HPC has been used in agricultural buildings where the impermeability of HPC provides durability against the aggressive agents from the animals and feed. Used in swine nurseries, HPC reduced infections by inhibiting the growth of harmful bacteria, which normally breed in the pores of conventional concrete. The result was heavier pigs and higher profits to the breeder (Gagné et al, 1994).
60 MPa HPC was also used for the floor slab of a poultry smokehouse.
In recent years there have been many large pig-breeding factories established on farmland. In these facilities, the pigs live on precast slats above a lower level where wastes are collected. Previously the precast slats were made with conventional concretes and these had a short service life in the aggressive environment.
Research has been carried out which confirms that precast slats made with HPC provide significantly longer service life than the quality of concrete usually used (Idriss et al, 2001). Two series of tests were made to determine the time to corrosion of steel reinforcement in mortar specimens exposed to hydrogen sulphide.
In the first series, the specimens were subjected to impressed voltage and electrochemical potential tests. For the impressed voltage tests, 95.5 mm diameter cylinders 200 mm long were cast with a 11.3 mm diameter 400 MPa reinforcing bar in the middle. After 28 days wet curing, the specimens (three replicates) were immersed in a solution of 2000 ppm of H2S for a year. The reinforcement was then connected to a 40-volt direct current to make
it anodic with respect to an external cathode, which was connected to the negative pole of the DC source. The time to develop a longitudinal crack was noted.
For the electrochemical potential tests, specimens 100 mm by 100 mm by 30 mm thick were cast with an 11.3 mm diameter reinforcing bar in the middle. Five stainless steel wires were attached to the reinforcing bar, three of which protruded from the specimen. The other two were attached at the ends of the bar outside the test specimen. The specimens were wet cured for 28 days. The specimens (three replicates) were immersed in H2S for 650 days and half-cell readings taken every 10 days starting at day 60. Once
the readings indicated active corrosion, the specimens were tested to determine the concentration of sulphides that initiated corrosion.
Failure time Susceptibility to Mix Tc min Corrosion, µA-1min-2
Type 10 w/c ratio 0.55 165 62.5
In the second series of tests, diffusion tests were made on specimens immersed in H2S
and epoxy coated on all but one surface. The time to reach the threshold value for corrosion in the Type 10E-SF 0.35 w/c ratio mix was mo re than eight times as long as in the Type 10 0.55 w/c ratio mix.
From these tests, it was concluded that the life of slats made with the Type 10E-SF mix would be double that of slats made with conventional concrete.
In Ontario, all the precast concrete producers use HPC for farm applications. In addition to slats, other HPC products are manger feeder bottoms, kennel floors and bunker silos.
REFERENCES
Gagné, R., Chagnon, D. and Parizeau, R., "Utilization of High-Strength Concrete in the Agricultural Industry", Concrete Canada Technology Transfer Workshop, Sherbrooke University, October 1994. Idriss, A.F., Negi, S.C., Jofriet, J.C. and Hayward, G.L., "Corrosion of Steel Reinforcement in Mortar Specimens Exposed to Hydrogen Sulphide, Part 1: Impressed Voltage and Electrochemical Potential Tests", In publication, Journal of Agricultural Engineering Research.
Idriss, A.F., Negi, S.C., Jofriet, J.C. and Hayward, G.L., " "Corrosion of Reinforcing Steel in Mortar Specimens Exposed to Hydrogen Sulphide, Part 2: Diffusion Tests", In publication, Journal of Agricultural Engineering Research.
Chapter 8 Shotcrete
The development, in 1910, of a double-chambered gun enabled a sand-cement mortar called Gunite to be sprayed onto structures. This process, with coarse aggregate added in the 1950s, became known as dry-mix shotcrete. By 1960, the development of pneumatic equipment made wet-mix shotcrete practicable. Widely used for repairs, the shotcrete process exhibited practical deficiencies such as excessive rebound, and, even with the addition of polymers, was not durable.
During the 1980s, many improvements were made to material science, plant and procedures, which made the use of high performance shotcretes possible. These were summarized as follows (Morgan, 1992):
Materials:
• “The use of steel and polypropylene fibre reinforcement in lieu of conventional mesh reinforcement;
• The use of special high early-strength cements, in lieu of accelerated shotcretes, in projects requiring early-strength development;
• The use of supplementary cementing materials, such as fly ash and silica fume, as additions or partial replacements, for the special benefits that these materials impart to shotcrete.
Plant and Procedures:
• The use of dry-batched, premix materials, supplied in either small (typically 30 kg) paper bags or large (typically 1600 kg) synthetic cloth, bulk bin bags;
• The use of mobile volumetric batching equipment with special dispensers for materials such as steel fibre reinforcement and silica fume;
• The use of robotic placement equipment, particularly for repair in road, rail, sewer and water tunnels;
• Improvements in wet-mix pumps and dry-mix guns;
• Improvements in ancillary shotcrete equipment, such as shotcrete nozzles and predampening units for use with dry, premixed shotcretes;
• Special dispensing units for addition of silica fume slurry at the nozzle, with or without accelerators, in the dry-mix shotcrete process”.
