Behaviour of Self-Consolidating Concrete Two Way Slabs Under Uniform Loading

An experimental investigation was conducted to study the strength, behaviour and deflection characteristics of two way slabs made with both self-consolidating concrete (SCC) and conventional concrete (CC). Six concrete slabs were tested to failure under simply supported uniform by distributed loading conditions. The variables were concrete type and macro synthetic fibres ratio (0%, 0.07% and 0.14%). The performance was evaluated based on crack pattern, ultimate load, load-deflection response and failure mode. The results showed that the ultimate strength of SCC slabs was larger than that of their CC counterparts. The results also showed an improvement of the behaviour and strength of slabs by adding the synthetic fibres.


Introduction
Conventional reinforced concrete (CC) which is one of the most widely used construction materials worldwide is a composite material of steel bars embedded in a hardened concrete matrix. To enhance the properties of this material, many suggestions may be taken according to the problems that need to be improved. One of these suggestions is using self-consolidating concrete to enhance the fresh properties of concrete. Self-consolidating concrete (SCC) is the latest innovation in concrete technology. It is a highly flowable mixture that spreads readily under its own weight, without the use of vibrators for consolidation, and it achieves good compaction without segregation, even in members with congested reinforcement. SCC is produced by increasing the fine aggregate content through the incorporation of mineral admixtures and/ or viscosity modifying admixtures (Khayat et al., 1997;Sonebi and Khayat, 1999;EFNARC, 2000;Lachmi et al., 2004). The coarse aggregate content in SCC is usually lower than in CC, and the minimum slump flow value is limited to 550mm to enhance the workability and flowability. Lack of information regarding in situ properties and structural performance of SCC is one of the main barriers to its acceptance in the construction industry (Lachmi et al., 2005). There is some concern among researchers and designers that SCC may not be strong enough in shear because of some uncertainties in mechanisms resisting shear notably the aggregate interlock mechanism. Because of the presence of comparatively smaller amount of coarse aggregates in SCC, the fracture planes are relatively smooth as compared with CC that may reduce the shear resistance of concrete by reducing the aggregate interlock between the fracture surfaces (Hassan et al., 2010). However, only few studies have been conducted on the strength and behaviour of structural elements made by using fibre reinforced SCC (Pereira et al., 2004).
Many studies explain the effect of synthetic fibres on the serviceability of concrete and to control on the bleeding and plastic shrinkage cracking (Wang and Belarbi, 2005). The increasing use of macrosynthetic fibres for structural applications necessitates the understanding and the evaluation of the effect of fibre addition on the behaviour of concrete members. Unlike steel fibres, there are only few studies reporting results on member strength with synthetic fibres (Wang and Belarbi, 2005;Altoubat et al., 2009). The results revealed that simple material tests do not always successfully predict the contribution of fibres in cases where structural geometry and boundary considerations control redistribution of load (Roesler, 2004).
This study is focused on the behaviour of two way slabs made with both SCC and CC with macro-synthetic fibres. Results obtained from testing six two-way concrete slabs under uniform loading are presented. The slabs were reinforced with longitudinal flexural reinforcement in two directions and were designed to fail in shear under uniform loading while being simply supported on all four sides.

Experimental Programme
Six simply supported rectangular reinforced concrete slabs were tested to failure in an experimental program conducted at Al-Mustansiriya University. They consisted of two series CC, and SCC. The primary variable was the concrete materials; SCC or CC. Each group consists of three specimens with different percentages of synthetic fibres (polypropylene); 0%, 0.07%, and 0.14% by volume. Each slab was loaded directly on the top compression face with uniform distributed load (sand bags) and concentrated load at mid-span over sand bag (from universal machine) and supported at four edges at the bottom, see Plate 1. The specimens were rectangular of 900×450×80 mm and reinforced with one layer mash of #6@150 mm on the tension side of the slab as shown in Figure 1.

Cement
The cement used in this study was Ordinary Portland cement (Type I). This Cement conformed with the requirements of the ASTM C150 standards.

Aggregate
Natural siliceous desert sand was used as fine aggregate and crushed river gravel with maximum size of 14 mm was used as coarse aggregate. Both types of aggregate conformed to ASTM C33 requirements.

Superplasticizer
For the production of SCC as well as high strength CC a superplasticizer is needed. In this work Gelnium 51 superplasticizer was used. Its composition is based on polycarboxylic ether. This superplasticizer conformed to the requirements of types A and F of the ASTM C494 standard.

Limestone Powder
The filler powder used was crushed limestone with a fineness of 3100 cm2/gm (100% passing sieve 0.075mm). The particle size of the filler powder according to Ref's (EFNARC, 2000;Esping, 2008) must be less than 0.125mm to be most beneficial. The chemical composition of the limestone powder is shown in Table 1.

Steel Reinforcement
In this study the reinforcement in concrete slab used is deformed bar #6mm. The material properties are obtained from the tests for steel reinforcements. The reinforcing steel bars had an average yield strength of 433MPa and average ultimate strength of 471MPa.

Synthetic Fibres
High performance micro polypropylene fibre (Grace Cemfiber) was used. Table 2 shows fibre specification according to manufacture label.

Testing of Fresh SCC
Fresh concrete testing is of serious importance for the production of SCC. The main characteristics of SCC are the properties in the fresh state. Production of SCC is focused on its ability to flow under its own weight without vibration and the ability to obtain the homogeneity without segregation of aggregate. The slump flow, V-funnel and L-box are used for assessment of fresh properties of SCC in this study. The tests results of the fresh properties of the SCC mix are shown in Table 4.

Compressive Strength Test
Two types of specimen shapes were tested to determine the concrete compressive strength. The concrete compressive strength was determined in accordance to ASTM C39 and BS1881. These types of specimens investigated were: 1. 150x300mm. cylinders. 2. 150x150x150mm. cubes. The concrete compressive strength of each concrete mix, represents the average of three specimens, are tabulated in Table 5. The concrete specimens were tested at age 28 days of water curing.

Splitting Tensile Strength Test
Splitting tensile strength tests were conducted in accordance to ASTM C496 on cylinder of 150x300mm size. The splitting tensile strength of each mix is shown in Table 5 which represents the average of three specimens at the age of 28 days of water curing.

Static Modulus of Elasticity
The modulus of elasticity was determined according to ASTM C469-02. The modulus of elasticity of each mix was the average of three (150x300mm) cylinder specimens, tested at age 28 days of water curing and tabulated in Table 5.

Slab Test Setup and Loading Procedure
The slabs were simply supported for all sides and tested under uniform load condition. A hydraulic jack was used to gradually apply the concentrated load at mid of the sand bag (the sand bag putting over the slab and cover all surface area of slab) until slab failure. The sand bag has a suitable thickness to distribute the concentrated load as uniform load. Three dial gauges were installed directly under the mid and quarters span to measure the deflection.

Experimental Load Deflection Response and Strength Characteristics
The main experimental results for tested slabs are reported in Table 6. Ultimate capacity of two way reinforced concrete slabs was determined using the design approach proposed by ACI318M-08. The Figure 2 shows the experimental load-deflection curves for the CC slabs that have different values of fibres. It is possible to evidence the relevant difference between curves corresponding to the fibre percentage. The deflections of the 0.07%V f is higher than that of slab without fibres, while the deflection of the slab having 0.14%V f is the lowest. Also the Figure 3 explains the effect of fibres on SCC which is similar to CC behaviour. Figures 4 to 6, explain the difference between SCC and CC slabs with the same percentage of fibres.

Modes of Failure
For a two way slab simply supported on all four sides and subjected to a uniform load, failures are generally due to combination of the two failure modes with the least ultimate load governing the failure of the slab. Usually bending failure cracks are the first to develop underneath the slabs as applied loading is gradually increased and the concrete tensile strength is exceeded. This is because concrete is weak in tension. Typical crack patterns observed underneath the slabs are illustrated in Plates 2 to 7. Collapse may therefore occur either through flexural failure resulting from the crashing of concrete and/ or fracture of the tension steel bars, or shear. The final failure mode however depends on the concrete type (SCC or CC) and the amount of synthetic fibres. It was observed from the tests that SCC gives good resistance to the shear than CC and the addition of synthetic fibres also increase the strength of two way slab for both flexural and shear failure.

Discussion of Test Results
The compressive, splitting tensile strength and the modulus of elasticity of CC and SCC was found to be stay without any improvement by the addition of synthetic fibres as listed in Table 5. From the load-deflection curves, it has been noted that when synthetic fibres are added, energy absorption capacity increases for both CC and SCC as shown in Figures 2 and 3. In the case of CC the ultimate strength is enhanced by about 14% and 8% for 0.14% and 0.07% of volume fraction of fibres respectively as listed in Table 6. In SCC the ultimate strength is enhanced by about 7% and 4% for 0.14% and 0.07% of volume fraction of fibres respectively as listed in Table 6. For the same percentage of fibres the ultimate strength of SCC is larger than of CC by about 32%, 28% and 24% for 0%, 0.07% and 0.14% of volume fraction of fibres respectively.

Conclusion
A two-dimensional Euler-Bernoulli flexible beam undergoing rigid-body motions is modelled. The equations of motion are obtained using the extended Hamilton principle. An exact solution for the natural frequencies is obtained. The model can be extended to a three-dimensional beam with variable cross section. Based on the results of this experimental investigation, the following conclusions are drawn: For the same concrete grade and the same size aggregate, self-compacting concrete with limestone filling gives shear strength larger than conventional concrete. The ultimate load carrying capacity of the two way slabs, for both conventional and self-compacting concrete, increased by adding synthetic fibres. The stiffness has also increased gradually both in conventional as well as in self-compacting concrete due to the addition of fibres.