Re-using Steel Wire from Tyres for Fibre Reinforced Concrete

Transcription

1 Re-using Steel Wire from Tyres for Fibre Reinforced Concrete eter Waldron, anos apastergiou and Kelvin Graham Emeritus rofessor, University of Sheffield, UK Engineer, Twincon Ltd, Sheffield, UK Design Manager, Twintec Australia ty Ltd, Silverwater, NSW, Australia Abstract: Steel fibres used to reinforce concrete are normally manufactured from new high tensile steel wire. However, steel of a much higher quality is used in the manufacture of car and truck tyres and a significant volume of this material is recovered each year when end-of-life tyres are recycled. Tyre wire is often heavily contaminated with rubber and is usually either landfilled or, if sufficiently clean, sent to the steel mills for re-melting. rocesses have now been developed to further clean and screen the recovered steel to enable it to be used as reinforcement in steel fibre reinforced concrete (SFRC). Whilst tyre steel tends to be of smaller diameter and length than the steel fibres traditionally used in SFRC, research has identified the range of fibre geometries that are effective. Research has also demonstrated that there are benefits to be had from using hybrid blends of standard manufactured fibres and re-used tyre fibres, with the different fibre types and geometries each contributing in different ways to the overall performance of the composite material. In conducting this research problems have been encountered with the manufacture of specimens for the internationally recognised prism test which disadvantage fibres of this type and recommendations are made that address this issue. Tyre steel offers a sustainable option for reinforcing concrete and this paper will demonstrate some significant structural, environmental and economic benefits of using this newly available material. Keywords: fibre reinforced concrete; sustainable construction; re-used tyre steel. Introduction First introduced in the 9 s SFRC continues to gain market share particularly for applications such as industrial ground slabs, tunnel linings (sprayed or precast) and structural elements with complex geometries difficult to reinforce with conventional rebar. Although still viewed as a niche market, SFRC currently consumes an estimated, tonnes of steel fibre per annum so must now be considered a main stream structural material. Virtually all of the structural steel fibre currently available is sourced from cold-drawn high tensile steel wire cut to length and deformed to provide adequate distributed bond and end anchorage in the concrete matrix, Figure. The new steel consumed for this purpose requires a staggering million GJ of energy for its manufacture releasing about million tonnes of CO in to the atmosphere every year (). Figure. Typical geometries of fibres manufactured from cold-drawn high tensile steel wire. Steel of a much higher quality (with a typical tensile strength of, Ma) is used in vast quantities to manufacture the billion car and truck tyres that are consumed each year around the world. After a few years of use these tyres will become redundant resulting in a major environmental issue regarding their safe disposal. In most developed countries landfilling of tyres is now prohibited and the two main routes for disposal are incineration (energy recovery or as a fuel in cement kilns) and mechanical shredding. In the former process the steel is lost and simply becomes a trace element in the cement or furnace bottom ash. In the latter process, the principal focus is on the recovery of the rubber which, in crumb form, has some value and can be re-used in the construction of recreational sports surfaces and in other manufactured products. The tyre steel recovered as a bi-product from mechanical shredding is usually heavily contaminated (Figure ) and is often landfilled (if permitted) or further processed to reduce the rubber contamination to <% for it to be accepted by the steel mills for remelting.

