NINTH INTERNATIONAL RAIL TRACK CONFERENCE PERTH AUSTRALIA 1992.

SHOTCRETE IN RAILWAY ENGINEERING.

SUMMAR'

K.O. CAMPBELL. B.Tech !Civil Eng. l, M.I.E. Aust., C.P. Eng., F.P.W.I. Regional Manager-V ictoria , Cemen t & Concrete Association of Australia.

The shotcret;ng process has been developing continuously since first invented in 1907 from the use ot drymix machine to wet mix, normally premixed concrete for applications, which require a thin layer of protective or structural concrete, particularly on shapes which are costly to form.

Shotcrete is now a proven technique to use in civil engineering applications such as retaining walls and protection of rock faces from weathering. The recent developments in the building industry in Melbourne where one contractor sprayed in excess of 40,000 m3 in basement walls is some measure of the acceptance of this technique to reduce torrnwork costs, improve safety in the workplace and solve difficult construction applications.

In ral iway applications there is now a requirement to improve the visual appearance of exposed concrete. Shotcrete can be coloured and patterned to blend in with the environment.

1. INTRODUCTION

A common concern for the railway engineer is the weathering of rock in cuttings which may become un­stable and dislodge from faces and tumble onto the track. A solution is a protective structural layer of material free-formed to follow the contour of the wall face.

A cost-effective way to do this is to use shotcrete. This requires no formwork or falsework and with the addition of steel fibre, el iminates the necessity for reinforcing fabric and bars. In many of these situations the cost of forming the face would exceed 75% of the cost of the concrete in place. Another situation where shotcrete can be used is In tunnelling where formwork can be el iminated by its use.

2. DEFINITIONS (1)

Shotcrete - Mortar or concrete pneumatically projected at high velocity onto a surface; also known as air-blown mortar.

Dry-mix Shotcrete - Shotcrete in which most of the mixing water is added at the nozzle.

Wet-mix Shotcrete - Shotcrete in which the ingredients including water, are mixed before introduction into the delivery hose; accelerator, if used, is normally added to the nozzle.

Pneumatic Feed - Delivery equipment in which is con­veyed by a pressurized air stream.

Positive displacement - Wet-m ix shotcrete delivery equipment in which the material Is pushed through the material hose in a solid mass by a piston or auger.

3. HISTORY (13)

The pneumatic method of placing mortar was developed in the USA by Carl Akeley, in 1907. On introduction to the construction industry in 1910 this sand/cement . product was given the proprietary name "gunite".

Carl Akeley developed the concrete gun which held a supply of dry mortar which was forced to the nozzle by compressed air and then mixed with the proper amount of water as it was blown onto a wire frame. He achieved a strong, thin coating which did not slump off the frame and eliminated the problems in placing traditional mortar.

In the early 1930's the American Rai lway Engineering Association introduced the generic term "shotcrete" to describe the process. The next development was the introduction of coarse aggregate into the mix changing the product from a mortar to a concrete in the 1950's. With the development of the wet-mix process, there has been significant extension in end uses.

The fi rst reported practical appl ication of steel-fibre shotcrete was in a tunnel at Ririe Dana, Idaho for the US Corp of Engineers in 1972.

A number of Codes of Practice have been developed to assist the correct use of the product.

These include:

-Concrete Institute of Australia, Recommended Prac­tice Sprayed Concrete, February 1987.

-American Concrete Institute 506R.3R-85, Guide to Shotcrete, (1985).

-American Concrete Institute 506.2-77, Specifications for Materials, Proportion ing and Application of Shotcrete, (1977).

- American Concrete Institute 506.3R-82, Guide to Certification of Shotcrete Nozzlemen (1 982) .

-The Concrete Society, UK, Specification for Sprayed Concrete, (1979).

-The Concrete Society, UK, Code of Practice for Sprayed Concrete, (1980).

4. MATERIALS

4.1 Concrete

The materials should comply w ith AS 3600 Concrete Structures, 1988.

4.2 Silica Furne

In addition to the materials described in AS 3600, Con­crete Structures, silica fume is frequently added to im­prove the performance of shotcrete, during placing and insitu.

K.D. CAMPBELL

SHOTCRETE IN RAILWAY ENGINEERING

Figure 1.

2.

Silica fume is can be defined as the very fine pozzolanic mater ial , composed mostly of amorphous silica produced by electric arc furnaces as-a by product of the production of elemental silicon or ferro-silicon alloys. Particles from the filtered- off gas are processed to produce di fferent grades of condensed silica fume.

