THE EFFECT OF ADDITION OF STEEL FIBERS ON COMPRESSIVE AND TENSILE STRENGTHS OF STRUCTURAL LIGHTWEIGHT CONCRETE MADE OF BROKEN BRICKS

This research studies the effect of adding steel fiber in two percentage 0.5% and 1% by volume on plain structural lightweight concrete (SLWC) produced by using crushed bricks as coarse lightweight aggregates (LWA) in a lightweight concrete mix designed according to ACI committee 211-2-82 with mix proportion 1:1.12 :3.35 by volume .The w\c equal to 0.5 and cement content 550 kg\m3. Different tests where performed for fresh and hardened SLWC such slump test ,fresh and hardened unit weight ,compressive strength and two indirect tests of tensile strength (splitting tensile and flexural strength). The results demonstrated that the effect of addition of steel fiber was more pronounced on the tensile strength of SLWC than the compressive strength of such concrete .The maximum increase of compressive ,splitting tensile and flexural strengths at 28-days were 38.8,77.12 and 111.2 % in the SLWC containing 1% fiber. On the other hand the rate of strength gain between 3 and 28 days was constant on compressive strength of plain concrete and that containing steel fiber while this rate was clearly increase on tensile strength especially flexural strength.


1-DEFINITIONS
Fiber reinforced concrete : Fiber reinforced concrete (FRC) is a concrete mix that contains short discrete fibers that are uniformly distributed and randomly oriented.
Fiber material can be steel, cellulose, carbon, polypropylene, glass, nylon, and polyester [1]. The amount of fibers added to a concrete mix is measured as a percentage of the total volume of the composite (concrete and fibers) termed Vf. Vf typically ranges from 0.1 to 3%. Aspect ratio (l/d) is calculated by dividing fiber length (l) by its diameter (d).
Fibers with a non-circular cross section use an equivalent diameter for the calculation of aspect ratio.

2-INTRODUCTION
The advantages to using concrete include high compressive strength, good fire resistance, high water resistance, low maintenance, and long service life. The disadvantages to using concrete include poor tensile strength, and formwork requirement. Other disadvantages include relatively low strength per unit weight.
Tensile strength of concrete is typically 8% to 15% of its compressive strength [3].This weakness has been dealt with over many decades by using a system of reinforcing bars (rebars) to create reinforced concrete; so that concrete primarily resists compressive stresses and rebars resist tensile and shear stresses. The LWC having less compressive strength than NWC .as such, a form of additional reinforcement is needed to enhance the weakness of tensile strength in SLWC . This will achieved by using steel fibers .
Advantages of using SLWC [4 ] : • Reduction in dead weight of structure.
• Savings in steel reinforcement.
• Reduction in dead weight gives better resistance to earthquake loading.

3-1-1 Cement
ordinary Portland cement produced by Kubaisa cement factory was used throughout this study physical properties of used cement are listed in Table (

3-1-3 Coarse lightweight aggregate
A crushed bricks were used as coarse LWA .The bricks pieces which are considered as waste materials were crushed into smaller sizes by means of crusher machine (jaw crusher) . Table 2

3-1-4 Water
Potable water was used in all mixes.

3-1-4 Steel fibers
Straight high tensile steel fibers were used .The properties of the used steel fibers are illustrated in Table (6 ) .

3-2 Mixing of concrete
A pan mixer of 0.1% m3 capacity was used to mix the concrete ingredients .The mixer was firstly cleaned from the remaining lumps of concrete .The dry mixed ingredients were placed in the pan mixer and they were mixed for 2 minutes to ensure the homogeneity of steel fiber and to split the agglomerations of cement particle .The required quality of water was added to the mix and the whole constitutes were mixed for 3 minutes .

3-3 Preparation ,casting, curing and types of the test specimens
Steel molds were used for casting all the tested specimens .Before casting the molds were cleaned and oiled to avoid the adhesion of hardened concrete to the inside faces of molds. The fresh concrete was placed inside the molds with approximately equal layers of 50 mm for all the specimens and consolidated by the mean of vibrating table for a sufficient period .Care was taken to avoid segregation of LWA because the lightest particles of LWA tend to float on the surface of concrete causing segregation of the mix consistent .After casting ,the concrete surface was leveled and covered with nylon sheets to prevent evaporation of water so as to avoid the plastic shrinkage cracks.
On the second day the specimens were demolded ,marked and immersed in tap water until the test age. 100x100x100 mm cubes ,100x200mm cylinders and 100x100x400 prisms were used for compressive ,splitting tensile and flexural tensile strengths tests respectively.

3-4-2 Fresh and hardened unit weights
The test was performed according to ASTM C 567-85 [20]. ys were da The above tests in addition to compressive strength test at 28 Note : conducted to achieve the requirements for SLWC in ASTM C330 [2]

3-4-3 Compressive strength test
The test was conducted on 100 mm cube according to BS part 116 :1989 .The test was performed at ages 3,7 and 28 days .The recorded value represents an average of three readings measured on three specimens.

3-4-4.a Splitting tensile strength test
The test was conducted on cylinders of 100x200 mm according to ASTM C496-86 [22] .The splitting tensile strength was calculated using the following equation: l : length of test specimen (mm).

3-4-4.b Flexural strength test
The test was performed on prisms 100x100x400 mm according to BS

4-1-2 a Splitting tensile strength
The splitting tensile strength results of R,0.5% and 1%-SLWC up to 28 days of water curing are illustrated in Fig(2).The results clarify that there is general progressive gain in splitting tensile strength of R,0.5% and 1%-SLWC.Results indicate that the 0.5%-SLWC showed higher splitting tensile strength corresponding to R-SLWC . The .Nevertheless, addition of steel fibers does not significantly increase compressive strength, but it increases the tensile toughness, and ductility. It also increases the ability to withstand stresses after significant cracking (damage tolerance) and shear resistance [25].