A State-of-the-Art Review of
High Performance Concrete Structures
Built in Canada: 1990-2000
John A. Bickley
Denis Mitchell
A State-of-the-Art Review of
High Performance Concrete Structures
Built in Canada: 1990-2000
John A. Bickley
Denis Mitchell
John A. Bickley, P.Eng. is a Consulting Engineer in Concrete Technology and a Principal Investigator with Concrete Canada.
Denis Mitchell is a Professor of Civil Engineering at McGill University and is Scientific Director and a Principal Investigator with Concrete Canada.
This publication is intended for the use of professional personnel competent to evaluate the significance and limitations of its content and who will accept responsibility for the application of the material it contains. The Cement Association of Canada and the authors disclaim any and all responsibility for application of the stated principles or for the accuracy of the sources.
State-of-the-Art Review of the Durability, Economics and Constructability of High Performance Concrete Structures Built in Canada: 1990-2000.
Executive Summary
In the search for durability, researchers in Canada and in other countries sought for higher performance materials. Technology from other countries, notably France, Norway, Japan and Germany, was incorporated into developments in Canada in the 1980s. High Performance Concrete (HPC) was included in this research.
With the establishment of Concrete Canada (CC) in 1990, a co-ordinated and concentrated programme of research commenced. In 1994, this programme expanded to include demonstration projects to implement HPC technology on construction sites. Technology Transfer was a primary goal of CC. Many seminars, workshops and technology transfer days were held across Canada, by CC alone, in co-operation with American Concrete Institute (ACI) Chapters, the Cement Association of Canada (CAC) and its member companies, and for specific entities such as Provincial Highway Departments and Cities.
Between 1990 and 2000, CC researchers published over 400 Papers in scientific journals. It seemed appropriate, as the old millennium ended, to assess the practice in the use of HPC in Canada over the past 10 years. The extent of its use, the varying specifications, results, economics and problems encountered have been reviewed. Looking ahead, areas for ongoing research and development have been identified.
The study demonstrates that, for those who have correctly implemented this technology, HPC is the high quality concrete of choice for high strength, durability and optimum life-cycle costs.
Preface
In 1990, as a Principal Investigator in the newly established and funded Network of Centres of Excellence on High Performance Concrete, one of my main activities was to lecture on this research programme and its objectives. At the time, the term High Performance Concrete was virtually unknown. Now, ten years later, I am no longer asked, "What is HPC?"
A perception that the use of HPC is now widespread led to the decision to produce this review. It seemed timely, after a decade of the intensive development, promotion and implementation of HPC, to step back and survey the extent of the use of this material across Canada, the achievements, the problems encountered, and to suggest directions for future practice.
The task turned out to be greater than I had envisaged. Despite my close involvement with the implementation of this technology, I was surprised to find the extent and variety of uses for this quality of concrete. As the pieces of the puzzle fell into place, a comprehensive picture formed.
There are some inconsistencies in my classification of projects. For instance, the Confederation Bridge is detailed in Chapter 4 on Precast Concrete Products. This structure must be considered to be one of the most outstanding precast concrete projects in the world. It seemed more appropriate to place it in Chapter 4 than in Chapter 3. Similarly, some projects have incorporated more than one technology, i.e. Conventional HPC and HPC using a Ternary cement. In such cases, the project is reported in the chapter covering the more innovative technology.
With the help of Rico Fung of the Cement Association of Canada and Kevin Cail of Lafarge Canada Inc., both of whom helped co-ordinate input from the cement companies and monitored the progress of this report, a nation-wide net was cast for information. As shown by the acknowledgements, many across the country provided me with information. To all of them, I extend my sincere thanks.
This report is based on my own library and experience, and the input I received from the many who helped. No doubt I have missed many projects, but the end result does demonstrate that HPC is now a widely accepted construction material.
I hope that this review will contribute to the correct use of HPC, and that, overall, the durability and serviceability of our structures will be improved.
John A. Bickley Toronto, Spring 2001
Acknowledgements
Particular thanks are due to Denis Mitchell for his chapter on design, Rico Fung and Kevin Cail for their guidance and support, Don Hopkins and Joan Dawson for the preparation of the manuscript for publication, my wife, Jean, for proof-reading; and to the many, listed below, who supplied me with information.
Pierre-Claude Aïtcin, Université de Sherbrooke; Vic Anderson, DELCAN; Herve Bachelu, Saskatchewan Highways and Transportation; Beata Berszakiewicz, Ontario Ministry of Transportation; Ken Bontius, Hatch Mott MacDonald; Don Brennan, Department of Works, Services and Transportation Newfoundland and Labrador; Paul Carter, Reid Crowther; Barry Charnish, Yolles Partnership Inc.; Mike Chung, City of Toronto; Louis-Georges Coulomb, Ministère des Transports, Québec; Martin Darby, Lafarge Construction Materials; Savio DeSouza, AMEC; Walter Dilger, University of Calgary; Sal Fasullo, DAVROC Testing Laboratories Inc.; James Fletcher, Nova Scotia Power Inc.; Richard Gagné, Université de Sherbrooke; Richard Golec, Pre-Con; Malcolm Gray, AECL; Gilbert Haddad, Terratec; John Hart, Consultant; Jack Holley, Lafarge Construction Materials; Dale Hollingsworth, Lafarge Construction Materials; Henri Isabelle, Consultant; Harry Jagasia, UMA; Jan Jofriet, University of Guelph; Kamal Khayat, Université de Sherbrooke; Jana Konecny, Ontario Ministry of Transportation; Ulrich Kuebler, Sky Cast Inc.; Francois Lacroix, Cement Association of Canada; Gord Leaman, Jacques Whitford; Bill LeBlanc, Con-Force; Gene Lecuyer, Lafarge Canada Inc.; Michel Lessard, Euclid Admixtures Canada; Wib Langley, Consultant; Bob Loov, University of Calgary; Paul Lowe, Lafarge Construction Materials; Ron Lowther, BC Ministry of Transportation and Highways; Yves Malier, ENPC, Paris and ENS Cachan; P.K. Mehta, University of California, Berkeley; Mike Meschino, Yolles Partnership; Grant Milligan, Quinn Dressel Associates; Rusty Morgan, AMEC; Richard Morin, Ville de Montréal; Robert Munro, Lafarge Canada Inc.; K Nasser, University of Saskatchewan; Nikola Petrov, Université de Sherbrooke; Michel Pigeon, Université Laval; Tony Purdon, SAR Transit JV; Gary Pyke, Nova Scotia Transportation and Public Works; Bob Ramsay, UMA Group; Elizabeth Read, SEM; John Ryell, Trow Consulting Engineers Ltd.; Phil Seabrook, Levelton Associates; Hannah Schell, Ontario Ministry of Transportation; Fred Strang, Department of Transport, New Brunswick; Michael Thomas, University of Toronto; Jean-Francois Trottier, Dalhousie University; Jim Turnham, SAR Transit; Daniel Vezina, Ministere des Transport, Québec; Gary Winch, Lafarge Canada Inc.; Steve Zupko, Lafarge Canada Inc.
