mr vs fc hpc con fibras acero india

8/12/2019 Mr vs Fc HPC Con Fibras Acero India

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High–Performance Fiber Reinforced Concrete underCompression and Flexure

P. Ramadoss, Research Scholar and K. Nagamani, Professor, Structural Engineering Division

CEG, Anna University, Chennai.

Mechanical properties of fiber reinforcement concrete are needed to use the FRC in various

structural applications. This paper presents the experimental investigation carried out to study

the behavior of high performance fiber reinforced concrete (HPFRC) under compression and

flexure with compressive strength ranging from 60 MPa to 86 MPa and flexural strength ranging

from 6 MPa to 10 MPa. Steel fiber volume fraction ranges from zero to1.5 percent (39, 78 and

117.5kg) with aspect ratio of 80 were used. The influence of fiber content on the compressive

strength and flexural strength with w/cm ratios ranging from 0.40 to 0.25 is presented.

Equations are proposed using regression analysis to predict the strength of HPFRC effecting the

fiber addition in terms of fiber reinforcing index. Strength comparison analysis was carried out to

validate the empirical equation and the maximum absolute variation obtained was 4 percent.

Addition of 1.5 % volume of crimped steel fiber resulted in 10% increase in the compressive

strength while flexural strength increased by 37% and the RCSR value evaluated is between

0.107 and 0.149.

Introduction

In last 3 decades, numerous research and development studies have been taken up in the field

of steel fiber reinforced concrete (SFRC) for understanding the behavior of sound composites.SFRC has been used in several areas of infrastructure and industrial applications including

highway and airport pavements, bridge decks, sky scrappers, hydraulic structures, industrial

floors, tunnel lining, etc. [1, 2]. As noted by ACI Committee 544, the composite has great

potential for many use in civil Engg., applications, especially in the area of structural elements.

High performance concrete (HPC) is defined according to ACI 363-1992 [3] as concrete, which

meets special performance and uniformity requirements that can't always be achieved by using

only the conventional materials and normal mixing, placing and curing practice. Typical high



performance requirements specify high strength (above 41 MPa), enhanced impermeability

(permeability less than 10-10 m/s), and high tensile strength (above 4 MPa) and other special

requirements. HPC is achieved by using super plasticizen (SP) to reduce water/cm ratio and by

using SCM (supplementary cementing material) as silica fume having pozzolanic reaction and

filler effect, which usually combines high strength with high durability [4].

High-performance/highstrength concrete leads to the design of smaller sections and reduces thedead weight, allowing longer spans and more useful area of structures. Reduction in mass is also

important for economical design of seismic resistant structures. is responsible for the

enhancement of strength and durability of the concrete [4]. Strength, ductility and durability are

the important factors to be considered in the design of earthquake resistant R.C. structures. Due

to the inherent brittleness of HPC/ HSC, it lowers its post-peak portion of the stressstrain

diagram almost vanishes or descends steeply [5]. This inverse relation between the strength and

ductility is a serious drawback for the use of HPC/HSC and a compromise to this drawback can

be obtained by the addition of discontinuous short steel fibers in to the concrete. When concrete

cracks, the randomly oriented fibers arrest a micro cracking mechanism and limit the crack

propagation, thus improving the strength and ductility thereby enhances the durability of

structural elements. Wafa and Ashour [6] have reported that addition of steel fibers in to HSC

changes its brittle mode of failure in to a more ductile one and results in a small increase in

compressive strength and more increase in tensile strengths compared to plain concrete. Fiberswith 1 percent volume fraction and aspect ratio of 75 provide maximum stiffness to concrete and

results in maximum increase in compressive strength [7]. Although a number of researchers

investigated the effect of inclusion of discrete steel fibers on the compressive strength [8, 9,

10], research on HPC where fibers play a major role is lacking. The main objective of this paper

is to study the influence of crimped steel fibers on the compressive strength and flexural

strength of high performance- fiber reinforced concrete with varying w/cm ratios and 10% silica

fumereplacement.

Research Significance

High performance concrete with and without fibers possesses mechanical properties that are

significantly different from normal strength concrete materials. This paper presents an extensive

experimental investigation on the mechanical properties of HPFRC with w/cm ratios of 0.40,

0.35, 0.30 and 0.25 and studies the effects of inclusion of fiber contents on improving these

properties.

