fresh and hardened properties of steel fibres

International Journal of Applied Engineering Research ISSN 0973-4562 Volume 6, Number 13 (2011) pp. 1565-1578 © Research India Publications http://www.ripublication.com/ijaer.htm

Fresh and Hardened Properties of Steel Fibres Reinforced Self-Compacted Concrete

S.A. AlTaan¹ and Z.S. AlNeimee²

¹Former Professor, Dept. of Civil Engineering, Mosul University, Iraq Presently, Cultural Attaché, Iraqi Embassy, Kuala Lumpur, Malaysia

E-mail: [email protected] ²Post Graduate Student, Dept. of Civil Engineering,

Mosul University, Iraq.

Abstract

Fresh and hardened properties of steel fibrous self-compacted concrete were studied in this investigation. One reference self-compacted concrete mix is used with ten percent of the cement replaced by limestone powder. Three steel fibres percentages by volume of concrete are used (0.35, 0.7, and 1.05). The used steel fibres were a shelled Harex type with irregular cross-section, equivalent diameter of 0.78 mm, and 16 mm long. Super plasticizer was used to improve the workability and flow ability of the mixes. The test results showed that the presence of steel fibres decrease the flow ability, and increase the time of spreading, segregation, and passing ability of the fresh concrete. For the fibres percentages used, the fresh properties were within the recommended specifications for the self-compacted concrete. The test results showed, an early strength development rate more that for plain normal concrete due to the presence of the fine materials. As for normal concrete, the test results showed also that the increase in the splitting strength is more than the increase in the compressive strength due to the presence of the steel fibres. The splitting strength of the cylindrical specimens is about eighty four percent of that for the cubic specimens. The brittle mode of failure of the plain un-reinforced specimens changed to a ductile one due to the presence of the steel fibres.

Keywords: Compression, limestone powder, self-compacted concrete, splitting, steel fibres, strength.

1566 S.A. AlTaan and Z.S. AlNeimee

Introduction Fibre reinforced concrete (FRC) is a concrete with a tensile strength, flexural strength, flexural toughness, cracking resistance, and strain capacity more than those of plain concrete. Moreover, it is characterized by multiple cracking, well-defined post cracking behaviour, and a ductile mode of failure under all types of loading (ACI Committee 544, 1988, 1996). Self-compacting concrete (SCC) is a concrete with good flow, spreading, and passing abilities and a good resistance to segregation (EFNARC, 2002, European Guidelines, 2005). Fibre reinforced self-compacting concrete (FRSCC), can be considered as a high performance hybrid of fibre reinforced and self-compacting concrete, which retain the properties of both concretes. It was shown that self-compacted steel fibre reinforced concrete (Miao, et al., 2003) can be produced with a good fluidity and without bleeding and segregation by using 52% and 70% of the weight of cement, slag and fly ash respectively. Three volume percentages (0.5, 1, and 1.5) of hooked steel fibres (aspect ratio =60) were used. The test results showed that, there was an improvement in the compressive strength up to 20% over the plain concrete, and the fibre / plain concrete strength ratio decreased with time. The flexural strength and toughness of the produced mixes increased with the fibres percentage. The hybridization of high modulus, two lengths of steel fibres (20 mm and 30 mm long and a rectangular cross-section of 1.6×0.03 mm) and low modulus 50 mm long polypropylene with a cross-section of 1.6×0.4 mm was tested (Oucief et al., 2006). Six self-compacting concrete mixes were tried:

1. SCC without fibres. 2. SCC with 10 kg/m³ steel fibres (20 mm long). 3. SCC with 6.75 kg/ m³ steel fibres (30 mm long). 4. SCC with 4.5 kg/m³ polypropylene fibres (50 mm long). 5. SCC with 10 kg/m³ steel fibres (20 mm long) and 4.5 kg/m³ polypropylene

fibres (50 mm long). 6. SCC with 6.75 kg/m³ steel fibres (30 mm long) and 4.5 kg/m³ polypropylene

fibres (50 mm long). The test results showed that the 30 mm long steel fibres gave the highest compressive strength, while the 50 mm long polypropylene fibres gave the lowest strength. The modulus of rupture was increased for all the types of fibres, but the steel fibres gave the highest increase. The flexural toughness was increased for all the types of fibres, but the polypropylene fibres gave the highest increase for all the indices. In hybrid form, the mix with 20 mm long steel fibres and 50 mm long polypropylene fibres gave the highest toughness. The energy was also computed from the load-deflection curve up to 5 mm deflection. The polypropylene and the hybrid fibres gave the highest energy. An investigation was performed (Rao and Ravindra, 2010) to compare the properties of plain self-compacting concrete (SCC) with (SCC) with steel fibres having a diameter of 0.92 mm. One control (SCC) and ten (SCC) mixes with steel fibres were investigated. Thirty percent of the Cement was replaced by Class F fly ash, and two types of plasticizers were used. The variables were the aspect ratio (15,