2 The total mass of tyres scrapped worldwide has been estimated to be about million tonnes per annum () of which about one-third are recovered by mechanical shredding. With a steel content of about -% for car tyres and -% for truck tyres, this suggests about, tonnes of tyre steel is recovered each year, a figure not dissimilar to the total current demand for steel fibre for SFRC applications. However, the recovered steel in its current form (Figure ) is clearly unsuitable for immediate use as concrete reinforcement, and considerable research has been conducted at the University of Sheffield over the past years to remove the residual rubber contamination, identify the effective fibre lengths and establish their structural performance when used in SFRC. This research has resulted in a process which allows about % of the steel from mechanical shredding to be recovered in a form which makes it suitable for use in SFRC applications (Figure ). Figure. Heavily contaminated tyre steel. Figure. rocessed tyre steel suitable for SFRC applications. Initial estimates indicate that the industrial process for producing Re-used Tyre Steel Fibre (RTSF) only uses about -% of the energy required for the manufacture of existing fibre products from new steel. In addition to the significant environmental benefits of using RTSF, the manufactured cost is also less than for existing manufactured fibre which will further encourage the growth of SFRC by providing an economic and environmentally sustainable alternative to standard reinforced concrete in an increasing range of applications.. Re-used Tyre Steel Fibre. Fibre length distribution The flexible steel cord used to reinforce tyres is in the form of a cable of about. -.mm diameter comprising a number of smaller filaments each about. -.mm diameter twisted together. During the mechanical shredding process the cord is cut in to random lengths and invariably unravels to leave a twisted, tangled mass of individual filaments. One of the key findings from the early research was that only a range of filament lengths is suitable for use as concrete reinforcement: too short and the fibre easily de-bonds and has limited structural value; too long and the fibres tend to ball together during mixing with the fresh concrete and are not uniformly dispersed throughout the mix as required (, ). For the filament diameters typically found in tyre steel the preferred range of filament length was established from extensive testing to be about -mm. Unlike manufactured fibres where the length can be maintained within a tight tolerance, RTSF contains fibres of many different lengths and it is important for this distribution of lengths to be established in order to confirm the suitability of the fibres for structural use. To this end a technique has been established whereby a small representative sample of RTSF is spread out on a light box and photographed (Figure ). The equivalent straight length of each fibre is then obtained automatically by software developed in house enabling a length distribution histogram to

3 8 8 8 Frequency (mass %) be produced (Figure ). As can be seen, fibre lengths vary considerably in this typical sample from - mm and the production challenge is therefore to maximise the proportion within the preferred - mm range. Although ideally all fibres should be >mm this is simply not achievable in practice and a realistically achievable figure of % >mm has been adopted as a minimum value for RTSF used in all research and commercial applications to date. 8% 7% % % % % % % % Frequency Cumulative % % 9% 8% 7% % % % % % % % Fibre length (mm) Figure. Fibre images. Figure. Histogram showing fibre length distribution.. RTSF integration within the concrete Although not mechanically deformed during production, RTSF tends to be randomly twisted and bent as a result of the shredding process. Whilst this helps provide mechanical anchorage to the concrete it can also result in the fibres becoming tangled during boxing, transportation and storage. Once removed from their packaging the fibres tend to have taken the form of a loosely formed briquette (Figure ) which needs to be broken up before placement in the concrete to ensure uniform dispersal. This has not proved to be a problem in practice and proprietary integration machines currently used by SFRC contractors for blowing fibres into ready-mix concrete trucks have been found to be effective without the need for modification. The briquettes are simply broken down by tumbling action within the machine s rotating drum allowing the fibres to be integrated uniformly by high pressure air into the fresh concrete (Figure 7). Figure. RTSF in briquette form. Figure 7. roprietary integration machine for blowing RTSF into ready-mix concrete.

4 . RTSF performance Research conducted by the University of Sheffield has considered all aspects of RTSF performance including bond, shear and flexural strength, durability, freeze-thaw and fatigue (-9). Much of this work was undertaken as part of a major international study investigating the use of RTSF-reinforced Roller Compacted Concrete (RCC) for constructing concrete roads (-). The ECOLANES project brought ten industrial and academic partners together and culminated in four highly successful fullscale concrete road construction trials subjected to very different levels of vehicular loading in countries with diverse climatic conditions.. Test procedure Only tensile strength testing will be presented in detail here from which extensive results have been obtained for mixes with % RTSF and many different hybrid blends of RTSF and manufactured fibres. The simple prism test adopted here was first introduced in Japan (), amended over the years by RILEM (), and has now been widely adopted as an international standard () and in recognized design guides (). Test specimens comprising a number of nominally identical notched SFRC prisms mm square in section are loaded in flexure over a simply supported span. The notch is formed at mid-span with a saw cut to a depth of mm and load to failure is recorded continuously together with central deflection and crack mouth opening displacement (CMOD) (Figure 8). Because the structural performance of SFRC is inherently variable, relatively large numbers of identical tests are required to provide statistically meaningful results.. Specimen preparation Figure 8. Experimental set-up for the standard prism test. For the test series reported here, a standard C concrete mix was employed supplied by a local ready-mix concrete supplier. The constituent materials and mix design are given in Table. Slump was measured on arrival and additional water and the specified super-plasticiser were added to achieve the required slump of mm. Small batches of the concrete were weighed and loaded into a smaller laboratory mixer and re-mixed with measured quantities of the RTSF and manufactured fibres to enable several different fibre fractions and blends of fibres to be obtained from the same concrete. The re-mixed batches of concrete now containing fibre were then poured into a range of standard prism and cube moulds. All specimens were vibrated on a vibrating table for the same interval, stored for hours under damp hessian and plastic sheeting before being stripped and stored at % relative humidity for 8 days prior to testing. Although the prism test is widely used internationally to confirm SFRC structural performance, the use of individual prism moulds (as recommended) for specimen preparation has proven to be problematic. revious research has shown that the side walls of the prism moulds tend to re-orientate the fibres during placement and vibration of the SFRC towards the axial direction thereby enhancing the flexural strength. This effect is particularly significant for long, stiff fibres but is generally neglected since most manufactured fibres used in practice have a similar aspect ratio and benefit from this effect to a similar

