Singapore Paper

8/10/2019 Singapore Paper

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BEHAVIOUR OF REINFORCED CONCRETE BEAMS STRENGTHENEDIN SHEAR

S.F.A.RAFEEQI 1, S.H.LODI 2, Z.R.WADALAWALA 3

ABSTRACT Non-engineered construction in rural areas of developing third world in recent years has

gained attention of many agencies mainly due to the alarming damage to property and lifecaused by earthquakes. Ferrocement having excellent ductility may become one of the main

structural materials for retrofitting and strengthening of modest span reinforced concrete beamsin such construction, specially, in earthquake prone areas.

Shear mode of failure in beams is undesired mainly being a brittle failure. An attempt,therefore, has been made to explore the potentials of Ferrocement in transforming brittle modeto ductile mode, through an experimental study presented in this paper.Test results of 4 beams in which shear spans were strengthened by providing a complete

Ferrocement warp and equally spaced strips, with one and two layers of woven square mesh are presented and compared with reinforced beam having identical material and sectional properties. The beams were loaded up to service load, de-loaded and strengthened prior to testup to failure. The sections were designed for brittle shear-compression failure by keeping shear

stirrups, which provided 3.5 to 4 times the shear capacity of concrete alone. The flexuralreinforcement was kept as the minimum that is normally provided in expected non-engineeredconstruction.

The strengthened beams showed a marked improvement in performances at service load, greatly improved ductility at ultimate with either a ductile shear failure or seemingly a transition from shear to flexure mode of failure.

Key words : ferrocement wrap; ferrocement strips; shear; strengthening; non-engineeredconstruction.

INTRODUCTIONReinforced concrete in one of the most abundantly used construction material not only in the

developed world, but also in the remotest parts of the developing world. In the rural areas of thedeveloping world, however, due to transference of expertise and technology know how,reinforced concrete poses threat due to its abuse rather than use, and majority of the houses areconstructed in traditional manner using indigenously developed techniques preferably followingsimpler and economical procedures. Unfortunately such non-engineered construction is mostly

prevalent in earthquake prone areas of the developing world e.g. Turkey, Pakistan, India andIran. The rural population in the developing world have mostly to rely on local skill, material andtechnology. The transformation of non-engineered construction into an engineered one, thereforeneeds to be such that it could be sustained. The methodology should be simple in execution, offer

better performance even when handled by less experienced workers, must involve materials,which are readily available, and yet durable, strong and economical. Ferrocement is one such

1 Professor and Co-Chairman, Dept. of Civil Engrg. , NED Univ. of Engrg. & Tech., Karachi, Pakistan.2 Professor, Dept. of Civil Engrg. , NED Univ. of Engrg. & Tech., Karachi, Pakistan.3 Assistant Professor, Dept. of Civil Engrg. , NED Univ. of Engrg. & Tech., Karachi, Pakistan.


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material which could afford to offer answer to such a situation and hence the present study is partof a programme under taken at NED University where the potentials of Ferrocement areexplored for its utilization in non-engineered construction, for improved performance in theevent of an earthquake.

LITERATURE REVIEWA 30 years record of major Turkish earthquake shows almost 0.2 million dwellings destroyed by earthquake [1], which is about 65% of the destruction caused by all other natural disasters.Dinar earthquake of 1995 and Kocoeli earthquake of 1999 have substantially increased the

percentage.Almost all post earthquake studies and damage assessment have led to the conclusion that a

building should be designed and constructed such that in the event of the probable maximumearthquake intensity: the building should not suffer total or partial collapse; it should not suffersuch irreparable damage which would require demolishing and rebuilding and in case it sustainsuch damage it could be repaired quickly and easily to bring it to its usual functioning [2].

Ferrocement over the years have gained respect in terms of its superior performance and

versatility, and now is being used not only in housing industry but its potentials are beingcontinuously explored for its use in retro-fitting and strengthening of damaged structuralmembers [3,4]. Ductility requirements are the main feature of an efficient earthquake restraintdesign process, and Ferrocement being highly ductile material have led to its application inrehabilitation of houses damaged by earthquake and the effectiveness of its use has been reported

by many researchers [5,6,7,8]. Taking the lead from its potential use in enhancing earthquakeresisting abilities, 5 houses were built in Northern area of Pakistan, using indigenous materialsand local skills utilizing Ferrocement bands to improve the earthquake resisting of such houses in1990 [9]. The houses since then have performed remarkably well and have sustained low tomoderate shocks effectively. The detail were simple to follow and execute by the local skilledworkers, and materials were readily available from near by cities.