Experimental Program

Four basic mixes for plain concrete designated as FC1-0.0, FC2-0.0, FC3-0.0 and FC4-0.0

according to the w/cm ratios of 0.4, 0.35, 0.30 and 0.25 were selected.

Materials and Mixture Proportions



Ordinary portland cement-53 grade satisfying the requirements of IS: 12269–1987 [11] and

silica fume supplied by Elkem India Ltd. having specific surface area of 23,000 m2 /kg, a specific

gravity of 2.35 and contained 88.7% of SiO2 were used in the ratio of 9:1 (1:1 partial

replacement of cement) in all the mixes. Fine aggregate of locally available river sand

conforming to IS: 383-1970 [12] with fineness modulus of 2.55 and a specific gravity of 2.63

and coarse aggregate of blue granite crushed aggregates conforming to IS: 383- 1970 with 12.5

mm maximum size and fineness modulus of 6.4, a specific gravity of 2.70 were used.

Fibers used are crimped steel fibers of diameter =0.45 mm and length = 36 mm, giving an

aspect ratio of 80. The properties of fibers used are given in Table 1. Mixture proportions used in

the test programme are summarized in Table 2 [13, 14, 15]. For each water to cemetitious

material ratio three fiborus concrete mixes were prepared with fiber volume fractions, V f of 0.5,

1.0 and 1.5 percent (39, 78 and 117.5kg/m3). Due to the inclusion of the fibers some minor

adjustments in terms different ingredients had to be made as shown in Table 2. A naphthalene

sulphonated formaldehyde (NSF) as HRWR admixture (super-plasticizer) conforming to ASTM

Type F with dosage range of 1.75% to 2.75% by weight of cementitious materials has been used

to obtain the adequate workability of plain and fiber reinforced concrete. Dosage of super

plasticizer was arrived based on the workability tests conducted on trial mixes.



Mixing and Curing

Silica fume was mixed with cement uniformly and thoroughly till homogeneity is attained.

Concrete was mixed using a tilting type mixer and specimens were cast using steel moulds,

compacted with table vibrator. For each mix at least three 150 mm cubes and three 100 x 100 x

500 mm prisms were produced. Specimens were demolded 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

water tank to maintain uniform curing.

Results and Discussions

The results of this investigation are applicable to the material and the type of fibers used.

Compressive Strength

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.

Figure 1(a) Silica fume concrete cubespecimens after failure in compression test

Figure 1(b) Fiber concrete cube specimens afterfailure in compression test

Behavior under Compression

Under uniaxial compressive loading, extensive crack was produced in the concrete during pre-

peak stage and then failed suddenly at peak load. Figure 1(a) reveals the failure mechanism of

Silica fume concrete and indicates that it increases its compressive strength and makes it more

brittle, fails violently and suddenly. It is observed from Figure 1(b) that when fibers in discrete

form present in the concrete, propagation of crack is restrained which is due to the bonding of

fibers in to the concrete and it changes its brittle mode of failure in to a more ductile one and

improves the post cracking load and energy absorption capacity.



Test results show that addition of fibers has a moderate effect on the improvement of

compressive strength values. Table 3 and Figure 2 show that the addition of fiber volume

fraction from 0 to 1.5% increases the compressive strength by about 10 percent compared to

zero 20 percent given in the literature [5, 7, 9, 10, 17] for normal strength concrete. It is

observed from the test results that for 1.0 percent fiber content, the increase in strength is

about 10 percent but beyond 1.0% fiber content, there is only marginal increase in strength.Based on the test results, using linear regression analysis, the compressive strength of HPFRC

may be estimated in terms of compressive strength of plain concrete, f c and Reinforcing index,

RI and volume fraction, Vf respectively, as follows:

Figure 2: Effect of steel-fiber reinforcing index(RI) on compressive strength

f cf = f c + 1.84 (RI) ----------- (1)

f cf = f c = + 4.74 (Vf ) ---------(2)

Where f cf = compressive strength of fiber reinforced concrete, MPa; f c = compressive strength of

plain concrete, MPa;



RI= steel fiber reinforcing index; Vf = volume fraction of steel fiber, percent

RI = wf * (l/d); average density of HPFRC = 2415 kg/ m3

Weight fraction (wf ) = (density of fiber/ density of fibrous concrete) * Vf

Aspect ratio (l/d) = length of fiber/diameter of fiber.

