Lab comparison of steel vs synthetic FRC segments Mar 2015

Structural design of fibre reinforced concrete (FRC) elements is based on the post-cracking residual strength provided by fibre reinforcement. For structural use, a minimum mechanical performance of FRC must be guaranteed.

Fibres can be used to improve the behaviour at SLS (service limit state) since they can reduce crack spacing and crack width, thereby improving durability. Fibres can also be used to improve the behaviour at ULS (ultimate limit state) where they can partially or totally substitute conventional reinforcement.

The mechanical properties of a cementations matrix are modified when fibres are added. However, elastic properties and compressive strength are not significantly affected by fibres, unless a high percentage of fibres are used.

Fig 2. Flexural test according to European code EN 14651

Fig 3. Bending test: typical nominal stress - crack opening

Steel fibre concrete has proven over the years to be a reliable construction material for precast segmental linings with many reference applications worldwide. After 30 years of experience, design guidelines for FRC using steel fibre are well validated and as described and explained in the Model Code 2010 edited by fib, the fédération internationale du béton or International Federation for Structural Concrete.

New fibre concretes, such as macro-synthetic fibre concrete, are not yet fully understood, but gain attention for some applications including spray concrete, or shotcrete, in the mining industry. Creep data, shear resistance, crack control, durability, design methods, references etc are lacking at the moment for macro-synthetic fibre concrete to be used for precast segments, but will evolve with experience.

In a comparison test, conducted by the Material and Structural Laboratory of the Civil Engineering Department of the University of Rome Tor Vergata. results were obtained for precast segments produced with DRAMIX® steel fibre reinforced concrete and with concrete reinforced with Barchip macro-synthetic fibre.

The segments were produced with 40kg/m³ of DRAMIX® steel fibre and 8kg/m³ of Barchip macro-synthetic fibre. The segments were cast using segment moulds typically used in water tunnels.

The laboratory tests were led by Professor Alberto Meda and Professor Zila Rinaldi.

Two different kinds of tests were performed. First a test simulating the point load effects on the segments produced by a TBM machine during the excavation phase, and secondly flexural tests simulating the behaviour of the segments when loaded under bending.

The tests have been carried out on 250mm thick precast segments with a length of about 1,850mm and a width of about 1,200mm (Fig 1). Reinforcement was by fibres alone with no traditional steel reinforcement rebar included.

Fig 4. Bending test comparison between DRAMIX® steel fibre and Barchip synthetic fibre segments

Point load and flexural tests

The point load tests were performed by applying two point loads at the segment to simulate the same hydraulic thrust ram shoes of a TBM.

A uniform support is considered, as the segment is placed on a stiff beam suitably designed. Two 2,000kN rackets were used for every steel plate for testing DRAMIX® steel fibre whereas two 4,000kN rackets were used for every steel plate for testing Barchip macro-synthetic fibre. The dimension of the loading shoes is the same for both the tests.

The load was continuously measured by pressure transducers. Four wire transducers, two located at the intrados and two at the extrados, to measure the shoes displacement, while one LVDT transducers is applied between the load shoes, to measure the crack openings (Fig 5).

Bending test

The tests were performed with the loading, in displacement control, by adopting a 1,000kN electromechanical jacket, with a PID control and by imposing a stroke speed of 10 µm/sec.

During the test, the following measures were continuously registered:

• The load F, measured by means of a 1,000kN load cell with a precision of 0.2%;
• The midspan displacement measured by means of two potentiometer wire transducers placed along the transverse line; and
• The crack opening at midspan, measured by means of two LVDTs.

• 

  Fig 5. Compression test set up

• 

  Fig 6. Bending test set-up with load distribution system

Furthermore, the crack pattern was recorded at different steps, with the help of a grid plotted on the intrados surface (100 x 100mm).

The segments were placed on cylindrical supports with a span of 1,200mm and the load, applied at midspan, was transversally distributed be adopting a steel beam (Fig 6).

Results reveal that the crack pattern at 2,000kN load is similar with a crack zone between the steel shoes and a crack in the middle of the segment.

Test results also illustrate that the crack pattern of the segment reinforced with DRAMIX® steel fibre is more limited with crack lengths and crack openings smaller.

The maximum crack opening for DRAMIX® RC80/60BM steel fibre reinforced segments is lower than 0.05mm whereas it is 0.4mm for the macro-synthetic fibre reinforced segments.

For bending test comparisons, two segments were tested for DRAMIX® steel fibre and one segment for Barchip macro-synthetic fibre (Fig 8).

It is worth remarking that the test was carried out in displacement control, with an electromechanical jack, up to the collapse. The cracking phase is highlighted during the test.

Table 3. Comparison of the point load test results
	Dramix® Test 1	Dramix® Test 2	Macro- synthetic
First crack load	130 kN	150 kN	122 kN
Maximum load	160 kN	200 kN	122 kN
Crack pattern	Multi cracks	Multi cracks	One crack

Fig 9. Comparison of the load versus displacement curves

In the comparison of the load versus displacement curves (Fig 9), it can be noticed that reaching the load level of 120kN or the first cracking level, the macro-synthetic fibre segment exhibited a softening behaviour. By contrast, the DRAMIX® steel fibre reinforced segment exhibited a hardening behaviour with increasing load and up to a maximum load level equal to 200kN or 65% more than for the macro-synthetic fibre reinforced segment.

Fibre specification recommendations for precast segment

Fibre should comply with the new ISO 13270 standard for steel fibres for concrete. This is important for following reasons:

• It is a reference document for steel fibres for concrete
• In countries where EN 14889-1 is not applicable, or in countries where no other national standard on fibres is issued, this standard can be adopted by the specifier
• National standard commissions can use the ISO 13270 standard as a blue print for the next national standard.

• Air content is introduced in the ISO 13270 standard and should be measured for all fibres (not only glued fibres) in the reference concrete, and should be mentioned on the label. The air content with fibres may not differ more than 2% versus plain concrete.
• When using fibres with a zinc or zinc/aluminium coating, an inhibitor is recommended.
• Two classes are prescribed for tolerances on diameter and length. One is more relaxed and the same as EN 14889-1 standard, and the other is more stringent. For structural precast segments, the standard product quality should conform to the more stringent category Class A.

For cold-drawn wire steel fibres (Group 1 ISO 13270), a minimum tensile strength of wire must be consistent with that of the matrix and recommended at fy ≥1.5 GPa when using a concrete mix > C40/50. The steel fibres should have a:

• Rating according to the CE label system 1, and
• Dimensional tolerances as per Table 2 of ISO 13270.

It is essential to stress the importance in specifying that the tolerances of the fibres must be according to Class A for structural reinforcement in elements such as precast segments. A maximum network effect (m/m^³) should be sought to ensure that the selected mode of use includes checking the proper orientation of fibres with regard to mechanical behaviour using l/D ≥ 65. l/D equal to 80 is recommended to optimise the anchor system, with the hooks at each end anchoring the fibre in the concrete matrix.

Fibre conditioning must take account of:

• Introduction of the fibres using an automatic dosing system which are validated for glued fibre by the dosing equipment producer
• Arraying the fibres in the concrete to obtain a uniform distribution
• Complete elimination of the appearance of fibre balls for fibres with l/D >65
• The use of bonded fibres are recommended to fulfil the above requirements

Minimum recommended dosage for precast segments using SFRC alone is 30kg/m³. Nevertheless, in order to obtain the desired results, accurate study of the material, including the fibre typology, should be carried out to develop suitable dosages to a peculiar concrete matrix.