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0Tunnelling and Underground Space Technology
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Tunnelling and Underground Space Technology
journal homepage: www.elsevier.com/locate/tust
Age-dependent changes in post-crack performance of fibre reinforced shotcrete linings
CrossMark
Erik Stefan Bernard *
TSE Pty Ltd, PO Box 763, Penrith, NSW 2750, Australia
ARTICLE INFO
Article history:
Received 30 December 2014
Received in revised form 12 April 2015
Accepted 8 May 2015
Available online 22 May 2015
Keywords:
Post-crack performance Fibre reinforced shotcrete Macro-synthetic fibres Steel fibres
Time-dependent behaviour
ABSTRACT
It is commonly assumed that when a mix achieves satisfactory performance in Quality Control tests at 28 days this result will translate into satisfactory performance throughout the design life of the corresponding concrete structure. While this is generally true of the compressive strength of concrete it is not necessarily true for other parameters. The post-crack performance of fibre reinforced concrete (FRC) differs from that of conventionally reinforced concrete in that the post-crack performance of fibres is related in a complex manner to the characteristics of the concrete matrix. Age-dependent changes in the characteristics of the concrete matrix can effect changes in the post-crack behaviour of fibres. The present investigation has examined how the post-crack energy absorption of fibre reinforced shotcrete (FRS) changes with aging and has found that some types of fibre exhibit dramatically different performance characteristics at late age compared to that displayed at 28 days. This change can have significant consequences for the design of ground support based on fibre reinforced shotcrete. Tunnel linings required to resist sustained ground stresses, or which may be subject to deformation associated with seismicity or ground movement at later ages, should be designed with consideration of a possible long-term loss of ductility exhibited by some types of fibre reinforced shotcrete.
© 2015 The Author. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://
creativecommons.org/licenses/by/4.0/).
1. Introduction
In the construction of public infrastructure such as tunnels it has become common to specify a design life of 100+ years (Franzen et al., 2001). This places very high demands on shotcrete used for ground stabilisation given the concurrent requirement on this material to be pumped and sprayed without blockages, gain strength rapidly, and then remain firmly in place while resisting the effects ofenvironmental exposure. Requirements for resistance to deterioration have generally promoted the use of high binder contents, including pozzolanic additives, and low water/binder ratios leading to low permeability and good chemical inertness. The long-term consequence of this tends to be high strength development at late ages (Malhorta, 1993). Most shotcrete used in underground applications is reinforced with fibres. Unlike conventional steel bar reinforcement, the post-crack performance of fibres is strongly influenced by the characteristics of the concrete matrix. Post-crack performance characteristics therefore change with the evolution of strength and hardness of the concrete matrix.
* Tel.: +61 418 407 892. E-mail address: s.bernard@tse.net.au
The ability of fibre reinforced shotcrete (FRS) to rapidly achieve and then retain ductility (commonly referred to as 'toughness') as it ages is generally recognized as being important to the successful use of this material, especially for ground stabilization. This is because ductility is critical to the re-distribution of load when, for example, ground movement causes localized cracking of the concrete matrix within a shotcrete lining. If ductility diminishes with age or exposure to aggressive agents, then the ability of a structure such as a tunnel lining to maintain stability may be compromised. This is why minimum levels of ductility are considered mandatory for moment re-distribution requirements in conventional above ground structures, made of, for example, reinforced concrete (Beletich et al., 2013; Nielsen, 1998) and is also the reason why FRS performance is specified in terms of energy absorption in widely used design approaches to ground stabilization such as the Q-system (Barton and Grimstad, 2004). The ability of FRS infrastructure to achieve and then maintain a high level of energy absorption over the life of a structure can be considered a valid and relevant indicator of fitness-for-purpose and 'durability'.
The vast majority of FRS is used to stabilize excavated ground, particularly hard rock, thus the focus of the present investigation has been the performance of FRS in this application. When FRS is first sprayed on to freshly excavated ground, the ability of the shot-crete to stabilize the ground will initially depend on the adequacy
http://dx.doi.org/10.1016/j.tust.2015.05.006 0886-7798/© 2015 The Author. Published by Elsevier Ltd.
This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Table 1
Mix details for shotcrete sets examined.