Fresh and Hardened Properties of Steel Fibres 1567

25, and 35) and the volume percentage of the fibres (0.5, 1.0, and 1.5). The test results showed that all the mixes can be considered as SCC, although all the mixes satisfy the suggested lower and upper limits [EFNARC, 2002]. The fibres inclusion did not significantly affect the measured compressive, splitting, and flexural strength. However, the strength and ductility of the fibre reinforced (SCC) specimens increased substantially in the case of specimens with fv =1% and aspect ratio of 25.

The effect of the fibre type on the rheological and fresh properties of self-compacting concrete was studied (Ozbay et al., 2010). One percent by volume of steel, polypropylene, and polyvinyl alcohol fibres were used, all the three types were of relatively similar sizes. Forty percent of the binder was replaced by fly ash. It was found that steel fibre reinforced SCC can be produced by increasing the amount of high range water reducer. While, polypropylene and polyvinyl fibre reinforced SCC cannot be produced even if the amount of the high range water reducer is increased. The effect of using steel fibres on the fresh and hardened properties of (SCC) was studied (Eren and Alyousif, 2010). One control (SCC) mix without fibres, and six mixes reinforced with hooked end steel fibres of aspect ratio (60, 65, and 80), and two weights (25 and 50 kg/m³; i. e., =fv 0.32 and 0.64% respectively) were cast. The

w/c was kept constant for all the seven mixes, but the admixture dosage was adjusted for the fibrous mixes to satisfy the workability requirements of (SCC). It was found that the steel fibres decreased the flowing and passing abilities of the fibrous mixes, and the mixes with long fibres showed blockage tendency. The improvement in the splitting strength was more than that for the compressive strength. The aim of this study is to assess the possibility of producing medium strength (SCC) with low fines percentage reinforced with steel fibres of low aspect ratio and varying volume percentages. The study aimed also at assessing the effect of the steel fibres on the fresh and hardened properties of the produced mixes. Experimental Programme Materials The mix proportions used were [AlNeimee, 2008], as shown in Table 1. Ordinary Portland cement, ten percent by weight of the cement was replaced by limestone powder passing sieve No. 200 (75 microns). The limestone powder used to act as filler, increase the flow ability, and decrease friction between the mix constituents. Medium size sand with a fineness modulus of 2.77, and gravel with maximum aggregate size of (12.50 mm) were used. The mix proportions were chosen to produce a nominal cube compressive strength of 40 MPa. Sika Viscocrete - 5w was used as a plasticizer whose physical properties are shown in Table 2. The dosage was adjusted as shown in Table 1 to satisfy the SCC specifications. Harex steel fibres of shelled deformed cross section, Figure 1, with a length lf of 16 mm, equivalent diameter of 0.78 mm, and aspect ratio of (lf / df =20.5), and three volume percentages (vf = 0.35, 0.7, and 1.05) were used. To get optimum strength improvement, the fibres length should be more than the maximum aggregate size, which in this case 12.5 mm.


Table 1: Mix Proportions.

Table 2: Properties of the Plasticizer.

Specimens Description The following tests were used to measure the properties of the fresh concrete; the slump flow test for the spreading and filling abilities, J-ring test for the passing ability, V-funnel test for the segregation resistance. The description of these tests and the corresponding limitations can be cited in References (EFNARC, 2002, European Guidelines, 2005). The specimens used for measuring the properties of the hardened concrete were; three (100 mm) cubes and three cylinders (100×200 mm) for the compressive strength, and three cylinders (100×200 mm) and three (100 mm) cubes for the splitting strengths.

Figure 1: Harex Steel Fibres.