5 degree. However, this re-orientation effect places the relatively short, flexible RTSF fibres at a significant disadvantage in a direct comparison of flexural performance with the longer manufactured fibres. For this reason an additional mm deep x 7mm x mm rectangular slab was cast for all three fibre mixes from which, after curing, six additional prismatic specimens were obtained by slicing with a diamond saw, as shown in Figure 9. By cutting the notch on the side face of each sawn specimen and rotating the prism to make this the bottom face, the effect of fibre re-orientation on the important bottom face is largely eliminated. Table. Constituent materials and concrete mix design. Material Type kg/m Cement Cem I CIIIA+SR Replacement GGBS Fine aggregates Sand 7 Coarse aggregates Oolitic limestone (<mm) 8 Water Mains 9 Admixture lasticiser. W/C Ratio -. Figure 9. Sliced slab specimen to reduce the fibre re-orientation effect caused by the individual mould side wall boundaries. Three different fibre blends are reported here comprising different ratios of RTSF and a standard sinusoidally deformed steel fibre of.8mm dia. and mm length denoted AFT 8/, as detailed in Tables and. Series E, H and N specimens were cast in individual moulds whereas the specimens for the corresponding series E s, H s and N s were obtained from the sawn slabs. Series comprises plain, unreinforced concrete specimens as a control on tensile strength at first cracking. Table. Steel fibre geometries and volume fractions used in the test series. Mix risms Cubes Industrial fibres Dosage Re-used Dosage kg/m fibres kg/m E AFT.8/ - - E s - AFT.8/ - - H AFT.8/ RTSF H s - AFT.8/ RTSF N - - RTSF N s RTSF

6 Bending stress (Ma) Bending stress (Ma). Test Results Table. Steel fibre physical properties. RTSF AFT.8/ Unit Tensile strength 8 Ma Diameter. ±..8 ±. mm Length ± ± mm Typical aspect ratio Flexural test results of all six prisms within Series E s are plotted in Figure (a) as an example of the significant variation in performance typically observed when testing SFRC. It is sometimes difficult to comprehend how specimens manufactured from the same concrete and tested under identical conditions can display such marked variation. However, this is a reminder that SFRC is an inherently variable material and its measured performance in this simple flexural test depends crucially on the location and orientation of just a few large fibres bridging the single macro-crack formed at the predefined saw cut position. Interestingly, the same level of variation is not observed in the results from Series N s (Figure (b)) which contains % RTSF. Although the concrete mixes used in Series E s and N s both have the same volume fraction of fibre, a single industrial fibre has the same mass as about - RTS fibres. rovided the RTSF are well dispersed throughout the concrete matrix, the much greater number of individual fibres in the Series N s specimens clearly reduces dependency on fibre orientation and location at the induced crack. Since consistency in the structural properties of SFRC is of crucial importance in industrial applications, this reduction in variability is perhaps sufficient justification for replacing at least some of the industrial fibres with RTSF. Es: (Average): AFT.8/ () AFT.8/ () Ns: (Average): RTSF () RTSF () (a) Series E s kg/m AFT.8/ fibres (b) Series N s kg/m RTSF Figure. Variation in mid-span deflection vs bending stress results for individual specimens. Given the significant variation in individual test results observed in Figure (a) the different mixes can only be meaningfully compared once the results of all nominally identical specimens from the same series have been combined in to a single plot of average values. Average plots of all six specimens in Series E, H, N and (all based on individually moulded specimens) are shown in Figure. In comparing the performance of different fibre types and fibre fractions, two key features need to be examined. The first is the area under the experimental curve since this is a measure of toughness or rather the energy absorbed by the fibres as the cracks open and extend; and the second is the performance at first cracking which provides an indication of how the tensile stresses in the concrete are transferred to the fibres through the mechanisms of bond and mechanical anchorage as the microcracks first develop under increasing load. In terms of energy absorption, it is apparent from Figure (a) that the average performance of the prisms reinforced with kg/m AFT.8/ industrial fibre (Series E) displays significant strain hardening after first cracking which results in an increase of about % in the peak bending strength and increases the area under the curve considerably. This effect is much less marked for either the + kg/m blend (Series H) or the % RTSF mix (Series N).

