RCP: Coulombs Mix Element Air %
Kg/m³ Max mm* mm† Min Max
HPC 1 All decks and slabs 5-8 340 0.37 70 150 50 1000 HPC 2 Barriers and medians 5-8 340 0.37 70 150 40 NA
All mixes contain 8% silica fume and a corrosion inhibitor. *Before superplasticizer, † after superplasticizer.
HPC cannot be placed if the anticipated air temperature is expected to exceed 22°C. Temperature of concrete at discharge must be between 10°C and 18°C, using ice or liquid nitrogen if needed to not exceed 18°C.
Immediately after finishing fog misting, evaporation retarder or special curing compounds shall be applied to the concrete surface.
Penalties are the same as the Calgary specification except for compliance with maximum RCP values for which the penalties are as follows: 1001- 4000 coulombs: $40/m³, > 4000 coulombs: $250/m³ or remove.
The Read Crowther specification contains the following key criteria:
Slump Mix Element Admixtures
Air % Cement Minimum Kg/m³ W/C Ratio Max mm mm HPC 1 Deck, superstructure, barriers, toppings CI 5-8 340 0.37 70 150
HPC 2 Barriers, toppings Fibres, SRA 5-8 340 0.37 70 150 HPC 3 Barriers, toppings Fibres, CI 5-8 340 0.37 70 150
CI: Corrosion inhibitor, SRA: Shrinkage reducing admixture.
Mix 28 Day Strength: MPa
Tendency to crack: Width (mm) x Length (m) per m² of Surface Area*
RCP: Max Coulombs
HPC 1 50 Comparative evaluation: 0.25-0.35 700 +/- 35 % HPC 2 40 Comparative evaluation: 0.25-0.35 700 +/- 35 % HPC 3 40 Comparative evaluation: 0.25-0,35 700 +/- 35 %
*Cracking for the area under consideration will be determined by measuring the estimated total crack length of cracks greater than 0.2 mm wide multiplied by the average crack width of each crack and then divided by the area under consideration in square metres, as defined by a perimeter 500 mm beyond the nearest crack under consideration.
HPC will not be placed if the air temperature is anticipated to exceed 22°C. Concrete temperature, as delivered, shall be between 10°C and 18°C, and ice or liquid nitrogen shall be used to ensure that the concrete temperature does not exceed 18°C.
Curing with pre-wetted burlap shall begin no more than 30 minutes after final finishing, and be continued for at least 7 days.
Penalties in this specification are as follows:
3.5 MPa to 4.5 MPa below $ 50/ m³ Strength
> 4.5 MPa below $ 100/m³or remove Up to 0.2 % outside range $ 60/m³
Air Entrainment
> 0.2 % outside range $ 150/m³or replace 1001-2500 coulombs $ 250/m³
Permeability
> 2501 coulombs No payment, plus acceptable protection or replace
For cracking, as measured by the definition shown above, the penalties are as follows: 0 to 0.30 No deduction
0.31 to 0.6 $ 100/m³ plus specified repair 0.61 to 1.0 $ 200/m³ plus specified repair
> 1.0 No payment plus acceptable protection system or replace
British Columbia
As in other provinces, there was not suddenly a day when all concrete became HPC. Requirements for high strength and durability became imperatives before 1990. The Annacis Bridge is a case in point (Taylor et al, 1986). In order to meet strength, weight and durability criteria, the deck was cast in a High-Strength HPC, it just was not called HPC.
The precast panels used the following mix: Type 10 cement: 425 kg/m3 Water-cement ratio: 0.19-0.24 Air content: 4-6 %
Compressive strength: MPa At 16 hours: 40-50 At 56 days: > 75
The cast-in-place deck topping contained fly ash, had a water-cement ratio of 0.28, and contained a superplasticizer. Compressive strength at 56 days averaged 63 MPa with a standard deviation of 3.3 MPa.