Reinforced concrete elements are designed to fail in a ductile manner by emphasizing on thedetailing requirements due to the brittle nature of concrete. Shear failure are also classified as

brittle and shear zones are therefore reinforced by provision of stirrups for transformation toductile failure, however, a limit is imposed on the provision to avoid brittle shear-compressionfailure. In the event of an earthquake, however, the shear loads can exceed shear capacities, anddamage in shear zones may lead to catastrophic failure of such members.

Many experimental studies has been conducted in recent years to strengthen flexure members by using various materials [10,11,12,13,14,15].

Andrew and Sharma [10 ] in an experimental study compared the flexural performance ofreinforced concrete beams repaired with conventional method and Ferrocement. They [10]concluded that beams repaired with Ferrocement showed superior performance both at serviceand ultimate load. The flexural strength and ductility of beams repaired by Ferrocement wasreported to be greater than the corresponding original beams and the beams repaired byconventional method.

Al-Farabi et al [11] while investigating the effectiveness of Fiberglass bonded plates forcapacity enhancement, reported increased strength and reduced ductility. Premature failure by

plate separation was also identified as a potential problem at the plate curtailment place.Steel plates bonded by epoxy were used to repair shear cracked beams utilizing various forms

of plate bonding by Basunbul et al [12]. The experimental investigation clearly demonstrated thatthe effectiveness of the repair primarily depends on how effectively the diagonal tension cracksin the shear-damaged beams were trapped. Flexural mode of failure was observed surpassingshear capacity for only those specimens where full encasement of the shear zone was carried,


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while for all other forms of bonding the failure either occurred due to separation or ripping ofsteel plates. Enhanced ductility was, however, reported for all methods.

In a similar study where fiberglass plates were bonded instead of steel plates, Al-Sulaimani etal [13] reported almost similar results. The shear capacities for beams where full encasement ofshear zone was carries, was enhanced to such an extend that the failure occurred by flexure, with

enhanced ductility.An experimental study in strengthening of reinforced concrete beams using Ferrocementlaminates was conducted by Ong et al [14], where eight beams were strengthened with a 20 mmthick laminate attached on the tension face. Three different series with different methods ofattachments including Ramset nails, Hilti bolts and epoxy resin adhesive were used. The

performance of the strengthened beams were compared to the control beams with respect tocracking, deflection and ultimate strength. All the strengthened beams exhibited higher ultimateflexural capacity and greater stiffness. Use of epoxy resin adhesive and Ramset nails at closerspacing of shear connectors were able to ensure composite action. A decrease in the volumefraction of reinforcement of Ferrocement laminates from 3.25% to 2.36% resulted in a reductionof strength. The presence of the Ferrocement laminates had an inhibiting effect on the tensile

crack, and the crack spacing and crack width were reduced after strengthening.Paramasivam et al [15] reported tests on six damaged RC beam repaired by Ferrocementlaminates. Repair of cracks was done by epoxy resin injection and the beams were thenstrengthened by Ferrocement laminates attached to the soffit by means of L-shaped barconnectors, to ensure composite action between the Ferrocement laminates and the in-situconcrete. The results showed that substantial improvement in the ultimate capacity and stiffnessmay be obtained, provided the shear connectors are adequately spaced and the cracks are wellgrouted.

Paramasivam [16] reported that a total of six T-beams were tested inverted and simplysupported under static and cyclic loads applied at mid span by Aurellado [17]. Except for control

beam all beams were strengthened with Ferrocement laminates as encasements onto the beamweb and soffit. Three methods of attachment were examined with bar shear connectors installedthrough web or through flanges of beams. This was compared with conventional methods ofstrengthening where additional U shaped stirrups were inserted through predrilled holes in theflange before bending the ends in to form closed links. The results showed that the beams weresubstantially strengthened and stiffened due to the provision of the Ferrocement laminates.

OBJECTIVES AND SCOPEThe objective of the study is to provide the rural population especially in earthquake prone

areas, with a simple and easily applied technology to upgrade and strengthen non-engineeredstructural members. It has been observed that most of the time the rural learned in an effort to

provide sufficient shear strength and a flexure mode of failure, fails to recognize the danger ofover-reinforcing shear zones and thereby paving way to catastrophic failure.

The scope of the study was thus confined to study the effect of confining shear spans of such beams with Ferrocement, where shear stirrups provides 3.5 to 4 times the shear capacity ofconcrete alone. The main parameters of the study were confined to one and two layers of wovensquare mesh and application of Ferrocement by way of providing wraps in full shear span andequally spaced strips in shear span.

