CHAPTER 3 BEHAVIOUR OF FERROCEMENT HOLLOW SLABS

Transcription

1 30 CHAPTER 3 BEHAVIOUR OF FERROCEMENT HOLLOW SLABS 3.1 INTRODUCTION There are numerous similarities between ferrocement and reinforced concrete. Both use similar matrix and reinforcement materials. They obey the same principles of mechanics and can be modeled according to the same theories. They can be analysed using similar techniques and can be designed according to the same philosophy. Hence, the ferrocement hollow slab was designed using the conventional reinforced concrete theory. Ten ferrocement ribbed hollow slab panels were cast, out of which five had circular hollow cores and the other five had rectangular hollow cores. The five ferrocement ribbed hollow slabs under each model were cast with various percentages of reinforcement designed for reinforced concrete one way slabs. They were tested to observe the effect of increased lever arm on the bending strength. Two slabs made of reinforced concrete designed as one way slabs under simply condition were cast for comparison of their bending behaviour with ferrocement hollow slabs and to establish the possibility of using ferrocement hollow slabs for roofing purpose. 3.2 MATERIALS The following materials were used for casting the ferrocement hollow slabs and reinforced concrete slabs:

2 31 Cement: Ordinary Portland cement of 43 grade, conforming to Indian standard specifications, was used for making the cement mortar. Fine Aggregate: River sand as per Indian standard specifications was used for making the cement mortar. Wire mesh reinforcement: Galvanised steel hexagonal wire mesh of 0.5mm wire diameter with a wire spacing of 17mm center to center was used to impart tensile strength to the mortar matrix and to ensure crack control. Steel Bars: Form and rigidity are imparted to ferrocement elements by skeletal reinforcement. High yield strength deformed steel s of 8 mm diameter were used as longitudinal reinforcement in the flanges. Mild steel s of 4mm diameter were used as distribution s and stirrups in the ribs. Water: Ordinary potable water was used for mortar and concrete making and also for curing the specimens. Admixture: Roff Superplast 820 of 6 ml per kg of cement was used to achieve good workability and quick setting. Coarse Aggregate: Crushed granite aggregate of nominal size 20 mm was used as coarse aggregate for making M20 grade concrete mix for reinforced cement concrete slab. 3.3 PREPARATION OF CEMENT MORTAR AND CONCRETE The ratio of cement to sand for making the mortar was 1:1.5. A water cement ratio of 0.43 was adopted. The proportion of the constituents was so adjusted that the mortar gives the required strength, workability, water tightness and finish. Roff Superplast 820 was used in the quantity of 6 ml per kg of cement for quick setting.

3 32 The reinforced cement concrete slabs were cast with a nominal mix of 1:1.5:3 and a water cement ratio of 0.43 as designed for M 20 grade concrete. 3.4 CASTING OF REINFORCED CEMENT CONCRETE SLABS Reinforced cement concrete slabs of size 1800 mm 100 mm and 2000 mm 100 mm were cast with M 20 grade concrete using a nominal mix ratio 1 : 1.5 : 3 with a water cement ratio of 0.43 as shown in Figure 3.1. Roff Superplast 820 was added as admixture in the proportion of 6 ml per kg of cement to the water used for making the concrete. Five 8 mm diameter steel s were provided with a spacing of 130 mm center to center in the tension face of the slab along the longitudinal direction. Mild steel s of 6mm diameter were provided in the transverse direction at a spacing of 200 mm center to center. The details of the test specimen are given in Table mm 8mm dia steel 6mm dia Figure 3.1 Cross section of reinforced concrete slabs SS1 and SS2

4 33 Slab ID Size of slab (mm) Table 3.1 Details of test specimen Thickness of ribs (mm) Number of layers of meshes in each flange Skeletal Reinforcement (mm 2 ) Spacing of stirrups (mm) Type of loading A UDL A UDL A UDL A UDL A UDL SS UDL B Line Load B Line Load B Line Load B Line Load B Line Load SS Line Load 3.5 CASTING OF FERROCEMENT RECTANGULAR HOLLOW SLABS The ferrocement rectangular hollow slab panels of size 1800 mm 135 mm were cast in wooden moulds. The cross section of the ferrocement rectangular hollow slabs A1, A2, A3, A4 and A5 are shown in Figure 3.2. The mould was oiled before placing the mortar. Cement mortar was spread evenly within the mould and the first layer of hexagonal wire mesh of size 1800 mm 540 mm was spread. The 8 mm diameter s of length 1790 mm was placed along the length of the slab with distribution s of 4 mm diameter tied at a spacing of 160 mm center to center. Two more layers of wire meshes were placed on the top of the s to cover the entire area. Also, 4 mm diameter s were provided for one legged stirrups at a spacing of 160 mm center to center. Mortar was spread evenly and compacted to form the lower flange.

