Durability Characteristics of High-Strength Steel Fiber Reinforced Concrete
Ramadoss, P, Research Scholar and Nagamani, K. Professor, Structural Engineering Division, Department of Civil Engineering, Anna University, Chennai
IntroductionCement-based materials such as concrete have long been used for the construction of civil infrastructure. However, the deterioration of civil infrastructure all over the world, has led to the realization that cement-based materials must be improved in terms of their engineering property and durability. The use of admixture as silica fume-a highly pozzolanic mineral, is a relatively effective way in improving these properties. ACI 318-95 has recently revised, specifying that structural concrete should have high durability, as presented by water to binder ratio (W/B). A concrete structure is said to be durable if it withstands the conditions for which it has been designed, without deterioration, over the past years. The term durability characterizes the resistance of concrete to a variety of physical and chemical attacks due to either internal or external causes.
Balaguru and Shah 1992, and ACI Committee 544 (ACI 544.1R-96) have reported that the addition of steel fibers in concrete matrix improves all engineering properties of concrete such as flexural strength, tensile strength, compressive strength, and toughness.
HPC is achieved by using super-plasticizer to reduce water-binder ratio and by using supplementary cementing materials (SCM) such as silica fume (CSF), which usually combines high–strength with high durability. Silica fume concrete has been reported to possess lower water permeability (ACI 226-1987). Steel fiber reinforced concrete (SFRC) is a cement-based composite material reinforced with randomly distributed discrete steel fibers of small diameter. It contains pozzolans and admixtures commonly used with conventional concrete. The demand for HSC/HPC has been growing at an ever-increasing rate over the past years, which lead to the design of smaller sections. Reduction in mass is also important for the economical design of earthquake resistant structures (ACI 363-92).
This paper presents an experimental investigation on thedurability properties such as water absorption, coefficient of absorption, air (void) content and sorptivity of HPC with w/cm ratios ranging from 0.25 to 0.4 and silica fume replacement at 5%, 10%, and 15%, and studies the effect of inclusion of crimped steel fibers (volume fractions Vf = 0.5 %, 1% and 1.5 %) on these properties.
Materials, Mixture proportions, and Preparation of specimensOrdinary Portland cement-53 grade having 28-day compressive strength of 53.5MPa, satisfying the requirements of IS: 12269–1987 and condensed silica fume (Grade 920-D) contained 88.7% of SiO2,having specific surface area of 23000 m2/kg, a specific gravity of 2.25, fineness by residue on 45micron of 2% were used. The chemical composition of OPC and CSF are given in Table 1a &1b respectively.
Coarse aggregate: Crushed blue granite stone with 12.5mm maximum size, conforming to IS: 383-1978 was used. The characteristics of coarse aggregate are:
Specific gravity = 2.70; Fineness modulus = 6.0; Water absorption= 0.65 % @24hrs.
Super-plasticizer: Sulphonated naphthalene formaldehyde (SNF) condensate as HRWR admixture conforming to ASTM Type F (ASTM C494) and IS: 9103-1999 was used. Specific gravity of SNF = 1.20.
Fibers conforming to ASTM A820-01 have been used, are crimped steel fibers of diameter =0.45 mm and length = 36 mm, giving an aspect ratio of 80, ultimate tensile strength (fu) = 910 MPa and elastic modulus (Ef) = 200 GPa.
Mixtures were proportioned using guidelines and specifications given in ACI 211.1–1991 and ACI 211.4R–93, recommended guide– lines of ACI 544-1993. Mixture proportions used in the test program are summarized in Table 2. This aspect of work was carried out elsewhere (Ramadoss and Nagamani 2006). Water present in super-plasticizer is excluded in calculating the water to cementitious materials ratio. For each water-cementitious materials ratio, 6 fibrous concrete mixes were prepared with three fiber volume fractions, Vf= 0.5%, 1% and 1.5 % by volume of concrete (39, 78 and 117.5 kg/m3). Super-plasticizer with dosage range of 1.75 % to 2.75 % by weight of cementitious materials has been used to maintain the adequate workability of plain andiber reinforced concrete.
Concrete was mixed using a tilting drum type mixer and specimens were cast using steel moulds, compacted with table vibrator. For each mix at least three 150mm x300mm cylinders and three 100 x 100 x 500mm prisms were prepared. Specimens were demoulded 24 hours after casting and water cured at 27±2oC until the age of testing at 28 days. All the specimens were cured in the same curing tank to maintain uniform curing for all the specimens.
Compressive Strength TestCompressive strength tests were carried out according to IS: 516-1979  standards using 150 mm cubes loaded uniaxially. The tests were done in a servo-controlled compressive testing machine by applying load at the rate of 14 MPa /min. Minimum of three specimens were tested to assess the average strength.
Durability StudiesThe water absorption test was performed according to ASTM C 642  and air (void) content, coefficient of absorption and sorptivity were also evaluated based on the water absorption test.
Coefficient of AbsorptionPowers (1968) suggested the use of co-efficient of absorption as a measure of the permeability of water in to the hardened concrete. This is measured by the rate of intake (capillary absorption) of water by dry concrete for the period of 60 minutes.