Reinforcement Quantity Nominal UCS UCS UCS
(kg/m3) grade 28 days 5 years 5 years
(MPa) Cyl. Cores Cyl.
(MPa) (MPa) (MPa)
Barchip 10 32 42 64 59
Kyodo,
SL62 WWFa - 50 58 - 72
Dramix RC 65/ 50 50 57 82 77
Dramix RC 65/ 50 40 45 83 78
Dramix RC 65/ 50 25 29 50 43
Novotex 0730, 50 40 43 61 61
Enduro 600, 7 40 48 65 62
Enduro 600, 7 50 53 60 61
Barchip BC54, 7 50 55 88 85
a SL62 denotes 6 mm deformed steel bars welded on a 200 mm orthogonal grid placed at mid-depth.
of the bond established between the freshly sprayed shotcrete and the ground. Early-age competency does not depend on fibre reinforcement because macro-fibres (whether made of steel or polymers) are largely ineffective over the first few hours after spraying (Bernard, 2008a). Moreover, most early-age lining failures are governed by punching resistance rather than flexure and fibres contribute very little to shear resistance in young concrete (Bernard, 2011). However, as the shotcrete gains strength the pre-dominant mode of failure changes from shear to flexure and it is at this point that the performance of fibre reinforcement becomes important. Fibre reinforcement provides substantial post-crack energy absorption and load re-distribution capacity to a material that is otherwise quite brittle and ineffective in ground control (Bernard, 2002; Naaman, 1985). It thereby allows a concrete lining to accommodate a degree of ground movement without loss of structural competency. Maintenance of this property is important to the long-term functionality of FRS linings should later-age ground movement occur.
It is generally recognized that the more unstable the ground, the greater is the requirement for energy absorption capacity in a FRS lining. The relationship between ground stability and minimum energy absorption requirement has been expressed in several widely used design guidelines for shotcrete linings such as the Q-system (Barton and Grimstad, 2004), the Australian Recommended Practice for Shotcrete (Shotcreting in Australia: Recommended Practice, 2010), and the Norwegian Concrete Association Publication Number 7 (Concrete for Rock Support, 2011). In each of these documents the minimum energy absorption requirement for FRS is related to the expected degree of ground movement. In Australian and North American practice (Bernard,
Table 2
Mix design for shotcrete used in each trial.
Component Nominal grade and quantity (kg/m3)
25 MPa 32 MPa 40 MPa 50 MPa
Coarse aggregate (10/7 mm CRG) 600 600 610 620
Coarse sand (2 mm) 372 372 350 330
Fine sand 720 680 680 640
Binder (Cement/fly ash/silica) 385 425 445 495
Water reducer (L/m3) 1.0 1.0 1.1 1.2
50 MPa Shotcrete Dramix RC 65/35
0 10 20 30 40
Deflection (mm)
Fig. 1. ASTM C1550 Load-deflection curves for specimens reinforced with Dramix RC65/35BN in 50 MPa shotcrete tested at various ages after spraying showing an increase in load resistance at small deflections with aging but a fall in resistance at large deformations.
2013; Decker et al., 2012), energy absorption is specified on the basis of FRS performance using ASTM C1550 round panels (ASTM, 2012). ASTM C1550 panels have therefore been used to assess FRS performance throughout this investigation. As an indication of performance requirements for underground applications, a minimum energy absorption of 400 J at 40 mm central deflection in the ASTM C1550 panel test has been found to be adequate for the majority of tunnelling and mining projects (Papworth, 2002).
Aging of concrete in underground environments normally gives rise to concerns about durability. The ability of fibres within uncracked FRS to resist corrosion under conditions of normal atmospheric exposure has been demonstrated through several long-term exposure trials (Schupack, 1985; Hara et al., 1992; Mangat and Gurusamy, 1985). While carbonation may promote corrosion and loss of structural performance, including ductility, for near-surface steel fibres (Schupack, 1985), any fibres that corrode due to proximity to a concrete surface have been shown to exert insufficient expansive pressure to disrupt the enveloping concrete (Hoff, 1987; Lankard and Walker, 1978). Localized surface corrosion therefore does not develop into structurally-threatening
Fig. 2. Pulled-out Dramix RC65/35BN fibres on crack face of 3 day old shotcrete of 50 MPa nominal grade.
