Effect of Surface Roughness on the Steel Fibre Bonding in Ultra High Performance Concrete (UHPC)

T. Stengel1

Abstract. Micro steel fibres are currently added to UHPC to improve ductility. In UHPC under load, the high fibre strength is only utilised partly when the fibres are pulled out of the UHPC matrix. Using steel fibres with a nano- and micro-roughened surface is one measure to enhance the bond between the UHPC matrix and the fibre. In this contribution the effect of surface roughness on the bond behavior of steel fibres is shown. Single fibre pull-out tests were performed to in-vestigate the bond behavior. Aim is to improve the bond behavior so that fibre pull-out takes place under a high load level with respect to the fibre strength.

1 Introduction

Ultra high performance concrete (UHPC) typically has a compressive strength of 200 to 250 MPa. The high compressive strength, the high durability as well as a reduced maximum grain size diameter of about 2 mm opens up a field of new con-struction methods and applications for concrete. However, the extreme brittleness of UHPC, characterized by explosive failure under critical load, restricts the ex-ploitation of the otherwise good properties of this material. Currently, 1.5 to 3.0 % by volume of micro steel fibres 0.15 -0.20 mm in diameter with tensile strength of up to 3.500 MPa are used to improve ductility. Short fibre length is necessary for the typically slender and light UHPC structures. Therefore the micro steel fibres usually have an aspect ratio (Lf / Øf) of about 50 to 100. High costs as well as a considerable contribution to the environmental impact of UHPC are caused by the aforementioned amount of micro steel fibres [1, 2]. It is therefore aim of a re-search project to reduce the amount of micro steel fibres necessary or to use more economic and environmental friendly fibres. The amount of micro steel fibres

T. Stengel Centre for Building Materials (cbm), Technische Universität München (TUM) e-mail: stengel@cbm.bv.tum.de www.cbm.bv.tum.de

372 T. Stengel

could be reduced e.g. by increasing the efficiency of every single fibre. If the fi-bres could be utilized to a higher extent a smaller amount of fibres would lead to the same composite performance. This can be achieved for example with an opti-mized bond between the UHPC matrix and the fibres when the fibres are pulled out under a load near the fibre breaking force. Fibres with a larger diameter of e.g. 1.0 mm, which are much more economic, would lead to unfeasible fibre length of 50 to 100 mm when the same aspect ratio as mentioned before is considered. In that case an optimized bond behavior may lead to aspect ratios lower than 50 pro-viding for a feasible application of these fibres. This contribution deals with the effect of surface roughness on the bond behavior of steel fibres in UHPC. Prelimi-nary single fibre pull-out tests using steel fibres 0.15 and 0.30 mm in diameter with a roughened surface have shown that the bond between the UHPC matrix and the fibre could be enhanced significantly [3]. Following that in this study three dif-ferent surface roughnesses and two different UHPC compositions were investi-gated together with steel fibres 0.98 mm in diameter.

2 Investigations

The fibre surfaces were mechanically treated by abrasive papers with three differ-ent roughnesses (grit 180, 240, 400). The surface roughening was done rubbing a 2 cm piece of abrasive paper for ten times along the fibre axis. Surface roughness parameters of the fibre surfaces were determined according to ISO 4287 with a confocal LED microscope using a fifty fold objective (vertical accuracy 2 nm). The compressive and splitting tensile strength of the plain UHPC was tested on cylinders with a diameter of 50 mm and an aspect ratio of 1 and 2 respectively. The load controlled tests were done at a rate of 0.5 MPa•s in the case of compres in the case of compres-sive strength and 0.5 kN/s in the case of splitting tensile strength. The pull-out tests were performed according to [3] but using a laser sensor (accuracy: 0.001 mm) for the displacement measurement. An embedded length of 7 mm was chosen for all pull-out specimens. Plain fibres as well as roughened fibres (grit 180 and 400) together with two different UHPC compositions were tested. From the pull-out curves the bond strength (assuming constant bond stress over the whole embedded length) as well as the bond energy (integral of load slip curve be-tween a slip of 0 and 0.5 mm) was derived. The steel fibres used in this study had a diameter of 0.98 mm and a tensile strength of 2,600 MPa. All single fibres were alcohol cleaned when fixed in the moulds. Two different UHPC compositions – with and without PVA fibres – were used, Table 1. The PVA fibres (tensile strength appr. 780 MPa) had a diameter of about 15 μm and a length of 12 mm. For both mixtures a well-cement strength class 42.5 and a water cement ratio of 0.28 was chosen. The mixing was done according to [3] but applying a vacuum (50 mbar) during the last 60 s. Within 60 s after the end of mixing, the workability was tested with a mortar cone (Hcone = 50 mm, Øupside = 70 mm, Øbottom = 100 mm) on a glas plate. The workability was 32 cm and 26 cm for C4 and C5 respectively. Specimens were casted without any compaction.

Effect of Surface Roughness on the Steel Fibre Bonding in UHPC 373

Table 1 Composition of the two UHPC mixtures [kg/m³]

Concrete Cement Silica Fume

Quartz Powder

Quartz Sand

Water Super- plast.