CONTENTS
Executive Summary………..……….……….... Preface……… Acknowledgements……… iii vi v CHAPTER 1 Introductions Introduction………... Background and History………...1 1
CHAPTER 2 Standards
National Standards for Structural Design………. CSA A23.3 and CSA S6 Bridge Code………. National Standards for Materials……….. CSA A23.1 & 23.2……… CSA A 3000……….. CSA A 413……… CSA S 438……… 4 4 19 19 19 20 20
CHAPTER 3 Review of Bridges
General……….. Alberta……….. British Columbia……….. Manitoba……….. New Brunswick……… Newfoundland and Labrador……… Northwest Territories……… Nova Scotia……….. Ontario……….. Prince Edward Island……… Quebec……….. Saskatchewan……… 21 21 27 30 31 33 34 34 37 45 47 51
CHAPTER 4 Precast Concrete Products
Confederation Bridge……… Bridges……….. Concrete Pipe……… Precast Concrete Slabs……….. Hollowcore Slabs……….. Tunnel Segments……….. Spun Concrete Poles………. Parking Structures………. Precaster Experience………. 58 60 63 63 64 64 65 67 67
CHAPTER 5 Buildings and Parking Structures
Buildings……….. Parking Structures……….
70 71
CHAPTER 6 Marine Applications
Hibernia Offshore Platform……….. Grand Manan Wharf……….
75 76
CHAPTER 7 Agricultural Applications……….…….. 79
CHAPTER 8 Shotcrete……….. 82
CHAPTER 9 Emerging Technologies
Self-Consolidating Concrete………. Reactive Powder Concrete……… Use of Ternary Cements……… High Performance Roller Compacted Concrete………....
88 91 92 93
CHAPTER 10 Discussion and Recommendations
Discussion………. Cementitious Materials………
Supplementary Cementing Materials……….. Admixtures……….. Mix Design……….………. Testing………..
Constructability……….…...………. Monitoring………..…..……… Conclusions and Recommendations……….
Cementitious Materials….…..………... Admixtures….……….……….. Testing……….…….………. Specifications…….……….……….…………. Constructability…….……….………... Curing…………..………. Service Life Predictions……...…...……….. Summary………..…. 98 98 98 98 98 99 103 105 108 108 108 108 108 108 109 109 109
Chapter 1 Introductions
Introduction
Mehta (1994) stated that his holistic model of concrete deterioration "provides a clear justification why impermeability of concrete should be the first line of defence against any of the physico-chemical deterioration processes described earlier. If adequate attention in concrete making and processing is paid to hold the first line of defense, why would one need epoxy-coated reinforcement for protection against corrosion, ASTM Type V cement for protection against sulfate attack, and low-alkali cement or non-reactive aggregates for protection against expansion associated with the alkali-silica reaction?" He went on to say "From the standpoint of life-cycle costs and conservation of resources, the ecological implications of the foregoing proposition cannot be ignored for too long".
In Canada, experience with concrete durability problems has led to a similar conclusion: that the impermeability of the cover concrete is paramount. With ternary mixes and available admixtures, high strength is relatively easy to achieve. The current fixation in research and practice is the construction of durable structures. Owners are demanding an extended service life and want reassurance prior to construction, that it will be achieved. As a consequence, increasing effort is focusing on the development of predictive life-cycle models that can provide assurance of the specified service life, and can be used to calculate life-cycle costs. This approach is particularly relevant since first costs for the use of HPC are often higher than for conventional concrete. It is widely accepted that life-cycle costs for HPC structures will be lower than those of similar structures using conventional concrete.
It is now commonplace for construction contract specifications to require some form of permeability test, and the trend is to carry out this test on samples taken from the finished structure. The need to provide assurance of contract quality and the increasing use of performance or end-result specifications highlights the need for quicker and more reliable test procedures.
Continuing research and site implementation of the correct procedures are ongoing construction industry objectives.
Background and History
Credit for the term "High Performance Concrete" must go to the French. It was coined in 1980 by Roger Lacroix and Yves Malier (Aïtcin, 1998). In 1986, the French project "New Ways for Concrete" brought together 36 researchers from France, Switzerland and Canada. Leading the Canadian group was Pierre-Claude Aïtcin. The research, findings and field applications of all the members of this group formed the contents of the first book published that was solely devoted to High Performance Concrete (Malier, 1990).
Towards the end of 1988, Pierre-Claude Aïtcin, assisted by Denis Mitchell and Michael Collins, wrote the successful proposal for the Network of Centres of Excellence on High Performance Concrete, funded under the Federal Government "Centres of Excellence Programme".
This research programme started in 1990, and, in its second phase, starting in 1994, the Network became known as Concrete Canada. The researchers who comprised Concrete Canada were not the only Canadians researching and using HPC, however, they were the pre-eminent and most active group in this field. By virtue of many publications in scientific journals, a Newsletter sent to 7,000 persons world-wide, the organization of technology transfer days and seminars, and the construction of demonstration projects, Concrete Canada played the major role in establishing HPC as a widely accepted construction material in Canada.
In the United States, the Strategic Highway Research Programme (SHRP) sponsored a project on High Performance Concrete. In 1990, "High Performance Concretes", an annotated bibliography, 1974-1989, was published as SHRP-C/WP-90-001.
The definition used by SHRP for HPC was as follows: 1. "It should meet one of the following criteria
a) A 3-hour strength not less than 3,000 psi b) A 24-hour strength not less than 5,000 psi c) A 28-day strength of not less than 10,000 psi
d) A water-cement ratio (including pozzolans) less than 0.36
2. It should also have a durability factor not less than 80 after 300 cycles of freezing and thawing".
Many of the entries in this bibliography met the above criteria, but the authors of only two of the 2204 publications included in this document used the term High Performance in the titles of their Papers, and these were dated 1986 and 1987.