Fig. 3. Ruptured Dramix RC65/35BN fibres on crack face of five year old shotcrete of 50 MPa nominal grade.
through-corrosion of the kind that is commonly observed in conventionally-reinforced concrete (Phan et al., 2001).
In contrast to the relatively good durability of uncracked FRS, the presence of cracks is recognized as leading to rapid degradation in the performance of FRS when reinforced with steel fibres. Most laboratory and field tests have shown that exposure of cracked steel FRC surfaces to aggressive environments can degrade post-crack performance (Hoff, 1987; Kosa and Naaman, 1990; Bernard, 2004). Nordstrom (1999, 2001) found that the rate of corrosion of steel FRS increased with crack width but that late-age hydration may have the effect of overcoming some of the deterioration in performance at small crack widths caused by corrosion of fibres. Nordstrom also noted that negligible corrosion and deterioration occurred for steel FRS exhibiting narrow (<0.1 mm) cracks. The effect of cracking on the performance of macro-synthetic fibres is not as well documented as for steel fibres, but the complete absence of corrosion and immunity to salt ingress characteristic of these fibres has been noted by several researchers (Bernard, 2004; Chernov et al., 2006; Zhang et al., 1999). The advantages of macro-synthetic fibres have also been recognized in industry. Corrosion of steel fibres was found to cause frequent FRS lining
failures within the Australian underground mining industry in the 1990s. This was one of the contributing factors that drove the industry to adopt macro-synthetic FRS in underground mines throughout Australia (Bernard et al., 2014). As a consequence of the corrosion risk associated with steel fibres, macro-synthetic fibres are mandated as the only acceptable form of reinforcement for subsea tunnel linings in Norway (Concrete for Rock Support, 2011) and now constitute the primary reinforcement in the majority of new tunnels recently completed in that country.
While durability issues in FRS related to matrix degradation and corrosion have been relatively widely addressed, changes in ductility with aging of intact FRS has only recently entered the consciousness of the construction industry. Early studies of the effect of matrix strength on fibre performance (Naaman and Najm, 1991) indicated that resistance to pull-out increased for hooked-end steel fibres as the compressive strength of the concrete increases. Naaman and Al-Khairi (1996) also noted that the shape of the load-deflection response for hooked-end steel FRC in flexure changed as age increased from one to 28 days, with load resistance increasing more rapidly at smaller crack widths than at larger crack widths as the concrete aged.
A study of the age-dependent behaviour of FRS on the M5 motorway tunnel in Sydney, Australia (Bernard and Hanke, 2002) revealed a loss of toughness at late age (termed 'embrittlement') for shotcrete reinforced with steel fibre. This fall in post-crack performance was unrelated to corrosion or any mechanism of deterioration in the concrete matrix. Age-dependent changes in post-crack performance were also investigated by Lange and Lee (2003) who examined energy absorption in FRC between one and 28 days at narrow crack widths and found differences in the pattern of performance development for different types of fibre. Another investigation involving tests out to four years age was undertaken by Sustersic et al. (2000) who observed that post-crack performance across very narrow cracks increased with aging. Preliminary results for the tests conducted in the present investigation were also reported by Bernard (2008b) who noted the early onset of performance loss for some FRS mixes. Recent work by Bjontegaard et al. (2014) has demonstrated performance losses for uncracked steel FRS over a period of three years in Norwegian road tunnels, while (Kaufmann, 2014) observed similar losses for steel FRS over one year in Switzerland. The majority of studies described above involved tests between early age and, at most, one years age. This is far shorter than the required design life of most FRS structures. The current study was therefore developed to examine performance over a longer (five year) period, and include a comprehensive range of fibres representative of international shotcreting practice.
Fig. 4. ASTM C1550 Load-deflection curves for specimens reinforced with Barchip BC54 in 50 MPa shotcrete tested at various ages after spraying showing an increase in load resistance across all deformations with increasing age.
Fig. 5. Pulled-out Barchip BC54 fibres on crack face of 28 day old 50 MPa shotcrete.
Fig. 6. Pulled-out Barchip BC54 fibres on crack face of 5 year old 50 MPa shotcrete.
