Development of Ultra High Strength Concrete Using Silica Fume and its Mechanical & Durability Properties - Experimental Investigation

This experimental investigation is aimed to develop an ultra-high strength concrete with minimum of 100 MPa as compressive strength.In order to obtain this, twenty different concrete mixes have been tried, using cement, river sand, coarse aggregate, water, silica fume and super plasticizer. During the preparation of trial mixes of concrete, the water / binder ratio of 0.2, silica fume of 10% to the weight of cement, super plasticizer of 10 litres per cubic metre of concrete and coarse aggregate of 1000 kg/m 3 were kept as constant. The amount of cement content (as 600-, 650-, 700-, 750- and 800 kg/m 3 ) and the ne aggregate content (as 500-, 600-, 700- and 800 kg/m 3 ) was varied. Totally 300 specimens were cast and tested in this investigation.The100 x 100 x 100 mm size of cubes, 150 x 300 mm size of cylinders, 100 x 100 x 500 mm size of prisms, 100 x 200 mm size of cylinders, 60 x 100 mm size of cylinders were used to test compressive, split tensile, exural strength, chloride penetration and water penetration tests respectively at the age of 7-, 14- and 28 days. Based on the test results, a suitable mix proportion to produce an ultra-high strength concrete has been identied. Subsequently, from this investigation, the maximum cube compressive strength of 130 MPa, split tensile strength of 6.94 MPa, exural strength of 21.39 MPa, chloride penetration 36 Coulombs which is lesser than 100 and sorptivity coecient valueof 0.582 has been achieved.


Introduction
Due to the paucity of space in cities, there is a huge demand for multistoried buildings. As the number of stories increases, the size of the column members also increases, thereby, requiring additional cost for foundation. To overcome this, nowadays high strength concrete is being adopted in beam and column members. Therefore, in this investigation it is mainly focused to develop the mix for Ultra High Strength Concrete (UHSC) using basic materials.
As per ACI Committee 211 [1], production of high strength concrete is based on suitable selection of constituents. In this report it is stated that addition of High Range Water Reducing Admixture gives better workability therefore no need for modifying the coarse aggregate content and there is a need to modify the cementitious content. For obtaining high strength concrete, optimum mix of cement, y ash, aggregates should be selected. The type of admixtures, dosage rate, and mix proportions also an important consideration in high strength concrete production.
According to Nageh N et al [2] UHSC is designated by higher dosage of silica fume and cement content with lowest water to cement ratio results in prominent mechanical and durability properties have been obtained. Reactive Powder Concrete with steel bres gives positive results in mechanical and durability properties.
Adel A et al [3] have been used cement, ne aggregate of maximum size 4.75 mm, steel bres, water, mineral admixtures such as high reactivity metakaolin, silica fume, and chemical admixture as super plasticizer for making ultra high performance concrete. Coarse aggregate was completely eliminated for casting the specimens.Further, it is concluded that percentage of increment in silica fume and steel bres lead to improve the compressive strength of cube specimen.
Prabhat Ranjan Prem, et al [4] proposed that Ultra High Performance Concrete made by cement, ne aggregate, silica fume, super plasticizer, quartz powder, steel bres with lower water to cement ratio and by complete expulsion of coarse aggregate shows better homogeneity. By the mineral and chemical admixtures gives a better gel formation in hydration process.
Mohammad Abdur Rashid, et al [5] investigated that high strength concrete have been produced by lower water to binder ratio, higher ratio of coarse to ne aggregate and optimum dosage of y ash, silica fume, super plasticizer. Their research also stated that there is an improvement in mechanical property of concrete by obtaining denser packing of solid constituents.
Srinivas Allena et al [6] have tried the development of UHSC using local materials. In their research type I / II cement, silica fume, locally available sand, water reducing admixture and steel bres were used. The curing of specimens made by moist curing, water bath curing and cured with oven, among which oven dried curing gives more strength than other methods of curing.
G.Venkatesan et al [7] recommended that the Ultra High Strength Concrete which was obtained by lesser water cement ratio and higher binder content.The materials Portland cement, ne aggregate, coarse aggregate (of size 12 mm and 6 mm with saturated and dry condition), silica fume, super plasticizer and water have been used. The results shows that the concrete specimens made by 6 mm aggregate with saturated condition contributes better compressive, split tensile and modulus of elasticity value.
Chong Wang et al [8] suggested that the improvement in strength, reduction in porosity and low water binder ratio have been achieved by addition of silica fume and quartz. Selected dosage of super plasticizer provided better ow ability to concrete without eliminating the coarse aggregate. P.C. Aitcin [9] conveyed that UHSC is derived by removing the coarse aggregate and improving the density with speci c grain size. Further, the usage of glass and iron powder in concrete contributed to attain the strength of 270 MPa and 800 MPa respectively. Therefore, this study is proposed with an aim to develop a UHSC having compressive strength more than 100 N/mm 2 without bres, glass and iron powder and to examine the mechanical and durability properties of developed UHSC mix.