A major rehabilitation project in 1999 was the deck overlay replacement on the Columbia River Bridge at Revelstoke (Morgan, 2000) using a steel fibre reinforced HPC. The original construction in 1959 was concrete infill to a metal T-grid deck surfaced with asphalt. By the 1990's, the bonded concrete overlay that replaced the asphalt in the 1970's had delaminated and spalled, resulting in potholes.
The existing concrete was removed to within 10 mm of the T-grid using high-pressure water jets. The concrete mix was dry batched in Vancouver in 1600 kg bulk bin bags, discharged into transit mixers near the site, water and admixtures added and the concrete delivered after 30 minutes batching and mixing time. Travel and discharge time was 45 minutes to 1.5 hours.
Just prior to placement, a sand-cement bonding agent was scrubbed into the T-grid cells. A Bidwell machine compacted and finished the overlay. The concrete was placed at night, fog sprayed prior to covering with wet burlap and plastic sheeting for 3 days followed by 4 days of wet curing.
The concrete mix design was as follows:
Material Proportions Kg/m³ Type 10 cement Silica fume 375 30 Coarse aggregate Fine aggregate 1010 750 Water 135 Steel fibres 50 Water-reducing admixture Superplasticizer Corrosion inhibitor Air-entraining agent 1.2 l 4.0 l 10.0 l 0.2 l
Air content of the fresh concrete was 5-8%, slump 70 +/- 20 mm, water-cement ratio 0.35 and steel fibre content 0.64 % by volume.
The properties of the hardened concrete were as follow: Compressive strength: MPa
3 days 35 7 days 55 28 days 68 Flexural strength and toughness at 7 days
Flexural strength: MPa 4.8
Toughness (ASTM c 1018) Level III to level IV JSCE SF-4 Toughness factor: MPa 2.7
Round determinate panel test
In view of the results obtained on this project, a similar specification and procedure was used on the Laforme Creek Bridge in Revelstoke in 2000.
The SkyTrain Millennium Line Project is a 20.4 km extension of the existing LRT line. The 16 km elevated guideway is comprised of 5,675 precast HPC segments, and used 135,000 m³ of HPC. It is the largest precast segmental construction contract undertaken in North America. The use of segmental construction allowed for different spans.
The segments were cast in a facility in Port Moody established for the project. The 17 month construction time mandated a one day cycle for most segments resulting in a 14 hour strength requirement of 18 MPa. Where segments were to be erected within 48 hours of casting, a 35 MPa strength was required prior to stressing.
The project specification was for a 100-year service life with "required maintenance no greater than ordinarily required for similar structures". The specification for the contract was independently evaluated. This review took a holistic view by asking two basic questions:
a) What are the mechanisms that could cause deterioration of the reinforced concrete construction prior to the 100 year design service life? and
b) How does the specification address these potential deterioration mechanisms, so as to provide a structure which meets the required service life?
Deterioration mechanisms evaluated and considered in the concrete mix designs included: frost damage, de-icing salt scaling, chloride ion and carbonation induced corrosion, alkali-aggregate reactivity, and cracking due to drying shrinkage and thermal stresses.
The criteria adopted for the segment concrete were as follows: Specified 28-day strength: 40 MPa* Type 10 cement: 378 kg/m³ Flyash: 42 kg/m³ Air content: 6% Maximum aggregate size: 14 mm Target water-cementitious ratio: 0.32 Maximum water-cementitious ratio: 0.36 *60 MPa for some segments.
Pre-construction tests were made to confirm air-void systems. Further analyses and modelling were made throughout the project to verify that "Time to Initiation of Corrosion" and "Time to Repair" were consistent with the 100 year service life specified. Tests were made on samples from the 20 year old existing LRT structure, as representative of the exposure to chlorides expected for the new structure.
A build-up rate of 0.032 kg/m³ was determined. RCP values of 2,600 coulombs were obtained from 28 day lab cured test cylinders, but 800 coulombs at 365 days were obtained from cores taken from a discarded concrete segment which had been field cured. Quality control was excellent with coefficients of variation generally between 4 and 6% and as low as 3%. Of 13,000 loads of concrete delivered, only two had to be rejected. Temperature probes in each segment form monitored the temperature of the hydrating concrete. These data, converted to maturity values, correlated closely to compressive strength, enabling form removal times to be determined accurately. The data also facilitated the fine-tuning of the mix designs.