EXPERIMENTAL PROGRAMIn all, five beams were cast where beam A was the control specimen while B1, B2 and C1, C2

were the beams to be strengthened by way of providing Ferrocement strips and wraps with oneor two layers of wire mesh respectively.


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All beams having 100 mm width, 200 mm over all depth and 170 mm effective depth, werecast with same mix having proportion of 1:2:4 with a w/c ratio of 0.55. The overall length of all

beams were kept as 915 mm whereas the simply supported span was 762 mm. The beams werereinforced with 2-12.7 mm dia. bars having yield strength of 445 N/mm 2 as main flexuralreinforcement, and 2-6 mm dia. bars were used as hanger bars, shear stirrups consisted of 6 mm

dia. bars spaced at 50 mm c/c. The yield strength of 6 mm dia. bars was 240 N/mm2

. Materialstrengths were obtained by testing control specimens in Material Testing Laboratory, NEDUniversity of Engineering and Technology , Karachi.

18 gauge woven square mesh with wire diameter of 0.9 mm and mesh opening of 4.6 mm wasused for Ferrocement, where 1:2 cement-fine sand mortar with a w/c ratio of 0.45 was used forall strengthened beams. The yield strength of the single wire was found to be 229 N/mm 2. Theaverage cylindrical crushing strength of concrete was found to be 22 N/mm 2. The average cubecrushing strength of mortar using 50 mm x 50 mm x 50 mm cubes was found to be 25 N/mm 2,while the tensile strength was evaluated through briquette test and was found to be 3.5 N/mm 2.

Testing of Specimen

All specimens were loaded under third point loading using Forney Universal LoadingMachine having a resolution of 0.025kN and capacity of 300 kN, at Material Testing Laboratory, NED University of Engineering and Technology, Karachi. One end of specimen rested on aroller arrangement allowing horizontal movement and the other on a simple support. All the

beam specimen were tested under load control with monotonically prescribed incremental loads.At each incremental load of 10kN, deflections, crack pattern, crack width and surface strainswere recorded.

Each specimen was loaded up to 70kN and de-loaded to represent a beam in service and thanthe prescribed strengthening method was applied, and reloaded to failure. The description of loadsequence and method of strengthening is presented in Table 1. Plate 1 and Plate 2 shows thespecimens of B and C series before application of 1:2 cement-sand mortar.

Table 1 - Testing sequence and method of strengtheningSpecimen Load Applied Method of Strengthening

A-iControl specimen loaded from un-cracked state to P 70 and de-loaded. N.A

A-ii Loaded from cracked state up tofailure N.A

B1-i

Loaded from un-cracked, un-strengthened beam to P 70 and de-loaded

N.A


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Table-1 ContdSpecimen Load Applied Method of Strengthening

B1-ii Loaded from strengthenedstate up to failure

After de-loading from P 70 i.e. as B1-i, 3-50 mmequally spaced strips in theshear span were chiseledfrom all around the beamup to outside of stirrups

One layer of mesh stripscut to size, wrapped andimpregnated with mortar

Left for 14 days curing before testing to failure

B2-iLoaded from un-cracked un-strengthened beam to P 70 andde-loaded

N.A

B2-iiLoaded from strengthenedstate up to failure

After de-loading from P 70 i.e. as B2-i, 3-50 mmequally spaced strips in theshear span were chiseledfrom all around the beamup to outside of stirrups.

Two layers of mesh stripscut to size, wrapped andimpregnated with mortar.

Left for 14 days curing before testing to failure.

C1-iLoaded from un-cracked, un-strengthened beam to P 70 andde-loaded

N.A

C1-ii Loaded from strengthenedstate up to failure

After de-loading from P 70 i.e. as C1-i beam, fullshear span on both sideswere chiseled from allaround the beam up tooutside of stirrups.

One layer of mesh strip cut

to size and wrapped allaround and impregnatedwith mortar

Left for 14 days causing be for testing to failure

C2-iLoaded from un-cracked un-strengthened beam to P 70 andde-loaded.

N.A


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Table-1 ContdSpecimen Load Applied Method of Strengthening

C2-iiLoaded from un-cracked un-strengthened beam to P 70 and de-loaded.

After de-loading from P 70 i.e. as C2-i,3-50 mm equally spaced strips in theshear span were chiseled from allaround the beam up to outside ofstirrups.

Two layers of mesh strips cut to size,wrapped and impregnated with mortar.

