0204kusterl-shotcrete

22 Shotcrete Magazine • Fall 2000

I n 1989, the Austrian Concrete Society pub-lished a Guideline on Shotcrete that subse-quently served as a reference work and a con-

tractual basis for shotcrete works in tunnel con-struction in Austria and abroad. Other standards,

such as DIN 18551, focused more strongly onshotcrete for concrete repair works. At the end of1998, after several revisions, a new edition of theAustrian Guideline was published that also con-siders recent developments in the field of shot-

crete technology. Dealing with awhole range of issues, from base ma-terials for sprayed concrete produc-tion to contractual aspects, the 80-page guideline will retain its value asa document of topical importance,even after publication of a EuropeanStandard (Working Group TC 104/WG 10). The text of the guideline isalso available in English.1

��

The mixture is the base product fordry-mix and wet-mix sprayed con-crete. Depending on the procedureapplied, three types of mixture can beused for dry-mix sprayed concrete(Table 1). Due to environmental con-siderations and efforts to increaseshotcrete quality, only non-alkalineaccelerators or special binders, calledspray cements, with short setting timeare now used in tunnelling. Dry-mixis still frequently used in central Eu-rope as many tunnels are constructedin poor rock conditions.

In dry-mix production, the risk ofsegregation during material transportshould be kept in mind; a maximumtemperature of 40 C (104 F) must notbe exceeded. Moist-mix for immedi-ate placing is produced in a continu-ous mixing plant upstream of thespraying machine. Accurate dosages,a uniform moisture content of the ag-gregate, and a short working time areimportant. Both moist-mix and wet-mix have to be used within no morethan 1.5 h of mixing.

�� “The Guideline on Sprayed Concrete”provides a reliable basis for everyconstruction site to select and apply

��

By WolfgangKusterle

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Table 1

Shotcrete mixture classification, abbreviations:TM: dry-mix for the dry-mix process with oven-dry aggregatesFM-L: moist-mix, storable, for the use in dry-mix process (conventional dry-mix process)FM-S: moist-mix, for immediate application in dry-mix process (for use with spray cement)NM: mixture for the wet-mix processTZ: tunnel cement, portland cement particularly suited for tunnellingSBM: spray cement — general term used to designate fast-setting binders that ensure the required rate of setting

and strength development in fresh sprayed concrete without addition of acceleratorSBM-T: spray cement for sprayed concrete with dry aggregatesSBM-FT: spray cement for sprayed concrete with moist aggregates (short contact time)EB-AF: non-alkaline accelerator

Shotcrete Magazine • Fall 2000 23

the most suitable spraying method. The advan-tages and disadvantages of non-alkaline sprayedconcrete technologies are summarized in Table 3.Descriptions of the individual procedures are ac-companied by instructions for proper application.These are quite similar to North American guide-lines, so there is no need to go into detail. It shouldonly be mentioned that special emphasis is laidon the proper metering of the accelerator. Thereis also experience in spraying shotcrete onto fro-zen substrates, requiring an increase in the sprayedconcrete layer thickness of 20 to 30 mm (0.8 to1.2 in.), and using a mixture with a minimum tem-perature of 13C (55.4 F).

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Unfortunately, the guideline does not give anydetailed specifications on nozzleman training orqualification.

��

�� Depending on the application and the structuralfunctions to be fulfilled, sprayed concrete is clas-sified as follows: sprayed concrete without struc-tural functions (Class SpB I), sprayed concretewith structural functions (Class SpB II), andsprayed concrete with special structural functions(Class SpB III).

Table 2: Indicative values for mix composition for SpB II and SpB III.*

*SpB = sprayed concrete; GK = maximum grain size; I,II,III = sprayed concrete classes; II being normally used for rock support uses, III,for example, for repair works).† With binder doses of less than 340 kg/m³, adhesion of the sprayed concrete to the substrate is noticeably diminished.**10 kg/m3 is approximately equal to 17 lb/y3 ( 16.86 lb/y3).

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Table 3: Comparison of non-alkaline sprayed concrete technologies.*

*The moist-mix procedure using a non-alkaline accelerator has not been included (this table is not part of the guideline).


This classification also determines the fre-quency of testing for quality control purposes. Fur-thermore, fresh sprayed concrete is classified onthe basis of early strength classes, J

2 being the

regular requirement for tunnelling with about 200mm (8 in.) layer thickness, J

3 being necessary only

under very difficult ground conditions or strongwater ingress.

The strength requirements and other proper-ties are based on the provisions of the existingconcrete standards.

��

Special procedures dealt with in the Guideline in-clude sprayed concrete under compressed air (un-changed from 1st edition), and steel-fiber sprayedconcrete. In Austria, steel-fiber concrete is notyet used very widely; hence, the requirementshave been used from other countries. An evalua-tion of the results obtained in the bending-beamtest according to R. Morgan has been includedin the Guideline.

By means of this method, the strength of

sprayed concrete can be evaluated even at an earlyage, which is essential for the use of sprayed con-crete for support in difficult rock conditions.