5 mm mm steel 20mm thick ribs steel with 3 layers of chicken mesh 4mm dia 160mm c/c steel 20mm thick ribs steel with 3 layers of chicken mesh 4mm dia 160mm c/c (a) (b) mm mm steel 20mm thick ribs steel with 3 layers of chicken mesh 4mm dia 160mm c/c steel 20mm thick ribs steel with 3 layers of chicken mesh 4mm dia 160mm c/c (c) (d) mm 20mm thick ribs steel with 3 layers of chicken mesh 4mm dia 160mm c/c (e) Figure 3.2 Cross section of ferrocement rectangular hollow slabs (a) Slab A1 (b) Slab A2 (c) Slab A3 (d) Slab A4 (e) Slab A5

6 35 Thermo cool pieces were placed in position to the cuts in the mould and the shear connectors were also fixed in position. The cement mortar was filled in between the thermo cool to form the ribs of 20mm thickness. The top flange was cast as the bottom flange. The companion cubes for all the slabs were cast for testing the compressive strength. The specimen was removed from the mould the next day and slab panels were cured under wet gunny bags for 28 days. Wet gunny bags were rolled and kept within the hollow rectangular cores. 3.6 CASTING OF FERROCEMENT CIRCULAR HOLLOW SLABS The ferrocement circular hollow slab panels of size 2000 mm 135 mm were also cast in wooden moulds. The cross section of the ferrocement circular hollow slabs B1, B2, B3, B4 and B5 are shown in Figure 3.3. The mould was oiled on the inner side and base and then a layer of cement mortar was spread to a thickness less than 10 mm. The first layer of galvanized steel hexagonal mesh reinforcement was placed and then cement mortar was spread evenly. Then the layer of reinforcement consisting of skeletal steel s of 8 mm diameter was placed along the length with the distribution s tied at a spacing of 150 mm center to center as shown in Figure 3.4. Another layer of galvanized steel hexagonal mesh was placed on top of the skeletal steel s and was covered by spreading cement mortar. The thickness was maintained in bottom flange and shear connectors of 6 mm diameter s were tied at a spacing of 150 mm center to center.

7 mm mm steel steel with 2 layers of chicken mesh (a) 75 mm dia circular grooves 6mmdia 150mm c/c steel steel with 2 layers of chicken mesh (b) 75 mm dia circular grooves 6mmdia 150mm c/c 550mm 550mm mm mm steel steel with 2 layers of chicken mesh (c) 75 mm dia circular grooves 6mmdia 150mm c/c steel steel with 2 layers of chicken mesh (d) 75 mm dia circular grooves 6mmdia 150mm c/c mm steel with 2 layers of chicken mesh (e) 75 mm dia circular grooves 6mm dia 150mm c/c Figure 3.3 Cross section of ferrocement circular hollow slabs (a) Slab B1 (b) Slab B2 (c) Slab B3 (d) Slab B4 (e) Slab B5 Figure 3.4 Casting the bottom flange

8 37 Four PVC pipes of 75 mm diameter with oil applied on the outer surface were placed in position with reference to the cuts in the mould as shown in Figure 3.5. The steel channels to form the end hollow portion of the slab were also oiled and placed in position. The cement mortar was placed in between the pipes and channel to form the 30 mm thick ribs. The top flange was cast similar to the bottom flange as shown in Figure 3.6. The depth of the slab was kept as 135 mm. After the final setting of cement, the pipes were withdrawn and the steel channels were also removed. The edge was given a smooth finish as shown in Figure 3.7. The mould was removed the next day and all the slab panels were cured for 28 days using wet gunny bags. Figure 3.5 Laying the PVC pipes to form circular cores Figure 3.6 Casting the top flange

9 38 Figure 3.7 Finished slab 3.7 EXPERIMENTAL SET UP The slabs were tested on a 500 kn capacity self straining loading frame. The ferrocement rectangular hollow slabs and solid slab SS1 were tested under simply supported condition with uniformly distributed load applied by means of a steel grill placed over the entire span of the slab. A 250 kn capacity hydraulic jack was used to apply the load. The deflectometers were placed at quarter span and at mid span for rectangular hollow slabs. Figure 3.8 Experimental set up for the flexure test

10 39 The ferrocement circular hollow slabs and solid slab SS2 were tested under line loads applied at one third span through a steel plate of 900 mm 300 mm 30 mm size. Mild steel round s of 25 mm diameter were used to apply the line loads fixed on the slab with plaster of Paris. The deflectometers were placed below the slab to measure deflections at one third span and at mid span as shown in Figure TEST PROCEDURE The load was applied by means of the hydraulic jack initially starting from zero and incremented by 2 kn. The deflections were measured for every load increment. The loading was continued up to the onset of ultimate load. The readings taken at one third span and at quarter span were used only for checking the correctness of mid span deflection while recording. 3.9 DISCUSSION ON THE TEST RESULTS The load deflection curves of all the rectangular hollow slabs and circular hollow slabs were plotted as shown in Figures 3.9 and 3.10 respectively to compare their flexural behaviour with the solid slab. The flexural behaviour was studied upto the onset of the peak load. Figure 3.9 Load deflection curves of FRHS and solid slab