In 1993, a study was made of four dams in British Columbia, which had previously been repaired with shotcrete (Heere et al, 1996). Recommendations for repair were made based on current high performance shotcrete technology.
In 1995, the multiple arches Littlerock Dam in California was repaired using silica fume steel fibre reinforced air-entrained shotcrete (Forrest et al, 1995), and Canadian technology. The shotcrete was used, as part of a seismic upgrading, to stiffen the arches of the dam.
The good practice measures taken to ensure a competent repair were as follows:
• Surface preparation of an adequate roughness profile (which was measured), suitable moisture condition, cleanliness and hardness.
• Anchors were grouted in a 1.2 m grid.
• A 28 day compressive strength of 41 MPa, 60 kg/m³ of steel fibres, 400 kg/m³ of cementitious material with 10 5 of the total silica fume, and an air content of 10- 12% at the pump to ensure about 5 +/- 1% in place.
An extensive preconstruction programme was carried out covering surface preparation, bond testing, core grading, shotcrete testing for physical properties including toughness, and anchor testing. Intensive construction monitoring included bond tests and coring to examine shotcrete thickness and compressive strength.
The bond test requirement of 1 MPa was met at all locations except two. At these locations adjacent retests passed. Compressive strengths of cores ranged from 40 MPa to 67 MPa, and all cores met grading requirements. A total of 4500 m² of shotcrete overlay were placed.
Berth faces in the Port of Montreal were repaired using steel fibre and polyolefin fibre shotcretes (Morgan et al, 1998). A total of 866 m2 were repaired. The object of the trial repairs was to compare performance and costs with the cast-in-place concrete repairs used before. The berth faces were built early in the 20th century and were in a seriously deteriorated condition despite previous repairs.
An extensive preconstruction testing programme was carried out, which included the qualification of all the nozzlemen allocated to the project. The compressive strength of cores taken from the finished work had compressive strengths from 50.6 to 61.4 MPa. The cost of the shotcrete repair was $515/m² compared to $550/m² for the cast-in-place repairs previously made. An inspection after one winter showed no deleterious defects. Berth faces at the Port of Saint John were also repaired with high performance shotcrete (Morgan et al, 1996). The berths had been constructed at various times from the 1900s to the 1950s. In 1986, a 10-year repair programme started using air-entrained, wet-mix, steel fibre reinforced, silica fume shotcrete.
In 1995, 9 years later, and after 2000 freezing and thawing cycles in a saturated condition, the shotcrete repair was inspected. No significant deterioration was found. Cores extracted from the repairs had compressive strengths from 53.5 to 60.1 MPa, slightly higher than cores from test panels nine years previous.
Costs for these repairs during the period 1989 to 1995 ranged from $126 to $270 per m² and averaged $172 per m².
HPC Shotcrete has been used in the repair of many marine structures including an oil- drilling platform (Morgan, 1997). Wet-mix shotcrete with a specified 28-day strength of 80 MPa and a direct tensile bond strength of 2 MPa was used.
In BC, the Ministry of Transportation and Highways has used fibre-reinforced HPC shotcrete to stabilize creek beds and protect bridge piers and abutments from scour.
In BC, an historic building, the Vancouver Block, was rehabilitated using HPC shotcrete (Chan and Morgan, 2000). Wind-driven rain had penetrated the walls and caused severe deterioration of the masonry walls and corrosion of the structural steel.
In 1997/98, at a cement plant in Bath, Ontario, a cement storage silo with a capacity of 68,000 tonnes was constructed of high performance shotcrete. The dome is 55 m in diameter and 29 m high. An inflatable form was attached to the foundation ring beam. After inflation, the interior was sprayed with polyurethane insulation and reinforcing steel fixed. The concrete was then sprayed on the inside in a single layer. Wall thickness varies from 0.51 m at the base to 0.15 m at the top of the dome. The inflatable form remains in place as permanent waterproofing.
The wet mix shotcrete used had the following proportions:
• Type 10E-SF cement: 480 kg/m³
• Coarse aggregate: 200 kg/m³
• Fine aggregate: 1340 kg/m³
• Water: 215 kg/m³
Water reducing and air entraining admixtures produced the desired set characteristics, a slump of 100 to 125 mm and an air content of 5-8%. The low rebound of 12% was attributed to the use of silica fume and a low coarse aggregate content in the mix. Test panels were made and cored. A summary of test data is as follows:
Concrete Cylinders Concrete Cores
7 days 28 days 7 days 28 days
No of tests 89 89 12 24
Mean strength: MPa 43.1 55.8 32.9 43.9 Standard deviation: MPa 3.43 4.68 6.85 5.35 Coefficient of variation: % 8.0 8.4 21.0 12.0
A major use of high performance shotcrete is the support of rock in tunnels, particularly in mines. A comprehensive review of this application was produced by an admixture supplier's underground construction group (Melbye, 1996). The need for the correct specification and application of specifications for shotcrete resulted from conflict between specifiers and contractors (Wood, 1992). The need for preconstruction qualification testing and ongoing QC testing during construction was emphasized. Two case histories are given in detail.
