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CHAPTER 4
EXPERIMENTAL INVESTIGATION
4.1
GENERAL
In this study, six numbers of quarter size, two bay-five storey reinforced concrete frames with different infill were tested under static cyclic loading. Proper care was taken to get the identical properties and strength for the frame. This was achieved by using steel mould, good quality materials, properly designed mix, adhering strict quality control during casting and curing. In this chapter, details of specimen, test setup and testing procedure are described elaborately.
4.2
DESIGN OF FIVE STOREY R.C FRAME
In the design, it is assumed that the plastic hinges form in all floor beams before the formation of plastic hinges in the columns. It is desirable that energy be dissipated by the floor beams before hinging of column commences. This will ensure that the columns remain in the elastic range of behavior till a very late stage of disturbances and thus, have a higher degree of protection against permanent damage. Loads were calculated as per IS 8751987 (Part 2 and 3) and various load combinations were considered for the design of five storey R.C. frame. The model RC frames were designed for lateral load assuming that the frames in the real structure are spaced at 4m interval.
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4.3
DETAILS OF SPECIMEN
The following quarter scale model frames were tested under static cyclic loading. The dimensions in metre are given in Figure 4.1 and the reinforcement
details
are
given
in
Table
4.1.
Figure
4.2
and
Figures 4.3(a) and 4.3(b) shows the reinforcement details of the model frame.
1.
Bare Frame-BF
2.
Brick Masonry infill Frame-BMF
3.
Reinforced Brick Masonry infill Frame-RBMF
4.
Hollow block Masonry infill Frame-HMF
5.
Reinforced Hollow block Masonry infill Frame-RHMF
6.
Reinforced Aerocon Block Masonry infill Frame-RABMF
Figure 4.1 Dimensions of model frame
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Table 4.1 Reinforcement details for columns and beams
Details
Size in mm
Columns
Main Longitudinal
10mm , 10 Nos.
Columns
10mm, 8 Nos. 3,4,5,8,9, 100 x 200 4 on either side 10,13,14, 15
1,2,3,6,7,8
Beams 4,5,9,10
100 x 150
10mm, 2 Nos. at top & bottom
100 x 150
Member Numbering
Reinforcement
1,2,6, 7, 11, 100 x 200 5 on either side 12
Beams
Lateral Ties
8mm, 2 Nos. at top & bottom
6mm 2 legged, 40mm c/c at junction region and 80 mm c/c at middle region 6mm 2 legged at 30mm c/c at junction region and 60mm c/c at middle region
Figure 4.2 Reinforcement arrangement of RC frame
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Figure 4.3(a) Reinforcement details of model frame
41
D-D
Figure 4.3(b) Reinforcement details in various sections
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4.4
MATERIALS USED
Hard blue granite metal available around Virudhunagar was used as coarse aggregate. Well graded river sand was used as fine aggregate. Ordinary Portland cement conforming to IS 269-1989 has been used for concreting and for masonry works. Potable water available in the college campus was used for concreting, curing and for construction of infill. High Yield Strength Deformed bars (HYSD) of various sizes were used as reinforcement of R.C. frames.M20 Grade concrete was used and the mix design was done according to B.I.S. method to achieve the required target strength. Good quality bricks, hollow blocks and aerocon blocks were used as infill materials.
4.5
CONCRETING AND CURING
Steel mould which was made up of rolled steel channel sections and plates were used for casting frames. The steel mould was fabricated to facilitate easy assembling and dismantling for repetitive works. The frames were cast in the college Structural Engineering Lab and sufficient precautions were taken for removing the specimen from the mould and for erection. Figure 4.4 shows the concreting of the specimen. Compaction was done by 20mm needle vibrator as shown in Figure 4.5. Control specimens such as cubes, cylinders and beams were cast. The side plates of the mould were removed after 24 hrs and the specimen was covered with wet gunny bags for continuous curing. It was kept moist by periodical springing of water for a period 28 days after casting. The control specimens were also cured under identical conditions as that of the frames.
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Figure 4.4 Concreting of the specimen
Figure 4.5 Vibrating the specimen
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4.6
DETAILS OF FOUNDATION BLOCK
For conducting experimental investigation a precast foundation block shown in Figures 4.6(a) and 4.6(b) was constructed to meet the following requirements.