Since then the term has become a popular buzzword. In 1993, the American Concrete Institute published the following definition:
"High-performance concrete (HPC) is defined as concrete which meets special performance and uniformity requirements that cannot always be achieved by using only the conventional materials and mixing, placing and curing practices. The performance requirements may involve enhancements of placement and compaction without segregation, long-term mechanical properties, early-age strength, toughness, volume stability, or service life in severe environments".
In most applications, the water-cementing materials ratio will not exceed 0.40, but, as will be noted later in this report, some truly high performance concretes do not meet any of the above definitions. Cast-in-place HPC will normally contain silica fume. Precast concrete may not, particularly where the main parameter needed is high strength.
In the decade 1990-2000, there has been an enormous amount of research on this subject, and thousands of Papers have been published (Zia, 1997).
Major research programmes have been carried out in many countries in Europe, Asia, Australasia, Japan and North America.
A unique feature of the Concrete Canada programme has been the use of demonstration projects to effect the implementation of the correct use of HPC on construction projects. Since 1990, many such projects have been carried out across Canada. The use of HPC has recently spread rapidly. Most Provincial Highway Departments and some major cities have adopted its use, or are in the process of doing so. As a result, many consultants are specifying it, and, consequently, many contractors are winning contracts which contain innovative features. This is resulting in some potential teething problems that are discussed later.
REFERENCES
Mehta, P.K., "Concrete Technology at the Crossroads-Problems and Opportunities", Concrete Technology Past, Present, and Future, ACI SP 144, 1994, pp. 1-30.
Aïtcin, P-C., " High-Performance Concrete", E & FN Spon, 1998, pp. 591.
Zia, P., "State-of-the-Art of HPC: An International Perspective", Proceedings PCI/FHWA International Symposium on High Performance Concrete, New Orleans, October 1997, pp. 49-59.
Malier, Y., "Les Bétons à Haute Performance", Presse de L'Ecole National des Ponts et Chaussées, 1990. English Edition E & FN Spon, 1992.
Chapter 2 Standards
National Standards for Structural Design
CSA A 23.3-94 Design of Concrete Structures and CSA S6-00 Canadian Highway Bridge Design Code
INTRODUCTION
This section presents the design requirements for high-strength concrete in the 1994 Canadian Standards Association (CSA) Standard A23.3-94 "Design of Concrete Structures" (CSA 1994). The differences between the provisions of the 1994 CSA Standard and those in the 1999 ACI Code are discussed. In addition, the requirements for high-strength concrete in the Canadian Standards Association (CSA) Standard S6, “Canadian Highway Bridge Design Code” are also discussed.
LIMITS ON SPECIFIED COMPRESSIVE STRENGTH OF CONCRETE
The provisions in the 1994 CSA Standard are applicable for designs having a specified concrete compressive strength not less than 20 MPa nor more than 80 MPa. The upper limit for the compressive strength of the concrete was chosen because of the insufficient amount of test data on the response of structural elements constructed with very high-strength concrete. The Standard permits designs with concrete compressive strengths greater than 80 MPa, provided that the structural properties and detailing requirements for these higher strength concretes are established for concretes similar to those to be used. Furthermore, it is pointed out that high-strength concretes vary in their brittleness and need for confinement.
The Standard cautions designers planning to use high-strength concretes to determine whether such concretes are available in their region and it also points out that it may be necessary to undergo prequalification of concrete suppliers and contractors.
The 2000 Canadian Highway Bridge Design Code (CSA S6, 2000) provides limits on the concrete compressive strength “unless otherwise approved”. These concrete compressive strength limits are:
• a minimum strength of 30 MPa for non-prestressed members;
• a minimum strength of 35 MPa for prestressed concrete; and
• a maximum strength of 85 MPa unless approved.
The Commentary to CSA S6 indicates that the minimum concrete strength limit of 30 MPa was chosen to provide a minimum level of durability. The Commentary also cautions designers about using concrete strengths greater than 85 MPa since “special consideration of the structural response of this material may be warranted”.
MODULUS OF ELASTICITY OF CONCRETE
The 1994 CSA Standard provides three methods for determining the modulus of elasticity, Ec, at the design stage, as follows:
• by testing cylinders of similar concrete.
• by determining the modulus of elasticity, Ec, for concrete with ãc between 1500
and 2500 kg/m3 as
where ãc is the density of concrete in kg/m3.
This equation is based on the work of (Carrasquillo et al, 1984) and gives a more appropriate expression for the modulus of elasticity, Ec, for concrete compressive
strengths greater than 40 MPa.
• by determining the modulus of elasticity, Ec, of normal density concrete with
compressive strength between 20 and 40 MPa as
This simpler expression is suitable for use with lower strength concretes and is about 5% lower than that given in the 1999 ACI Code.
The CSA Standard also cautions designers that the modulus of elasticity of high-strength concrete is highly dependent on the properties of the aggregates used and hence can vary in different regions of the country (Baalbaki et al, 1991). If the modulus of elasticity is critical to the design, then a minimum value of Ec shall be
specified and shown on the drawings.
SPECIAL CONFINEMENT REQUIREMENTS FOR HIGH-STRENGTH CONCRETE COLUMNS
Studies on high-strength concrete columns (by Yong et al, 1988, Cusson and Paultre, 1994, Ibrahaim and MacGregor, and Polat, 1992), have indicated the need to provide a greater degree of confinement to improve the ductility of high-strength concrete columns. Furthermore, there is a tendency for splitting cracks to form in high-strength concrete columns, resulting in premature spalling of the concrete cover and a reduction in the capacity as discussed by (Collins et al, 1993).
(
)
′ 2300 6900 + 3300 = c 1.5 γ c c f E (1) c c f E =4500 ′ (2)Tie Spacing Limits
The 1994 CSA Standard requirements for maximum tie spacings in columns made with normal strength concrete are the same as those in the 1999 ACI Code. However, the tie spacing requirements for columns made with concrete, having a specified compressive strength greater than 50 MPa, are reduced by multiplying the tie spacing limits of the ACI Code by 0.75. Thus, for higher strength concrete columns, the tie spacing shall not exceed the smallest of:
• 12 times the diameter of the smallest longitudinal bars or the smallest bar in a bundle;
• 36 tie diameters;
• 0.75 times the least dimension of the compression member; and
• 225 mm in compression members containing bundled bars.