SorptivitySorptivity (water sorption) measures the rate of penetration of water in to the pores of concrete by capillary suction. When the cumulative volume of water penetrated per unit surface area of exposure is plotted against the square root of time of exposure, the resulting graph could be approximated by a straight line passing through the origin. The slope of this straight line is considered as a measure of rate of movement of water through the capillary pores and is called Sorptivity (Hall 1993). This test evaluates the quality of concrete based on surface pores of the concrete specimens.
i = S t1/2---(1)
Where S [in mm/min1/2] is the water sorptivity of the concrete and i [in mm] is the cumulative absorbed volume per unit area of in flow for duration of time‘t’
Results and Discussion
Compressive StrengthCompressive strengths for various mixes are given in Table 3. The 28-day strengths obtained vary from 55.6 to 86.5 MPa depending upon the w/cm ratio, binder content and fiber volume fraction in percent. Table 3 shows that the addition of fiber volume fraction from 0.5% to 1.5% increases the compressive strength by about 13 percent of plain concrete.
Durability CharacteristicsFor the series of high-strength fiber reinforced concrete mixes contained silica fume having w/cm ratios varying from 0.40 to 0.25, water permeability was unmeasurable.
Water Absorption and Air (void) ContentThe results of water absorption and air content are presented in Table 3. Water absorption and Air content for concrete mixes investigated after 28-day are found to be in the range of 0.83 – 2.0% and 2.02 – 4.80% respectively. According to the CEB report of Concrete Society, United Kingdom (1989), concrete quality is classified as good if the saturated water absorption is around 3%. This indicates that the performance of the mixes developed is considered to be good from the point of view of the water absorption. Results show that water absorption and air content reduce as the w/cm ratio reduces and also increasing of SCM. Bharatkumar et al. (2001) obtained water absorption in the order of 4.91– 3.51 for fly ash concrete and% void from 11.98–8.47, which compare well with present results.
Coefficient of AbsorptionThe values of coefficient of absorption obtained are given in Table 3. For the concrete mixes investigated after 28 days, it is in the range 1.35x10-10– 0.632x10-10m2/sec. It is seen that there is reduction in the value of coefficient of absorption as the w/cm ratio reduces. Bharatkumar et al. (2001) have reported the coefficient in the range of 3.01x10-10– 0.89 x10-10m2/ sec for flyash concrete (HPC), which compares well with present results.
SorptivityResults of water sorption for various HSFRC mixes are given in Table 3 and are in the range of 0.0916 – 0.0427 mm/”min (< 0.77mm/”min) indicated that durability performance of concrete containing silica fume is excellent (Navy 1997). Taywood Engineering limited (1993) has suggested that good quality concrete has the sorptivity value less than 0.1mm/”min. Past findings (MacCarter et. al. 1992; Martys and Farraris 1997) indicated that sorptivity can be correlated to permeability, as is a function of porosity, pore diameter, and continuity of pores within the concrete matrix. Figure 1 shows the typical sorptivity plot for HSFRC at 28-days. Bharatkumar et al. (2001) have reported that sorptivity of fly ash concrete (HPC) is in the range of 0.0883 – 0.0627 mm/min 0.5(1.14 x10-5–0.81 x10-5m/s 0.5) which compares well with the present results. It is clear that concrete sorption is closely related to surface pores of the concrete paste. The surface pores reduce as C-S-H gel developed by the addition of silica fume, as a result which decreases the water sorption and hence the durability performance of mixes improves.
Sea Water Resistance
Acid ResistanceFor studying the resistance of HSFRC to acids, 150mm side cubes were weighed and immersed in sulphuric acid solution (containing 1% of sulphuric acid by weight of water) for 45 days continuously and then taken out and weighed. The percentage loss in weight and the reduction in compressive strength were evaluated and given in Table 3. The maximum loss in compressive strength obtained was found to be about 4.51% for non-fiber concrete and 4.42% for fiber concrete, which show that SFRC mixes are less attacked by acid. Less deterioration effect was noticed in the case of SFRC. From the tests, it could be observed that SF concrete/ SFRC greatly enhance the durability in aggressive environments.
ConclusionBased on the above experimental investigation, the following conclusions can be drawn:
- It is observed from the results of water absorption and air content that quality of concrete mixes is good and show that water absorption and air content reduce as the w/cm ratio reduces and also increasing of SCM.
- Coefficient of absorption of HSFRC mixes is found to be of the order 10-10m2/sec. and is found to reduce with reduction in w/cm ratio of the mix.
- Sorptivity of HSFRC mixes is found to be of the order 10 -5m/ min.0.5shows quality of concrete mixes is superior and therefore, HSFRC is a very less permeable concrete. The above two characteristics are comparable to the reported values of flyash based HPC. It is observed from the test results of sorptivity test that the quality of concrete mixes is superior and therefore, HSFRC is a less permeable concrete.
- HSFRCs exhibit better performance against the attack of sulphuric acid and sea water.
- The use of silica fume and low w/cm ratio resulted in particularly impermeable concrete.
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