42 TUNNELLING JOURNAL

MACRO-SYNTHETICS

MACRO-SYNTHETIC FIBRE REINFORCEDCONCRETE or shotcrete (MSFRC or MSFRS) isa technology that is becoming an acceptedform of reinforcement in the constructioncommunity and is seen by many designengineers as offering a viable alternative tosteel fibre and steel bar reinforcement inapplications such as tunnel linings. Thetechnology is now common place inapplications such as shotcrete for primary andsecondary ground support in both mining andcivil tunnel applications. For example it hasbecome the standard form of reinforcement inthe Australian mining industry, where 2014marked the end of steel fibre reinforcementuse in shotcrete[1]. Additionally, an increasingnumber of tunnels are adopting permanentsprayed concrete linings using macro syntheticfibres, examples of which include the A3Hindhead tunnel near Guildford in the UK andthe North Strathfield Rail Underpass in Sydney,Australia[2,3].

Applications such as permanent tunnellinings have required further research andinvestigation to confirm that macro synthetic

fibre reinforcement meets the strictperformance criteria. This is a consequence ofthe highly performance-based nature of theindustry in which engineers constantly monitorlining behaviour and adapt design andconstruction methods to achieve improvementsin outcomes. This research combined with anumber of major tunnel lining projects recentlycompleted using macro synthetic fibrereinforcement sees the future of this type ofreinforcement looking extremely bright.

For civil tunnel lining applications there are anumber of factors design engineers mustconsider when determining whichreinforcement product can meet the designperformance criteria. These include structuralcapacity, durability, corrosion, crack width,creep and embrittlement to name a few. It isalways important for a designer and the clientto understand the benefits of adding any typeof reinforcement to the concrete - in order toassess value for money. What are the benefitsin terms of durability or extended asset lifebecause of the use of macro synthetic fibre?

This article examines a number of these key

considerations with the aim of summarisingthe latest research and industry thinking as wellas presenting instances where MSFRC willdeliver a distinct advantage to the finishedproduct.

DurabilityThe durability of a tunnel concrete liningencompasses a number of factors including thepermeability of the concrete, concrete strength,reinforcement and control of cracks. Thedurability of the concrete matrix in FRC isaffected by the same parameters governingplain concrete when subject to the exposureconditions typical of an undergroundenvironment. When macro-synthetic fibres areused in concrete (in the absence of steel bars),there is no need to be concerned aboutchloride ion penetration, carbonation, and to alesser degree, water impermeability, since thereis no steel within the concrete which could besubject to corrosion. This frees the designerfrom the difficulties of trying to satisfy multipleperformance requirements related to corrosionresistance thereby allowing greater flexibility indesign. The designer can therefore focus onstrength gain, shrinkage, sulphate resistance,and post-crack performance without thecomplications of trying to satisfy otherparameters such as crack width control, whichlargely governs traditional design approachesfor the sake of protecting the incorporatedsteel against corrosion.

CorrosionIf steel bars or fibres are included within atunnel lining, the mix design becomes morechallenging and compromises must be madeto satisfy concerns over durability. Chloride ionpenetration and carbonation depth need to beconsidered thereby restricting the design of themix. Maximum allowable crack widths aremuch smaller when using steel bars or steelfibres since cracks, should they occur, act aspoints of rapid salt ingress to the

In-service performanceof Macro SyntheticFRC in tunnel linings

Left: The North Strathfield Rail Underpasstunnel in Sydney, Australia, incorporated asingle pass macro-synthetic fibre reinforcedpermanent sprayed concrete lining[3].

R. Winterberg, G. Sedgman of Elasto PlasticConcrete state the case for the use of MacroSynthetic Fibres in tunnel linings.

MACRO-SYNTHETICS

reinforcement. Maximum acceptable crackwidths are about 0.15mm for steel barreinforcement, and recent in-field tests byKaufmann[4] and Bernard[5], supported by earlierresearch by Nordstrom[6,7], all indicate amaximum acceptable crack width of only0.10mm for steel FRC. This places verystringent requirements on crack control whensteel reinforcement is used, especially for steelfibres since crack width control is much moredifficult and unpredictable for steel fibres thanfor steel bar reinforcement.