PVA-Fibres

C4 680 138 360 990 166 35 0

C5 674 137 352 975 158 54 3

3 Results

Figure 1a to Figure 1d shows parts of the confocal microscope pictures (each appr. 305 × 229 μm²) of the different fibre surfaces. The height scale lasts from 0 (white colour) to 10 μm (black colour). It can be seen that with increasing grit number the depth and the distance of the scratches is decreasing while the number of scratches is increasing.

a b

c d

Fig. 1 Confocal microscope pictures of fibre surfaces: a: plain surface, b: grit 180 rough-ened, c: grit 280 roughened, d: grit 400 roughened

The roughness parameters parallel and perpendicular to the fibre axis (meas-ured length: 250 μm) of the four fibre surfaces are given in Table 2. Only small differences in surface roughness were found parallel to the fibre axis while the

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Table 2 Roughness parameters of the different fibre surfaces parallel (║) and perpendicular (┴) to the fibre axis

Roughness parameter

Plain Surface

Grit 180 Roughened

Grit 280 Roughened

Grit 400 Roughened

Rq ║ [μm] 0.034 0.051 0.053 0.037

Rq ┴ [μm] 0.293 0.879 0.703 0.516

Rt ║ [μm] 0.224 0.379 0.351 0.326

Rt ┴ [μm] 3.241 5.556 4.793 3.254

Fig. 2 Effect of abrasive paper grit number on roughness parameters of steel fibre surfaces

roughness perpendicular to the fibre axis increased with decreasing grit number linear dependent, Figure 2. The root mean square roughness was between 0.293 μm for plain and 0.879 μm for grit 180 roughened fibre surface.

The mean compressive and splitting tensile strengths of the two concretes are shown in Table 3. Concrete C4 had a 9% higher compressive strength than con-crete C5; the splitting tensile strength was even 58% higher. This may be due to the diverse workability which causes different deariation behavior. Further inves-tigations will be done on that. The coefficient of variation was in every case less than 7% (three specimen tested).

Table 3 Mean Compressive and Splitting Tensile Strength of the UHPCs [MPa]

Concrete Compressive Strength

Standard Deviation

Splitting Tensile Strength

Standard Deviation

C4 235 3 15.8 1.1

C5 216 9 10.0 0.7

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Table 4 shows the derived mean bond properties for the two concretes and the different fibres.

Table 4 Mean bond properties evaluated from single fibre pull-out tests

Concrete / Fibre

Bond Strength [MPa]

Bond Energy [10-3 J]

C4 / plain 5.83 31.9

C4 / 180 9.10 78.7

C4 / 400 3.69 31.1

C5 / plain 4.27 39.9

C5 / 180 11.5 93.3

C5 / 400 5.94 45.2

For both concretes the grit 180 roughened fibres showed the highest bond strength as well as the highest bond energy. This is obviously due to the surface roughness which was the highest for the grit 180 roughened fibre surfaces. Despite the lower compressive and splitting tensile strength the addition of PVA fibres seems to affect the bond behavior of steel fibres beneficially when the fibre sur-face is rough. Using PVA fibres the bond behavior was enhanced significantly for both surface roughnesses.

4 Discussion

The surfaces of steel fibres were roughened with abrasive paper of different grits (180, 280, 400) along the fibre axis. The fibres roughened with the lowest grit showed the best bond properties. This is due to the highest surface roughness which was obtained with the lowest grit number. The grit 400 roughened fibres had a similar bond behavior as the plain fibres when using UHPC without PVA

Fig. 3 Reinforcing effect of PVA fibres within the UHPC matrix surrounding a steel fibre

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fibres. An addition of PVA fibres affected the bond behavior of all roughened fibres beneficially. The higher pull-out load in the case of roughened fibres may result in a higher volume of UHPC matrix involved in the stress transfer, Figure 3.

When using surface roughened fibres the stress transfer volume has an exten-sion beyond the mean roughness and lasts into the bulk matrix where the micro re-inforcing PVA fibres can be found, Figure 3. In that case the PVA fibres can bridge micro cracks and therefore strengthen the matrix volume involved in stress transfer. This affects the pull-out behavior in a positive way.

5 Conclusions

Surface roughening done with abrasive paper affects the bond behavior of steel fi-bres in UHPC significantly. The lower the grit number of the abrasive paper the higher is the roughness of the steel fibre surface. Surface roughness depends on the grit number in a linear way. Due to the higher surface roughness steel fibres treated with grit 180 paper showed the best bond behavior in this study. Bond properties were enhanced by roughly 200% compared to plain fibres. Even better bond behavior of the roughened steel fibres is achieved when PVA fibres are added to the UHPC matrix. Bond properties were enhanced by another 20% com-pared to the plain UHPC without PVA fibres. This may be because of a micro re-inforcing effect of the PVA fibres to the UHPC matrix surrounding the steel fibre. Due to higher pull-out loads when using roughened steel fibres the stress transfer volume around the steel fibre is significantly larger than in the case of plain steel fibres. Therefore PVA fibres can bridge microcracks and strengthen the matrix within the stress transfer volume. To verify this, further investigations on the ef-fect of PVA fibres like SEM analyses of the interfacial zone will be done.

Acknowledgments. The author would like to thank the German government for the finan-cial support within the NanoTecture research programme and the FRT GmbH for the surface measurements.

References

1. Stengel, T., Schiessl, P.: Sustainable Construction with UHPC – from Life Cycle Inven-tory Data Collection to Environmental Impact Assessment. In: Fehling, E., Schmidt, M., Stuerwald, S. (eds.) Proc. of the 2nd Int. Symposium on UHPC. Kassel University Press, Kassel (2008)

2. Stengel, T., Schiessl, P.: Life Cycle Assessment of UHPC Bridge Constructions: Sher-brooke Footbridge, Kassel Gärtnerplatz Footbridge and Wapello Road Bridge. In: Ko-tynia, R., Gawin, D. (eds.) Proc. of the 6th Int. Conference Analytical Models and New Concepts in Concrete and Masonry Structures AMCM 2008. University of Lodz, Lodz (2008)

3. Stengel, T.: Optimisation of Steel Fibre Bonding in Ultra High Performance Concrete. In: Gettu, R. (ed.) Proc. of the 7th Int. RILEM Symposium on Fibre Reinforced Con-crete: Design and Application BEFIB 2008. RILEM Publications S.A.R.L., Bagneux (2008)