Figure3: Relationship between Compressivestrength of fiber concrete and Plain concrete

Figure 4: Bar chart showing the variation ofCompressive strength on the effect of steel

fiber content for w/cm ratios

The effect of fibers on 28-day compressive strength of concrete may be evaluated form Figure 3.

In this figure, the cube compressive strength of all fibrous concrete irrespective of V f , is plotted

against the plain concrete. The least square line obtained using linear regression analysis with

correlation coefficient, r =0.98 is given by:

f cf = 1.087 -----------(3)

Figure5: Effect of steel-fiber reinforcing index(RI) on Modulus of rupture

Figure 6: Modulus of rupture, f rf as function ofCompressive strength, f cf

The predicted values as obtained by equation (1) and the equations proposed by the researchers

[5,7,17] were compared with the experimental values, are presented in Table 4. The proposed

equation (1) evaluates the compressive strengths with absolute variations less than 4%. Figure4 (Bar chart) shows the improvement of compressive strength on the effect of addition of fiber

content for different w/cm ratios. It may be seen from the Figure 8 that the predicted

compressive strength by the equation (1) and different researchers are having a good

correlation with experimental values.



Flexural Strength (Modulus of Rupture)

The flexural strength (Modulus of rupture) tests were conducted as per the specification of ASTM

C 78- 94 [18] using 100 x 100 x 500 mm prisms under third point loading on a simply supported

span of 400 mm. The tests were conducted in a 100 kN closed loop hydraulically operated

Universal testing machine. The load was applied at the rate of 0.1 mm/ min. Minimum of three

specimens were tested to compute the average strength.

Figure 7: Relationship between Modulus ofrupture, f r and Compressive strength (Plain

concrete)

Figure 8: Comparison of Predicted compressivestrengths (MPa) with Experimental values

(MPa)

Table 3 and Figure 5 present the variation of the modulus of rupture f rf , on the effect of fiber

content. Figure 6 shows the variation of the modulus of rupture f cf as a function of the

compressive strength f cf of the HPFRC. The equation obtained for predicting the modulus of

rupture, f rf of fiber reinforced concrete using non– linear regression analysis, as follows:



f rf = 0.019f cf 1.425, MPa ------------(4)

The Cement and Concrete association of Australia adopts the following relationship for FRC, in its

publication Industrial pavement-Guidelines for design, construction and specification.

f rf = 0.70 √f c,' MPa which yields lower value compared to the proposed equation (4).

Figure 7 shows the variation of the modulus of rupture, f r as a function of compressive strength

√f c of the HPC (plain concrete). The following equation is provided using regression analysis for

the test results.

f r = 0.861, √f c MPa ----------------(5)

This equation (4) yields the values less than that obtained by Wafa and Ashour [6] of 1.03'

√f c for HSC ACI 363-1992 [3] proposed the equation f r = 0.94 (ƒ'c)0.5 for concrete strength

range of 21 MPa < ƒ'c < 83 MPa which yields higher values to the predicted equation (4).

Rashid et al. 2002 have proposed the equation which was obtained from the correlation of data

by least square regression analysis (correlation coefficient = 0.94) as = 0.42 (ƒ' c) 0.68 for 5 MPa< ƒ'c < 120 MPa.

It is observed from the test results (Table 3) that there is a significant improvement in flexural

strength in increasing the steel fiber content from 0.0 to 1.5 percent for all the mixes, varying

from 16 to 38 percent of plain concrete. The increase in strength of 38 percent for 1.5% fiber

content and 29 percent for 1.0% fiber content reveal that toughness will be much more than

that of plain concrete. Using the tested results presented in Table 4, by performing linear

regression analysis, the peak values of modulus of rupture of HP-FRC may be expressed as a

function of RI and Vf respectively as follows.

f rf = f r +0.665(RI) -----------------(6)

f rf = f r + 1.695 ( Vf ) ---------------(7)

where f rf = modulus of rupture of HPFRC, MPa

f rf = modulus of rupture of concrete, MPa

RCSR values obtained, varied in the range of 0.107–0.149.

Where, RCSR is the ratio of rupture strength to compressive strength.

Conclusion

Based on the test results of the experimental investigation using crimped steel fiberreinforcement with an aspect ratio of 80, the following observations can be drawn:

High performance concrete (silica fume concrete) is a highly brittle material and fails

suddenly. The brittle mode of failure is changed by addition of steel fibers in to HPC, in to

a more ductile one. It was observed that SFRC improves the concrete ductility, its post-

cracking load carrying capacity.