In contrast, crack widths are not a concernwhen macro-synthetic FRC is used since there isthen no corrodible material present within thelining. Crack width limits are thereforegoverned by other considerations, such aswater-tightness. However, if significant in-planecompressive loads are present in the lining(which is almost always the case) flexural crackswill not penetrate through more than a smallfraction of the total lining thickness so wateringress to the tunnel is unlikely to occurthrough unplanned cracks.

Examples of where corrosion has been seenas a potential problem are the subsea tunnelsin Norway and Finland. Research on durabilityin these aggressive environments, due to thesaline water peculating through the rock to thetunnel lining, has brought about a completeban on steel fibre in these constructions andthe use of macro synthetic fibres for allshotcrete reinforcement[8].

The Ryfast Subsea tunnel, currently underconstruction (the longest subsea tunnel in theworld) near Stavanger, is a current example ofthis as is the Helsinki West Metro Extension.

Another example of the successfulapplication of macro-synthetic fibres in thisregard is the 4.47km Oliola water tunnel inNorth Eastern Spain, which has now been inservice for 5 years and was constructed using aBarChip fibre reinforced secondary concretelining to eliminate the risk of corrosion in thisenvironment[9].

Most concrete or shotcrete mix designs focuson durability and corrosion protection toprovide high resistance against chemical attackover their service life, which in tunnelling istypically 120 years. To achieve this, the mix

designs often contain large proportions ofpozzolanic binders, which in turn can showsignificant post-hardening of the concrete withage. This leads to embrittlement of the fibreconcrete matrix, which is responsible for post-crack performance loss when using steel fibres.

Embrittlement of FRCEmbrittlement of concrete or shotcrete withage due to post-hardening and its detrimentaleffect on the post-crack performance of steelFRC, has been known for nearly 20 years.Numerous investigations have indicated thataging can lead to a significant loss of post-crack performance for steel FRC[10,11,12].

The change in behaviour with age is due to achange from a high-energy pull-out mode ofpost-crack fibre behaviour to a brittle low-energy rupture mode. This leads to rupturingof steel fibres at crack widths in excess of theelongation capacity of the fibre. Thus, theperformance degradation primarily affectsresistance to late-age load conditions such asnearby underground construction, seismicloading, or changes in groundwater pressure.

Extensive research into post-crack behaviourof steel fibres, strand, and bar in concretesuggests that rupture is caused by excessivelyhigh friction between the steel andcementitious paste that is more closely relatedto the hardness of the binder paste. Pastehardness increases with the strength of the

concrete, but is also related to the elasticmodulus and composition of the paste. For thisreason, satisfactory performance at early ages(around 28 days) is not a guarantee ofacceptable performance at late ages for steelFRC. The performance of steel FRC at crackwidths in excess of 1.0mm can fall by as muchas 50% compared to the optimum exhibited atearly ages, thus a performance reduction factorshould be applied to the long-term flexuralresistance of steel FRC[5,12]

Macro-synthetic FRC is largely unaffected bythis phenomenon since changes in pastehardness make little difference to thebehaviour of the fibre within the composite

beyond the first few days ofhardening. The performance ofmacro-synthetic FRC evident at 28days can therefore be relied uponto be retained with age. Thus,designers can be more confidentabout the long-term retention ofpost-crack performance.

Crack width controlAn investigation into the potentialuse of BarChip macro-syntheticfibres was undertaken todetermine whether these fibreswere effective in reducing meancrack widths generated inconventionally reinforced concretemembers as a result of flexuralloading[13]. Use of macro-synthetic

fibres to reduce maximum crack widths inreinforced concrete flexural members subject toaggressive environmental exposure is morerational than the use of steel fibres because thelatter are more sensitive to the corrosive effectsof environmental exposure at cracks. The fibrewas an experimental variant of a standardBarChip fibre that has been developed forcrack width control purposes.