2. Experimental investigation
To assess the long-term post-crack performance of FRS, a comprehensive series of trials were conducted involving the production and testing of ASTM C1550 round panels to assess energy absorption between 1 day and 5 years age. Eight sets of FRS specimens were produced by spraying panels in an inclined position 45° to the vertical, and one set of specimens reinforced with SL62 steel mesh was produced by casting (see Tables 1 and 2 for reinforcement and mix data). The fibres used in the trials were selected on the basis of their widespread use in the underground industry and include 35 mm Dramix hooked-end steel fibres, 30 mm Novotex flattened-end steel fibres, 48 and 54 mm Barchip embossed macro-synthetic fibres, and 50 mm Enduro crimped macro-synthetic fibres. The specimens comprised ASTM C1550 round panels (ASTM, 2012) that, if sprayed, included a set accelerator at a dosage rate of about 4% bwc. Cast cylinders and cores extracted from concurrently sprayed 200 mm thick panels were tested in parallel with the round panels (Australian Standard). The ages at testing were 1, 2, 3, 7, 14, 28, 56, 91, and 180 days, and then 1, 2, 3, and 5 years. Since the specimen sets were produced in sequence over a period of three years, and some spares
were included, results out to 8 years were available for some sets at the time of writing. The exposure site was located under a wide viaduct to simulate the entrance of a tunnel.
3. Results
The results of the investigation comprised load-deflection curves obtained for each of the ASTM C-1550 panel specimens and UCS data obtained from cores and cast cylinders. An example set of load-deflection curves for Dramix RC 65/35 BN-reinforced specimens produced using 50 MPa shotcrete and continuously wet cured in the laboratory is shown in Fig. 1. These curves reveal that a dramatic change in post-crack performance characteristics occurred with increasing age for some of the presently examined mixes.
The hooked-end steel FRS specimens typically exhibited a relatively broad load-deflection curve at early-age with substantial post-crack capacity up to 40 mm central deflection (equivalent to 13 mm average maximum crack width (Bernard and Xu, 2008). However, as the concrete aged, load resistance increased slightly at small crack widths but fell substantially at wider crack widths leading to a dramatic fall in total energy absorption at 40 mm for the higher shotcrete strength grades (40 and 50 MPa). By an age of one year the specimens had lost about 70 per cent of load capacity at a central deflection of 5 mm (equivalent to an average maximum crack width of only 1.7 mm). The steady loss of ductility with age experienced by the hooked-end steel FRS appeared to be related to a change in post-crack fibre behaviour from pull-out to rupture (see Figs. 2 and 3).
The load-deflection curves for the macro-synthetic FRS revealed a very different pattern of performance change with age, as represented by the load-deflection curves for the FRS reinforced with Barchip BC54 fibres shown in Fig. 4. These curves exhibited a consistent shape but increasing magnitude of average load resistance as age increased. The consistent shape of the curves suggested that the mode of post-crack fibre behaviour did not change with age. Indeed, an examination of the pulled-out fibres revealed a similar appearance in both young and old specimens (Figs. 5 and 6). The only difference appeared to be a steady increase
Fig. 7. Energy absorption at 5, 10, 20, and 40 mm for Kyodo in 32 MPa shotcrete.
Fig. 8. Energy absorption at 5, 10, 20, and 40 mm for SL62 in 50 MPa shotcrete.
Fig. 9. Energy at 5, 10, 20, and 40 mm for Dramix RC65/35 in 50 MPa shotcrete.
in the degree of surface distress to each fibre as age increased; this probably indicated an increase in the degree of friction at the fibre-concrete boundary during pull-out.
The performance of all the specimens has been summarized in terms of energy absorption in the ASTM C1550 panel test at 5, 10, 20, and 40 mm cumulative central deflection. The change in energy absorption for each specimen set as a function of age is shown in Figs. 7-15. For clarity, each set of curves includes actual data points for 5 and 40 mm energy absorption, and curve-fits to energy absorption at 5, 10, 20, and 40 mm deflection. Curve fitting was performed with non-linear functions using a program called TableCurve 2D.
The energy absorption data revealed dramatically different patterns of post-crack performance for the three types of
reinforcement used. The SL62 mesh-reinforced specimens (Set 2, Table 2) demonstrated a close-to-maximum post-crack performance capacity within one day of casting and only small changes in energy absorption at 40 mm with age (Fig. 8). This variation may possibly have been due to discrepancies in the lever arm between the steel mesh and the concrete compression block in the concrete. The results obtained for the macro-synthetic FRS specimen sets indicated a steady increase in performance out to 5 years with none of these sets (1, 7, 8, 9, see Table 1) exhibiting a systematic fall in performance with age (Figs. 7 and 13-15).
