Durability Characteristics of High-Strength Steel Fiber Reinforced Concrete

The combined use of silica fume having pozzolanic reaction and filler effect, and super-plasticizer will improve the interface of the materials, thereby enhancing the strength of the concrete, can lead to economical high performance concrete (HPC) with enhanced durability. Addition of steel fibers in to the silica fume concrete alters their brittle character in to a more ductile one, and improves the mechanical properties. An experimental investigation was carried out to assess the durability performance of high-strength steel fiber reinforced concrete (HSFRC) for water to cement material ratios (w/cm) ratios ranging from 0.4 to 0.25. The variation in concrete strength from 55.6 to 86.5 MPa was achieved by varying the water-to-cementitious materials ratios of the mix from 0.40 to 0.25, and silica fume replacement at 5% and 10%. Crimped steel fibers of fiber volume fractions, Vf= 0.5%, 1.0% and 1.5%, with an aspect ratio of 80 were used in this study. Test results indicated that inclusion of steel fibers in to silica fume concrete improves the compressive strength by 13% and concrete mixes are having the better durability performance.

Introduction

Cement-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). Due to the inherent brittleness of HSC/HPC, it lowers its post-peak portion of the stress-strain diagram almost vanishes or descends steeply (ACI 363-92).This inverse relation between the strength and ductility is a serious drawback for the use of HSC/HPC and acompromise to this drawback can be obtained by the addition of discontinuous short steel fibers of small diameter in to the concrete matrix. Bharatkumar et al. (2001) have studied the durability characteristics such as water absorption (%), void content (%), coefficient of absorption and sorptivity, of fly ash concrete, found to improve by reduction of w/b ratio and further improved by addition of cement replacement materials (CRM). Chang et al. (2001) have studied the durability properties of HPC, found to improve by reduction of w/b ratio and further improved by addition of cement replacement material. But for the writers’ knowledge durability studies are very limited on the HSFRC. Designing concrete for low permeability results in restricted access of water or solutions from external sources. While permeability of good concrete can be difficult to measure, the study of porosity or air content, coefficient of absorption and water sorption can provide some qualitative insight in to permeability. In this study, the durability performance of high-strength steel fiber reinforced concrete with compressive strength ranging from 55.6 - 86.5 MPa was examined.

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.

Experimental Program

Materials, Mixture proportions, and Preparation of specimens

Ordinary 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.

Fine aggregate: Locally available river sand passing 4.75mm IS sieve, conforming to grading zone-II of IS: 383-1978 was used. It has fineness modulus of 2.65, a specific gravity of 2.63 and water absorption of 0.98 % @ 24 hrs.

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 Test

Compressive strength tests were carried out according to IS: 516-1979 [16] 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 Studies

The water absorption test was performed according to ASTM C 642 [21] and air (void) content, coefficient of absorption and sorptivity were also evaluated based on the water absorption test.

Coefficient of Absorption

Powers (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.

Sorptivity

Sorptivity (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.

Test Procedure

Immediately after the immersion of the cube in to water the start time of the test was recorded; at intervals of 30, 60, 90, and 120 minutes after the start of the test, the specimen was removed from the water tub; after the surplus water was wiped of with tissue papers, it was weighed to the nearest 0.01gm and then returned to the tub. The relationship between water sorption and time was determined by Power (1968) as

i = S t^1/2---(1)

Where S [in mm/min^1/2] 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 Strength

Compressive 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 Characteristics

For 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) Content

The 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 Absorption

The 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.

Sorptivity

Results 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

For studying the resistance of HSFRC in sea water, cubes of 150 mm side were weighed and immersed in sea water prepared artificially, (3 % of sodium chloride 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 about 3.84 % for non-fiber concrete and 2.53% for fiber concrete, indicating that HSFRC mixes have better and higher resistance against sea water. No cracks were formed on the surface of the specimens. This indicated that the reaction between the CaOH and NaCl solution is reduced due to the reduction of harmful lime content in the concrete matrix as CSF reacted with lime present in the paste matrix.

Acid Resistance

For 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.

Conclusion

Based 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.

References

• ACI Committee 318-1995 Building code requirements for structural concrete, ACI 318-95, American Concrete Institute, Detroit
• Balaguru, N., and Shah, S.P. 1992 Fiber reinforced concrete composites, McGraw Hill International edition, New York.
• ACI Committee 544-2006 State-of-the-art report on fiber reinforced concrete, ACI 544.1R- 82, American Concrete Institute, Detroit.
• ACI Committee 363-1992 State-of-the-art report on high-strengthconcrete, ACI 363-1992, American Concrete Institute, Detroit
• Bharatkumar, B.H., Narayanan, R., Raghuprasad, B.K., and Ramamoortht, D.S. 2001 “Mix proportioning of high-performance concrete,” Cement and Concrete Composites, 23, 71-80.
• Chang, P.K., Peng,Y.N., and H wang, C.L. 2001 “A design consideration for durability of high-performance concrete,” Cement and Concrete Composites, 23, 375-380.
• ACI Committee 211-1999 Standard practice for selecting proportions for normal, heavy weight and mass concrete, ACI 211.1-91 ACI Manual of concrete practice.
• ACI Committee 211-1999 Guide for selecting proportions for High strength concrete with Portland cement and Flyash, ACI 211.4R-93 ACI Manual of concrete practice.
• ACI Committee 544–2006 Guide for specifying, mixing, placing and finishing steel fiber reinforced concrete, ACI 544.3R- 95, American Concrete Institute, Detroit.
• Ramadoss, P., and Nagamani, K. 2006 “Investigations on the tensile strength of high-performance fiber reinforced concrete using statistical methods,” Computers and Concrete- An International Journal, 3(6), 389-400.
• IS: 516-1979, Indian standard methods of tests for strength of concrete, BIS 2002 Bureau of Indian Standards, New Delhi, India.
• American Society for Testing and Materials 1990 Standard test method for specific garavity, water absorption and unit weight of hardened concrete, ASTM Stand Concrete Aggregate, 4, 318-9.
• Power, T.C. 1968 Properties of fresh concrete, New York.
• Hall, C., 1993 “Water sorptivity of mortars and concrete- A review,” Magazine of Concrete Research, 419(147), 51-61.
• Nawy, E.G. 1997 Concrete Construction Engineering Hand Book. CRC Press, Boca Raton, New York
• Taywood engineering limited, 1993 Correspondence to MBT (Singapore) pvt.Ltd Australia,
• McCarter, W.J., Ezirim, H. and Emerson, M.1992 “Absorption of water and chloride in to concrete,” Magazine of Concrete Research, 44 (158), 31-37.
• Martys, N.S., and Ferraris, C.F. 1997 “Capillary transport in mortars and concrete,” Cement and Concrete Research, 27 (5, 747-760.