The additional confinement for high-strength concrete columns is aimed at providing greater confinement and improved ductility.
Anchorage Details for Ties
The 1994 CSA Standard also requires that ties in columns made with specified concrete compressive strengths greater than 50 MPa have standard tie hooks with a bend of at least 135o. The typical 90o bend anchorages are replaced with 135o bend anchorages for the ties in high-strength concrete columns in order that the ties remain effective, even if some concrete cover spalling takes place.
Confinement for Fire Resistance
While there are a number of ongoing research projects aimed at developing a better understanding of the influence of high-strength concrete on the fire resistance of columns, a number of important observations have been made (Phan). It has been found that high-strength concrete can experience "explosive spalling" when subjected to rapid heating. The rate of temperature increase, the type of aggregates, the details of reinforcement, the moisture content and the porosity of the concrete are all important factors which affect the spalling of the concrete. Due to the low permeability of high-strength concrete, it tends to retain moisture and, hence, rapid heating leads to the development of large internal vapour pressures, which results in explosive "spalling". Since the fire resistance is affected by loss of cover, it is hoped that the 1994 CSA Standard with more closely spaced ties with 135o bend anchorages should improve the fire resistance of high-strength concrete columns. Research is currently under way to study the effects of these new details and the influence of steel fibres on the fire resistance of high-strength concrete columns.
FLEXURAL AND AXIAL LOAD RESISTANCES OF HIGH-STRENGTH CONCRETE ELEMENTS
In determining the factored resistance for flexure and axial loads the 1994 CSA Standard prescribes a plane-sections approach using one of the following two methods:
Using Realistic Stress-Strain Relationships
The position and magnitude of the resultant compression in the concrete may be found by first assuming a realistic stress-strain relationship between the compressive stress and concrete strain and then integrating these stresses. This relationship must account for the fact that as the concrete strength increases the compressive stress-strain curves exhibit greater initial stiffness, greater linearity and decreased ductility. Fig. 1 shows plots of stress-strain relationships which illustrate these features (Collins and Mitchell, 1997).
In using this approach it is necessary to account for differences between the in-place strength and the strength of standard cylinders. In this regard, the CSA Standard requires that the compressive stress-strain curve for the in-place concrete be based on stress-strain curves with a peak stress no greater than 0.9f’c.
Using Stress Block Factors
The 1994 Standard provides an alternate approach using an "equivalent rectangular concrete stress distribution", together with a maximum strain at the extreme concrete compression fibre of 0.0035. A value of 0.0035 was chosen to better reflect the extreme concrete compressive fibre strain at flexural ultimate for a large range of concrete compressive strengths. The equivalent rectangular concrete stress distribution is defined by the following:
• a concrete stress of á1öcf shall be assumed uniformly distributed over an
equivalent compression zone bounded by edges of the cross section and a straight line located parallel to the neutral axis at a distance a = ß1c from the
fibre of maximum compressive strain;
• the distance c shall be measured in a direction perpendicular to that axis; and
• the factors á1 and ß1 shall be taken as:
These new stress block factors are more suitable for a wider range of concrete strengths than those in the previous 1984 CSA Standard (CSA A23.3-84). The stress block factors are intended to account for both the significant change in shape of the stress-strain curves as the concrete strength increases and the difference between the concrete cylinder strength and the in-situ strength of the column concrete. 0.67 0.0015 0.85 = fc′≥ 1 α (3) 0.67 0.0025 0.97 = fc′≥ 1 β (4)
Fig. 2 illustrates the two analysis procedures for determining the factored flexural resistance of members. Fig. 3 compares the factored moment resistances, Mr, using
the new stress block factors of the 1994 Standard, with those determined using stress block factors of the 1984 CSA Standard, for a 400 mm by 600 mm deep beam singly reinforced with 3 No. 30 bars. The two approaches are very similar for the case of pure moment. This figure also illustrates that for this example, doubling the concrete compressive strength results in only about a 6% increase in pure moment capacity.
Maximum Axial Load Resistance
The maximum factored axial load resistance Prmax of compression members is
determined from the following expressions: (a) for spirally reinforced columns:
Prmax = 0.85 Pro (5)
(b) for tied columns:
Prmax = 0.80 Pro (6)
where Pro is the factored axial load resistance at zero eccentricity defined as:
where
öc = resistance factor for concrete, equal to 0.60
Ag = gross area of section
Ast = total area of longitudinal reinforcement
At = area of structural steel shape
Ap = area of prestressing tendons
ös = resistance factor for reinforcing bars, equal to 0.85
öa = resistance factor for structural steel, equal to 0.90
öp = resistance factor for prestressing tendons, equal to 0.90
fpr = stress in prestressing tendons when concrete reaches limiting
compressive strain
fy = specified yield strength of reinforcement yield
Fy = specified yield strength of structural steel section
The value of Prmax in the 1994 CSA Standard is a function of the stress block
factor, á1, which varies with the concrete compressive strength. Fig. 4 compares
the Prmax values calculated using the 1984 (similar to the 1999 ACI Code
approach) and 1994 CSA Standards for a 450 mm by 450 mm column containing 8 No. 20 bars. For this column, containing 1.2% steel, the new provisions give about 4%, 9% and 13% lower capacities for concrete strengths of 30, 60 and 80 MPa, respectively. p pr t y st y s p t st g c c ro f A A A A f A F A f A P =α1φ ′( - - - )+φ +φa - (7)
Limit of c/d
Due to the change in the maximum strain at the extreme concrete compression fibre from 0.003 to 0.0035 in the 1994 CSA Standard, the tension reinforcement in flexural members shall not be assumed to reach yield unless:
where c is the depth of compression and d is the effective depth of the member. When c/d exceeds this limit, the stress in the tension reinforcement must be computed based on strain compatibility.
MINIMUM AMOUNT OF FLEXURAL REINFORCEMENT
The purpose of providing a minimum amount of flexural reinforcement is to ensure a ductile flexural response. An insufficient amount of flexural reinforcement can result in the flexural capacity being lower than the cracking moment, resulting in a brittle response after cracking.