To fix the test specimen, to ensure fixity and to simulate the real structural foundation system.
To avoid rigid body rotation of the test specimen with respect to foundation block and displacement of foundation block with respect to test floor.
The foundation was designed to resist the maximum base shear of 2000 kN considering all aspects to withstand the forces transferred from testing specimen. The reinforcement details of the foundation block and its construction are shown in Figures 4.7 and 4.8.
Figure 4.6 a) Cross Section of foundation block
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Figure 4.6(b) Plan of foundation block
Figure 4.7 Reinforcement details of foundation block
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Figure 4.8 Construction of foundation block
4.7
LIFTING AND ERECTION OF THE FRAME
The specimen was cured for 14 days using gunny bags. Then the specimen was cleaned and allows resting on 75mm diameter mild steel pipes places across the specimen at regular intervals. A steel wire rope tied at third storey level as shown in Figure 4.9 was used to lift the specimen. The frame was lifted using 5T capacity electrically operated overhead crane and then moved towards the foundation block. The foundation portion of the test specimen was inserted in the gap between the two webs of the foundation block as shown in Figure 4.10.
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Figure 4.9 Lifting of frame
Figure 4.10 Erection of frame
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4.8
CONSTRUCTION OF INFILL
The significance of infill in determining the actual strength and stiffness of the framed buildings subjected to lateral force has long been recognized. Despite rather intensive investigations inclusion of infilling walls as structural elements in the design is not yet common because of the design complexity and lack of suitable theory for analysis. Hence, in this study the RC frame with various infill materials such as brick masonry, reinforced brick masonry, hollow block masonry, reinforced hollow block masonry and reinforced aerocon block masonry were considered.
In the frame, construction of infill was carried out on the same day of erection. Suitable scaffolding arrangement was made for the construction of infill. The cement mortar 1:4 with water cement ratio of 0.45 was used. The panel sizes for all infilled specimens were 1000 x 600 mm with a thickness of 100mm.
4.8.1
Brick Masonry Infill
The stretcher bond type construction was done with mortar joint thickness of 10 mm and bed mortar thickness of 16mm. 190 x 90x 90 mm size bricks were used for infill. To find out compressive strength of brick masonry, brick prisms of size 200 x 200 x 500 mm were prepared and tested after 14 days curing as recommended by IS 1905-1987. The average compressive strength of the brick masonry was 1.975 N/mm 2. The same 1:4 cement mortar mix was used for this compression test to attain an identical condition.
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4.8.2
Reinforced Brick Masonry Infill
As per IS 2212 – 1991 – Code of practice for brick work cl.4.5, cl.10.8 to 10.10, cl.12.4., the available code provision regarding the construction of reinforced brick masonry are as follows:
The reinforcement for brick masonry may be in the form of mild steel bars or fabric or mild steel flats or expanded mesh or hoop iron. In case of round bar, the diameter of steel shall not exceed 8mm. Flat bars and similar reinforcement shall not have a thickness exceeding 8 mm. Generally the reinforcements are used in every 3rd or 4th courses of the brick work.
The thickness of reinforced brick work shall not be less than 10cm
The mortar used for reinforced brick work generally is rich, dense and C.M. of mix 1:4.
The crushing strength of bricks used in reinforced brick wall shall not less than 14 N/mm2
The mortar covering in the direction of joints shall not less than 15mm
The mortar interposed between the reinforcement bars and the brick shall not be less than 5 mm.
The inlaid steel reinforcement shall be completely embedded in mortar overlaps in the reinforcement, if any, shall not be less than 30 cm.
In case where reinforcements cross inside a joint, the diameter of reinforcement shall not exceed 5 mm.
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In hot and dry weather, the brick work is constantly wet for at least 7 days.
The construction of reinforced brick masonry infill was done according to the above codal requirement. The size of the brick used was 190 x 90 x 90 mm and its compressive strength is 14 N/mm2. Stretcher bond type construction was adopted to simulate the brick wall thickness of 100mm. One number of 6 mm diameter mild steel bar was embedded in the bed mortar of each course as shown in Figure 4.11. To find out the compressive strength of reinforced brick masonry, brick prisms of size 200 x 200 x 500mm with reinforcement were prepared and tested after 14 days and the compressive strength measured was 2.745 N/mm2.
