CHAPTER 2 SPECIMEN DETAILS, TEST SETUP AND TESTING PROCEDURE

Transcription

1 38 CHAPTER 2 SPECIMEN DETAILS, TEST SETUP AND TESTING PROCEDURE 2.1 GENERAL In the conducted experimental study, three two-dimensional partially infilled RC frames were cast and tested under quasi-static cyclic loads simulating seismic action. The specimens were constructed as 1/3 scale, two-bay, two-storey RC frames representing multi-storey system to fit the testing facility. Geometrical dimensions and reinforcement of all the specimen frames were kept the same. During design phase of the frames, commonly observed deficiencies in buildings such as insufficient confinement of concrete at columns and beam-column joints were taken into account. The frames were assumed to be representing two-storey RC structures typical of old construction built in India without specific provisions for earthquake resistance. Its design aimed at obtaining a gravity load designed frame and was performed using the concrete design code IS-456 (1978) enforced in India in 1980s. To ensure that the primary damage would occur in columns, as observed in many existing (non-ductile) concrete structures, the beams were designed conservatively and the columns were constructed with nonconforming transverse reinforcement. Sufficient care was taken to get identical properties and strength for the frames. The details of the specimens, material properties, casting and erection, test setup, and the testing procedure are presented in this chapter.

2 DETAILS OF TEST SPECIMENS Three types of experimental works have been carried out in this study with the following specimens: i. RC frame with partial infill S1BF (Control specimen) ii. RC frame with partial infill retrofitted using GFRP laminates S2FRP iii. RC frame with partial infill retrofitted using masonry inserts S3MF In the experimental study, height of bottom-storey masonry infill wall for the frames was chosen as 75% of the storey height which is commonly adopted for fixing ventilators in buildings and industries. The infill panel of the top-storey covered the entire frame opening. The specimens were designated as S1BF, S2FRP, and S3MF; in which S1BF was the control specimen tested to identify the captive-column behaviour, S2FRP was retrofitted using GFRP laminates, and S3MF was retrofitted using masonry inserts. The frames S2FRP and S3MF were casted in a similar way as the control frame S1BF and then retrofitted using GFRP laminates and masonry inserts respectively. The construction, erection, test setup, instrumentation, and the testing procedure were maintained to be the same for all the three specimens and are discussed in the following sections.

3 RC FRAME WITH PARTIAL INFILL The two-bay two-storey RC frame S1BF representing multi-storey system was designed in the conventional way without considering the effect of infill. Gravity loads alone were considered for the analysis. Test model was fabricated to 1:3 reduced scale by scaling down the geometric properties of the prototype. Brick masonry infill was provided for the full height in the topstorey and for 75% of the storey height in the bottom-storey. The foundation portion of the frame was provided with holes to anchor the specimen to test floor so as to test the frame as a vertical cantilever. This section describes the geometric dimensions, reinforcement details, construction, erection, test setup, and instrumentation details of the control frame S1BF Geometric Dimensions and Reinforcement Details The geometry of the frame model is shown in Figure 2.1. The width and height of each bay in the specimen were 1000 mm and 1200 mm, respectively. The columns and beams were constructed with dimensions 100 x 200 mm and 100 x 150 mm, respectively. Clay bricks with nominal dimensions of 195 x 100 x 75 mm were used for the infill and the thickness of the wall is 100 mm. The partial masonry infill was provided for a height of 900 mm in the bottom-storey. The geometric properties of the specimen frame are shown in Table 2.1.

4 41 Figure 2.1 Geometry of the Frame Model Table 2.1 Geometric Properties of the Frame S.No. Property Prototype Model Scale (mm) (mm) 01. Storey Height: Bottom-Storey Top-Storey Frame Span (Each bay) Beams 300 x x Columns 300 x x 200 3

5 42 The reinforcement details of the frame are shown in Figure 2.2. In columns and beams, four high yield strength deformed (HYSD) bars of 10 mm diameter were used as longitudinal reinforcement. Plain bars with a diameter of 6 mm spaced at 100 mm were used as closed ties in columns. Plain bars with a diameter of 6 mm spaced at 75 mm were used as stirrups for beams. The reinforcement grill for the RC frame is shown in Figure 2.3 and a closer view of the arrangement of reinforcement is shown in Figure 2.4. Since the main focus of the study was to examine the effects of masonry infill, bar slipping at lap splices was prohibited by making all longitudinal reinforcement continuous throughout the length of the columns. At the base, to ensure fixity, the column reinforcement was taken beyond the full depth of footing and bent to achieve adequate anchorage length. Similarly the beam rods were bent to the required development length and inserted into the column reinforcement grill.