Details of typical high-performance mixes and performance specifications are reproduced in the following tables:
Components Dry Mix Wet Mix
kg/m³ % of dry kg/m³ % of dry Materials Materials Cement 420 19 420 18.1 Silica fume 50 2.2 40 1.7 Aggregate (blended) 1670 75.5 1600 68.9 Steel fibres 60 2.7 60 2.6 Accelerator 13 0.6 13 0.6 Superplasticizer 6 litres 0.3
Water reducer 2 litres 0.1
Air entraining agent if required
Water 180 7.7
Characteristic Test Method Minimum Required
ASTM C42 5 Uniaxial compressive strength: MPa 8 hr (accl.) 1 day " 10 3 day " 20 " 30 7 day 28 day " 40 ASTM C1018 4
Flexural strength: MPa 7 day 28 day " 6 ASTM C1018 3.5 Toughness index I5 I10 " 5 Boiled absorption: % ASTM C 642 8 max Voids volume: % " 17 max Accelerated set: min ASTM C 403 Initial: 10
" Final : 30 Aggregate gradation ACI 506R-90 1,2 or 3
Note: The standards quoted in the above table have all been updated since 1992.
The paper concluded that the complex installation of brittle layers of shotcrete and wire mesh could be replaced with a single pass of steel fibre reinforced silica fume shotcrete. An outstanding example of the application of high performance shotcrete in the mining industry is Inco’s Stobie mine in Sudbury (O'Hearn et al, 1998). A number of extensive
in-situ trials were made to confirm that steel fibre reinforced silica fume shotcrete could safely replace the use of traditional shotcrete plus wire mesh reinforcement and rock bolts. The trials also served to optimize the wet mix that was used.
The final proportions per m³ were as follow:
• Type 10 cement: 400 kg • Silica fume: 40 kg • Water: 180 kg • Superplasticizer: 850 ml/100 kg of cement • Coarse aggregate: 350 kg • Fine aggregate: 1,275 kg • Steel fibre: 50 kg
A high degree of quality control was exercised during construction. Analysis of the results showed not only that the high performance shotcrete system was cheaper and easier to install than traditional shotcrete systems, but the safety margin of the finished installation was better.
REFERENCES
Morgan, D.M., "New Developments in Shotcrete for Repairs and Rehabilitation", Proceedings, Advances in Concrete Technology, Athens, May 1992, pp. 699-739.
Heere, R., Morgan, D., Banthia, N. and Yogendran, Y., "Evaluation of Shotcrete Repaired Dams In British Columbia", Concrete International, Vol 18, no 3, March 1996, pp. 24-29.
Forrest, M.P., Morgan, D.R., Obermeyer, J.R., Parker, P.L. and LaMoreaux, D.D., "Seismic Retrofit of Littlerock Dam", Concrete International, Vol 17, no 11, November 1995, pp. 30-36.
Morgan, D.R., Rich, L. and Lobo, A., "About Face-Repair at Port of Montreal", Concrete International, Vol 20, no 9, Septenber 1998, pp. 66-73.
Morgan, D.R., Bremner, T.W. and Gilbride, P., "Repair of Berth Faces at the Port of Saint John, New Brunswick", Concrete Canada Technology Transfer Day, University of Moncton, August 1996, pp.61- 72.
Morgan, D.R., "Shotcrete Repair of Infrastructure in North America", Beton-Instandsetzung '97, Igls, Austria, January 1997, pp. 21-37.
Chan, C. and Morgan, D.R., "Infrastructure Repair and Rehab: Shotcrete: Spraying on the Solution", CE News, March 2000, pp. 66-71.
Hopkins, D.S., Cail, K., Robert, N. and Thomas, M.D.A., "World's Largest Dome for Cement Storage Silica Fume Shotcrete", Canadian Society for Civil Engineering, Proceedings Annual Conference, Regina, June, 1999, pp. 117-133.
Melbye, T.A., "Sprayed Concrete for Rock Support", MBT International Underground Construction group, Switzerland, 1994.
Wood, D.F.,"Specification and application of fibre reinforced shotcrete", Rock Support in Mining and Underground Construction, Kaiser and McCreath, Editors, Balkerma, Rotterdam, 1992, pp. 149-156. O’Hearn, B., Buksa, H. and Walker, S., "Stobie signals shotcrete success", Engineering and Mining
Chapter 9 Emerging Technologies