Longitudinal Section
Cross Section
Figure 4.11 Reinforced brick masonry infill
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4.8.3
Hollow Block Masonry Infill
Hollow concrete blocks are used in place of bricks and they have been primarily designed to serve as a filler wall material in framed structures. In some cases they are also used to substitute brick work for bearing loads but such cases are limited in volume, magnitude and in height. Hollow concrete block masonry wall has one specific advantage over the brick masonry as the strength of the block can be increased so as to cater to the load carrying requirements subject to the limitations of slenderness effects. The second important factor is that wherever the load carrying requirements are larger, the concrete hollow block masonry can be further strengthened by filling the voids with plain cement concrete and if required by reinforcing the filler concrete to make the wall a composite material.
The hollow blocks were manufactured as per IS 2185-2005. The size of hollow block used was 400 x 200 x 100 mm. The hollow concrete blocks prisms of size 200 mm x 200 mm x 500 mm were prepared and tested after 14 days with identical curing and compressive strength of 3.56 N/mm2 was found.
4.8.4
Reinforced Hollow Block Masonry Infill
The size of hollow concrete block used was 400 mm x 200 mm x 100 mm. The reinforced hollow concrete block masonry was 100mm thick, HYSD bars of size 8mm diameter was inserted in the hole of hollow concrete block masonry vertically extending throughout the height as shown in Figure 4.12. The hole was filled with concrete of mix 1:2:3 and the coarse aggregate used for this mix was below 12mm size.
To find out the
compressive strength of the reinforced hollow concrete block masonry, prisms of size 200 mm x 200 mm x 500 mm with reinforcement was prepared and
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tested after 14 days of curing and was found as 5.96 N/mm2. Figure 4.13 shows the construction of infill panel. After the construction, the infill panel was placed in the RC frame and Figure 4.14 shows the construction stage of infill in the RC frame.
Elevation
Cross Section
Figure 4.12 Reinforced hollow block masonry infill
Figure 4.13 Construction of RHMF - Infill panel
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Figure 4.14 Construction stage of RHMF
4.8.5
Reinforced Aerocon Block Masonry Infill
Autoclaved aerated concrete block (AEROCON) is a high quality engineered building material that offers a unique combination of strength, light weight, thermal and sound insulation, fire resistance, resistance to water penetration and good workability. It is manufactured using cement, fly ash, lime, aeration agent and water conforming to IS 2185-2005. The aeration process generates minute non-inter connecting cells which forms the characteristics cellular or air-crete structures. The cellular structures make the block light weight, fire resistant high thermal and high acoustic insulation. The autoclaving process gives strength and stability to the block.
Autoclaved aerated concrete block is not just a substitute for conventional bricks in the wall. It represents a complete wall system with outstanding advantages like conventional clay bricks and hollow concrete blocks. Autoclaved aerated concrete blocks have more benefits due to its unique size in terms of mortar consumption, speed of construction, thermal insulation and ease of handling.
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Autoclaved aerated concrete blocks are easy to cut to achieve the desired size, shape and hence suitable for architectural elements also. The size of the block used is 600 x 100 x 100 mm having dry density of 651 kg/m3 which is approximately 1/3rd the density of clay brick. Mild steel bar of 6 mm diameter was embedded in the bed mortar of each course as shown in Figure 4.15. To find out the compression strength of reinforced aerocon block masonry, block prisms of size 200 x 200 x 500mm with reinforcement were prepared and tested after 14 days curing and the measured compression strength is 2.5 N/mm2.