In contrast to the shotcrete reinforced with macro-synthetic fibres or steel mesh, the hooked-end steel FRS exhibited performance that was very sensitive to the compressive strength and age of the shotcrete, with optimum post-crack performance occurring for concrete with a medium compressive strength (sets 3-5 in Table 1, Figs. 9-11). The 50 MPa mix exhibited near-maximum performance over the first few days following spraying, after which performance at larger deflections fell precipitously as the mode of fibre failure changed from pull-out to rupture. Energy absorption at 10 and 20 mm deflection also fell with increasing age. For the 40 MPa mix, the peak in performance occurred around 2856 days, while for the 25 MPa mix the peak performance reached a plateau around 180 days age. The shotcrete reinforced with Novotex 0730 steel fibres also exhibited a peak in performance followed by a slight fall at later ages, but the sensitivity of this mix to aging was less dramatic than exhibited by the hooked-end steel FRS (Fig. 12).
Note that none of the FRS mixes used in this investigation exhibited a fall in unconfined compressive strength (UCS) with age (see Table 1). The loss of toughness exhibited by the hooked-end steel FRS was therefore not attributable to degradation of the concrete matrix. Inspection of the crack surfaces after testing revealed that the loss of post-crack performance that occurred with age was due to a change in the mechanism of post-crack fibre behaviour as the concrete matrix became progressively stronger and harder (see Figs. 2 and 3). Examination of the relation between UCS and energy absorbed at 40 mm for the hooked-end steel FRS sets (Fig. 16) indicated that an optimum level of performance occurred for a UCS of about 40-45 MPa, beyond which
Fig. 10. Energy at 5, 10, 20, and 40 mm for Dramix RC65/35 in 40 MPa shotcrete.
Fig. 11. Energy at 5, 10, 20, and 40 mm for Dramix RC65/35 in 25 MPa shotcrete.
Fig. 12. Energy at 5, 10, 20, and 40 mm for Novotex 0730 in 40 MPa shotcrete.
Fig. 14. Energy at 5, 10, 20, and 40 mm for Enduro 600 in 50 MPa shotcrete.
performance suffered. Observations of fibre failure modes suggested that the transition from pull-out to rupture coincided with the progression of compressive strength through the 40-45 MPa region, but the age at which this occurred differed between mixes. It is suggested that further research is required into the effect of concrete matrix properties on the transition from pull-out to rupture in post-crack behaviour of hooked-end steel fibres.
All the steel FRS mixes exhibited an increase in energy absorption at small deformations with age, while concurrently exhibiting a fall in energy absorption at large deformations. This meant that the residual load resistance at large deformations was doubly diminished with age (see Fig. 1 for typical load-deflection curves). In contrast, the macro-synthetic FRS mixes exhibited an initial
increase in energy absorption with compressive strength up to about 40-45 MPa after which performance stabilised (Fig. 17). Energy absorption between 20 and 40 mm deflection in the macro-synthetic FRS panels reached a plateau at 40-45 MPa and thereafter remained stable. These results suggest that macro-synthetic fibres and steel mesh both offer superior performance in ground stabilisation applications requiring long term ductility compared to hooked-end steel fibres. The rupturing of the hooked-end steel fibres at late ages suggests that hooked-end steel fibres (of medium strength grade) offer long-term post-crack performance characteristics similar to that of low-ductility steel mesh (Gilbert and Sakka, 2007).
Fig. 13. Energy at 5, 10, 20, and 40 mm for Enduro 600 in 40 MPa shotcrete.
Fig. 15. Energy at 5, 10, 20, and 40 mm for Barchip BC54 in 50 MPa shotcrete.