��

A large part of the Guideline deals with testingand testing requirements. The tests referred to areshown in Table 4.

Tests are to be performed in in-dividual steps, starting with basematerials, up to and including in-spection of mixing and meteringequipment. Non-compliance withthe quality criteria to be met byhardened sprayed concrete is to beavoided through systematic estab-lishment of important parametersbefore and during production.

Table 11 of the Guideline con-tains a clear overview and a bind-ing list of the testing requirementsto be met. The individual tablesonly refer to the relevant require-ments for each spraying method.In regards to the testing of base ma-terials, the tables refer to prelimi-nary testing, self-inspection and in-ternal inspection on the construc-

Figure 1: Hand application of dry-mix shotcretewithout keeping nozzle within correct distanceto substrate.

Figure 2: Early strength requirements to be met by fresh sprayed concrete (between Aand B: class J

1, between B and C: class J

2, above C: class J

3).

Figure 3: Bound-aries of flexuraltoughness rangesfor fiber-reinforcedsprayed concrete(example).

Shotcrete Magazine • Fall 2000 25

tion site. Information on preconstruction testing,production testing, and structural testing is pro-vided (Table 5).

�� Many test procedures for base materials followthe national specifications, but it is very impor-tant to provide a reliable indication of the interac-tion of cement and accelerator on the behaviourof spray cement. This testing is done using a modi-fied Vicat test and tests on small cylinders orprisms [40 x 40 x 160 mm (1.6 x 1.6 x 6.3 in.)].

Special instructions are given to ensure fastmixing and placing of the samples. Moreover, anidentification test of the accelerator, includingchemical properties, is obligatory.

Testing of base materials for concrete produc-tion, though a meaningful approach, will neverbe able to simulate the spraying procedure com-pletely. Hence, testing of the reference sprayedconcrete, in addition to laboratory testing as speci-fied above, is provided for. A reference formulafor sprayed concrete for small laboratory spray-ing equipment is specified.

Following a tradition established in the firstedition, the Guideline devotes considerable atten-tion to the testing of fresh sprayed concrete. Itspecifies the needle penetration test for a strengthof up to 1.2 MPa (174 psi), and the stud-driving

method for the range of strength from1.0 to 16 MPa (145 to 2320 psi)(Fig. 4). The penetration needle isused to measure the force required topush a needle of 3 mm (0.12 in.) di-ameter and a tip with a taper angle of60 degrees to penetrate into the shot-crete to a depth of 15 mm (0.6 in.).The instrument indicates the resist-ing force through compression of aspring, from which an estimated com-

pressive strength can be derived from a conver-sion curve (Fig. 5).

For the stud-driving method, a threaded studis driven into the shotcrete and the depth of pen-etration is determined. Then, the stud is extractedand the pullout force measured. The ratio of pull-out force to penetration depth (or for low-strength,only penetration depth) can be used to obtain anestimated strength from conversion curves for dif-ferent maximum aggregate grain size (Fig. 6).

The stud-driving method has been extended tocover the entire range of strength development.However, values above 16 MPa (2320 psi) can onlybe estimated. For aggregates other than those nor-mally used in central Europe, mixture calibrationis recommended, which can be performed quicklyin any testing laboratory.

Testing of sprayed concrete after 28 days isperformed on test cores according to the concretestandards. Special tests for sulfate resistance andleaching of sprayed concrete are referred to. Sul-fate attack is more common in the Alps than al-kali aggregate reactivity. Leaching may be a prob-lem when using alkali aluminates or silicates asaccelerators. During sprayed concrete production,inspections of dosages (cement content, accelera-tor dosage), rebound, fine-dust concentration,thickness of the sprayed concrete layer, and fibercontent are essential. Fine-dust concentration is

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Table 4: Reference to testing in the guideline

Table 5: Overview of Table 11 in the Guideline: Testing of base materials, mix and sprayed concrete, inspection ofmixing and metering equipment


measured by light-scattering instru-ments. Conversion of the relativedust concentration into fine-dust con-centrations obtained by gravity-based samplers can be done by con-version factors ranging from 1.1to1.5.

References

1. Austrian Concrete Society,“Guideline on SprayedConcrete,” Vienna 3/99,e-mail: [email protected].

Wolfgang Kusterle received hisdiploma in civil engineering atthe University of Innsbruck in1980. He then worked for 2 yearswith a consulting engineerdesigning bridges and otherreinforced concrete structures. In1983, he started working on hisdoctoral thesis at the University

of Innsbruck, which he finished in 1984 with a PhD incivil engineering. He stayed at the Institute for Build-ing Materials and Building Physics as assistant pro-fessor and later as associate professor. His mainresearch areas are sprayed concrete and concreterepair. He is involved in national and internationalresearch programs and has published more than 60papers. Consulting in concrete technology, productdevelopment, and the organization of conferences aresome of his other areas of work.

Figure 4 : Measuring procedures for the testing of fresh-sprayed concrete (Hilti 450“L” being thestud-setting unit used).

Figure 6: Testing equipment for stud-driving method.Figure 5: Penetration needle in use.

0204kusterl-shotcrete

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