Manitoba
Up to 1998, the following bridges used HPC: Manitoba Highways
Bridge Component
Plum River 75 mm overlay
Assiniboine River Floodway 75 mm overlay Little Saskatchewan River Superstructure Assiniboine River Bridge 75 mm overlay
Boine River 100 mm overlay
All concrete contained 8% silica fume and met a 35 MPa 28 day strength requirement. City of Winnipeg
Bridge Component
Norwood Bridges, North & South
140 mm deck lift
Charleswood 75 mm overlay
LaSalle River, North & South 75 mm overlay
All concrete contained 8% silica fume and met a 35 MPa 28 day strength requirement. Key criteria in the current Manitoba concrete specifications are as follow:
Manitoba
Cement 10 SF
Cement content: kg/m³ 340 Silica fume content: % 8
Water-cement ratio 0.37 max
Rapid chloride permeability 1000 coulombs max
Air: % 5-8
28-day strength: MPa 45
Allowable cracking width (mm) x length (m) per square metre of surface area: 0.25 to 0.35
New Brunswick
During the last five years, over 150 HPC structures have been constructed in New Brunswick. A review of test data from 25 HPC projects showed average coulomb values were 730 while for 4 projects using normal strength concrete with Type 10 cement, the average coulomb value was 4024. No value is specified, but values less than 1000 are expected for the specified concrete prior to the addition of a corrosion inhibitor. A value of 1500 coulombs is expected after the addition of the corrosion inhibitor. The reinforcement is uncoated and the decks are waterproofed and paved.
The following summary of New Brunswick experience is based almost verbatim on a presentation made by Fred Strang of the New Brunswick Department of Transportation at the ACI Convention in Toronto in October 2000 (Strang, 2000).
High Performance Concrete - Specifications and Mix Design
The Department specifications are method based. The specifications are as follows:
• 28 day compressive strength of 45 MPa
• Type 10 SF (low alkali) cement
• 420 kg/m³ cement content
• 0.37 water-cementitious ratio
• Plastic air content of 5-8% after final discharge
• Slump is 125 +/- 50 mm after final discharge
• Maximum concrete temperature (at delivery) 25°C
• Corrosion Inhibitor
• The deck is then waterproofed and paved Construction Methods
Goals
The goals this department is striving to meet are as follows:
• Eliminate plastic shrinkage cracking and crazing
• Minimize transverse cracking
• Produce a surface texture suitable to receive waterproofing
• Meet specified surface tolerances
The department has had success meeting these goals following the practices described below:
Placing
• Place the concrete at night. At night there is less evaporation of water from the surface of the concrete because the relative humidity is higher and there is less wind. The temperature is also lower at night. This prevents rapid drying of the surface. The concrete properties such as air and slump remain more consistent.
• The newer superplasticizers EO/PO combination polymer (Ethylene oxide/Polyethylene oxide) gives better consistency on site. Less time is spent on site testing. Control of the water content at the plant is critical for consistency of plastic concrete properties on site.
• The sooner the screeding operation is performed after the concrete is placed, the easier the concrete will be to work with and to finish.
Finishing
• Success with a bullfloat has been limited. This concrete is sticky and will pull and tear. The bullfloat can be used but additional hand finishing is usually necessary.
• The newer superplasticizers EO/PO comb polymer (Ethylene oxide/Polyethylene oxide) finish better.
• Concrete with slumps between 125 mm and 150 mm seem to finish the best. Concrete with slumps over 150 mm does not produce such a good finish. New Brunswick decks have a 3% crown.
• The majority of surface imperfections such as aggregate tears and ridges are finished with the hand trowel. The closer the finisher is behind the screed machine the easier it is to finish.
• The area beyond the screed rails that is finished entirely by hand is more prone to problems with meeting the surface tolerance requirements.