Fiber volume fraction up to 1.0 percent (RI= 2.58) is effective in increasing compressive

strength which is by about 10% of plain concrete (silica fume concrete).



The addition of steel fibers by 1.50 percent volume fraction results in an increase of about

10 percent in the compressive strength, and results in an increase of 38 percent in the

flexural strength compared to no fiber matrix. The improvement in flexural strength

varying from 16 to 37 percent of plain concrete.

Empirical equations that predict the influence of fiber contents in terms of fiber reinforcing

index on mechanical properties of HPFRC are presented.

The tensile strength as measured by modulus of rupture of HPC (plain concrete) is closelyestimated by the equation f r= 0.861, f c, MPa

The validity of the proposed expressions is limited to the type of fiber used up to 1.5 %

volume fraction (RI= 3.88).

The high-performance fiber reinforced concretes obtained have higher RCSR values, which

varied in the range of 0.107–0.149.

Notations

HPC= high performance concrete

HPFRC= high performance fiber reinforced concrete

f c = compressive strength of plain concrete, MPa

f cf = compressive strength of HPFRC concrete, MPa

f r = modulus of rupture plain concrete, MPa

f rf = modulus of rupture of HPFRC concrete, MPa

Vf = volume fraction of steel fiber, percent

l/d= aspect ratio

RI= fiber reinforcing index.

References

Balaguru, N., and Shah, S.P., Fiber reinforced concrete composites, McGraw Hill

international edition, 1992, p.179- 214.

ACI Committee 544, "State-ofthe- art report on fiber reinforced concrete," ACI 544.1R-

82, American Concrete Institute, Detroit.

ACI Committee 363, State-ofthe- art report on High strength concrete, ACI 363R- 92,

American Concrete Institute, 1992.

Attcin, P.C., High performance concrete, 1st edition, E& FN, Spon, London., 1998.

Samer Ezeldin, A,. Balaguru, P.N., "Normal and high strength fiber reinforced concrete

under compression," ASCE, Journal of Mate. in Civil Eng., 4(4) (1992), p. 415- 429.

Wafa, F.F. and Ashour, S.A. (1992), "Mechanical properties of high strength fiber

reinforced concrete," ACI Materials Journal, 89(5) (1992) p. 445- 455.

Saluja, S.K., Sarma, M.S., Singh, A.P., and Kumar. S., "Compressive strength of fibrous

concrete," Indian Concrete Journal, 66(2) (1992), p. 99–102.

Hsu, L.S., and Hsu, C.T.T., "Stress–strain behavior of steel fiber reinforced high strength

concrete under compression," ACI Materials Journal, 91(4) (1994) p. 448-457.

Mansur, M.A., Chin, M.S., and Wee, Y.H., "Stress–strain relationship of high strength fiber

concrete in compression," ASCEJournal of mate. Civil. eng, 13(1) (1999), p. 21- 29.



IS: 12269-1987, Specification for 53-grade OPC, Bureau of Indian standards, New Delhi,

India.

IS: 383-1970, Specification for coarse and fine aggregates from natural sources for

concrete, Bureau of Indian standards, New Delhi, India.

ACI 211.4R-93., Guide for selecting proportions for High strength concrete with Portland

cement and Flyash, A C I Manual of concrete practice 1999.

IS: 10262-1992, Recommended guide lines for concrete mix design, Bureau of Indianstandards, New Delhi, India.

ACI Committee 544, "Guide for specifying, mixing, placing and finishing steel fiber

reinforced concrete," ACI materials Journal, 90(1) (1993), p. 94-101.

IS: 516-1979, Indian standard methods of tests for strength of concrete, BIS 2002

Bureau of Indian Standards, New Delhi, India.

Nataraja, M.C., Dhang, N., and Gupta, A.P., "Steel fiber reinforced concrete in

compression," ICJ, (July 1998), p. 353- 356.

Standard test method for flexural strength of fiber reinforced concrete, ASTM C78-1994,

Annual book of ASTM standards, American society for testing and materials, USA.

Bayasi, Z, and Soroushan, P., "Effect of steel fiber reinforcement on fresh mix proportions

of concrete," ACI materials journal, 89(4)(1992), p. 369-374.

Bhattacharya, G.K., Johnson, R.A. Statistical Concepts and Methods, Wiley, New York,

1977.

NBMCW May 2008

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mr vs fc hpc con fibras acero india

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