The 1994 CSA Standard gives three alternative approaches for satisfying the requirements for minimum reinforcement as given below:
(1) At every section of a flexural member where tensile reinforcement is required by analysis, minimum reinforcement shall be proportioned so that:
where the cracking moment, Mcr, is calculated using the modulus of rupture, fr.
(2) In lieu of (1) above, minimum reinforcement may be determined from:
where bt is the width of the tension zone of the section considered.
However, for slabs and footings only the minimum amounts for "temperature and shrinkage" need to be provided.
(3) The requirements (1) and (2) above may be waived if the factored moment resistance, Mr, is at least one-third greater than the factored moment, Mf.
y f + 700 700 d c ≤ (8) cr r M M ≥1.2 (9) h f 0.2 = y min t c s b f A ′ (10)
Older codes (CSA A23.3-84 and ACI 318-83) typically required a minimum reinforcement ratio that was a function of the yield strength of the reinforcement, but was not a function of the concrete strength. The 1994 CSA Standard and the 1995 ACI Code make the minimum amount of reinforcement a function of not only fy, but
also f’c to account for the higher cracking moment as the specified concrete strength
is increased. Fig. 5 shows the variation of the reinforcement ratio, ñ, with increasing compressive strength. With the 1994 CSA provisions, a smaller amount of minimum reinforcement is required than previous code provisions for concrete strengths below about 40 MPa. Larger amounts of minimum reinforcement are required by the 1994 Standard for flexural members with concrete compressive strengths above 40 MPa.
MINIMUM AMOUNT OF SHEAR REINFORCEMENT
The purpose of minimum shear reinforcement is to prevent brittle shear failures and to provide adequate control of shear cracks at service load levels (Collins et al, 1993). The 1984 CSA Standard (CSA A23.3-84), like the 1983 ACI Code (ACI 318-83), required a minimum area of shear reinforcement equal to 0.35bws/fy (i.e.,
stirrups to carry 50 psi) which is independent of the concrete strength. As the concrete compressive and tensile strengths increase, the cracking shear also increases. This increase in cracking shear requires an increase in minimum shear reinforcement such that a brittle shear failure does not occur upon cracking. The 1994 CSA Standard (CSA A23.3-94) makes the minimum amount of shear reinforcement a function of not only fy, but also f’c to account for the higher cracking
shear as the specified concrete strength is increased. Where shear reinforcement is required, the minimum area of shear reinforcement shall be such that:
Figure 6 compares the 1994 CSA and the 1999 ACI required amounts of minimum shear reinforcement. The CSA requirements provide a more gradual increase in the required amount of minimum shear reinforcement as the concrete strength increases. Tests carried out by (Yoon et al, 1996) on large beams with concrete strengths varying from 36 MPa to 87 MPa indicated that the amount of minimum shear reinforcement prescribed by the 1994 CSA Standard provides adequate control of diagonal cracks at service load levels and provides reasonable levels of ductility. CRACK CONTROL REQUIREMENTS FOR HIGH-STRENGTH CONCRETE ELEMENTS
As discussed above, the amount of minimum reinforcement required for both flexure and shear is a function of the concrete compressive strength, f’c. An important design
issue is whether or not the amount of uniformly distributed reinforcement required in
y w c v f b f A =0.06 ′ s (11)
disturbed regions is also a function of the concrete strength. These disturbed regions include deep beams, corbels and regions near supports. The 1984 CSA Standard A23.3 introduced the strut-and tie design procedure for these disturbed regions and these requirements remain unchanged in the 1994 Standard. In the strut-and-tie design procedure the tension ties and compressive struts are first designed and then uniformly distributed horizontal and vertical reinforcement is added to control cracking at service load levels. The CSA Standard requires a minimum amount of uniformly distributed reinforcement corresponding to a reinforcement ratio of 0.002 in both the horizontal and vertical directions. This maximum spacing of this crack control reinforcement is 300 mm. For bridge design (CSA S6, 2000 and AASHTO, 1994 and 2000), this minimum reinforcement ratio has been increased from 0.002 to 0.003. An important question to be answered is whether or not this minimum reinforcement ratio should be a function of f’c, in keeping with the philosophy for
minimum amounts of flexural reinforcement and shear reinforcement. Preliminary experimental evidence from testing carried out in full-scale bridge pier caps (Macleod et al, 1996) indicated the following:
• a reinforcement ratio of 0.0018 was sufficient to control service load cracking for the specimens having a concrete compressive strength of 38 MPa.
• a reinforcement ratio of 0.003 was adequate in controlling the service load cracking for the specimens having a concrete compressive strength of 79 MPa. TRANSMISSION OF COLUMN LOADS THROUGH FLOOR SLABS
When the specified compressive strength of concrete in a column is greater than that specified for a floor system, transmission of the column load through the floor system needs to be investigated. There are three methods of dealing with this issue in the 1994 CSA Standard as described below:
(1) "Puddled" High-Strength Concrete in Slab - One method is to provide concrete of the strength specified for the column, f’cc, "puddled" in the floor slab at and
around the column locations. The top surface of the column concrete placed in the floor shall extend at least 500 mm into the floor from the face of the column to be considered effective (see Fig. 7(a)). This usually requires that the higher strength concrete in the floor in the region of the columns be placed before the lower strength slab concrete is placed to avoid accidental placement of the lower strength concrete in the column area. In addition, the lower strength concrete must be placed while the higher strength concrete is still plastic and must be vibrated to ensure integration of the two concretes.
(2) Accounting for Confinement Effects - Another approach is to take account of the confining effect of the floor slab surrounding the column (see Fig. 7(b). The 1994 Standard prescribes an effective concrete compressive strength, f’e,
which differentiates between the different amounts confinement at interior, edge and corner columns as shown in Fig. 7(b).
(3) Use of Vertical Dowels - The third approach is to supplement the strength of the weaker concrete, "sandwiched" between the upper and lower column, by adding sufficient vertical dowels to enhance the capacity locally (see Fig. 7(c)), or by adding spirals or hoops in this critical region to increase the confinement and effective strength of the concrete in the slab region.
DEVELOPMENT LENGTHS OF REINFORCEMENT
The 1994 CSA Standard(CSA A23.3-94) has adopted the same approach as the 1995 ACI Code by limiting the maximum permissible value of the square root of f’c
to 8 MPa when calculating development lengths. More research is needed on this important topic since there is a tendency for more brittle bond failures in high-strength concrete members unless an adequate amount of transverse reinforcement is provided (Azizinamini et al, 1993, Abrishami et al, 1995).
