thus observed to cause a delay in beam failure. Hence, the observed experimental load and deflection values are higher due to the combined strength of concrete and ferrocement jacketing. The strengthened shear-deficient beams exhibited similar elastic behaviour as the strengthened shear-designed beams, and the beams were observed to fail in shear compression and diagonal tension.
Failure load detail for all the tested beam specimens is given in Table
The ultimate load-carrying capacity of all the strengthened beams considerably improved as compared to controlled beams at both
Ultimate loads of shear-designed beams tested at
The failure loads of strengthened shear-deficient beams were also considerably improved. This improvement was found to be lesser for the beams with a higher initial damage level of 95%. The initially stressed shear-deficient beams almost got damaged at 95% stress level, but the strengthening technique showed its worth to restore and enhance the total load-carrying capacity of such beams. The percentage improvement in the failure load of beams RSDB45-ad-1, RSDB75-ad-1, and RSDB95-ad-1 tested at
Ultimate loads of shear-deficient beams tested at
However, it was observed that some cracks appeared even through jacketing, which indicates a good bond of PMF jacketing with the existing concrete structure. However, in some cases, no cracks developed in the strengthened length where the jacketing was applied, and the failure cracks developed only in the vicinity of the jacketing edges. It reflected that the PMF jacketing behaved monolithically with the concrete specimens and helped to improve the ductility of beams after strengthening.
For comparative study, the deflection values under the load point are only discussed in this article. The load-deflection plots for all the beam specimens are shown in Figures
A few number of cracks of varying width and spacing were observed in both types of control beams. The beams ultimately failed because of the widening of any of these cracks. On the other hand, the predamaged strengthened beams showed fewer hairline cracks during testing. The strengthened beams ultimately failed due to the formation of inclined shear cracks. These beams showed minor cracks when compared with the control beams of that particular segment. Plates
Shear-designed beams tested at
Shear-designed beams tested at
Shear-deficient beams tested at
Shear-deficient beams tested at
The tested beams got cracked, apparently due to two different conditions. In some cases, the cracks appeared diagonally near the support mostly because of shear forces, and in others, the cracks appeared towards centre due to shear forces together with some flexure moment. Most of the beams tested at
In the case of both shear-designed and shear-deficient beams when tested at
The shear mode of failure was observed in the tested beams, and most of the cracks developed near the support and loading point (refer Plates
The behaviour of all the strengthened beams with different levels of initial damages was observed experimentally, and it was found that the jacketing neutralised the opening of cracks. The strengthened beams behaved stiffer as compared to the corresponding beams without strengthening, and the rate of crack development was also reduced. PMF-strengthened beams further displayed a lesser number of cracks when loaded to failure. This strengthening technique helps to improve the ductility of beams and causes to delay the formation of cracks. As a result, the figurative deflection of predamaged strengthened beams also enhanced. Spalling of concrete in the vicinity of the support point also reduced because of strengthening. PMF jacketing acted monolithically with the RCC beam specimens, and no bond failure was observed during the testing process. However, some cracks were found near the edge of jacketing which are attributable to the differential characteristics of strengthened beams at the newly created intersection of jacketed and unjacketed sections.
The shear failure mechanism of RCC beams is a complicated phenomenon because of interlinking of many factors such as loading pattern, shear-span ratio, beam section, the strength of concrete, and the quantity of shear and bending reinforcement. The characteristic of shear failure is abrupt as compared to the flexural failure. However, the PMF jacketing technique leads to achieve the ductile failure of strengthened beam specimens. From the abovementioned experiments, it is concluded that the PMF strengthening technique has improved the deformation behaviour, the cracking pattern, and the ultimate shear load-carrying capacity of the initially damaged beams.
The following conclusions have been drawn from this experimental study: PMF is fully effective to restore and enhance the original strength of initially stressed beams even after 95% damage. All the strengthened beams showed a complete restoration of original strength irrespective of stirrup spacing, the level of initial damage, and PMF jacketing causes to delay the direct shear failure of beams and apparently increases the contribution of stirrups to resist more loads. This technique is found to be more efficient in case of shear-deficient beams. The PMF strengthening technique increased the ductility of predamaged beams and caused to delay the shear failure by resisting and distributing the applied loads. The beams behaved more elastically, and deflection behaviour also improved after strengthening. The rate of crack development was also reduced, and the strengthened beams displayed a less number of cracks as compared to the corresponding beams without strengthening. No bond failure of jacketing was observed which reflects the proper bonding and compatibility of polymer-modified ferrocement with the concrete structures.
The authors declare no conflicts of interest.
The authors pay the heartiest gratitude to Sika India Pvt. Ltd. for supplying the SBR latex polymer.