Longitudinal Section
Cross Section
Figure 4.15 Reinforced aerocon block masonry infill
4.9
TEST SETUP
The static cyclic load involves loading a structure to failure by applying a load at a constant rate in a series of load-unload cycles. The schematic diagram of test setup is presented in Figure 4.16 with the following arrangements to measure load, deflection, strain and crack pattern. The static cyclic load was applied at first, third and fifth storey levels by means of
1. Foundation block 2. Hydraulic jack 3. Test specimen
4. Infill panel 5. Distributor (loading) 6. Distributor (unloading)
7. Oil pump unit (loading) 8. Oil pump unit (unloading) 9. Strong test floor
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Figure 4.16 Schematic diagram of test setup
10. Loading frame 11. LVDT 12. Hose pipes
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hydraulic jacks of 500 kN capacity. All the three jacks were actuated by two hydraulic pumps for loading and unloading. The load applied was measured by means of load cell placed between loading frame and the jack. The reading was taken through load indicator having 10 channels. The linear variable displacement transducers (LVDT) were used to measure deflection. The LVDT was fixed to square mild steel tubes that were in turn fixed to H type reaction frame. The LVDT were connected to a common digital displacement meter having 10 channels. Proper provisions were made to anchor the specimen to the testing floor to avoid the rigid body rotation. However, the deflection due to rigid body rotation any was measured by dial gauges which were provided on the base slab of the foundation block. Demountable mechanical strain gauge (DEMEC) was used to measure strain at different locations while testing the frame. The response of the structure was monitored and developments of cracks were traced using crack detection microscope. Figure 4.17 shows the hollow block masonry infill frame during testing. Figures 4.18 to 4.20 show the details of testing arrangements for various infill frames.
Figure 4.17 Testing of HMF
Figure 4.19 Testing arrangements of RBMF
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Figure 4.18 Testing arrangements of BMF
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Figure 4.20 Testing arrangements of RABMF
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4.10
TESTING PROCEDURE
The models were tested as a vertical cantilever by varying shears under a cyclic loading program. To start with, the frame was loaded with small loads and then released to check the effectiveness of the instrumentation setup and loading. This process was repeated till readings were constant. A load increment of 5 kN was applied in each cycle. At ultimate load of each cycle, deflection, strains and crack pattern were measured. The load cycles were continued till the final collapse occurred.
4.11
TESTING OF CONTROL SPECIMEN
The concrete cubes were tested in compression testing machine to find out the compressive strength of concrete. The concrete cylinders were tested under compression in universal testing machine to find out the modulus of elasticity of concrete. To find out the modulus of elasticity of steel and yield strength, the tensile test was conducted in universal testing machine. The beam of size 150 x 150 x 750mm was tested in universal testing machine to determine the flexural strength of concrete. The masonry prisms were tested for different infill to find out the modulus of elasticity of masonry. The summarized properties of materials used for model are given in Table 4.2.
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Table 4.2 Properties of materials used for model frames
Description
BF
BMF
RBMF
HMF
Cube Compressive Strength of Concrete in N/mm2 (fck)
32.35
36.45
32.35
36.05
32.80
36.45
Split Tensile Strength of Concrete in N/mm2 (fct)
3.13
3.28
3.13
3.65
3.53
3.28
4.39
5.46
4.39
5.17
5.07
5.46
8 mm Ф
496
491
496
628.48
628.48
491
10mm Ф
468
473
468
465.53
465.53
473
Flexural Strength of Concrete in N/mm2
RHMF RABMF
(fcr) Yield Strength of Steel in N/mm2 (fy)
Modulus of Elasticity of Steel in 2.10 x 105 2.06x 105 2 x 105 N/mm2 (Es) Compressive Strength of Masonry for C.M 1:4 in
-
1.975
2.745
2 x 10 5 1.96 x 10 5 1.98 x 10 5
3.56
5.96
2.50
N/mm2 (fm) Modulus of Elasticity of Concrete (Ec) in
2.84 x 104 3.02 x 104 2.84 x 104 3 x 10 4 2.86 x 10 4 3.02 x 10 4
N/mm2 Modulus of Elasticity of masonry (Em) in N/mm2
-
1890
2550
3153
5655
2354
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4.12
CLOSURE
In this chapter, the details of specimen for doing experimental study on multistory R.C. frame, experimental set up with various measuring arrangements and details of testing procedures are discussed. The properties of various materials used for this study are clearly explained. In this study, six numbers of infilled frames were tested under lateral cyclic loading and their structural behaviours are discussed in the next chapter.