CONCRETE CAST IN CSA CERTIFIED PRECAST PLANTS
The 1994 CSA Standard employs material resistance factors rather than capacity reduction factors when calculating the factored member resistance. For elements produced in certified manufacturing plants, the concrete material resistance factor, öc, may be taken as 0.65, rather than 0.60. This increased value of öc, reflects the
better quality control achieved in certified precast plants.
SEISMIC DESIGN
The "Special Provisions for Seismic Design" in the 1994 CSA Standard (CSA A23.3-94) limits the specified compressive strength, f’c, used in design to 55 MPa.
These special provisions contain the seismic design and detailing requirements for ductile and nominally-ductile structural elements. This conservative approach was deemed necessary by the code committee because of the lack of test results of high-strength concrete elements subjected to reversed cyclic loading. The 1995 New Zealand Standard (NZS 3101:1995) limits the specified concrete compressive strength to 70 MPa for ductile elements and elements of limited ductility. The 1999 ACI Code (ACI 318-95) does not have an upper limit on the specified concrete strength for the design of ductile elements.
CONCLUSIONS
The 1994 CSA Standard (CSA A23.3-94) introduced special provisions for the structural design of high-strength concrete members which have some significant differences with the ACI Code (ACI 318-95). It is clear that more research is required in many different areas in order to develop codes for the future. The Canadian Highway Bridge Design Code (CSA S6, 2000) currently does not give any additional benefit for the use of high-performance concrete in determining cover requirements for different types of construction (prestressed and non-prestressed) and for different exposure conditions. It is hoped that future research will take account of the improved durability of high-performance concrete.
REFERENCES
Canadian Standards Association, "CSA A23.3-94 Design of Concrete Structures", Rexdale, 1994, pp. 199. American Concrete Institute, Building Code Requirements for Structural Concrete (ACI 318-95) and
Commentary, Detroit, MI, 1999, pp. 391.
Canadian Standards Association, CSA S6, “Canadian Highway Bridge Design Code”, Rexdale, 2000, pp. 724. Carrasquillo, R.L., Nilson, A.H., and Slate, F.O., "Properties of High Strength Concrete Subject to Short-Term Loads", ACI Journal, V. 78, No. 3, May-June 1981, American Concrete Institute, Detroit, pp. 171-178. Baalbaki, W., Benmokrane, B., Chaallal, O. and Aï tcin, P.-C., "Influence of Coarse Aggregate on Elastic Properties of High-Performance Concrete", ACI Materials Journal, Vol. 88, No. 5, Sept.-Oct. 1991, pp. 499-503.
Yong, Y.K., Nour, M.G., and Nawy, E.G., "Behavior of Laterally Confined High Strength Concrete Under Axial Loads", Journal of Structural Engineering, Vol. 114, No. 2, Feb. 1988, ASCE, pp. 332-351.
Cusson, D. and Paultre, P., "High Strength Concrete Columns Confined by Rectangular Ties", Journal of Structural Engineering, Vol. 120, No. 3, Mar. 1994, ASCE, pp. 783-804.
Ibrahaim, H.H.H. and MacGregor, J.G., "Flexural Behaviour of High Strength Concrete Columns", Structural Engineering Report No. 196, Dept. of Civil Engineering, University of Alberta, pp. 197.
Polat, M.B., "Behavior of Normal and High Strength Concrete Under Axial Compression", Master's thesis, Department of Civil Engineering, University of Toronto, 1992, pp. 175.
Collins, M.P., Mitchell, D., and MacGregor, J.G., "Structural Design Considerations for High-Strength Concrete", Concrete International, Vol. 15, No. 5, May 1993, American Concrete Institute, Detroit, pp. 27-34.
Phan, L.T., "Fire Performance of High-Strength Concrete: A Report of the State-of-the-Art", NISTR 5934, National Institute of Standards and Technology, Gaithersburg, Maryland.
Collins, M.P. and Mitchell, D., "Prestressed Concrete Structures", Response Publications, Montreal/Toronto, 1997, pp. 766.
Canadian Standards Association, "CSA A23.3-84 Design of Concrete Structures for Buildings", Rexdale, 1984. American Concrete Institute, Building Code Requirements for Reinforced Concrete (ACI 318-83), Detroit, MI,
1983, pp. 111.
Yoon, Y.-S., Cook, W.D. and Mitchell, D., "Minimum Shear Reinforcement in Normal, Medium and High-Strength Concrete Beams", ACI Structural Journal, V. 93, N. 5, Sept/Oct, 1996, pp. 576-584. “AASHTO LRFD Bridge Design Specifications and Commentary”, American Association of State and
Highway Transportation Officials, Washington, D.C. 1994 with updates in 2000, pp. 109.
MacLeod, G.D., Cook, W.D. and Mitchell, D., “Full-Scale Tests of Bridge Pier Caps”, Proceedings of Graduate Student Seminars, Concrete Canada, Moncton, 1996, pp. 10.
Azizinamini, A., Stark, A., Roller, J.J. and Ghosh, S.K., "Bond Performance of Reinforcing Bars Embedded in High-Strength Concrete", ACI Structural Journal, V. 90, No. 5, Sept-Oct 1993, pp. 554-561. Abrishami, H.H., Cook, W.D. and Mitchell, D., "Influence of Epoxy-Coated Reinforcement on the Response of
Normal and High-Strength Concrete Beams", ACI Structural J., V92, N2, Mar/Apr. 1995, pp. 157-166. 16. New Zealand Standard, NZS 3101:1995, "Concrete Structures Standard", Standards Council of New
Figure 1 Variation of compressive stress-strain curves with increasing compressive strength. Adapted from Collins and Mitchell, 1997.
Figure 2 Assumptions for determining flexural resistance (1994 CSA Standard).
Figure 3 Variation of Mr with increasing concrete compressive strength.
Figure 5 Variation of ñ with increasing concrete compressive strength.
Figure 6 CSA 1994 and ACI minimum amounts of shear reinforcement (note that the 1989 ACI requirements remain unchanged in the 1999ACI Code.