Densified silica fume is handled like cement and is the most common grade used in premixed concrete. When mixed with Portland cement, silica fume behaves as a pozzolan similar to fly ash.

The fineness of s il ica fume assists In performance of shotcrete by:

- greatly improving cohesion and adhesion, improving overhead spraying and providing better finish. - allowing thicker layers to be used without set accelerators. - reducing wastage of the shotcrete. • reducing bleeding of the concrete.

4.3 Reinforcement (7)

Reinforcement materials should comply with:

-AS 1302-1991 , Steel Reinforcing Bars for Concrete.

St eel Fibre Shotcreting and Curing -AS 1303-1991, Steel Reinforcing Wire for Concrete.

a) ...

d)

~ ~ L .. , NOTlemJ

g)

2Smm with line ooorogoto (gvnite)

-AS 1304-1984, Welded Wire Reinforcing Fabric for Concrete.

• • ;. . . b)

e)

h)

• •

.

' .... • NOTIHSthJ

50mm with c:oars.e o00,e,gotc (shotcroto)

Figure 2. Location of Reinforcement

C)

I)

j)

I NOTle .. than 65mm

K.D . CAMPBELL SHOTCRETE IN RAILWAY ENGINEERING

a) MAS0NIW OA Fl$$UAEO ROCK Pins 1n10 ,oinis

O> TIM.BEA w,,es1.a:ple-s

0 1 l1s$uros

ct CONCRETE OR ROCK $cfCW$ 3nc:t washe<s. Of nads. ,nto drilled ~nd P'V99fcl hOl•.s

di CONCRETEOAAOC:K Sho1·.fi,ed pins with wnhers or llexi~ clips

Figure 3. Fastening Devices for Reinforcement

Steel fibre should comply with the manufacturer's recom­mendations. Steel fibres are generally manufactured by one of three processes:-

(a) Cutting cold-drawn wire (b) Slitting steel sheet (c) Extracting them from a pool of molten steel

The dimensions are typically In the range:

Length Diameter

6.4 to 76 mm 0.25 to 0.76 mm

Steel-fibre shotcrete incorporates steel fibres up to 2% by volume of the total mixture. The improvements In flexural strength, ductility and toughness are sufficient to enable it to be used as a replacement for steel mesh reinforced shotcrete on rock-slope stabilisation and tunnel linings.

4.4 Concrete Mix Design (16)

A typical shotcrete concrete mix design for bank retention would be:

Cement Type GP Silica Fume Slump (Specified nominal)

Water-cement Ratio*

Aggregate (Max size)

350-400 kg/m3

25-40 kg/m

60-90mm

0.57-0.50

7-10 mm

Admixtures Water reducer or retarder and air entrainers

* cement includes silica fume.

3

5. DESIGN

The reinforced shotcrete should be designed according to AS 3600 Concrete Structures. It is general practice to use steel reinforcement fabric in shotcrete th icker than 50mm. Recommended fabric sizes are 100mm x 100 mm which have the least interference with placement. When reinforcement bars are used they should be less than 20 mm diameter and spaced greater than 2 diameters apart.

Design of steel-fibre shotcrete for structural uses is similar to the design of shotcrete reinforced with fabric or bars. However, material properties are significantly different allowinQ different th icknesses to be achieved. Most design data available is for ground support such as tunnel linings.

6. EQUIPMENT (4)

6.1 General

The equipment is required to supply concrete to ensure continuous placement and a continuous supply of clean dry air with adequate nozzle velocity for all parts of the face. Al lowance should be made for a blow pipe to clear away the waste concrete, ie rebound material, from the base of the project.

NOZZLEMAN BLOWPIPE OPERATOR

I ' , Sptayed ooncre!it $!fl}(lfn :i't i

, ','1,.

FllllOMd en<l

BLOWPIPE DETAILS

Figure 4 .

Rct>ovnCI ·-~·~···~--, ·• ••

, Blowp,po \\ ~f•"9 awoy

~oouno

Correct Use of the Blowpipe

X:. D. CAMPBEU.

SHO'l'CRETE IN RAILWAY ENGINEERING

6.2 Comparison of Processes

6.2.1 Drymix

6.2.1 .1 Advantages

- Equipment is small, easily manoeuvrable Into con­fined space, less expensive, easier to operate and less plagued by break-down.

- Running cost lower.