7 Bending stress (Ma) Bending stress (Ma) Bending stress (Ma) Bending stress (Ma) As regards the performance at first cracking, the close-up shown in Figure (b) for the first.mm of mid-span deflection shows very similar behaviour for all three reinforcement arrangements with the % RTSF reinforced concrete performing similarly or slightly better at first cracking than those containing manufactured fibre. This supports the observation that the inclusion of a large number of shorter, finer RTS fibres may have a beneficial role in delaying and controlling crack initiation. E: AFT.8/ () H:.8/ () + RTSF () N: RTSF () : lain E: AFT.8/ () H:.8/ () + RTSF () N: RTSF () : lain E E H N N H (a) Full test range (b) Close-up of first.mm deflection Figure. Average mid-span deflection vs bending stress for individually cast specimens. The results for Series E s, H s and N s, in which all specimens were obtained by slicing from a single slab, are shown in Figure. These slabs were cast from the same concrete mixes as used for the individually cast prisms corresponding to Series E, H and N. All specimens were cast at the same time, with similar levels of vibration and identical curing conditions leaving the method of specimen manufacture as the only variable. In Figure (a) the wide variation in average performance noted in Figure (a) for the different fibre blends has largely disappeared and the bending stress vs. mid-span deflection plots are all now very similar. The very significant strain hardening effect observed in Figure (a) for the specimens reinforced with industrial fibres is no longer apparent and all specimens now display strain softening as mid-span deflection increases. Indeed, Figure (a) now shows a significant drop off in load carrying capacity for those specimens from Series E s and H s immediately after first cracking which does not occur for those specimens in Series N s with % RTSF. Once again, the large number of RTS fibres well dispersed throughout the concrete matrix seems to be having a positive effect in controlling the transfer of tensile stress from the concrete to the fibres as the initial micro-cracks develop. This observation is confirmed in the close-up of the initial crack development stage of the test presented in Figure (b). Ns (a) Full test range Es: AFT.8/ () Hs: AFT.8/ () + RTSF () Ns: RTSF () : lain Hs Es (b) Close-up of first.mm deflection Figure. Average mid-span deflection vs bending stress for sawn prism specimens. The results from Figures (a) and (a) for all three test series for both the individually cast specimens and sawn specimens are superimposed in Figure. From this, it is clear that the method Hs Es: AFT.8/ () Hs: AFT.8/ () + RTSF () Ns: RTSF () : lain Ns

8 Bending stress (Ma) of specimen production (individually cast or sawn) has no apparent influence on the performance of the RTSF reinforced specimens (Series N and N s ). Indeed, the only two anomalous results are those for Series E and H containing the industrial fibres in individually cast specimens. E: AFT.8/ () Es: AFT.8/ () H:.8/ () + RTSF () Hs: AFT.8/ () + RTSF () N: RTSF () Ns: RTSF () : lain E H Hs Es N Ns Figure. Average mid-span deflection vs bending stress for all series. These results provide clear evidence that the recommended method of specimen manufacture for the internationally accepted prism test is flawed. For concretes reinforced with long, stiff industrial fibres the structural performance indicated by this test methodology is not representative of that attainable in normal industrial applications and could give designers an unrealistic expectation of material performance. The move towards the adoption of sawn specimens sliced from a single slab of representative SFRC material is recommended to be adopted by the international research community.. Conclusions Re-used Tyre Steel Fibre, suitably cleaned and selected for length, is an effective alternative to normal steel fibre manufactured from new cold-drawn steel wire. Since each industrial fibre is replaced by about - shorter, finer fibres, RTS fibres are likely to be better dispersed in the concrete matrix and seem to be particularly effective in delaying and controlling crack initiation. Once macro-cracks have formed, the relatively short length of RTSF makes it marginally less effective than standard industrial fibre. These observations suggest that a blend of RTSF and industrial fibre may provide the best of both worlds: delayed crack initiation leading to a larger number of narrower micro-cracks together with effective control of macro-cracks, once established, by the larger industrial fibres. The internationally accepted prism test used here is simple and cost effective although, due to the inherent variability associated with SFRC, requires many nominally identical specimens to be tested in order to provide statistically meaningful results. The recommended method of specimen manufacture in individual moulds is, however, flawed as the side walls have the effect of re-orientating the fibres towards the axial direction thereby enhancing the measured flexural strength. This is particularly the case for long, stiff fibres typical of many industrially produced fibres used in practice where an apparent % increase in peak bending strength has been observed. This appears to be due entirely to fibre re-orientation at the boundary walls of the moulds used in the manufacture of the test specimens. These boundaries do not occur in practical ground slab applications and may lead to overestimates of material capacity. The alternative method of specimen preparation suggested here of slicing the prisms from a single cast slab is no more expensive, provides prisms of equal accuracy, and substantially eliminates this re-orientation effect.

