Effect of Elevated Temperature on Punching Shear of Regular and Irregular Shaped Self-Compacted Concrete Slabs

The exposure of reinforced concrete slabs to high temperature causes changes in their structural behavior. This work aims to study the post-heating behavior of self compacted concrete (SCC) regular (square) and irregular (trapezoidal) shaped flat plates and assessing the residual pu nching shear strength of these slabs. Twenty four reduced scale rei nforced concrete slab specimens divided into two main groups (square and trapezoidal slabs), each of which consists of twelv e slab identical in size and s hape but different in concrete compressive strength (30,50,70) MPa. These specimens are subjected to different temperature levels (100, 300, 500) 0 C and still heate d in that temperature level for one hour using a n electric furnace and the results are compared with specimen s tested at room temperature 25 0 C. The effect of specimen shape, heating level, concrete compressive strength on the punching shear resistance of slabs a re discussed in details. Results indicate that the reduction in punching shear strength is ranged between (0 and 16) %, for s quare slab, at maximum temperature exposure (500) 0 C, while this rate is ranged between ( 6.3 and 40.4) % for trapezoidal slabs. This indicates that the punching shear resistance of regular sha ped slabs is hig her than that of irregular shaped slabs.


Introduction
Flat plats are very common and competitive structural system for cast in place slabs in buildings since no beam is involved.The means that the formwork of these slabs is very simple and economic.Using this type of structure presents however a serious disadvantage because of the risk of a brittle punching failure at the connection between the slab and the column .Large scientific efforts have been done in the past to predict the punching shear strength of the slabcolumn connection.However, some punching failures that occurred during the past decades showed that it is necessary to improve design methods to avoid this type of failure .Technological advances in this field came with new materials and techniques to improve punching shear resistance of flat plates such as, adding fibers into the concrete [2] or using special types of concrete like self compacted concrete [3] and high strength concrete In the last few years, irregular shaped flat plate slabs are commonly used in high rise residential buildings.
Architectural shape, interior layouts and requirements often dictate the use of irregular shaped flat slabs [6] .However, the currently available building codes don't provide specific guidelines for designing irregular shaped flat slabs or building.Therefore, the design and testing of the specimens in this investigation were done experimentally.
The well-known capacity of concrete to withstand high temperature and fire is put to the test by the most recent high and ultrahigh performance concrete There are several reasons to explain such sources of high temperatures, such as strong winds or droughts heat waves.But, according to several scholars, the most critical fire trigger is represented by isolated homes and buildings . The exposure of structural members to high temperature causes change in the properties of their constituents, namely concrete and steel, and in structural behavior.So, in this study the effect of exposure to elevated temperature on the punching shear strength of regular and irregular shaped SCC slabs are tested, evaluated and discussed.

Experimental Programme Materials
Ordinary Portland cement (type 1) was used in all mixtures.The cement is (Taasloja).Limestone powder LSP with surface area of 3100 cm 2 /g was used as filler.The superplasticizer based on polycarboxylic ether was used, it has the trade mark Glenium 51 (G 51) [12] .While, tap water was used for casting and curing all the specimens.The steel reinforcement deformed bar mesh of Ø 5mm was used as bottom mesh reinforcement with 10 mm concrete cover.Yield strength of the wires is 310 MPa while the ultimate strength is 550 MPa. Figure (1) shows the arrangement of the wires in regular shaped slabs (square) and irregular shaped slabs (trapezoidal).All the specimens were designed to fail in punching.

Mix Design
Several mixtures were tested in order to find the desired strength.Then three mixtures (S30, S50, S70) were chosen with cube compressive strength of (30, 50, 70) MPa.The composition of mixtures used in this investigation is shown in Table (4)

Fresh Properties
Self compacted concrete is defined as a high performance concrete that has excellent deformability in the fresh state and high resistance to segregation and can be placed and compacted under its self weight without applying vibration.In this study, the slump flow and L-box tests were done.The results of fresh properties are shown in Table (5

) Preparation of Specimens
After testing the fresh properties, the concrete was poured in the moulds.For testing the compressive strength 12 cube of size (150*150*150) mm for each mixture were cast, while, 8 beams of size (100*100*500) mm for each mixture were cast for testing in flexure.For testing punching shear resistance, 4 regular shaped (square) slab and 4 irregular shape (trapezoidal) slab were cast for each mixture.Then, all the specimens were covered with polythene sheets for 24 hours.After that, the specimens were demolded and were put in water for 28 days for curing.After the period of curing, the specimens (cubes, prisms and slabs) were put in an electric furnace to study the effect of elevated temperature (100, 300 and 500) 0 C on compressive, flexural, punching shear strengths respectively, and these specimens were tested and compared with the specimens still at room temperature 25 0 C.
The electric furnace used for heating all the specimens is shown in Figure (2), the applied duration of heating for each level of temperature was 1 hour.This duration is enough to heat the specimens completely.At the end of the exposure time, the furnace was switched off and the PDF created with pdfFactory Pro trial version www.pdffactory.comspecimens were allowed to cool for 24 hours to ensure complete cooling of the specimens.

Results of Compressive and Flexural Strengths
In order to obtain a better understanding of the behavior of reinforced concrete slabs after exposure to high temperature, it is important to study the properties of concrete (compressive and flexural strengths) subjected to high temperature in first place.
In this study three cubes of concrete of (150)mm size and two prisms of (100*100*500) mm, with each level of heating and for each mixture, were tested to obtained the compressive and flexural strengths respectively.Table ( build up and decrease thermal diffusivity [13] .

Results of Punching Shear Strength of Slabs Exposed to Different Level of Temperature
Punching test was carried out on two series of slabs as follows: Series 1: this series consists of 12 regular shaped (square) slabs with dimensions (45*45*5) cm.Series 2: this series consists of 12 irregular shaped (trapezoidal) slabs with dimensions (45*20*70) cm.
All the slabs were supported along their perimeter and tested by pushing down square steel column of dimensions (4*4) cm on the center of gravity of each slab.While the deflection was measured using a dial gauge of 0.01 mm sensitivity.Figure (5) shows the loading test system.
is a very important advance in concrete technology in recent time.The specific properties of SCC help to improve the quality of concrete structures of today [5].

F
Figure (1) The Arrangement of the Wires in Regular Shaped Slabs (Square) and Irregular Shaped Slabs (Trapezoidal).

Table (
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Table ( 2) Grading of Fine Aggregates Sieve size (mm) % Passing by weight IQS 45-84 limits zone
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Table ( 6): Results of Compressive and Flexural Strengths Mixture No. Compressive Strength (MPa)* Modulus of Rupture (MPa)**
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