- The drymix can be conveyed In the flexible hose over much greater distances than wetmix.

- Greater impact velocity ensures greater adhesion, especially to overhead substrate.

- Hose pressure Is less, therefore, a burst hose is less dangerous than with wetmix.

- Material already In the hose is not wasted if work has to stop.

6.2.1.2 Disadvantages

- Dust problem is a health hazard especially in con­fined spaces.

- The make-up water at the nozzle can not be metered accurate ly, therefore the water-cement ratio is liable to variation.

- Less out-put, more clean up.

6.2.2 Wetmix

6.2.2.1 Advantages

- Heavy-duty equipment provides large output.

- Water-cement ratio Is more consistent.

- No dust problem.

- Several hoses (up to three) nozzles can work simul-taneously off one pipeline from the pump.

6.2.2.2 Disadvantages

- Hose pressure Is very high, dangerous In case of bursting.

- Equipment is very expensive; requires more skill to operate; more expensive to maintain/repair.

- Impact velocity is less than in case of drymix.

- No opportunity to adjust water-cement ratio at the nozzle.

- Material in the pipeline and hose Is wasted when work stops.

7.

7.1

SHOTCRETE PLACEMENT (4)

General

The use of competent operators Is essential because the quality of shotc rete is dependent on the machine operator and the nozzle man. The nozzle should be held close to perpendicular to the surface being sprayed, at a distance of 0.6 to 1.2 metre.

It is normal to commence spraying from the bottom and work up the wall.

7.2 Surface Preparation

Surfaces of the substract should be trimmed and graded before applying shotcrete.

Soils may need to be compacted to prevent erosion. Chip or scarify any area which may cause an abrupt change in the thickness of the shotcrete otherwise pro­vide suitable reinforcement.

Rock, concrete, brickwork or masonry surfaces should be cleaned of loose or unsound material. All surfaces should be damp prior to the application of shotcrete.

Immediately before shotcreting the surface may need to be scoured clean with an air/water Jet.

7.3 Preparation for Succeeding Layers

After the layer has taken its initial set, it should be cleaned of laitance, loose material and by brooming.

If difficult to remove use wet-sand-blastlng. The surface should also be checked for drummy areas which need to be cut out and replaced.

Surfaces to be shotcreted should be damp.

7.4 Construction Joints

All joints should be deformed or cut back, ie taper con­struction Joints to allow edge form about 25 mm thick . Clean and wet Joint prior to shotcretlng . Make joints perpendicular and continue reinforcement across joints.

~ ~/n ~//;;,;;;/,J a) UNFORMED JOINT (NOT RECOMMENDED)

~//h ~////07v//J c) STOP-ENO JOINT

Fisure S. End-of-day Joints

K.D. CAMPBELL SHOTCRETE IN RAILWAY ENGINEERING

7.5 Finishing

The surface of shotcrete Immediately after placing is rough and undulating and generally requires finishing by one of the following techniques:

• broomed • floated - trowelled · sponge floated · flush finish

Avoid trowelling thin sections of shotcrete unless moist curing can commence immediately.

Do not scrape surface to remove high spots unti l the shotcrete has become stiff enough to withstand the pull of the cutting device.

7.6 Curing

Shotcrete is in a relatively dry condition when applied. Proper curing is required especially because of the th in sections used. The surface should be kept continuously wet for at least seven days. Clean water which will not stain the surface should be used if the shotcrete finish is to have an aesthetic appearance.

Liquid membrane-forming curing compounds may be used if the drying conditions are not severe, no surface treatment is required and the compound is aesthetically acceptable.

.,.)

In underground work, tunnels etc, when the temperature and humidity are more favourable curing may not be as critic:=il.

8.

Figure 6 . Complet ed Shotcret e Pr otection for

Rock Face from Weathering APPLICATIONS

8. 1 General

Sprayed concrete is traditionally used for concrete con­struction to replace expensive formwork. The cost of placing the concrete can be more expensive than by using traditional placing methods eg pumping, but with the significant saving in formwork and the ability to place thinner sections there are considerable cost advantages.

In railway engineering shotcreting is commonly used for:

- Tunnel linings as structural elements and for protec­tion, eg. water proofing.

5

- As a repair material, to strengthen the brickwork, con­crete and steel elements, or to protect them from the environment or to improve the appearance due to deterioration of the exposed face.

- For stabilisations and/or protection of earth and rock faces.