In this first part of the research, the focuswas on the effect of the presence of specialmacro-synthetic fibres on the crackingdevelopment and the developing crackspacings. Through a set of experimental trialsinvolving 32 beams reinforced with normallevels of steel bar reinforcement, and between0 and 6kg/m3 of BarChip macro-synthetic crackwidth control fibres, it has been demonstratedthat these fibres are indeed capable ofreducing mean crack spacings by up to 30%compared to plain reinforced concretemembers.

This result is useful in the field because,unlike steel fibres, macro-synthetic fibres areentirely immune to the effects of corrosion andthus their potential location close to the surfaceof a member (in a crack or otherwise) will notcompromise durability. Inclusion of up to6kg/m3 of BarChip crack width control fibresmay therefore prove a useful tool for reducingmaximum crack widths in RC members inaggressive environments where maximum

TUNNELLING JOURNAL 43

Left: Sprayingof MSFRS atthe HelsinkiWest MetroExtensionproject

Below: Macro-synthetic fibrereinforced castin situ in thefinal lining ofthe Oliolawater tunnel,Spain[9].

44 TUNNELLING JOURNAL

acceptable crack widths of about 0.10-0.15mm must be achieved.

The reduced crack widths will offer higherdurability and therefore longer asset life andless maintenance over the design life. Malaga

Airport High Speed Rail was a project whichbenefited from this type of reinforcement.Macro synthetic fibre was used in combinationwith the traditional reinforcement cage. Theaddition of macro-synthetic fibre was able toassist in keeping crack widths, which may havebeen produced from either the TBM rams or inservice loads, much smaller than if there wasno fibre. This design reduced repair andmaintenance costs and increased asset life inthe long term.

Creep considerationsCreep has always been a question for designersas they see the material properties betweensteel and macro synthetic fibre as very different.However, in segmental linings hoopcompression forces typically govern theoccurring stresses and tensile stresses areminimal after installation and grouting to thesubstrate. Potential cracks, which havedeveloped during hauling, handling andinstallation of the segments, or during TBMpropulsion, are expected to close over time dueto the natural creep of concrete under highcompressive forces. The creep of the type ofreinforcement plays a negligible role herein.

The particular requirement for long-termtesting of macro-synthetic FRC is only necessarywhen long-term tensile stress is expected to beimposed on a cracked section in service. Thisalmost never occurs in reality as the section istypically under compression due to high axialforces from surrounding groundwater andearth pressure. In service, the tensile behaviouris not as significant as during production andtransient stages. If transitory tensile stresses areexpected, then there is no need for long-termtest data because macro-synthetic FRCperforms very similarly to steel FRC in the shortterm.

However, concerns have been raised aboutthe long-term performance of macro-synthetic

fibres in respect of creep and the associatedconsequences for crack width developmentwith time under sustained flexural loads. Toaddress these concerns, a method todetermine the effects of creep and shrinkage

on the time-dependentbehaviour of cracked,macro-synthetic fibrereinforced concrete cross-sections containingconventional barreinforcement subjected to asustained bending momentand axial force has beendeveloped[14]. The results ofthis analysis show that theinclusion of macro-syntheticfibres in the concrete hasonly a minor effect on theflexural strength of thecross-section, but the fibresreduce time-dependent in-service deformations andsignificantly reduce

maximum crack widths when used incombination with conventional reinforcingbars. This research work will be presented atthe World Tunnelling Congress (WTC 2015) inDubrovnik, Croatia this year in May.

The physical properties of different macro-synthetic fibres vary greatly and for the majorityof the research and the completed projectspresented in this article only highly engineeredmacro synthetic fibres with a tensile strength of> 600MPa and a Modulus of Elasticity of>10GPa has been used. These fibrecharacteristics are required to achieve andmaintain the presented performanceoutcomes.

ConclusionsHighly engineered macro-synthetic fibres proveto be effective in reducing crack widths in RCmembers and therefore significantly add to atunnel lining’s durability.