6 Figure 2.2 Reinforcement Details of the Frame 43

7 44 Figure 2.3 Reinforcement Grill for the RC Frame (a) A Closer View of Foundation Beam Reinforcement (b) A Closer View of Beam-Column Joint Figure 2.4 Arrangement of Reinforcement Materials Mix design In order to approximate the strength of concrete typical of older construction in the specimen frame, a target 28-day cube compressive strength of 20 N/mm 2 was selected. Ordinary Portland cement of grade 53 conforming

8 45 to IS (1987) was used. Well-graded, crushed hard blue granite metal of maximum size 10 mm available around Coimbatore was used as coarse aggregate. Uniformly graded sand was used as fine aggregate. Potable water was used for concreting, construction of brickwork, and curing of the specimen. Good workability was required to achieve proper consolidation due to potentially difficult placement of the concrete into small column crosssections with little reinforcement cover. A reinforcement cover of 12 mm necessitated the use of aggregate limited to 10 mm maximum size. To provide cover to the reinforcement, precast cover blocks of 12 mm thickness were used for columns and beams. The mix design followed the guidelines of ACI (1991) American Concrete Institute guideline. The details of mix design and the final mix proportions used for this study are given in Table 2.2. Table 2.2 Details of Mix Design S.No. Description Data/Results 1. Required Strength 20 MPa 2. Water-Cement Ratio Slump 50 mm 4. Maximum Size of Coarse Aggregate 10 mm 5. Shape of the Aggregate Angular 6. Specific Gravity of Cement Specific Gravity of Fine Aggregate Specific Gravity of Coarse Aggregate Fineness Modulus of Fine Aggregate Fineness Modulus of Coarse Aggregate Dry Rodded Density of Coarse Aggregate 1573 kg/m Cement Required 13. Fine Aggregate Required 14. Coarse Aggregate Required 15. Water Required 345 kg / m 3 of concrete 848 kg / m 3 of concrete 881 kg / m 3 of concrete 200 litres / m 3 of concrete

9 Concrete and steel properties Compressive strength tests and split cylinder tests were performed as per IS-516 (1959) and IS-5816 (1999) respectively, on samples cast from each pour of the concrete frame. Table 2.3 gives the results of tests conducted during the testing of frame to assess the actual strength of the frame concrete. The testing of cube and cylinder specimens is shown in Figure 2.5. Table 2.3 Test Summary for Tests Conducted During Frame Testing 9 Samples Cube Compressive Strength (MPa) Cylinder Compressive Strength (MPa) Split Tensile Strength (MPa) Average (a) Testing of Cube for Compressive Strength (b) Testing of Cylinder for Split-Tensile Strength Figure 2.5 Testing of Cube and Cylinder Specimens

10 47 The reinforcement grill was prepared using two types of reinforcing bars. The two different types of reinforcing steel used in the concrete frame were tested in tension to determine their yield strength, ultimate strength, and elastic modulus. For each bar type, three tensile specimens were tested and the test results are summarised in Table 2.4. Table 2.4 Average Properties of Reinforcing Bars Yield Ultimate Elastic Bar Diameter Strength Strength Modulus (mm) (MPa) (MPa) (MPa) Type Deformed Plain Masonry testing Good quality, burnt clay bricks of average size 195 x 100 x 75 mm available at Coimbatore were procured and used for masonry infill in the frames. The bond used was English bond with the thickness of the wall as 100 mm. Compression tests were conducted on single blocks, mortar cubes, and brick prisms according to IS-1905 (1987). The tests were carried out upto final failure of the specimen. Results from these tests are summarised in Table 2.5. The testing of brick prism is shown in Figure 2.6.

11 48 Table 2.5 Compressive Strengths of Clay Bricks, Mortar and Brick Prism Compressive strength (MPa) Sample No Average Clay Brick Mortar Brick Prism Figure 2.6 Testing of Brick Prism Description of Formwork Rolled steel channels and plates were cut to the required width and length, and fitted together to make the formwork for beams and columns. For the sides and bottom of the mould, 6 mm thick plates cut to the required width and length were used. The beams and columns were connected using equal angles at the corners of the joints by means of bolts and nuts. The foundation mould was made with steel plates and rolled steel channel sections. The arrangement of the frame mould was made in such a way that the frame and its foundation portion can be cast monolithically. The formwork used for casting the frame is shown in Figure 2.7.