National Standards for Materials
CSA A 23.1-00 Concrete Materials and Methods of Concrete Construction and A23.2-00 Methods of Test for Concrete
The main text of the latest edition of this standard has been reviewed to ensure that it is compatible with the material and construction needs of HPC. This edition also includes an Appendix J that provides guidance on the use of HPC. Subjects on which guidance is provided are:
Introduction General
Cement and Supplementary Cementing Materials (SCMs) Water
Aggregates Admixtures Reinforcement Formwork
Fabrication and Placement of Reinforcement Mix Proportions Durability Requirements Concrete Quality Production of Concrete Placing of Concrete Curing References
CSA A 3000-98 Compendium of Cementitious Materials
This document brings together the latest editions of all the CSA standards for cementitious materials, viz.:
A 5 Portland Cement A 8 Masonry Cement
A 23.5 Supplementary Cementing Materials A 362 Blended Hydraulic Cement
A 363 Cementitious Hydraulic Slag
A 456.1, A 456.2 and A 456.3 which cover all physical and chemical testing of the materials in the above five standards
The new edition of A 362 provides for a large range of binary and ternary blended cements, incorporating blast furnace slag, fly ash and silica fume in various percentages and combinations. This new concept offers the buyer a wide range of potential choices. For example, it is possible to buy blended cement containing Portland Cement, Slag and Silica Fume interground or blended in specific proportions. Already this option has been exercised on a number of contracts – see Chapter 9 Emerging Technologies.
CSA S 413-94 Parking Structures
In the current edition, HPC is not allowed in cast-in-place floors and roofs. In precast prestressed roofs and floors, low-permeability concrete is allowed. This concrete is specified to have a maximum water-cementitious ratio of 0.40 and a maximum coulomb rating (ASTM 1202)* of 1500 at 28 days. Later ages, up to 91 days, may be specified for these tests where fly ash or slag is used.
* ASTM C 1202-97 "Test Method for Electrical indication of Concrete's ability to
resist Chloride Ion Penetration". This test is commonly called the Rapid Chloride Permeability Test (RCP).
CSA A 438-00 Concrete Construction for Housing and Small Buildings
The new edition contains an Appendix R: Premium Quality Residential Basements. This appendix is intended for use where new basements are designed for residential use. Desirable properties for the concrete forming the walls of these basements are:
Minimal evaporative water to reduce noxious and harmful mould growth High impermeability to water and gas
Reduced shrinkage High-quality finish
Criteria are provided for the use of self-consolidating concrete in these structures to meet the above properties.
A demonstration building and other projects using this technology are described later in Chapter 9 Emerging Technologies.
Chapter 3 Review of Bridges by Province
General
It is in the bridge field that HPC has found the widest and earliest use. The following are summaries of HPC use in the Provinces, in alphabetical order.
Alberta
As in other Provinces, durability problems with normal quality concrete led to efforts to improve the serviceability of bridges. Alberta Transportation and Utilities (ATU) enlisted the assistance of the University of Calgary to research a number of materials improvements, some of which would meet today's criteria for HPC (Carter, 1998).
An excerpt from this reference summarizes well the philosophy driving changes in Alberta practice:
"Analysis of primary highway bridge rehabilitation costs showed that after a certain bridge condition level was reached, the rehabilitation costs increased exponentially in relation to time. Bridge decks that were in poor condition consumed more money than if they had been repaired a few years earlier. It was concluded that most of the existing major bridges would benefit economically from protection systems, and that new and less expensive protection systems that were applied at the right time, and that were intended to keep the condition of the bridges good, would be less expensive, produce better repair service life, and result in a higher number of annual repairs than the current policy of focusing repairs to bridges in bad condition”.
During the 1980s some of the bridge management changes used high performance materials. These included the use of fibres and supplementary cementing materials. Some milestones of innovation were as follows:
• Silica fume shotcrete in a culvert repair, 1983
• Steel fibres in three bridge deck repairs and eight shotcrete repairs, 1984
• Superplasticized steel fibre reinforced overlays, 1984
• Silica fume steel fibre shotcrete in prebagged proprietary mixes for ten bridge repairs, 1985. Five more in 1986
• Fly ash in overlays, 1985
• Silica fume superplasticized overlay, 1985
• Proprietary overlays without superplasticizer, 1987
• Proprietary 12% silica fume superplasticized overlay, 1988
In the 1990s research at the U of C investigated the use of superplasticizers and SCMs with local materials to improve durability (Johnston, 1993).
During the 1990s, silica fume overlays became the preferred protection system. They replaced membranes below asphalt paving on bridges. Recently, most overlays have incorporated steel fibres. The lessons learned in the use of these high performance overlays were summed up as follows (Carter, 1998):
• "It is possible and practical to produce a consistent mix for silica fume concrete on a large scale,
• Superplasticizer is a must with silica fume mixes,
• In general only compatible admixtures should be used,
• Avoid cement/admixture compatibility problems by trial batching,
• Silica fume concrete with its lack of bleed water and susceptibility to surface crusting from evaporation should be placed and finished in low evaporation conditions,
• Prevention of cracking is a major concern in Alberta's climate,
• Mild weather conditions are essential to preventing cracking,
• Mix temperature is best in the range of 12°C (ATU specifications call for rejection if > 18°C), for prevention of cracking and for keeping slump and air retention properties to manageable levels,
• Ice is needed for temperature control on many ATU pours,
• Best time to pour is at night because hot decks absorb water from mix, making finishing hard, night time pours do not conflict with the mix supplier’s other customers’ jobs,
• Use of compatible retarder/superplasticizer combination has benefits,
• Transit mix HPC can be used acceptably for pours within 1 hour travel time, and this means most Alberta sites; dry truck batching is not as good as plant mixing,
• Pre-bagged mixes are harder to work with than transit mix, partly because oven dried aggregates create slump variability from batch to batch,
• Today's pours are done with a workable, 120 mm slump,
• Mono-monecular curing compound is recommended for use immediately after texturing and prior to wet burlap placement to reduce surface cracking,
• A second generation superplasticizer, when properly matched with an air entraining agent, can meet the recognized requirements for air void spacing factor (<0.23 mm),
• Add superplasticizer after batching and mixing for 5 minutes; adding the superplasticizer during batching can result in unmixed balls in the mix,
• HPC can be used for bridge deck construction and replacement".
Developments were also made in the use of HPC for precast bridge girders, and these are described in chapter 4 - Precast Concrete Products.