• It is easier to finish the concrete under the screed rails when the rails are raised. The screed rails are removed when the concrete is still plastic and the voids are filled with fresh concrete. The use of shorter lengths of screed rail pipe allows the filling and finishing of the voids in a timely manner.
Misting
• The timing of when to mist and how much to mist is important. It will change with the time of day and other weather conditions.
• If concrete is placed at night, early morning, or on a cool day, the misting system will be used sparingly.
• As the placing operation progresses, the misting is heavier and becomes the transition to water curing (which is continued for a minimum of 7 days using a burlap or non-woven geotextile fabric).
Surface Finish
• The surface finish needs to meet the requirements of the waterproofing manufacturer.
• Surface projections are to be ground.
• A delay in finishing the plastic concrete will result in voids in the surface. Trying to remove these voids, by working water into the top surface, is unacceptable.
Newfoundland and Labrador
About 15 bridges have been constructed in HPC since the introduction of an HPC specification in 1998. The change to HPC is considered a success. Plastic and other cracking is no more than in bridges cast with normal concrete. Finishing, as elsewhere, has been more difficult than with normal concrete, but, with experience, the finishers have become accustomed to the challenge. Meeting the 1000 coulomb requirement has not proven to be difficult.
Criteria for HPC are included in the Department of Works, Services, and Transportation specification Section 904 of January 1999.
Key criteria are as follows:
Superstructure
45 MPa at 28 Days 40 MPa at 28 Days Water/Cement Ratio 0.36 max 0.37 max
Slump as per mix design* as per mix design* RCP: coulombs 1000 max 1000 max Air content: % 6+/- 1 % 6+/-1 % Air void system:
Spacing factor: µm 230 max 230 max Specific surface: mm²/mm³ 25 min 25 min
* Maximum slump after addition of superplasticizer shall be 230 mm. The mix design
shall state the slump before and after the addition of superplasticizer and appropriate tolerances.
Type 10E-SF cement or Type 10 plus silica fume is specified, and the silica fume content must be 7-10% by mass of cement. Cementitious content for all superstructure concrete is 420 kg/m³. Air content for severe exposure (decks, curbs, endblocks, barriers and grade separation columns) shall be 7+/-1%.
With regard to curing, decks are floated, straight edge and broom finished, and then coated with evaporation retardant. The concrete is then kept moist for at least 7 days at a temperature of at least 10°C.
The penalty for failure to meet strength requirements is: $ (Adjusted concrete price) = $ (Bid price) - $ (10 (Specified strength-Tested strength)).
Northwest Territories
The department of Transportation specification for cast-in-place concrete was revised in January 2000. Class SF concrete contains 7.5% silica fume and meets a specified 28 day strength of 35 MPa. Minimum cement content is 350 kg/m³ and maximum water-cement ratio 0.38. Where fibre is added, the requirements are the same. Air void system requirements are as per CSA A23.1. Temperature at the time of discharge must be between 10 and 18°C. Curing is by the placing of filter fabric or burlap on the finished surface as soon as the surface will not be marred as a result. A fine water spray is then applied and the concrete wet cured for at least 7 days.
For bridge deck repairs, requirements are similar with the following exceptions: cement content: minimum 360 kg/m³, 10% silica fume, 60 kg/m³ steel fibres. Curing by wet burlap is carried out for a minimum of 3 days.
Penalties for lower than specified strengths are as follow: New construction:
28 Day Compressive Strength Penalty: $/m³
35 MPa and above Nil
34 to 35 MPa 15 33 to 34 MPa 30 32 to 33 MPa 45 31 to 32 MPa 60 30 to 31 MPa 80 29 to 30 MPa 110 28 to 29 MPa 150 27 to 28 MPa 200
Below 28 MPa Reject
Repairs:
28 Day Compressive Strength Penalty: $/m³
35 MPa and over Nil
32.5-35 MPa 25
30-32.5 MPa 50
27.5-30 MPa 100
Below 27.5 MPa Reject
Nova Scotia
In 1996, Concrete Canada received a request from the Deputy Minister of the Department of Transportation and Public Works (NS TPW) for assistance in the design and construction of a bridge in HPC.
