HISTORICALLY, PERMANENT concrete lin-ings for underground structures have beeninstalled using form and pour methods, butthe use of shotcrete or sprayed concrete forstructural linings is becoming increasinglycommon. Although cast in place methodsare well proven, they do have their down-sides, especially where non-uniform shapesare required.

Although form and pour methods can beused for virtually every combination ofshapes and spaces, there are drawbacks toits use especially when non-uniform crosssections and junctions etc. are required. De-signing and installing custom built form-work is time consuming and depending onproject logistics can cause pinch points inthe schedule. As Clients strive to managethe scarce capital they have to manage ex-isting and build new facilities, designers andconstructers are increasingly being chal-lenged to minimize the excavation and lin-ing quantities. This brings new challenges tothe use of cast in place concrete due to thecomplex nature of the shapes being de-signed.

The use of shotcrete or sprayed concretefor the installation of the permanent struc-tural lining for non-uniform openings is awell-established process, but in the last fewyears the boundaries of its use, especially inthe US have been stretched. Mott MacDon-ald in conjunction with Superior Gunitehave been at the forefront of expanding theuse of this application method in the under-ground environment.

Freeform concreteSo what are the benefits of the use offreeform concrete linings, how do they dif-fer from shotcrete final linings, and whatare the potential drawbacks?

Freeform concrete, also referred to asPneumatically Applied Concrete (PAC), asdeveloped for use on many of the majorcapital projects under construction in NewYork, involves the application of structuralconcrete utilizing high velocity pneumaticprojection from a nozzle as the means for

achieving consolidation, compaction, and auniform distribution of the concrete con-stituents.

The end product is a one half inch, minusPortland Cement Concrete (PCC), capableof achieving conventional and highstrengths, while maintaining or exceedingrequired end properties by design. Materialsare pumped wet to the nozzle where air isadded at high pressure to achieve the re-quired spray pattern and high velocity forthe concrete application. This applicationmimics ACI 506 and sites the ACI specifica-tion as a reference.

How is this different to a shotcrete finallining (SFL)? A typical SFL lining involves the

use of lattice girders to support the steel re-inforcement and assist in controlling theprofile/geometry of the tunnel cross sectionand is applied in layers to build up the con-crete thickness of the final linings. Rein-forcement in such applications is usuallysmall bar diameter (five or less) and well-spaced out, to minimize the opportunity forshadowing of the shotcrete around thegirders and rebar.

It requires a high level of application skill,workmanship, and a rigorous quality controlprocess. It is also increasingly being installedusing robotic spraying, which therefore lim-its the finish that can be achieved to that ofa nozzle finish. For example, a typical se-

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Andy Thompson, Vice President of Mott MacDonaldin New York, and Frank Townsend, Vice President ofOperations for specialty contractor Superior Gunite,discuss innovations and issues that have beenencountered when using sprayed concrete for theinstallation of permanent structural linings for non-uniform openings in underground construction

PAC in use

SHOTCRETEInnovations

quence for SFL may include: (1) installationof lattice girders at 5ft centers with a rebarreinforcement mat placed against the water-proofing membrane at the extrados side ofthe girders, and partial spray-ing of the lattice girders;(2) shotcreting of an in-fill first layer be-tween thelattice girders;(3) shotcretingof a secondlayer; (4) instal-lation of rebar re-inforcement on theintrados side of the lining;and then (5) installing a finalshotcrete layer to provide theminimum cover over the reinforce-ment. The number of shotcrete layer instal-lations would depend on the total designthickness of the final lining.

Freeform concrete or PAC, by contrast, isutilized with the same rebar design thatwould be used for a form and pour liningand no specific provisions need to be madeto accommodate its use. It can be usedaround extremely heavy and congested rein-

force-ment and

against PVC orspray applied waterproof-

ing membranes. It is applied inlayers and can be hand finished to

achieve any standard required, includingtextured architectural finishes. It does how-ever require highly skilled nozzle men andsupport crews to ensure a safe and highquality finish is achieved as well as an ex-tremely rigorous quality control process bothbefore application and during.

PAC excels in tunnel applications whereconventional forming methods are difficultlogistically as well as costly to construct.Where conventional methods use large,

heavy,and in most

cases steel formsthat have limited flexi-

bility in final position, PACfinds its most effective uses.

The benefits the use of PAC bringsinclude no need to engineer, fabricate,

install and remove a form system in a re-stricted underground space, which meansthe forms are also not going to block thetunnel during concrete placement opera-tions. Scaffolding is needed, but typicallythere is a need for scaffolding for rebar in-stallation anyway, and in any case scaffold-ing is lighter and easier to transport andinstall than a form system. PAC can be usedwith or without waterproofing, be it sheetmembrane or spray applied, although en-hanced QC will be required for sheet mem-brane systems especially in overheadapplications to ensure the membrane is tightagainst the substrate.

PAC has been successfully used for cav-erns, wyes, cross passages, vent shafts, airplenums, inclined escalator shafts, TBMcrossovers and tunnel junctions, all of whichare locations that render uniform linear ap-plications vulnerable to customization re-quirements. In these locations, PAC affords amonolithic placement process to be utilizedwhile allowing the Designer and Contractorto achieve the needed variations in conform-ing to the ever changing conditions of aproject, which would not otherwise beachievable with a fixed forming system. Fora tunnel system PAC is an ideal placementmethod for the many and varied geome-tries.

While PAC is extremely versatile it is not aprocess that can be used in all locations, forexample, repetitive uniform TBM lining oper-ations are better served using a traditionalform and pour approach as the rate ofplacement of PAC can never equal that of aform.