11 40 Figure 3.10 Load deflection curves of FCHS and solid slab The first crack strength, ultimate strength, deflections at first crack strength and onset of reaching ultimate load were recorded and tabulated as shown in Table 3.2. Table 3.2 Deflections at first crack load and ultimate load of Type of Slab ferrocement hollow slabs At first crack load Load in kn Mid span deflection in mm At Ultimate load Load in kn Mid span deflection in mm Stiffness W/ at first crack kn/mm SS A A A A A SS B B B B B

12 BEHAVIOUR OF FERROCEMENT RECTANGULAR HOLLOW SLABS (FRHS) From Table 3.2, it is observed that the first crack load of the ferrocement hollow slab A1 is more than that of solid slab SS1. Also, the ultimate load carrying capacity of slab A1 is more than that of the solid slab SS1 due to the effect of increased lever arm in hollow slab and the presence of reinforcement in the top flange. There was decrease in ultimate load carrying capacity for the other slabs due to reduction in percentage of reinforcement than the solid slab. The deflection at first crack load was observed as 2.88 mm for slab A1 compared to a deflection of 2.28 mm for solid slab. The mid span deflections of FRHS at first crack load was found to be closer but on the higher side than the solid slab and were within permissible limit of 4.57 mm (L/350=1600/350) as suggested by the Indian standard code for reinforced concrete IS The self weight of FHS specimen is 176 kg against 252 kg of solid slab. A reduction in self weight is about 30% BEHAVIOUR OF FERROCEMENT CIRCULAR HOLLOW SLABS (FCHS) The first crack load of ferrocement hollow slab B1 was found to be same as that for solid slab SS2. The ultimate load carrying capacity of slab SS2 was 40 kn and that of slab B1 was 48 kn against a theoretically found load carrying capacity (excluding self weight) of kn. There was decrease in load carrying capacity for the other slabs due to reduction in tensile reinforcement. This experimentation also proves that the reduction in

13 42 reinforcement in ferrocement hollow slab leads to decrease in ultimate load carrying capacity than the solid slab. The deflection at first crack was observed as 1.52 mm for slab B1 compared to a deflection of 1.58 mm for solid slab. The mid span deflection of FCHS at first crack load was found to be closer to that of solid slab and also within permissible limit of 5.14 mm (L/350=1800/350). It is observed that increased lever arm in hollow slab B1 and the reinforcement in top flange FCHS give more ultimate load than that of solid slab. The self weight of FHS specimen is 229 kg against 280 kg of solid slab. A reduction in self weight is about 18% MODE OF FAILURE The flexural crack in the tension face of the slab started from one longer edge and extended throughout the width of the slab. Also, the cracks appeared on the side ribs and propagated upwards towards the compression zone. The solid slab failed at an ultimate load of 40 kn with a deflection of mm with few widened cracks on the tension face at large distance apart. The FRHS panels A1 and A2 reached an ultimate load of 46 kn and 36 kn respectively showing many cracks on the tension face of the slab, the cracks being spaced closely and of lesser crack width. No crack was observed on the compression face of the slab. A5 collapsed suddenly at a load of 12 kn as no skeletal steel reinforcement was provided in it. FCHS panels showed multiple cracks concentrated on the central zone of the panel and diagonal cracks initiated over the support. The final failure took place when diagonal cracks developed at support propagated upwards towards the loading line as shown in Figure 3.11.

14 43 Figure 3.11 Failure mode of ferrocement hollow slabs 3.13 CONCLUSION 1. In the present investigation it has been found that the prefabricated ferrocement slabs can be used as replacement of one way reinforced concrete slabs for roofs and floors. 2. Deflections at mid span of ferrocement hollow slabs at service load are within permissible limits. 3. The deflections are less for FCHS due to the stiffness offered by the increased thickness and number of ribs. 4. The ultimate load carrying capacity is increased with increase in lever arm in ferrocement hollow slab with the same percentage of longitudinal steel as in the solid slab of reinforced concrete. 5. The decrease in percentage of reinforcement in ferrocement hollow slab leads to decrease in ultimate load carrying capacity than the reinforced concrete slab. 6. A reduction in 18% of self weight in the FCHS and 30% of self weight in the FRHS leads to overall economy of the

15 44 structure by reducing the loads over beams, columns and foundations. 7. The number of ribs, thickness of ribs and the presence of web reinforcement play an important role in developing full moment capacity. 8. Stiffness of ferrocement hollow slab decrease with reduction of reinforcement in tension and compression zone. But the ferrocement hollow slab with same percentage of reinforcement as solid reinforced concrete slab has more stiffness.