The inherent isolated creep properties ofmacro-synthetic fibre reinforcement play asubordinate role in the long-term performanceof tunnel linings where compression forcestypically govern.

High-performance macro-synthetic fibrereinforcement is ideal for aggressive exposureconditions and guarantees durableperformance over the design life cycle withoutsuffering matrix embrittlement andperformance loss with age.

…making it an ideal construction materialfor tunnel linings.

MACRO-SYNTHETICS

[1] E. Stefan Bernard, Matthew J.K. Clements,Stephen B. Duffield, and D. Rusty Morgan, 2014.Development of macro-synthetic fibre reinforcedshotcrete in Australia, 7th InternationalSymposium on Sprayed Concrete – Modern Useof Wet Mix Sprayed Concrete for UndergroundSupport – Sandefjord, Norway, 16. – 19. June2014[2] Ireland, T.J., Stephenson, S., 2010. “Designand construction of a permanent shotcrete lining– The A3 Hindhead Project, UK”, Shotcrete: MoreEngineering Developments, Bernard (ed.), pp143-152[3] Gonzalez, M., Kitson, M., Mares, D., Muir, B.,Nye, E., Schroeter, T., 2014. “The North StrathfieldRail Underpass – Driven Tunnel Design andConstruction”, 15th Australian TunnellingConference 2014, Sydney, 17-19 September, pp369-374. [4] Kaufmann, J. P., 2014. Durability performanceof fiber reinforced shotcrete in aggressiveenvironment, Proceedings of the World TunnelCongress 2014 – Tunnels for a better Life. Foz doIguaçu, Brazil.[5] Bernard, E.S. 2008. “Embrittlement of FiberReinforced Shotcrete”, Shotcrete, Vol. 10, No. 3,pp16-21, American Shotcrete Association[6] Nordström, E., 1999. “Durability of steel fibrereinforced sprayed concrete with regard tocorrosion”, Proceedings, 3rd Int. Symposium onSprayed Concrete, Gol, Norway, 26-29September.[7] Nordström, E., 2001. “Durability of steel fibrereinforced shotcrete with regard to corrosion”,

Shotcrete: Engineering Developments, Bernard(ed.), pp213-217, Swets & Zeitlinger, Lisse. [8] Hagelia, P. 2008, “Deterioration mechanismsand durability of sprayed concrete in Norwegiantunnels”, Fifth International Symposium onSprayed Concrete, Lillehammar, Norway, 21-24April, pp180-197.[9] “Spain’s synthetic reinforcement”, Tunnels andTunnelling International, April 2009, pp 25-27[10] Bjontegaard, O., Myren, S.A., Klemtsrud, K.,and Kompen, R., 2014. “Fibre Reinforced SprayedConcrete (FRSC): Energy Absorption Capacityfrom 2 days age to One Year”, SeventhInternational Symposium on Sprayed Concrete,Sandefjord, Norway, 16-19 June, pp 88-97.[11] Kaufmann, J.P., 2014. “Durabilityperformance of fiber reinforced shotcrete inaggressive environment”, World TunnellingCongress 2014, (Ed. Negro, Cecilio and Bilfinger),Iguassu Falls Brazil, p279. [12] Bernard, E.S., 2014. “Age-dependentChanges in Post-cracking Performance of Fibre-Reinforced Concrete for Tunnel Segments”, 15thAustralian Tunnelling Conference 2014, Sydney,17-19 September, pp229-235.[13 Bernard, E.S., 2015. “Investigation of CrackWidth Control Using Barchip Fibres in ReinforcedConcrete Beams”, TSE Report 248, Penrith, NSW,Australia, January 2015[14] Gilbert, R. I. and Bernard, E. S., 2015. Time-dependent Analysis of Macro-synthetic FRCSections with Conventional Bar Reinforcement, (inpress) World Tunnelling Congress (WTC 2015) inDubrovnik, Croatia, May 2015

REFERENCE

Above: Malaga Airport High Speed Rail segmental lining used acombination of steel cage and macro-synthetic fibre reinforcement