Test Results and Discussion Properties of Fresh Concrete Flow and filling abilities Table 3, and Figures 2 and 3 shows that the steel fibres increased the time 500T and decreased the average spreading diameter avgD for the three fibres reinforced SCC

mixes. However, both 500T and avgD for the four SCC mixes are within the specified limitations (EFNARC, 2002, European Guidelines, 2005), which means that the four mixes have a reliable flow and filling abilities. Figures (2-3) show the overall limits for 500T and avgD .

Table 3: Spreading Ability of the self-compacted fibrous concrete mixes.

Passing ability Table 4, and Figures 4 -6 show the test results for the passing ability of the four SCC mixes, the average diameter avgD , the time required to reach the 500 mm diameter

500T , and the difference j

S between the heights of concrete at the periphery and at


the centre of the J-ring. The test results show that the presence of the steel fibres decreased the average diameter avgD , increased the time required for concrete to

reach the 500 mm diameter 500T , and increased the difference j

S between the

heights at the periphery and at the centre of the J-ring. All the test results are within the specified limits (EFNARC, 2002, European Guidelines, 2005), except Mix 1.05-1.9 for which

jS was, slightly greater the 20 mm upper limit, however, no

conglomeration of the concrete was noticed.

Table 4: Resistance to Passing of the self-compacted concrete mixes.


Filling and segregation resistance Table 5 and Figures (7-8) show the results of the V-funnel test results, which are an index of the filling, segregation resistance, and viscosity of the SCC mixes.

Table 5: Separation and Segregation Times of the Self-Compacted Concrete Mixes.

The test results show that all the oT values are within the specified limits. The results show also, that the 5T values for all the mixes are more than oT , and this may be attributed to the more hydration that may take place between the time intervals oTand 5T .


Figure (9) shows the difference in times oTT −5 for the four mixes, except Mix M1.05-1.9 with fibres volume percentage of 1.05, all other test results comply with the specification limits (EFNARC, 2002, European Guidelines, 2005). Figures (10-11) show the percentage increase in the times oT and 5T for the fibrous mixes over those of the plain mixes. This may be attributed to the higher friction between the concrete constituents that is induced by the steel fibres presence.

Figure 9: Variation of the Time Difference (T5-To) with the Steel Fibres

Fresh and Hardened Prop

Properties of Hardened CStrength development inThe strength developmentcubes (without fibres) and

Figure 12: Strength D A regression analysisEquation was obtained:

9.0'

28=

f

f

c

ct

perties of Steel Fibres

Concrete n compression t is investigated in this study, by casting a n

d testing them at different periods as shown in

Development of Normal and Self-Compacted

s was performed for the test results and

39 +× tt

1573

number of SCC n Figure 12.

d Concrete.

the following

(1)


where ctf = the strength at age t, '28cf = the strength at the age of 28 days. The test

results, as shown in the Figure and Table 6 revealed that SCC develops strength at the early ages faster than the normal concrete as shown by the following Equation (ACI Committee 209, 1992):

485.0'

28 +×=

tt

f

f

c

ct (2)

The faster strength development may be attributed to the greater percentage of fines that is present in the SCC, which in turn increase the rate of hydration, the other factor is the presence of the super plasticizer that separates the cement particles and distributes them more uniformly in the concrete mix.

Table 6: Ratios of the Concrete Strength to the Strength at 28 days

Compressive strength Compression strength is one of the important properties of hardened concrete. Table 7, and Figure 13 shows the variation of the cube compressive strength with the steel fibres volume, and Figure 14 show the percentage increase in the cube strength due to the steel fibres addition. Both Figures show that the steel fibres enhanced the compressive strength and changed the brittle and sudden mode of failure of plain concrete to a gradual and ductile one, and retain the integrity of the failed specimens. The strength enhancement depends of course on the mix proportions, fines percentage, and the percentage and fibres dimensions. Table 7 shows that the average ratio of the cylinder to the cube strength is 0.76, while the computed ratio from


Equation (3) for normal concrete is o.84 (Neville, 1996):

6.19

log76.0

'cu

cu

c fff

+= (3)

Table 7: Cylinder and Cube Compressive Strength of the Four SCC Mixes*.