The following specification was used in a number of bridges:
• Specified 28-day strength: 55 MPa
• Type 10 cement plus 7.5-10.0% silica fume
• Minimum cement content: 380 kg/m³
• Maximum water-cement ratio: 0.35
• Delivered temperature: 10-18°C
• Maximum coulomb value: 600
Bridges constructed to this specification include the Fish Creek and Canyon Meadows 1995, Stoney Trail CPR 1997 and Ogden Road Overpass, Elevated Roadway at Calgary Airport and Center Street in 1998.
Details of some other recent HPC bridges are as follow:
Bridge Date Specified 28 Day Strength Maximum RCP Value Components MPa Coulombs
Stoney Trail 1997 55 600 C-I-p box girders
Centre Street rehabilitation 1999 45 600 C-I-p deck
35 600 C-I-p barrier walls
Anderson/McLeod 2000 50 1000 C-I-p decks
Interchange 40 1000 C-I-p barrier walls
Deerfoot Trail/ 22X 2000 50 1000 C-I-p decks
Interchange 40 1000 C-I-p barrier walls
Fish Creek LRT 2000 50 1000 C-I-p decks
40 1000 C-I-p barrier walls Some recently rehabilitated or reconstructed bridges have incorporated stainless steel reinforcement:
Cadotte River Bridge and Marten River Bridge
These two bridges were rehabilitated in 1999. Each bridge consisted of three 10.7 m spans of prestressed concrete box girders (no deck). The spans were made continuous for live loads through the addition of a 130 mm thick silica fume concrete overlay reinforced with stainless steel reinforcing.
A total of 14.4 tonnes of stainless steel reinforcing was used. It was supplied in stainless steel grade 316N and strength grade 420 MPa. Bar sizes used were Imperial #4, #5 and #6.
Sturgeon River Bridge
This bridge replacement project was constructed in the Fall of 2000. It is a 40 m simple span "Bulb-Tee" girder bridge (25.4 span/depth ratio) with a HPC deck. The girders have an ultimate concrete strength of 65 MPa and release strength of 45 MPa.
A total of 9 tonnes of stainless steel clad reinforcing was used in the top mat of the deck and in the curbs. The stainless steel cladding was stainless steel grade 316L and strength grade 420 MPa.
In 1999, the Province of Alberta adopted a specification for HPC in the form of Special Provisions. In 1999, these provisions were used for two bridge deck overlays and for the
top slab of a box culvert. In 2000, they were used for three bridge deck overlays and for the deck of the Sturgeon River Bridge. HPC is designated as Concrete-Class SF (Modified). Key criteria for this concrete are as follows:
• Minimum 28 day compressive strength: 50 MPa
• Maximum water/cementing material ratio: 0.36
• Minimum cement content (excluding SCMs): 350 kg/m³
• Silica fume: 7.7% to 9.5%
• Sum of silica fume and fly ash: 25% of cement content
• Slump at discharge: 120 +/-30 mm
• RCP at 28 days: <1000 coulombs
• Slump of trial mix 45 minutes after batching to be at least 50% of initial slump
• Determine air void system of hardened concrete
• For 7 days after casting, the temperature differential between the centre and surface of the concrete shall not exceed 20°C
• Fog misting to be applied from time of screeding until concrete is covered with white filter fabric or burlap.
• Cracks to be measured in width and length. Those over 0.3 mm in width to be repaired.
• Penalties are charged for understrength concrete down to 42 MPa, below which the concrete is unacceptable, as follows:
Test results: MPa Penalty: $/m³
50 or over Nil 49 to 50 20 48 to 49 40 47 to 48 60 46 to 47 80 45 to 46 100 44 to 45 130 43 to 44 180 42 to 43 240 Below 42 rejected
Trial batches are to be made at least 35 days before concrete placement. A trial batch placement of at least 3 m³ is required to simulate anticipated placing procedures.
Where stainless steel is used as reinforcement, it is specified to meet the requireme nts of ASTM A955M-96 "Deformed and Plain Stainless Steel Bars for Concrete Reinforcement" and shall be deformed, stainless steel Grade 316LN or 2205 Duplex and strength Grade 420. Chairs or bar supports shall be non-metallic. Tie wire shall be Grade 316L stainless steel.
Comprehensive specifications were recently adopted for HPC by the Cities of Calgary and Edmonton, and by Reid Crowther. Key criteria are listed in the following table:
Calgary Reid Crowther Edmonton Date January 2000 August 2000 November 1999
Cement: kg/m³ Min 360 Min 340 Min 340
Silica fume: % Max 8 Max 8 Max 8
W/c ratio: Max 0.37 Max 0.37 Max 0.37
Air: % 5-8 5-8 5-8
RCP: coulombs Max 600† 700 +/- 35 % Max 1000¶
Air void system CSA A23.1* CSA A23.1 -
28-day strength
Decks/girders 50 MPa 50 MPa #
Barrier walls 40 MPa
50 MPa with
corrosion inhibitor‡ 40 MPa # Scaling test 0.4kg/m²/30 cycles 0.4 kg/m²/30 cycles 0.4 kg/m²/30 cycles
† Decks and girders ‡ 40 MPa with fibres and either corrosion inhibitor or shrinkage reducing admixture. ¶ For 50 MPa concrete. # 50 MPa for decks, abutment deck slabs and approach slabs; 40 MPa for Barrier walls and medians. * Air void system limits can be relaxed if 300 cycles of freezing and thawing tests to ASTM C 666 produce a durability factor greater than 90%.
Curing with fog mist or evaporation retarder to start immediately after finishing. Curing to continue for at least 5 days (3 days for overlays) with wet burlap covered with vapour proof sheeting.
The City of Calgary requires tests in advance of construction to determine the cracking potential of the proposed mix. Cracks over 0.2 mm in width in the structure are repaired. A number of penalties are imposed for failures to meet specification requirements:
Unit Penalty: $/m³
3.5-4.5 MPa below specification 70 Strength
> 4.5 MPa below specification 150 or remove
0.2 % outside limits 60
Air content
> 0.2 % outside limits 150 or remove
RCP 601-1200 coulombs 40
>1200 coulombs 250 or remove
For overlays, a corrosion inhibitor is specified. The RCP value is given as 600 coulombs plus a 20% tolerance. Consequently, the penalties shown in the above table are different, only for RCP values, as follows:
The City of Edmonton adopted a specification for HPC in November 1999. Key criteria are as follows:
Min Cement
W/C
Ratio Max Slump
28 Day Strength: MPa 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.