Separate meetings were held to discuss design and construction, and a pre-tender workshop was held for bidders. An industry committee consisting of representatives from NS TPW, Dalhousie University, CPCA, the Atlantic Provinces Ready Mixed Concrete Association, Concrete Canada, Jacques Whitford and Lafarge developed the special provisions for HPC (Bickley, 1998).
A preconstruction test programme was carried out at Dalhousie University, (Trottier, 1997). Extensive tests were made on suitable locally available aggregates. Six trial mixes were then made, and tested for strength at ages up to 91 days, modulus of elasticity, air void system, scaling resistance and rapid chloride permeability. The early creep of one mix was determined.
The bridge is a two span 215 mm thick continuous deck 70.59 m long and 8.85 m wide on three 1900 mm bulb tee girders per span. Uncoated reinforcing steel was 400 MPa grade to CSA G30.12-78. Prestressing strand was 16 mm nominal 1860 MPa grade to ASTM 415 or CSA G279.
The specified strength for all cast-in-place concrete in the bridge was 60 MPa, and 65 MPa for the girders. The design in HPC allowed a wider spacing of girders with a consequent reduction in cost. The use of HPC resulted in the elimination of waterproofing and paving and the use of uncoated reinforcing steel.
Prior to construction, an economic analysis was made to compare costs for alternative designs (Fletcher, 1997).
The results were as follows:
Construction Costs Life Cycle Costs
Normal strength bridge $484,697 $578,827
HPC bridge-paved $470,317 $525,070
HPC bridge- exposed deck $444,815 $454,587
As a result of the extensive pre-construction testing, the contract document gave the bidders the option of using the mix developed at Dalhousie or, after carrying out equivalent testing, proposing an alternative mix. The successful contractor chose to use the proven mix, which was as follows:
kg/m³ Type 10E-SF cement (low alkali) 450
Class F fly ash 30
Will-Kare fine aggregate 690 Will-Kare coarse aggregate 1045
Water 144 mL/100 kg of cement
Micro-Air 350*
Daratard 17 and/or WRDA-82 250*mL/m3
WRDA 19 5000*
Specification criteria for the concrete were as follows:
• Slump: before superplasticizer: 60 mm, after superplasticizer: 190 +/- 30 mm
• Air content at discharge from truck: 7 +/- 1 %
• Minimum compressive strength at 28 days: 60 MPa
• Spacing factor: average not greater than 230 µm, no value to exceed 260 µm
• Rapid chloride permeability at 91 days: 600 coulombs maximum
• Maximum delivered concrete temperature: > 2m: 18°C, < 2m: 25ºC
• Maximum concrete temperature in situ: 70ºC, maximum gradient: 20ºC
"Curing: Apply an evaporation retarder immediately after initial screeding and/or between finishing operations as needed to aid in bullfloating and final texturing. Curing compound shall be applied at twice the manufacturer's suggested rate immediately after texturing, within 20 minutes of initial screeding and prior to covering with burlap. The second application of curing compound shall be applied at 90º to the first. Two layers of burlap, pre-soaked for 24 hours, shall be placed over the curing compound immediately after initial set of the concrete, kept continuously wet for 7 days, and covered with a layer of moisture vapour barrier immediately following the placement of the burlap".
Shortly after the bridge was completed, the construction team met to review the project. With respect to design, the expectations had been that the use of HPC would lead to significant increases in girder spans. In fact, the potential increase was found to be one metre. The reduction in the number of girders, the elimination of the waterproofing and paving, and the use of uncoated steel resulted in cost savings. The service life study confirmed this and contract prices confirmed the study figures.
Because a pre-tested mix was specified and time constraints did not allow for the development of an alternative, the specified mix was used. It was felt that, in future, the contractor should propose the mix. It was agreed that testing laboratories retained by the contractor could provide adequate proof of compliance with the specification criteria.