12 49 Figure 2.7 Formwork Used for Casting the Frame The reinforcement grill was lifted using an over-head crane and placed inside the formwork with suitable arrangements. The mould arrangements with reinforcement grill ready for casting is shown in Figure 2.8. PVC pipes were inserted in the foundation beam as shown in Figure 2.9 for making holes in the beam to insert the bolts while erecting the frame in the foundation block. Figure 2.8 Mould Arrangements with Reinforcement Grill

13 50 Figure 2.9 PVC Pipes for Making Holes in the Foundation Beam Mixing, Placing and Curing of Concrete The frame was cast in the structural engineering laboratory and sufficient precautions were taken so that the specimen could be easily removed from the casting place and erected for testing. For mixing of concrete, an electrically operated concrete mixer was used and the concrete was placed immediately after mixing. Needle vibrator of 25 mm diameter was used for the compaction of concrete. The casting was done at a stretch. 12 numbers of bolt holes of 50 mm diameter were provided in the foundation beam of the frame at the same locations as that in foundation block. The top surface of concrete in the specimen was hand troweled. Companion specimens such as cubes and cylinders were cast for all the mixes. The process of mixing, transporting, placing, and compaction of concrete is

14 51 shown in Figures 2.10 (a), 2.10 (b), 2.10 (c), and 2.10 (d). The side planks of the mould were stripped after 24 hours and the specimen was covered with wet gunny bags and cured by periodical sprinkling of water for a period of 21 days from the day of casting. The companion specimens were also cured for the same period as that of the frame. The method of curing is shown in Figure Figure 2.10 (a) Mixing of Concrete Figure 2.10 (b) Transportation of Concrete

15 52 Figure 2.10 (c) Placing of Concrete Figure 2.10 (d) Compaction with Needle Vibrator

16 53 Figure 2.11 Curing of the Cast Specimen Foundation Block Details A pre-cast foundation block available in the Structural Engineering Laboratory was used for fixing the specimen for testing. It was cast earlier in such a way that the same unit could be used for testing any number of frames. The foundation block had twelve numbers of bolt holes of 50 mm diameter to enable the fixing of frame specimens to be tested. The fixity of foundation block was obtained by fastening the block with the structural test floor using twelve numbers of 50 mm diameter rods Lifting and Erection The gunny bags were removed after curing the specimen for 21 days and the specimen was cleaned. The specimen was supported on

17 mm concrete cubes to carry out the lifting operations. At first, the frame was fastened securely to the chain block of the over-head moving crane after covering the beam-column junctions of the middle column with gunny bags to avoid even minor damages. Then the frame was lifted using the crane and moved towards the foundation block on the test floor. After that, the foundation beam of the test specimen was inserted in the gap between the two webs of the foundation block. The bolts were inserted in the foundation block before the crane was released and then fastened. The lifting and erection process of the frame is shown in Figures 2.12 (a) and 2.12 (b). Suitable scaffolding arrangements were made at required levels on both sides of the specimen. They act as a platform both for the preparation of specimen for testing as well as for taking measurements of displacement during testing. Figure 2.12 (a) Lifting of the Frame Using a Crane

18 55 Figure 2.12 (b) Erection of the Frame in Foundation Block Construction of Brick Infill Brickwork construction was carried out on the next day of the erection. For infilling the frame with brick masonry, cement mortar 1:3 with a water cement ratio of 0.45 was used. The construction of brick infill in the frame is shown in Figure The panel size at the bottom-storey of the infilled specimen was 1000 x 900 mm (partial infill) and it was 1000 x 1200 mm at the top-storey. The thickness of the brick masonry panel was 100 mm. To study the properties, 230 x 230 x 460 mm size brick prisms as recommended by IS-1905 (1987), were prepared as control specimens using the same cement mortar mix used for the construction of brick infill. The curing of brickwork was done for 7 days. Finally, the frame was white-washed for easy identification of cracks during the test. The frame and the control

19 56 specimens were tested after the stipulated period of curing. Figure 2.14 shows the control frame S1BF ready for testing. Figure 2.13 Construction of Brick Infill in the Frame

20 57 Figure 2.14 Control Frame S1BF Ready for Testing Test Setup The two storey, two-bay RC frame with partial infill in the bottomstorey and complete infill in the top-storey was tested as vertical cantilever under a quasi-static cyclic loading programme. The schematic diagram of test setup is presented in Figure It consists of the following arrangements: Loading arrangement Instrumentation for measuring deflection Rigid body rotation of foundation block

21 58 Figure 2.15 Schematic Diagram of Test Setup Loading arrangement Two load points were located at bottom-storey and top-storey levels in line with the beams. The reaction frame, which is used for the loading arrangements, was rigidly fixed to the test floor. Hydraulic jacks of 500 kn capacity were placed at the required levels. Both the jacks were controlled by a common console. Pressure gauges were used to measure the applied load and the same was counter-checked by a load cell. Two numbers of handoperated oil pumps were used for the application of load through the jacks. Unexpected lateral movements of the test frame during the ultimate load stage were avoided by providing suitable guides using mild steel pipes. The loading jack arrangement with load cell is shown in the Figure 2.16.