As noted above the use of PAC requires arigorous Quality Control Process. For exam-

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Typical example of PAC lined structure

Complex Tunnel network lined using PAC.

in the US

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ple when PAC was introduced to a recentproject in New York it became clear that ad-ditional measures would need to be in-cluded in the Quality Control process, bothto ensure a safe and high quality installa-tion, but also to satisfy the requirements ofthe New York State Building Code (NYSBC).The NYSBC includes requirements that needto be met to permit the installation of shot-crete as a structural component. These re-quirements include #5 rebar as themaximum size of rebar to be used, a “6inch” minimum rebar spacing and a prohibi-tion on the use of full contact lap splices.Crucially, however, the NYSBC includes aprovision for a waiver to these requirementsshould the designer be satisfied that full en-capsulation of the designed rebar can beachieved. So to satisfy this requirement thefollowing process was put in place.

In addition to vertical and overhead testpanels for proving the shotcrete mix designto be used, a full size preconstructionmockup of the lining was required to be un-dertaken, utilizing the approved mix designsand the equipment proposed for use in theworks. Working with the designer, the mostheavily congested rebar sections – both ver-

tically and horizontally – were identified andinstalled together with any embedded ele-ments required for the final lining togetherwith the waterproofing system. All nozzle-men were required to demonstrate theirability to completely encapsulate the rebarand provide the requisite compaction ofconcrete required prior to the use of PACbeing authorized for use in the permanentworks. Once the mockup had been sprayedsections were cored and saw cut to demon-strate that the encapsulation had beenachieved.

During production operations regulartesting of the design mix was undertaken,but limited testing of the finished productwere performed. The nature of the applica-tion method, plus using experienced ACICertified nozzle men, ensures that shadow-ing and voids are dealt with as the shotcreteis being placed, due to the close proximityof the nozzleman with shooting from thenozzle taking place mostly from inside thereinforcement. Coring through the finishedproduct was minimized and was typicallyundertaken in early applications only, using“sacrificial” additional rebar to check theencapsulation.

PAC is typically used with a waterproofingmembrane which can either be a PVC orspray applied. In both cases a layer of meshis installed approximately 1 to 2 inchesaway from the waterproofing and hung onthe waterproofing suppliers proprietary an-chor system. This mesh is spaced off thewaterbarrier using bulsters to help hold themembrane back and to give the shotcrete asurface to grip against. This enables over-head applications to be undertaken with lit-tle difficulty. Where a PVC membranesystem is utilized, all waterbarriers used aspart of the waterproofing sectioning systemare equipped with re-groutable hoses to en-sure adequate embedment of the waterbar-riers with the PAC. After the concrete lininghas gained its 28-day compressive strength,grout is injected through the re-groutablehoses to fill any voids between the water-barrier and the PAC final lining.

Similar to form and pour and shotcretefinal linings, contact grouting is requiredwhen PAC is used to fill any voids betweenthe waterproofing membrane and the con-crete final lining. This contact grouting isnot limited to roof sections only, but also aradial and more frequent distribution ofgrouting ports and pipes around the crownand above the spring line. This was imple-mented with the injection of low viscositycementitious grouts between the final PAClining and the membrane to ensure a tightcontact between the initial and final lining.

Application examplesThis method of concrete placement hasbeen used in many different applications in-cluding a 30 inch thick 60ft SEM tunnelconstructed through frozen ground (seetwo pictures below):

Preconstruction vertical (a) and overhead (b) test panel

Pre-construction field trialPAC mock-up and hand

spraying of vertical walls

Encapsulation around reinforcement, embed-ments and encasement of a waterbarrier (onright lower side)

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Client benefitsFor the Client the benefits of using the PACmethod are mainly associated with scheduleand quality. This method is synonymous withACI 506R-16 Guide, using an ACI Certifiednozzleman in the underground which is notcurrently a requirement by ACI.

As no forms are used, there is no need togo through a drawn out process for their de-sign, fabrication, delivery installation and re-moval. As such, the PAC method can be usedthroughout the duration of the project, en-abling the final lining to be installed relativelyquickly after excavation. This can enable fol-low on contracts to enter into these com-pleted sections for access or for completionwork earlier than would be the case with aform and pour lining. In addition, the lack offorms means there is no blocked accessroutes through the area to be lined, althoughthe scaffolding required to install the controlwires and to undertake the concrete place-ment may cause some blockage, it is of a farlesser duration.

The finished space is not now limited bythe need to build and install forms. Continu-ously changing cross sections can be devel-oped that minimize excavation, liningthickness and schedule as the PAC methodcan be used to match the lining to the spacerequirements and the challenge is now backwith the designer to economize on these ele-ments knowing that PAC is a tool in his ar-mory.

With regard to Quality the finished productcan be seen as the work progresses, there is

no waiting until the form is struck to discover,voids, honeycombing etc. these are fixed asthe work progresses with PAC, thereby mini-mizing the need to go back and undertakeremedial works in completed sections of tun-nel. In fact, the PAC method is often used torectify areas where problems have been en-countered with the use of formed concrete.

SummaryForm and pour concrete will continue to bethe prime method of placement of final lin-ings in underground structures. For repeti-tive lining operations such as lining a TBMtunnel over several thousand feet this is in

reality the most practical method of con-crete placement. The PAC method thoughoffers a viable alternative placementmethod for use in non-uniform cross sec-tions, shafts and other areas where the in-stallation of a form would be problematic. Itis certainly not a panacea and requires a rig-orous engineered approach to the design ofthe structures and methods to take advan-tage of its flexibility and quality benefits.The challenge moving forward is to take ad-vantage of this method to provide efficientand economic designs that take into, andaccept the limitations and benefits of thePAC placement method.

And also used toform circularcolumns as partof a ventilationplenum

It has also been used for the creation of aninclined escalator shaft