In many of these applications shotcrete has an advantage over conventional concrete if access to the work face is difficult, thin layers are required or there is a necessity to use varying layer thicknesses.

In slope stabi lisation or rock protection the recent intro­duction of steel fibre shotcrete has eliminated the neces­sity to prefix steel reinforcement fabric to the substrate. Steel fib re shotcrete has proven to be cost effective In slope protection, ground support in tunnels and concrete repair.

8 .2 Rock Stabilisation • Sweden (7)

At an oil refinery on the west coast of Sweden about 4500 m2 of rock face was stabilised.

A layered construction was used.

5 to 10 mm of plain sllotcrete followed by 20 mm of steel fibre shotcrete covered by a top layer of 5 to 10 mm of plain shotcrete.

Mix details for Steel Fibre Shotcrete were:

Mix Proportion

Cement 10 mm Aggregate Coarse sand Fine sand Steel fibres

Kg/m3

439 362

1,143 223 59

8.3 British Rail - Arch and Tunnel Relining (7)

Steel fibre shotcrete is used to strengthen tunnels and brick arches under bridges by British Rail in England. It is applied up to 150 mm thick. A 13 mm coat of plain shotcrete is used to cover exposed fibres. One example of rail tunnel work is that the scaffolding required for steel fabric installation can be eliminated and traffic interrup­tion is limited.

The technique used was to bulk excavate 1.2 to 1.5 metres in depth and bench to facil itate construction, placement of shotcrete and reinforcement. The reinforce­ment fabric of F82 was held off the earthworks by 50 mm plastic bar chairs. These chairs were used in preference to steel chairs which would have penetrated the soft sand or clay.

The excavation was in tertiary sand over silurian clays. Block outs were used on the face for later soil drilling for the soil nailing. The soil nails were either Y24 or Y20 bars, ranging in length from 3-7 metres and spaced at 1.5 metres in the sand and 1.2 metres in the clay.

K.D. CAMPBELL

SHOTCRETE IN RAILWAY ENGINEERING

The steel reinforcement was encapsulated in polyethylene tube in 130 mm diameter holes and then grouted internally and externally.

There were drilled drains, 75 mm diameter, 1.5 m spac­ing, and lined with agricultural pipe, slotted PVC tube with a filter sock located 300 mm above the sand clay interface at 10 ° to the horizontal to release water trapped above the c_lay.

The maximum number of lifts or benches were seven with the maximum depth being 9 metres. The area covered exceeded 3,000 m2.

There was a no-fines drain at the base to prevent under­mining.

The concrete used was a low slump, pumped concrete of 3:> MPa characteristic strength, with 14 mm maximum sized aggregate.

Since the cutting was in a prestigious residential area, the shotcrete was lined with coloured precast concrete panels for aesthetic reasons.

8.4.2 Alexander Avenue Cutting

The road cutting was excavated through limestone in the 1930's, over the next 50 years the rock weathered result­ing In small and large boulders falling on to the roadway.

To stabilise the rock, tension bolts were cast into the larger boulders on the face or the cutting. The rock bolts were 3-5 metres long.

A network of drainage holes were drilled through the intercepted fissures. A nominal steel mesh was curved to the shape of the rock faces and welded to the rock bolts.

A concrete parapet was cast at the top of the wall. The face was sprayed with shotcrete, using ofl white cement and brown aggregates to match the surrounding stone cutting.

An agricultural drain was constructed about 0.5 metre down the face to prevent water staining the shotcrete.

There are periodic inspections for cracking of the shotcrete and to clean out the drainage channels.

8.4.3 Jacana Underpass

Rock bolts and steel fibre shotcrete were specified for the stabilisation of the terrace walls.

The terrace walls were excavated in stages such that the· cut excavation does not exceed 1.5 m below any rock bolt or natural surface prior to the installation and grouting of the next lower rock bolt.

The rock bolts were galvanised Y24 bars in lengths of 3.5 or 7.5 metres and spaced at 1.3 m centres both horizon­tally and vertically and inclined at 15° to the horizontal.

The rock bolts were placed in pre-drllled 115 mm diameter holes with plastic bar spacers and grouted with 25 MPa grout.

Strip drains (100 mm x 40 mm thick "Nylex") were placed at 1.3 m centres. F81 galvanised steel fabric was used in the shotcrete.

Shotcrete of 35 MPa compressive strength and a maxi­mum aggregate size of 9.5 mm was sprayed on the cut face to achieve a minimum thickness of 11 O mm.