9 The volume of tyre steel currently generated from end-of-life tyres each year broadly matches the volume of cold-drawn steel wire used in SFRC. The wide-spread introduction of RTSF in fibre reinforced concrete construction would provide economic benefits as a cheaper, more sustainable alternative to industrially produced fibre as well as making a significant impact in reducing the million tonnes of CO currently generated each year in steel fibre production.. Acknowledgements The authors are pleased to acknowledge the financial and technical support of Twintec (UK) Ltd and the on-going support of academic and technical staff from the Department of Civil and Structural Engineering, University of Sheffield, UK.. References. World Steel Association, Sustainable steel - olicy and indicators, WSA, p.. The European Tyre Recycling Association, Introduction to tyre recycling, ETRA,, Belgium, p.. Tlemat, H., ilakoutas, K. et al., ull-out behaviour of steel fibres recycled from used tyres, roceedings of International Symposia on Celebrating Concrete: eople and ractice (in Role of Concrete in Sustainable Development),, Dundee.. ilakoutas K., Neocleous K. et al., Reuse of Steel Fibres as Concrete Reinforcement, roceedings of the Institution of Civil Engineers, Engineering Sustainability, 7 (ES), Sept, pp -8.. Tlemat, H., ilakoutas, K. et al., Stress-Strain Characteristics of SSFRC Using Recycled Fibres, Materials and Structures, (RILEM) Journal, Vol. 9 (),, pp -.. Graeff, A., ilakoutas, K. et al., Analysis of the Durability of SSFRC against Corrosion of Fibres and Freeze-thaw Cycles, th Annual International fib Symposium - Concrete: st Century Superhero: Building a Sustainable Future, London, UK, June 9, pp Graeff, A., ilakoutas, K. et al., Corrosion durability of recycled steel fibre reinforced concrete, Concrete Communication Conference Liverpool UK, - September 8, pp Graeff, A, ilakoutas, K et al. Fatigue resistance and cracking mechanism of concrete pavements reinforced with recycled steel fibres recovered from post-consumer tyres, Engineering Structures, Vol., December, pp Neocleous, K, ilakoutas, K. et al Design Issues of Concrete Reinforced with Steel Fibres Recovered from Tyres, ASCE Journal of Materials in Civil Engineering, Vol. 8 (),, pp European Commission, ECOLANES - Economical and sustainable pavement infrastructure for surface transport, F STRE, -, Neocleous, K., Angelakopoulos, C. et al., Fibre-reinforced roller-compacted concrete transport pavements, roceedings of the Institution of Civil Engineers, Transport, Volume Issue TR,, pp Achilleos, C, Hadjimitsis, D, et al, "roportioning of Steel Fibre Reinforced Concrete Mixes for avement Construction and Their Impact on Environment and Cost" Sustainability, Vol., No. 7: pp Japanese Society of Civil Engineers, Methods of Tests for Flexural Strength and Flexural Toughness of Steel Fibre Reinforced Concrete, Concrete Library of JSCE, SF, No. 98, pp 8-.. RILEM TC -TDF, Test and Design Methods for Steel Fibre Reinforced Concrete: Bending test - Final Recommendation, Journal of Materials and Structures, vol.,, pp British Standards Institution, Test method for metallic fibre concrete Measuring the flexural tensile strength, BS EN, July, London, p.. Concrete Society, Concrete industrial ground floors - A guide to design and construction, Technical Report, Third edition,, Berkshire, UK, 7p.