Left for 14 days curing before testing tofailure.

P70 = Estimated service load of 70 kN

Plate 1- Detail of beam strengthened by ferrocement strips in shear span.

Plate 2- Detail of beam strengthened by ferrocement wraps in shear span.


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TEST RESULTS

The load-deflection curves of second cycle of loading for control specimen and strengthened beams are presented in - Fig. 1.

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9

Measured mid span deflection (mm)

M e a s u r e

d l o a d

( k N )

A - ii

B 1- ii

B 2 - iiC 1 - ii

C 2 - ii

Fig. 1- Measured load-deflection curves for control and strengthened specimen.

The state of strengthened specimens is presented in Plate 3 through Plate 6. Plate 3 shows thecontrol specimens loaded to failure in a cracked state where failure occurred due to shear-compression failure. Specimen B2-ii representing beam strengthened with Ferrocement having

two layers of wire mesh applied in equally spaced strips in the shear span is presented in Plate 4.Specimen C1-ii represent beam strengthened with complete wrap of one layer of wire meshalong full shear span, Plate 5, while C2-ii represents beam strengthened with complete wrap oftwo layers of wire mesh along full shear span as shown in Plate 6 receptively.

Plate 3 - Typical brittle shear-compression failure of controlled specimen A-ii.


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Plate 4 - Specimen B2-ii at failure.

Plate 5 - Specimen C1-ii at failure.

Plate 6 - Specimen C2-ii at failure.

The failure loads of the test specimen and their deflections at cracking load, service load andultimate load along with the failure mode are presented in Table 2.


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Table 2 - Measured results at cracking, P 70 and ultimate.Specimen

testReference

Crackingload

(kN)

Mid-spanDeflectionat Crack-ing Load

(mm)

Mid-spandeflection

at P 70

(mm)

Mid-spandeflectionat ultimate

load

(mm)

Ultimateload

(kN)

Mode ofFailure

A-iA-ii

B1-iB1-ii

B2-iB2-ii

C1-iC1-ii

C2-iC2-ii

28-

29-

28-

30-

26-

1.18-

1.15-

0.98-

1.21-

1.32-

2.312.71

2.232.50

2.182.41

2.342.45

2.262.35

-5.55

-6.49

-7.22

-7.90

-8.20

-120

-122

-124

-125

-127

-Brittle, shear-compression

-Brittle, shear-compression

-Shear-compression,

more cracks anddelayed failure

-Seemingly transition

to ductile-

Ductile, shear

DISCUSSION OF RESULTSAt service stage the strengthened beams showed improved performance, as is evident from -

Fig. 1 and column (3), column (4) of Table 2, where increased stiffness due to Ferrocement stripsand wraps and relatively decreased deflection in comparison with control specimen is evident.

Increased number of cracks with decrease in crack width was observed for strengthenedspecimen, however, no significant difference in pattern was noticed between the two methods ofFerrocement application.

The mode of failure as described in Table 2 is also evident from Plate 3 to Plate 6.The load that the test specimen could sustain theoretically in shear was calculated to be 119.5

kN, using ACI Code model without using capacity reduction and load factors.The increase in shear capacity varied between 1.5% to 5.8% , the maximum being for C2-ii,

where the specimen was wrapped with 2 layer of wire mesh. The increase in shear capacity,therefore, is not substantial, however, an evidence of transformation of brittle, shear-compressionfailure to ductile shear failure is evident from - Fig. 1 and Plate 6. Almost all the failures wereshear failures, however, the presence of Ferrocement strips and wraps delayed the failure giving

ample warning before failure, which is considered as the desired mode of failure by almost allcodes. Greater number of cracks and sufficient ductility is apparent from - Fig. 1 and Plate 3through Plate 6 of the strengthened beams. This is a significant result, which may allow beamsthat are over-reinforced in shear to fail in a ductile mode. In earthquake zones where largermoments had to be resisted by beam on small openings, the use of Ferrocement wrap may allowuse of shear stirrups of capacity more than 4 times the capacity of the beam.

CONCLUSIONSThe following conclusions may be drawn from the present experimental study.1. Confining concrete in the shear zone by Ferrocement have the potential of transforming

brittle shear-compression failure to ductile shear failure.


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2. Ferrocement wraps are more effective than Ferrocement strips.3. The enhancement in load carrying capacity is not substantial, however, is present. In

service range increased stiffness of strengthened beams reduces the crack width anddeflections in comparison with the un-strengthened beam.