4. Discussion
The shotcrete mixes used in the present investigation had a nominal UCS at 28 days of between 25 and 50 MPa. Due to the widespread inclusion of Supplementary Cementitious Materials (SCMs) for reasons of sprayability and improved environmental resistance, the strength of shotcrete continues to increase well beyond 28 days. The long-term UCS will therefore be substantially greater than the nominal 28 day strength. Moreover, the hardness of the paste continues to increase as the concrete ages irrespective of the strength. The results of this investigation indicate that in-place concrete compressive strengths in excess of 40-45 MPa at 28 days can promote a change in the mode of behaviour for some types of steel fibre that otherwise appear to be optimized for performance in lower-strength concrete. The change from a high energy pull-out mode of failure to a low-energy rupture mode is manifested as a dramatic fall in ductility. Ductility is as important to the maintenance of satisfactory load resistance in structures as the maximum strength of materials, hence the loss of ductility with age is a cause for concern.
The fall in ductility exhibited by the steel FRS examined in this investigation may possibly reduce the ability of a tunnel lining to redistribute loads should ground movement occur at late ages. This is of concern because these fibres have been widely used throughout the world for this application. Late age ground movement may occur, for example, in the event of seismicity, roadway widening, nearby excavation, or change in groundwater level many years after the FRS lining is initially installed. It may also occur as a result of loosening of rocks at joints in flat roof or shallow-arched tunnels (Pells, 2002). If any of these circumstances were to arise, inadequate ductility may increase the risk of lining collapse.
If high toughness is required throughout the life of a FRS structure, then it is necessary to look beyond the performance achieved at 28 days and consider the effects of strength development, paste hardening, and embrittlement on late age performance. The present data indicate that fibres intended to provide resistance to persistent loading or late-age transient loads should be selected on the basis of demonstrated performance at late ages. The naive assumption that
Fig. 16. Energy at 40 mm for Dramix RC65/35BN as a function of UCS based on cores.
Fig. 17. Energy at 40 mm for macro-synthetic FRS as a function of UCS based on cores.
satisfactory post-crack performance obtained in 28 day QC specimens will automatically translate to similar levels of performance throughout the design life of a structure could lead to serious over-estimates of structural ductility in long-life infrastructure.
The evolution of energy absorption over time observed in these trials indicates that performance characteristics appeared to stabilize by an age of one year for most of the FRS mixtures examined. For FRS reinforced with macro-synthetic fibres or steel mesh, performance simply reached a plateau, but for the steel FRS long-term performance loss was clearly sensitive to the strength of the concrete matrix. This suggests that when steel FRS is used there is a need to provide proof that the long term performance will not fall relative to the 28 day result. If long-term performance cannot be guaranteed, a performance reduction factor should be applied to steel FRS to account for the very real possibility that long-term performance will be substantially lower than observed at 28 days. The common minimum energy absorption requirement of 400 J in the ASTM C1550 panel test (equivalent to 1000 J in the EN14488 square panel (Bernard, 2002; BS-EN 14488, 2006) may therefore have to be higher for steel fibres to account for the possible loss of performance that these fibres suffer with aging. Based on the present trials, the performance at 28 days may have to be as much as 50% higher for shotcrete reinforced with medium strength steel fibres compared to macro-synthetic fibres if equivalent performance is to be obtained at late ages. Claims have been made by manufacturers that high strength steel fibres do not suffer performance loss with age, but this has yet to be verified.
5. Conclusions
A clear conclusion from this investigation is that satisfactory post-crack energy absorption observed in QC tests at 28 days does not guarantee satisfactory performance at later ages, at least for some types of steel fibre reinforced shotcrete. The effects of strength gain and hardening of the cement matrix in aging shot-crete can change the mechanism of failure for some types of steel fibre leading to a loss of post-crack performance at large deformations. This may reduce the ability of FRS linings composed of such
concrete to redistribute loads in response to ground movement. It is therefore necessary to consider the ultimate compressive strength of a shotcrete matrix and its likely long-term effect on the failure mechanism of fibres contained in the matrix when assessing the most suitable fibre to use as reinforcement in long life tunnel linings. If steel fibres are used in shotcrete of greater than 40 MPa nominal grade, post-crack performance requirements in quality control tests at 28 days may have to be higher than for alternative forms of reinforcement to account for possible falls in performance with aging.
Acknowledgements
The author gratefully acknowledges the Roads and Maritime Service of New South Wales, Transurban Roads Ltd, EPC P/L, Propex Concrete Systems, and Holcim Australia Ltd for their generous support of this investigation.
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