The shotcreted surface was then screeded and !rowelled to a surface fin ish such that the maximum irregularity in the surface does not exceed 10 mm in 2.5 metres.

The shotcreted terrace was clad with a smooth rock facing consisting of 300 mm broken basalt fixed to the wall and mortared.

8.4.4 Basements - Melbourne (16)

During the construction boom in the Central Business District of Melbourne there have been considerable developments In the use of shotcrete with the addition of silica fume which have shown the potential of earth con­tainment to considerable depths. The deepest structural shotcrete wall is 8 basement levels in depth. There was extensive investigation of the quality of shotcrete for the Southgate project. A 40 MPa concrete was used as shotcrete on the first basement.

Cores were taken and tested at 1 O days showing com­pressive strength of 33 MPa average and unit weight averaging 2250 kg/m3. Standard test cyclinders were made on one batch; compressive strength was 32 MPa at 5 days and 37 M Pa at 7 days.

Visual inspection showed compaction around the water stop and steel reinforcement was excellent. However, compaction In a test area to trial the use of expanded metal mesh was not good and has not been used with shotcrete since.

This method of constructing basements has several ad­vantages, including:

a. Reduced risk of core in from the loose, crumbly earth.

b. Cave in would be noted and cleared prior to shotcreteing.

c. Faster Construction In addition to routine standard test samples, a steel box was sprayed full of concrete on each placement. This was done to verify the effectiveness of shotcreting and operators. These samples were taken to the laboratory after one day demolded and placed in the curing tank. At age 7 days the block was cored; the cores were given standard moist curing for 28 days, then compression tested.

K. D. CAMPBELL

SHOTCRETE IN RAILWAY ENGINEERING

Test Cylinders

Slump (mm) Unit weight (kg/m3) 28 day of compressive strength (MPa)

84 2230

52.4

84 2280 kg/m3

48.1

These results confirmed the excellent quality of silica fume shotcrete.

Shotcrete Mix Proportions

Max. Aggregate size

Water/(Cement + silica fume)

Retarder

9. REFERENCES

7 mm

0.43

370ml/100 kg

1. American Concrete Institute Committee 116, "Cement and Concrete Terminology• Manual of Concrete Practice Vol. 1. American Concrete Institute Detroit, 1990.

2. Standards Australia, AS 3600:1988, "Concrete Struc­tures", SAA, Sydney, 1988.

3. Concrete Institute of Australia, "Recommended Prac­tice for Sprayed Concrete•, Concrete Institute of Australia, Sydney, February 1987.

4. American Concrete Institute 506R-85, "Guide to Shotcrete", American Concrete Institute, Detroit, 1985.

5. American Concrete Institute 506.2-77, "Specifications for Materials, Proportioning and Applicat ion of Shotcrete", American Concrete Institute, Detroit, 1977.

6. American Concrete Institute 506.3R-82, "Guide to Certification of Shotcrete Nozzle men", American Con­crete Institute, Detroit, 1982.

7. American Concrete Institute 506.1 R-84, "State-of-the­Art Report on Fibre Rein forced Shotcrete", Amercian Concrete Institute, Detroit, 1984

8. The Concrete Society, UK, "Specification for Sprayed Concrete•, The Concrete Society, U.K., 1979.

9. The Concrete Society, UK, "Code of Practice for Sprayed Concrete", The Concrete Society, U.K., 1980.

10. Moore, J.A., "Dry and Wet-mix Process Shotcrete", Concrete Constructjon, Vol. 29 No. 7, July 1984, pp. 629-630

11. Crom, T.R., "Nozzling Dry-mix Shotcrete", Concrete Construction, Vol. 29 No.7, July 1984, pp. 641-647

12. Ramakrishnan, V., et al, "A Comparative Evaluation of Fibre Shotcrete", Concrete International, Vol. 3 No-.1, January 1981 , pp. 59-69

7

13. Allentown Pneumatic Gun Inc., "Shotcrete", PA, USA

14. Morgan, D.R., ''High Early Strength Blended Cement Wet-mix Shotcrete", Concrete International, Vol. 13 No.a, May 1991 , pp. 35-39.

15. Seegebrecht, G.W., "Durability of Dry-mix Shotcrete, Concrete International. October 1989, pp 47-50

16. Burnett, I.D., "Silica Fume Concrete In Melbourne, Australia, Concrete International, August 1991, pp. 18-24