22 59 Figure 2.16 Arrangement of Loading Jack with Load Cell Instrumentation for measuring deflection The specimens were instrumented with linear variable differential transducers (LVDT) of least count 0.01 mm to measure storey displacements. When the use of LVDT needed frequent resetting, the LVDT was removed and disc-type displacement meters of least count 0.1 mm were used. The LVDT/displacement meters were connected to slotted angles that were in turn connected to the fixed type of steel reaction frame available in the laboratory. The test setup of LVDT arrangement is shown in Figure LVDT 1, LVDT 3, and LVDT 4 were used to monitor displacements at the base, bottom-storey, and top-storey levels, respectively. An additional LVDT (LVDT 2) was placed adjacent to the top level of partial infill in the bottomstorey to identify the behaviour of column portion in that region (Figure 2.15).

23 60 Figure 2.17 Closer View of LVDT (To Measure Nodal Displacement) Rigid body rotation of foundation block Proper care was taken to avoid displacements due to rigid body rotation of the frame with respect to foundation, and the foundation block with respect to test floor. Mild steel channels and horizontal rods were used to arrest the rotation of the frame. However, the deflection due to rigid body rotation, if any, was measured by using deflectometers provided on the sides of the footings (Figure 2.18 (a)). The displacement due to rigid body rotation (lifting) of the foundation block with respect to the test floor was measured by the deflectometers as shown in Figure 2.18 (b). The observations on these deflectometers provided the necessary data for working out the error caused by the rigid body displacements.

24 61 Figure 2.18 (a) Deflectometer at the Bottom of the Frame Figure 2.18 (b) Deflectometer at the Base of Foundation Block 2.4 RC FRAME WITH PARTIAL INFILL RETROFITTED USING GFRP LAMINATES The two-bay two-storey RC frame S2FRP representing multi-storey system was designed in a similar way as the control frame S1BF without considering the effect of infill. It was constructed exactly identical to the control frame S1BF with the same geometric dimensions and reinforcement. The construction, erection, test setup, and the instrumentation were maintained

25 62 to be the same as the control frame S1BF and are discussed in Section 2.3 of this chapter. After construction, curing, and erection of the frame, it was retrofitted using GFRP laminates. Then the brick infill was constructed for 75% of storey height (900 mm) in bottom-storey and for full height (1200 mm) in the top-storey. The foundation portion of the frame was provided with holes to anchor the specimen to test floor so as to test the frame as a vertical cantilever. This section describes the properties of GFRP composites and the retrofit scheme adopted to strengthen the frame S2FRP using GFRP laminates Properties of GFRP Laminates Glass fibre-reinforced polymer (GFRP) laminates namely chopped strand mat (CSM) with unit weight of 240 g/m 2 were used in the retrofitted frame S2FRP. The mechanical properties of the GFRP sheets as provided by the manufacturer are listed in Table 2.6. Table 2.6 Mechanical Properties of GFRP Sheets Modulus of Elasticity (GPa) 70 Ultimate Tensile Strength (MPa) 1400 Ultimate Strain Allowable Strain Thickness of Dry Fibre (mm/ply) 0.30

26 Retrofit Scheme Using GFRP Laminates The proposed retrofit scheme using GFRP laminates was aimed at optimising the benefits of the externally bonded GFRP wraps along the direction of dominant stresses by increasing the column confinement and shear capacity of beam-column joints thereby avoiding brittle collapse modes. Balasubramanian et al (2007) reported that in situations where the structures have to be retrofitted to meet recent seismic design provisions (IS ), it is possible to enhance the performance of the compression members of those structures by providing them with a single layer of GFRP wrap. Sheikh (2002) found that one layer of GFRP wrap enhanced the column s energy dissipation capacity by over 100 times. Thus in this study, to meet modern seismic provisions (IS ) for partially infilled buildings and to provide adequate confinement to the captive columns, a layer of GFRP mat was wrapped around the bottom-storey columns for the entire height as shown in Figure The wrap was extended to the top-storey columns for a distance of development length of the longitudinal bar in the columns and to the adjoining beams for a distance of twice the effective depth of the beam to improve the shear capacity of beam-column joints.