*Average of three specimens

Splitting strength Table 8 and Figure 15 show the cylinders and cubes splitting strength of the four SCC mixes. For all the fibres percentages, the cylinders splitting strength are less than the corresponding values of the cubes by around 16%. This may be attributed to the fact that, in a splitting test a greater percentage of the volume of the cylinder is subjected to tension than the cube as shown by the elastic analysis that was conducted in Reference (AlNeimee, 2008). The following equation was used to estimate the splitting strength of the cube (Krishnaswamy, 1969):


2L

PKfsp = (4)

where P = splitting load, L= cube side length, and K = constant = 0.569 when the indentor is 25 mm wide (Krishnaswamy, 1969). Figure 16 shows the percentage increase in the cylinders and cubes splitting strength, the Figure shows that the percentage increase for the cylinders is about 17 – 30% more than that for the cubes, and the strength enhancement is increasing with the fibres volume.

Table 8: Cylinder and Cube Splitting Strength for the Four SCC mixes.


Conclusion Fibre reinforced self-compacted medium strength concrete can be produced using low percentage of limestone powder. However, the steel fibres have adverse effect on the spreading, passing, and segregation resistance, which can compensated for by adjusting the superplasticizer dosage. The strength development in compression of the SCC mix at the early ages is more than the usual rate of normal concrete. The maximum increase in the compressive strength due to fibres addition was 12.8%, while the maximum increase in the splitting strength was up to 23.9%. The splitting strength for cylinders is more than that for cubes. The mode of failure of the fibrous specimens was changed to a ductile one, and the compressive specimens retained their integrity at the ultimate stage. More test results are required for this type of concrete so that they can be used as guidelines for future mix design and fresh and hardened properties. References

[1] ACI Committee 544.4R-88, [1988], "Design Considerations for Steel Fibre Reinforced Concrete", Re-approved 1999, Manual of Concrete Practice, 18 pp.

[2] ACI Committee 544.1R-96, [1996], "State-of the-Art report on Fibre Reinforced Concrete", Manual of Concrete Practice, 66 pp.

[3] ACI Committee 209R-92, [1992], "Prediction of Creep, Shrinkage, and Temperature Effects in Concrete Structures”, ACI Manual of Concrete Practice, (ACI 209R- 92), (Re approved 1997), 47 pp.

[4] AlNeimee, Z. S., [2008]," Shear Strength of Steel Fibers Reinforced Self Compacted Concrete Beams", M.Sc. Thesis, Mosul University, Iraq,

[5] Eren, O., and Alyousif, A., [2010], "production of Self-Compacting Fibre-Reinforced Concrete in North Cyprus", Design, Production and Placement of Self- Consolidating Concrete, Sixth Inter. RILEM Symposium on Self-


Compacting Concrete (4th North American Conference on the Design and Use of SCC, Montreal (Canada), September 26- 29, pp. 1323- 1331.

[6] Krishnaswamy, K. T., [1969], "Strength of Concrete under Combined Tensile- Compressive Stresses", Materials and Structures, Vol. 2, No. 9, pp. 187-194.

[7] Miao, B., Chern, J. C., and Yang, C. A., [2003], “Influences of Fibre Content on Properties of Self-Compacting Steel Fibre Reinforced Concrete", Jour. of the Chinese Institute of Engineers, Vol. 6, No. 4, pp. 523-530, 2003.

[8] Neville, A. M., [1995], "Properties of Concrete", Fourth Edition, Prentice Hall. [9] Oucief, H., Habita, M. F., and Redjel, B., [2006], "Hybrid Fibre Reinforced

Self - Compacting Concrete: Hardened Properties", Inter. Jour. of Civil Engineering, Vol. 4, No. 2, pp. 77-85.

[10] Ozbay, E., Cassagnebere, F., and Lachemi, M., ”[2010], "Effects of Fibre Types on the Fresh and Rheological Properties of Self-Compacting Concrete", Design, Production and Placement of Self-Consolidating Concrete, Sixth International RILEM Symposium on Self-Compacting Concrete (4th North American Conference on the Design and Use of SCC, Montreal (Canada), September 26-29, pp. 435-443.

[11] Rao, B. K., and Ravindra, V., [2010], "Steel Fibre Reinforced Self-Compacting Concrete Incorporating Class F Fly Ash", International Jour. of Engineering Science and Technology, Vol. 2, No. 9, pp. 4936-4943.

[12] Specification and Guidelines for Self-Compacting Concrete, [2002], EFNARC, Association House, UK, 32 pp.

[13] The European Guidelines for Self-Compacting Concrete: Specification, Production and Use, [2005], 68 pp.

fresh and hardened properties of steel fibres

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