4. The method is simple to understand and apply, and, therefore, may be utilized in non-

engineered rural construction and could achieve maximum results.

ACKNOWLEDGEMENTThe authors wish to acknowledge the financial support provided by the Department of

Civil Engineering, NED University of Engineering and Technology, Karachi, and wish to thankall those students and staff of the laboratory who helped in carrying out work in the laboratory.

REFERENCES1. Gulkan, P.; Wasti, S.T.; and Karaesmen, E., Some Aspects of Earthquake Engineering

Research in Turkey, Research Report , Middle East Technical University, Ankara, Turkey1980.

2. A Manual of Earthquake Resistant Non-Engineered Construction, Book Published by Indian Society of Earthquake Technology, University of Roorkee, 1981 . 155 pp . 3. Singh, K.K., Kaushik, S.K.; and Parakash, A., Strengthening of Brick Masonary Columns

by Ferrocement, Proceedings of the Third International Symposium on Ferrocement. Roorkee,University of Roorkee, 1998, pp. 306-315.

4. Waliuddin, A.A.; and Rafeeqi, S.F.A., Study of the Behaviour of Plain Concrete Confinedwith Ferrocement, Journal of Ferrocement , V. 24, No.2, April 1994, pp. 139-151.

5. Shake Table Tests Conducted in India, News letter, Earthquake Hazard Center, V. 2, No.2, Oct. 1998, pp. 2-3.

6. Giesecke, L.Z..; Bartolome A.S.; and Quiun, D., Reconditioning of Existing AdobeHousing to Mitigate the Effects of Earthquakes, Proceedings of the 11 th World Conference in

Earthquake Engineering , Mexico, 1996.7. Desia, R .; Field Shake Test Programme at Latur, Western India, News letter, Earthquake

Hazard Centre , New Zealand, V. 3, No.2, Oct. 1999, pp. 4-5.8. Wasti, S.T.; Erberik, M.A.; Sucuoglu, H.; and Kaur, C., Studies on Strengthening of Rural

Structures Damaged in the 1995 Dinar Earthquakes, Proceedings of the Eleventh EuropeanConference on Earthquake Engineering, CD-ROM, Paris, France, 1998.

9. Non-Engineered Construction in Earthquake Prone Areas and Earthquake Mitigation withSpecial Reference to Pakistan, Project Report , Department of Civil Engineering, NEDUniversity of Engineering and Technology, Karachi, Pakistan, 1998, 103 pp.

10. Andrews, G.; and Sharma, A.K., Repaired Reinforced Concrete Beams ACI, Concrete International, Detriot , April 1998, pp. 47-50.

11. Al-Farabi, M.S.; Baluch, M.H.; Al-Sulaimani, G.J.; and Basunbul, I.A., Repair ofDamaged R/C Beams using Externally Bonded Fiber Glass Plates, Fourth InternationalConference Structural Failure, Durability and Retroffiting, Singapore, July 14-16, pp. 621-628.

12. Basunbul, I.A.; Husain, M.; Sharif, A.M.; Al-Sulaimani, G.J.; and Baluch, M.H.,Repair of Shear Cracked R/C Beams with Bonded External Steel Plates, Fourth InternationalConference on Structural, Failure, Durability and Retro-fitting, Singapore, July 14-15, 1993,

pp. 629-634.13. Al-Sulaimani, G.J.; Al-Farabi, S.; Basunbul, I.A.; Baluch, M.H.; and Ghaleb, B.M.,

Shear Repair for Reinforced Concrete by Fiber Glass Plate Bonding, ACI Structural Journal, V. 91, No. 3, July-August 1994, pp. 458-464.


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14. Ong, K.C.G.; Paramasivam, P.; and Lim, C.T.E., Flexural Strengthening of ReinforcedConcrete Beams using Ferrocement Laminates, Journal of Ferrocement, V. 22, No. 4, October1992, pp. 331-342.

15. Paramasivam, P.; Ong, K.C.G.; Lim, C,T.T., Repair of Damaged RC Beams usingFerrocement Laminates, Fourth International Conference on Structural, Failure, Durability

and Retrofitting,, Singapore , July 14-15, pp. 613-620.16. Paramasivam, P., Recent Research and Application of Ferrocement in Singapore, Proceedings of the sixth International Symposium on Ferrocement, University of Michigan AnnArbor, June 7-10 , 1993, pp. 53-64.

17. Aurellado. P. G. Jr., Performance of Repaired Reinforced Concrete T-beams, M.Sc.Dissertation, National University of Singapore, Sig., 1994, 103 pp.

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