Repairing and Strengthening of RC beams using coir fiber
Sonti girish babu Jonna rohan reddyAsst professor department of civil engineering ellanki college engineering patancheru Hyderabad Telangana
Asst professor department of civil engineering Vardhaman College of engineering Shamshabad Hyderabad Telangana
Repair and rehabilitation are one of the critical problems in civil engineering.
Currently repair and or rehabilitation have become necessary because of distress in structures due to various reasons. Conventional repair such as increasing the section and providing reinforcement with concrete overlaying are difficult because of various field restrictions.
Repair with coir fiber reinforced polymer sheets easy, convenient and effective.
Bonding of CoFRP sheets with epoxy resin is an easy and effective in Strengthening of structures. Beams, slabs, and columns can be easily repaired rehabilitated with CoFRP sheets with minimum effort. Strength in shear and Flexure, Weathering, Torsion Resistance, Resistance to temperature, axial compression is various advantages of this method.
Under reinforced beams (150mm x 150mm x 700 mm) were casted and cured for 28 days. The beams were then left to environment for a year. A year later beams were repaired with wrapping technique using CoFRP was bonded with the help of resin. Now the beams were loaded until failure.
Strength regained by single layer bonded shear beam and full wrapped shear beam when compared to the shear strength of control beam of 107.7 KN were 104.5 KN and 121 kN Percentage regained strength by single layer bonded shear beam and full wrapped shear when compared to control beam were 97.03% & 113.18%.
Strength regained by single layer bonded shear beam and full wrapped flexure beam when compared to control beam of 103.23 kN were 103.3 kN and 92.8 kN Percentage regained strength by single layer bonded shear beam and full wrap shear beam when compared to control beam were 99.93% and 89.9%.
The maintenance and retrofitting of structural members is one of the major problems in civil engineering applications. Many structures constructed previously using the old design codes in different parts of the world are structurally unsafe according to the new design codes. Therefore replacement of such structural members is required. But this requires large of amount of funds and time. This leads us to the strengthening methods for improving the load carrying capacity and extending their serviceability. So strengthening may be required due to the reasons listed below.
Additional strength may be required to allow higher loads to be placed on the structure. This usually happens when the use of structure changes and higher load carrying capacity is needed.
Strengthening may be needed to allow the structure to resist loads that were not anticipated in the original design. This may be encountered when the structural strengthening is required for loads resulting from wind and seismic forces or to improve resistance to blast loading.
Additional strength may be needed due to a deficiency in the structures ability to carry the original design loads. Deficiencies may be the result of deterioration (e.g., corrosion of steel reinforcement and loss of concrete section), structural damage (e.g., vehicular impact, excessive wear, excessive loading and fire).
Y.C.Wang & C.H.Chen 2003 conducted an analytical study on the behavior of reinforced concrete beams retrofitted with CFRP plates for a flexure and GFRP plates for a shear.
Hui Li,Zhi-qiang Liu, & JIN-Ping Ou2008 conducted tests on a simple reinforced concrete beam temporarily strengthened by SMA wires recovery forces can decrease deflections CFRP plates
Ines G. Costa & Joaquim A.O.Barros 2010 conducted research to experimental, numerical and analytical investigations have revealed that carbon fiber reinforced polymer CFRP strips with larger cross section height improve the effectiveness of the near surface mounted (NSM) technique for the flexural strengthening of existing reinforced concrete (RC) beams
Alaamorsy & EI Tony Mahmood 2013 conducted experimental study on bonding techniques for flexural strengthening of RC beams using CFRP laminates by the way of fasting steel rivets through the FRP laminate and concrete substrate.
weichenXue Yuan Tan &, Lei Zeng 2010 conducted tests on flexural response predictions of reinforced concrete beams strengthened with prestressed CFRP plates experimental studies showed that the reinforced concrete (RC) beams strengthened with pre stressed carbon fiber reinforced polymer (CFRP) plates had three possible flexural failure modes 9including the compression failure, tension failure and debonding failure) according to the CFRP reinforcement ratio.
1.2 STRENGTHENING WITH CoFRP COMPOSITE.
Only a few years ago, the construction industry started using FRP for structural reinforcement. FRPs exhibit many properties like strength, lightweight, flexibility in design and ease of application. Many researchers had investigated on FRP sheets or plates bonded to concrete beams. Adding strength with adhesive bonded fiber reinforced polymers was identified as effective method applicable to many types of concrete structures like columns, beams, slabs, and walls. As the FRPs are non-corrosive and non-magnetic they are widely used. From the previous studies it has been proved that externally bonded fiber reinforced polymers can enhance the flexural and shear capacity of beams.
1.3 ADVANTAGES AND DISADVANTAGES OF FIBER COMPOSITE STRENGTHENING
Fiber composite strengthening materials have higher ultimate strength and lower density than steel. When taken together these two properties lead to fiber composites having a strength/weight ratio higher than steel plate in some cases. The lower weight makes installation easier than steel. The availability of material also simplify the installation.
The main drawback of external strengthening structures with fiber composite materials is the risk of catching fire, accidental damage unless the strengthening is protected.