27 64 Figure 2.19 GFRP Retrofit Scheme The RC column and beam surfaces to be retrofitted were first cleaned, hacked, and sand blasted. The corners of the sections were smoothened to provide a transition zone between adjoining faces. The resin system used in this study was made of two parts namely resin and hardener. The components were thoroughly hand mixed for at least five minutes before application. A thin layer of primer coating was first applied on the concrete surface and cured for two hours under room temperature. The two component resin was then applied and the GFRP sheet was wrapped over the sections as shown in Figure The outside layer of the sheet was extended by an

28 65 overlap of 100 mm to ensure the development of full composite strength. For proper adhesion between the concrete surface and the wrap, a roller was applied over the wrap. Special attention was taken to ensure that there was no void between the GFRP sheet and concrete surface. Finally a layer of resin was applied over the wrapped surfaces and allowed to cure for seven days. Once the retrofitting process is complete, the masonry infill was constructed with partial infill in the bottom-storey and a complete infill in the top. Finally, the frame was white-washed on the concrete surfaces without the GFRP wrap for easy identification of the cracks. The frame was tested after the stipulated period of curing. Figure 2.21 shows the retrofitted frame S2FRP ready for testing. Figure 2.20 Wrapping of GFRP Sheet

29 66 Figure 2.21 Retrofitted Frame S2FRP Ready for Testing 2.5 RC FRAME WITH PARTIAL INFILL RETROFITTED USING MASONRY INSERTS The two-bay two-storey RC frame S3MF representing multi-storey system was designed in a similar way as the control frame S1BF without considering the effect of infill. It was constructed exactly identical to the control frame S1BF with the same geometric dimensions and reinforcement. The construction, erection, test setup, and the instrumentation were maintained to be the same as the control frame S1BF and are discussed in Section 2.3 of this chapter. After construction, curing, and erection of the frame, brick infill was constructed for 75% of storey height (900 mm) in bottom-storey and for full height (1200 mm) in the top-storey. Then the frame was retrofitted by providing additional masonry inserts. The foundation portion of the frame was

30 67 provided with holes to anchor the specimen to test floor so as to test the frame as a vertical cantilever. This section describes the retrofit scheme adopted to strengthen the frame S3MF using masonry inserts Retrofit Scheme Using Masonry Inserts The retrofit scheme using masonry inserts was proposed to find a solution from the non-structural element point of view by adding masonry inserts that would close the opening near the column. These masonry inserts are additional brick infills of certain length that defend the column by allowing the compression strut in the masonry wall to travel along the wall plane, thus diverting away the critical shear force from the RC column. The size of these inserts were obtained in such a way that the diagonal compressive strut falls completely within the length of insert. Accordingly the additional masonry inserts were constructed in both the bays along the loading direction with a horizontal length equal to 1.5 times the height of the opening (0.45 m) as shown in Figure 2.22 to induce a complete diagonal strut action along the wall plane. However the opening was not completely closed. Finally the frame was white-washed and tested under quasi-static loads. Figure 2.23 shows the retrofitted frame S3MF ready for testing.

31 68 Figure 2.22 Masonry Insert Over the Partial Infill Figure 2.23 Retrofitted Frame S3MF Ready for Testing

32 TESTING OF FRAMES Figure 2.15 shows the test setup adopted for testing all the frame specimens. The effectiveness of instrumentation setup and the loading were checked in the beginning by loading and unloading the frame with small loads (of the order of 0.5 kn at the two load points) till all the readings were repeatable. The three frames were tested under uni-directional lateral cyclic loads in a quasi-static pattern simulating seismic action. In the experiments, the lateral load called base shear was applied at the beam levels using hydraulic jack and the applied load was measured using load cell. Since the main purpose of the experiment was to observe the frame s behaviour under lateral loading, no vertical load was applied on the specimens except for the self-weight of frames and walls. In each cycle of loading, 20% of the base shear was applied to the bottom-storey and the remaining 80% was applied to the top-storey. Initially a base shear of 5 kn was applied and the loading was progressively increased by 5 kn base shear in successive cycles until the maximum load-carrying capacity of the specimen was reached. During the tests, storey displacements and the lateral loads were monitored. After each cycle, new initiated cracks and crack propagations were marked on the specimens and failure mechanisms were observed. The deflectometer readings for calculating the error due to rigid body rotation of foundation block were also recorded.

33 SUMMARY This chapter summarised the details of test specimens, materials, test setup, and the testing procedure used for carrying out the experimental investigations. The procedure adopted for manufacturing the test specimens was explained with illustrations on formwork and reinforcement, concreting and curing, lifting and erection of frames, and the construction of infill. The two different retrofit schemes using GFRP laminates and masonry inserts for strengthening the captive columns of partially infilled RC frames were also discussed.