Materials Used
In this research, the river sand is used as ne aggregate (which obeys to Zone -II as per IS: 383-1970 [10]), 12 mm coarse aggregate and grade 43 OPC cement, silicafume, super plasticizer were used as constituent materials to prepare trial mixes of concrete. The properties of cement and aggregates are given in Table 1 and for silicafume and super plasticizer are as follows.
Silica Fume: The Elkem 920D grade micro silica is improves the concrete performance and mortar formulation.
Physically it optimises packing of particle in cement mortar/concrete and chemically act as an extremely responsive pozzolanic. The speci c gravity of 2.25 and bulk density between 500-700 kg /m 3

Super plasticizer:
Master Glenium Sky 8233 is a type of super plasticizer, based on modi ed polycarboxylic ether, to achieve high workability concrete with greatly reduced water content. The properties of the super plasticizer used in this investigation are speci ed in Table 2.

Experimental Investigations
In order to develop an UHSC, twenty different proportions of trial mixes tried are reported in Table 3. During the trial mix preparation, the water-binder ratio of 0.2, silica fume of 10% by weight of cement, super plasticizer of 10 litres per cubic metre and coarse aggregate of 1000 kg/m 3 were kept as constant parameters. The amount of cement content was varied as 600-, 650-, 700-, 750-and 800 kg/m 3 and the amount of ne aggregate was varied as 500-, 600-, 700-and 800 kg/m 3 .

Preparation of Specimens
In this investigation, to develop and optimise the mix proportion for an UHSC, a total number of 300 specimens were prepared; pan mixer was used to mix the concrete are as shown in Fig. 4. The 100 x 100 x 100 mm size of cubes, 150 x 300 mm size of cylinders and 100 x 100 x 500 mm size of prisms, 100 x 200 mm size of cylinders, 60 x 100 mm size of cylinders were cast to test the compressive, split tensile, exural strength, chloride penetration and water absorption tests at the age of 7-, 14-and 28 days. Table vibrator was used to compact the concrete. After 24 hours, the samples were detached from the steel moulds and positioned for immersion curing into the curing tank as shown in Fig. 6.

Testing Of Specimens
After mixing, the fresh concrete tests have been performed then the concrete mix poured into the mould.
The cast and cured specimens have been tested after 7-, 14-and 28 days of curing to determine its mechanical and durability properties.

Characteristics of fresh concrete
The slump cone test was performed to check the workability of the concrete. It was observed that with zero slump value as shown in Figure 5. Since the silica fume was a very ne particles it absorbs more amount of water. The specimens prepared for this investigation were tested using a 4000 kN capacity of CTM as per IS 516: 1959 code [13]. The test results of concrete cube are reported in Table 4, Figure 1 and test setup is as shown in Figure 7. The cylinders were tested using computerised UTM of 1000kN capacity as per IS 5816: 1999 code [14]. The test results of cylinder are described in Table 5, Figure 2 and test setup are as shown in Figure 8. The prisms were tested using the exural testing machine of 1000 kN capacity as per IS 9399:1979 code [15].The test results of prism are reported in Table 6 and Figure 3 and test setup are as shown in Figure 9. Based on 20 trial mixes reported in Table 3, the optimum mix to achieve an UHSC has been identi ed. The recommended design mix to produce UHSC is given in Table 7.
It is observed from Table 7, that the C series mixes of C1, C2, C3 and C4 yields compressive strength value more than 100 N/mm 2 .
A signi cant rise in the concrete compressive strength is witnessed by the addition of cement content.
The cement content was increased at the rate of 50 kg/m 3 for A, B, C, D and E series of cubes respectively. This high strength is attributed due to the packing of ne silica fume and the improved strength of cement paste between aggregates.
The C series of trial mixes provides higher strength since the cement content is optimum. In series A & B cement content is less and, in series D& E cement content is more than optimum value. The in uence of the higher dosage of cement beyond 700 kg/ m 3 (Series C) may deliver increased amount of heat due to the reaction with water, which may be the indication for the development of internal small cracks even it is controlled by silica fume and super plasticizer content.
A notable rise in split tensile strength of concrete are obtained from series A to series C with the increase in cement content. From the experimental study, the following four mix proportions are recommended to obtain an UHSC with the compressive strength of more than 100 N/mm 2 .

Investigation of Durability Properties:
In order to study the durability of series C mixes rapid chloride penetration and water sorptivity tests were carried out.

Rapid Chloride Penetration Test (RCPT):
Rapid Chloride Penetration Test (RCPT) was completed as per ASTM C1202-97 [16]. A cylinder was cut three equal parts as top, middle and bottom with the diameter of 100 mm and height of 200 mm. Each part of the cut specimen was kept in the acrylic cell. Sodium chloride (3%) and sodium hydroxide (0.3 N) solutions were used as negative and positive terminals respectively subjected to 60 Volts potential for 6 hours as shown in Figure 10. For every 30 minutes, the results were recorded to calculate the current passed up to 6 hours. The penetration of the ions in the concrete specimen is depends on the permeability of concrete. The test results are as presented in Table 8. The more permeable concrete shows the higher Coulombs and the less permeable concrete, shows the lower Coulombs.

= 36 Coulombs
The current owing through the cells have the value of 36 Coulombs which was less than 100, therefore it is concluded that the chloride permeability rating is negligible, concrete particles have been closely packed.