Experimental Study on the Behaviour of High Strength Concrete with Silica Fume under Monotonic and Repeated Compressive Loads

The main objective of this paper is to determine the effect of silica fume on the compressive strength of concrete under two types of loading, monotonic and repeated compressive loads. Mixes were carried out by replacing cement with different percentages of silica fume of (0, 5,10,15,20) % by wt. of cement with a constant waterbinder ratios 0.25. Tests on standard cylinders (300*150) mm for the different ratios of silica fume were carried out to study the behavior of concrete under monotonic and repeated loads, the load was applied for many cycles up to failure with rate of loading (0.3) N/mm 2 /sec. The results indicated that the addition of silica fume improves the properties of concrete giving high compressive strength, high stress and strain, stresses carried by the concrete increased by (46.3,68.8,63.1,57.5)% also the strain increased by (58.5,97.8,52.8,22)% for silica fume percentages (5,10,15,20)% for monotonic loading, and stresses increased by (44.4,68.5,63.7,56.5)% and strain increased by (45.8,68.4,51.5,26.3)% for the same percentages under repeated loads. Compressive strength, dissipated energy, and modulus of elasticity also increased with increasing silica fume percentage.


Introduction
Silica fume is a bye product of silicon metal or ferrosilicon alloys. One of the most beneficial uses for silica fume is in concrete. Because of its chemical and physical properties, it is a very reactive pozzolan. Concrete containing silica fume may have very high strength and may be very durable. Silica fume is available from suppliers of concrete admixtures and, when specified, is simply added during concrete production. Placing, finishing, and curing silicafume concrete require special attention on the part of the concrete contractor.
Prior to the mid-1970s, nearly all silica fume was discharged into the atmosphere. After environmental concerns necessitated the collection and land filling of silica fume, it became economically viable to use silica fume in various applications, in particular high-performance concrete [1]. Silica fume has many effects on properties of fresh concrete: a) Workability: With the addition of silica fume, the slump loss with time is directly proportional to increase in the silica fume content due to the introduction of large surface area in the concrete mix by its addition. Although the slump decreases, the mix remains highly cohesive. b) Segregation and Bleeding: Silica fume reduces bleeding significantly because the capillary water is consumed in wetting of the large surface area of the silica fume and hence the capillary water left in the mix for bleeding also decreases. Silica fume also blocks the pores in the fresh concrete so capillary water within the concrete is not allowed to come to the surface. Silica fume may improve concrete in two ways the basic pozzolanic reaction, and a micro filler effect. Addition of silica fume improves bonding within the concrete and helps reduce permeability, it also combines with the calcium hydroxide produced in the hydration of Portland cement to improve concrete durability. In (2005) Mostofinejad and Nozhati [2] made many attempts to propose some experimental models to predict the modulus of elasticity of high strength concrete based on some known characteristics of the concrete mix. To do so, 45 mix proportions including 5 different ratios of silica fume, i.e., SF/CM = 0,5,10,15 and 20 percent, 3 water to cementitious materials ratios, i.e., w/cm = 0.24,0.3 and 0.4, and 3 types of coarse aggregates, i.e., limestone, quartzite and andesine were selected. 540 cylindrical specimens were cast, cured and tested after 7, 28 and 91 days. Regarding different ratios of silica fume and different ratios of w/cm, the relationship of modulus of elasticity of concrete at different ages was discussed and some empirical equations were proposed. The maximum compressive strength of HS concrete was achieved by a 10% substitution of silica fume for cement when w/c was 0.4, and by a 15% substitution of silica fume for cement when W/C was 0.3 or 0.24. The optimum silica fume percentage does not seem to be constant, and increases when the ratio of w/c decreases. To achieve the maximum modulus of elasticity of HS concrete, the optimum silica fume percentage is not necessarily equal to that for achieving the maximum compressive strength. In (2006) Ali Bashah [3] studied the effect of silica fume on the properties of high performance ground granulated blast furnace slag concrete to reveal the potential furthest. It was found by compressive strength test, that high strength concrete can be achieved. At the age of 28 days, concretes containing 5,7.5, and 10% silica fume gave compressive strengths of 65.6, 64.6, and 67.1 Mpa, respectively. At aged 56 days concrete containing 10% silica fume had the highest strength. However the concrete containing 5% of silica fume had low permeability thus may enhance the durability. In (2007) George et al. [4], carried out for two grades of concrete ie. M20 and M30. For the grades of concrete the cement content, sand, grit, coarse aggregate and w/c ratio are kept constant. Only the percentage of silica fume has been changed ie. 0%, 5%, 10% and 12.5%. The results have shown the considerable improvement in compressive strength, modulus of elasticity and durability with respect to sulphate. Splitting tensile strength is observed to be nearly same with or without the use of silica fume for both the grades. During cyclic loading, the concrete, is subjected to loading, unloading, and reloading processes. This will cause initiation of micro-cracks and will sometimes lead to the fatigue limit of the materials. This, thus resulting in reduction in the stiffness. Hence, it is necessary to evaluate degradation of stiffness in the concrete subjected to cyclic or repeated loading. In (2002) Daniel and Loukili [5] investigated the behavior of high-strength fiber-reinforced concrete (HSFRC) beams under cyclic loads. They illustrated that HSFRC with tensile reinforcement ratio of 0.55% exhibited behavior similar to that of a HSC beam with a tensile reinforcement ratio of 0.97% and the fibers have no influence on strength deterioration during loading cycles at a given displacement.