2. MATERIAL & METHODOLOGY
The concrete is a mixture of cement, fine aggregates, coarse aggregates and water. The mix ratio of M40 is used in this investigation .Three cube specimens were casted and tested at the time of beam test (at the age of 28 days) to determine the compressive strength of concrete. The average compressive strength of concrete was 49N/mm2 .
Cement is the important material in the concrete that helps in imparting binding property with the other constituents of the mixture. Cement show its behavior only when it comes into contact with water. The most widely used of the construction cements is Portland cement. The specific gravity is at least 3.10.
Table 2.1 properties of ordinary Portland cement 53 grade
cement Fineness (m2/Kg) Setting time Compressive strength MPainitial final 3 7 28
1 OPC 53 GRADE 225 75 125 32 37.8 53
Locally available natural river sand with specific gravity 2.63 and zone II is used, confirming to IS 383:1970.
Table 2.2 Sieve analysis report
I.S SEIVE NO WEIGHT RETAINED CUMULATIVE WEIGHT RETAINED PERCENTAGE OF CUMULATIVE WEIGHT RETAINED PERCENTAGE WEIGHT RETAINING
10 0 0 0 100
4.75 0 0 0 100
2.36 59 59 59 94.1
1.18 285 344 34.4 65.6
600 210 554 55.4 44.6
300 185 739 73.9 26.1
150 190 929 92.9 7.1
<150 71 1000 100 0
2.4 COARSE AGGREGATE:
Locally available crushed stone angular aggregate of size 20mm was used. Specific gravity of 2.67 confirming to IS 383:1970.
Water fit for drinking is considered fit for preparing concrete mixture.
Water should be free from acids, oils, alkalis, vegetables or other organic impurities. In this investigation potable water is used.
The longitudinal reinforcement used here are HYSD bars of 12mm diameter. The stirrups were made from mild steel bars with 6mm diameter. The yields strength of steel reinforcements used in this experimental program was determined by performing the standard tensile test on three specimen of each bar. Average proof stress at 0.2% strain of 12 mm bars was 437 N/mm2 and that of 6 mm bars was 240 N/mm2.
Fig 2.1 Reinforcement detailing
A fiber is a material made into long filaments with diameter. The aspect ratio of length and diameter can be ranging from thousand to infinity in continuous fibers.
The main functions of the fibers are to carry load and provide additional stiffness, strength to member.
Fibers in FRP must have the following properties:
Low variation of strength
stability during handling
High uniformity of diameter and surface dimension among fibers
The fiber used in this investigation is coir fiber composite i.e. COIR MAT
Table 2.3 Physical properties of coir fiber
Ultimate length Diameter
Single fiber Breaking elongation
Moisture regain at 65% RH Swelling in water
length density Tenacity 0.6mm 16 microns 6 to 8
inches 1.4 g/cc 10 g/ tex 30% 10.5% 5% in diaTable 2.4 Chemical composition of coir
Water soluble Pectin & related compounds Hemi cellulose cellulose Lignin
5.25% 3.30% 0.25% 43.44% 45.84% 2.22%
2.8 EPOXY RESIN:
Epoxy resins are relatively low molecular weight pre-polymers capable of being processed under a variety of conditions. Two important advantages of these resins are over saturated polyesters resins are first partially cured and stored in that state, and second they exhibit low shrinkage while curing. However, the viscosity of conventional epoxy resin is higher and they are more expensive compared to polyester resins. The cured resins have high chemical, corrosion resistance, good mechanical and thermal properties, outstanding adhesion to a variety of substance, and good and electrical properties.
Epoxy resin with trade name “SYSBOND 757” under manufactures of m/s SYSCON CHEMICALS available locally, for bonding new cementations materials to existing cementations surfaces, was used in this investigation. Ideal for extensions and repairs to structural concrete in buildings, loading bays, bridges, roads, bonded or granolithic floor toppings etc.
SYSBOND 757 is based on solvent free resin system containing pigment and filler. It is supplied as two part pre-packed material. The unique formulation gives excellent bonding properties with an extended pot life.
The hardener has to be totally poured in to the base container. Two parts have to be thoroughly mixed with a low speed mixing, has to be done until a uniform color is obtained. The mixture should be applied with the help of a brush on the dust clean surface.
Table 2.5 Properties of the SYSBOND 757 resin
Properties Pot life 2-3 hours at 30 degrees centigrade
Curing period 48 hours
Over coating time 6-9 hours
Compressive strength 51 N/mm2
Flexural strength 35 N/mm2
Tensile strength 20N/mm2
Shear strength 25N/mm2
2.9 MIX DESIGN OF CONCRETE:
Ordinary Portland cement (OPC) -53 grade (Birla A1 premium cement) was used for the investigation. The fine aggregate used in this investigation was clean natural river sand passing through 4.75 mm sieve with specific gravity of 2.63. The grading of fine aggregate was zone II as per Indian standard specifications. Machine crushed angular stones were used as coarse aggregate. The maximum size of coarse aggregate was 20 mm with specific gravity 2.67. Ordinary clean potable water free from suspended particles and chemical substances was used for both mixing and curing of concrete.
Concrete mix design of 1:2.26:2.029 by weight is used to achieve the strength of 40 N/mm2. The water cement ratio 0.43 is used. Three cube specimens were cast and tested at the time of beam test (at the age of 28 days) to determine the compressive strength of concrete. The average compressive strength of the concrete was 49 N/mm2.
I) Characteristic compressive strength=40N/mm2.
ii) Maximum size of coarse aggregate=20 mm.
iii) Degree of quality control=good
iv) Type of exposure=mild
Cement required=420 kgs
I) specific gravity of fine aggregate=2.63
ii) Specific gravity of coarse aggregate=2.67
iii) Sand confirming to zone=II
Target mean strength=48.250 N/mm2.
W/C for target mean strength=0.43
Water content for cubic meter of concrete = 180.6 lit
Fraction of coarse aggregate in total aggregate=0.460
Correction of coarse aggregate=0.014
Fraction of coarse aggregate required after correction=0.474
Quantity of material in volume(m3)
a) Total quantity = 1.000
b) Cement = 0.133
c) Water = 0.181
d) Volume of aggregate = 0.686
e) Weight of coarse aggregate = 852.012
f) Weight of fine aggregate = 949.091
3. TEST PROCEDURE
The beams are divided into three groups and named them as A, B, C.
Group A-control beams
Group B-beams tested in flexure
Group C-beams tested in shear
Figure 3. SEQ Figure * ARABIC 1 Detailing of beam with shear reinforcement
Figure 3.2 detailing of beam with flexure reinforcement
3.2 PREPARATION OF BEAMS:
After oiling the moulds the cage (reinforcement) was placed in the moulds. As per the design mix concrete was mixed in concrete mixer and placed in the moulds and compacted with the help table vibrator. The moulds were de-moulded and immersed in water for curing for 28 days.
Beams were left in environment for a year so as they get proper exposure to possible harsh environmental effects.
Here 12 beams were casted i.e. 4 beams under each category.
All beams were designed as under reinforced beams. All beams were tested on UTM 100 ton capacity.
Group A beams were again divided into two groups where two beams were tested under two point loading at third point where a region of pure flexure was obtained, two beams were tested under point loading at mid-point where a maximum shear case was obtained
Two point load
3.3 SURFACE PREPARATION:
The surface was cleaned by using a sand paper under continuous running water. The dust settled on the surface was cleaned and beams were left under the sun for a day to get dried. Then resin is applied on the surface and sheet was warped according to desire pattern
Fig 3.3 preparation of beams
3.4 WRAPPING SYSTEM:
Two types of wrapping is followed for this study
One side wrapping
All around wrapping
ONE SIDE WRAPPING:
Wrapping was done only on tension face.
ALL AROUND WRAPPING:
Wrapping was done in such a way that sheet covers all the four faces of the beam.
3.5 TEST PROCEDURE:
Before testing members were checked dimensionally, and a detailed visual inspection was made with all information carefully recorded. After fixing supports at required position in the UTM, beam was placed and load was applied and gradually increased.
Point load at mid span
Two pint load at third point
4. RESULTS AND DISCUSSION
Table 4.1 Test results for the shear control beams
Load in kN Deflection in mm
Graph 4.1shear control beam load vs. deflection
Table 4.2 Test results of flexure control beam
Load in kN Deflection in mm
Graph 4.2 flexure control beam load vs. deflection
Table 4.3 Test result of shear sample with single wrap fiber.Load in kN Deflection in mm
Graph 4.3 load vs. deflection of single wrap shear beam
Table 4.4 Test results of flexural beam with full wrap system
Load in kN Deflection in mm
Graph 4.4 load vs. deflection of full wrap flexure beam
Table 4.5 Test results of shear beam full wrap
Load in kN Deflection in mm
Graph 4.5 load vs. deflection of full wrap shear beam
Table 4.6 Test results of single wrap flexure beam
Load in kN Deflection in mm
Graph 4.6 load vs. deflection of single wrap flexure beam
Graph 4.7 load vs. deflection of control shear beam, single wrap shear and full wrap shear beam.
Graph 4.8 load vs. deflection of flexure control beam, single wrap flexure and full wrap flexure beam.
Fig 4.1 showing the peak loads under one point loading
Fig 4.2 Showing peak load under two point loading.
Fig 4.3 showing percentage regains under one point loading
Fig 4.4 showing percentages regain under two points loading
Beams were exposed to harsh natural environment for one year, so that they experience loss of strength and surface wear & tear compared to newly casted beams in lab to stimulate field conditions.
From this investigation even though beam surfaces was exposed to harsh environment deboning of fiber was not observed in most of the beams except in case of full wrap flexure under two point load.
Strength regained by single wrap shear beam and full wrap shear beam when compared to shear control beam of 107.7 kN were 104.5 kN and 121 kN
Percentage regain in strength by single wrap shear beam and full wrap shear when compared to control beam were 97.03% & 113.18%.
Strength regained by single wrap flexure beam and full wrap flexure beam when compared to control beam of 103.23 kN were 103.3 kN and 92.8 kN
Percentage regain in strength by single wrap shear beam and full wrap shear beam when compared to control beam were 99.93% and 89.9%.
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