Research Significance:
The objective of this study was to determine the effect of silica fume on compressive strength, stress and strain of concrete under monotonic and repeated loads.

Materials:
Materials used in this research for ordinary concrete were as follow: 1. Cement: locally manufactured cement was used. The physical and chemical analysis for this type of cement showed in table (1) and (2)

Casting and test procedure:
The mixed proportion was (1:2:3/0.25), Concrete cylinders were prepared, the materials were placed in mixer the placement order was coarse aggregate, fine aggregate , cement, silica fume (mixed in dry condition), then the mixing water was added with the Super plasticizer , and then the samples were cast in the molds and after casting the samples were left for 24 hours and then cured in the water (standard curing). After 28 days the samples were taken out of water, then were tested in compression as shown in Fig. (1).

Results:
It is observed that when silica fume is added in concrete as an admixture, it modifies most of the properties of concrete such as workability, compressive strength, modulus of elasticity.

WORKABILITY:
The slump test is used for the indication of consistency of fresh concrete. The test is an empirical test that measures the consistency of fresh concrete. More specifically, it measures water contents between batches. Fig.(2) given below show that consistency (workability) varies with the different silica fume percentages with added a super plasticizer, The effectiveness of super plasticizer is enhanced in the presence of silica fume. With increase in silica fume content in concrete, the value of slump decreases. which can be attributed to high specific surface of silica fume.     (3) and (4) under monotonic and repeated loads. It is observed from the figures that there is an improvement in compressive strength for all the percentages of silica fume. When a super plasticizer of 2% of cement weight was added to the mix, the maximum strength obtained is for 10% of silica fume. It can be noticed that the percent increase in compressive strength is (46.3,68.8,63.1,57.5)% for the silica fume (5,10,15,20)% under monotonic load, and the percent increase in compressive strength is (44.4, 68.5, 63.7, 56.5)% for the same percentages of silica fume under repeated load. So the percentage (10) of silica fume gave the optimum value of compressive strength under monotonic and repeated loads. This was because of the hydration of Portland cement which produces many compounds including Calcium Silicate Hydrates (CSH) and Calcium Hydroxide (CH). When silica fume is added to fresh concrete, it reacts with the CH to produce additional CSH which improves the bond between the cement and the surface of the aggregate [10]. The relationship between stress and strain that a material displays is known as a Stress-Strain curve. It is unique for each material and is found by recording the amount of deformation (strain) at distinct intervals of tensile or compressive loading. These curves reveal many of Similarly when testing standard cylinder (300*150) mm, of concrete for the same ratio of silica fume under repeated loads, the results show from figs. (6,7,8,9,10)   Modulus of elasticity, or elastic modulus, It is defined as the ratio of the uniaxial stress over the uniaxial elastic strain in the range of stress in which Hooke's Law holds [11]. It can be experimentally determined from the slope of a stress-strain curve created during compressive tests conducted on a sample of the material. Fig. (11) shows that the Modulus of elasticity of the concrete under monotonic loads is maximum between 10% and 15% of silica fume. with the knowledge that the Modulus of elasticity is calculated at 40% of maximum stress, according to ASTM C469-02  One of the principal challenges in structural engineering concerns the development of modern design concepts to a better protecting of structures, with the occupants and contents, from the damaging effects of destructive environmental forces including those due to winds, waves and earthquakes. Passive energy dissipation devices, when incorporated into a structure, absorb or consume a portion of the input energy, thereby reducing energy dissipation demand on primary structural members and minimizing possible structural damage[13]. Fig. (13) show the result of Energy absorption which is defined as (area under the stress-strain curve). The Energy absorption increased for all silica fume percentages and giving the optimum percentage at (10)% under monotonic and repeated loads.
Summary for the obtained results are shown in table (8):

Conclusions:
The following conclusions can be stated: