HPC Defined for Highway Structures
By Charles Goodspeed
The Strategic Highway Research Program (SHRP) investigated more than 60 concrete and structures products . To stimulate State Highway Agency (SHA) use of selected products, the Federal Highway Administration (FHWA)is using "Showcases" to demonstrate these and other new product technologies. Products selected for showcasing include those contributing to the production and performance evaluation of higher quality concrete. To establish a clear understanding of High Performance Concrete (HPC), the FHWA is proposing to define HPC using long-term performance criteria. The proposed definition consists of four durability and four strength parameters. Associated with each definition parameter are performance criteria, testing procedures to measure performance, and recommendations to relate performance to adverse field conditions. To specify an HPC concrete mixture using the FHWA HPC definition, a user states, based on field conditions, the level of performance desired for each performance characteristic. Updates will have to be made to keep the definition current with improvements in technology and field experience.
FHWA's primary purpose in offering the HPC Showcase and preparing the HPC definition is to stimulate the use of higher quality concrete in highway structures. A recent study conducted in the Chicago area evaluated the performance characteristics of commercially available concrete ranging in strength from 70 MPa (10 ksi) to 140 MPa (20 ksi). This study demonstrated that a significant improvement in concrete durability resulted from an increase in strength which is not being regularly specified for highway structures. That HPC is not being specified more frequently may be because engineers do not have confidence that higher strength concrete is more durable, that it can be reliably achieved in the field, that the higher strength cannot always be used, or combinations thereof.
The FHWA HPC Showcase addresses these issues by illustrating cost-effective state of the art procedures for producing, evaluating, and designing with HPC. The FHWA is sponsoring State Highway Agencies, SHA, demonstration projects over the next several years to illustrate the use of HPC.
This paper presents the performance definition using three tables: Table 1 gives the parameters and performance criteria, Table 2 identifies standard tests to evaluate performance, and Table 3 relates recommended performance to exposure conditions.
A SHRP study defined HPC for concrete as:
- a maximum water-cementitious ratio (W/C) of 0.35;
- a minimum durability factor of 80%, as determined by ASTM C 666, Procedure A; and
- a minimum strength criteria of either
- (a) 21 Mpa (3,000 psi) within 4 hours after placement (Very Early Strength, VES),
- (b) 34 MPa (5,000 psi) within 24 hours (High Early Strength, HES), or
- (c) 69 Mpa (10,000 psi) within 28 days (Very High Strength, VHS).
The American Concrete Institute defines HPC as concrete that meets special performance and uniformity requirements that cannot always be obtained using conventional ingredients, normal mixing procedures, and typical curing practices. These requirements may include the following enhancements:
- ease of placement and consolidation without affecting strength,
- long-term mechanical properties,
- early high strength,
- volume stability, and
- longer life in severe environments.
The SHRP definition uses W/C as a mixture proportion criterion to define HPC. The ACI cites fresh concrete properties, and both cite long-term performance parameters. By restricting the definition to long-term performance parameters, concrete mixture designers may be more willing to incrementally modify the use of ingredients, change concrete curing procedures, and use admixtures and alternate hydraulic cements such as granulated ground blast furnace slag (gbfs). Use of a performance definition alone cannot, however, address all deterioration mechanisms. There is insufficient experience to relate laboratory test results with resistance to the wide range and combination of field conditions. Deterioration stemming from poor quality materials subjected to an adverse environment can also represent a quality control and quality assurance problem.
For bridge engineers to adopt a HPC performance definition it must include adequate durability and strength parameters . The proposed HPC definition uses eight parameters and relates each to deterioration resistance and gives standard tests to evaluate the performance of each parameter.
DURABILITY AND STRENGTH PARAMETERS
The definition has an adequate number of performance parameters to facilitate the definition's use as a guide when specifying concrete mixtures. The HPC definition resulted from an investigation of general field conditions that cause concrete structures to deteriorate. Field conditions can be divided into three categories: climate, exposure effects, and loads (Table 4). Climatic conditions include temperature fluctuations, cycles of freezing and thawing, and relative humidity. Exposure conditions include the presence of salts (applied for deicing or suspended in water) and aggressive chemicals (sulfates, acids, and carbon dioxide). Loading conditions include traffic, wind, earthquake, and other applied loads.
Climate may cause adverse thermal expansion, an undesirable moisture content, or a deterioration of strength due to cycles of freezing and thawing. Exposure to aggressive chemical agents may cause scaling, destructive expansion within the concrete, or corrosion of reinforcing steel. Stresses due to loading may result in unacceptable creep, deflection, capacity or cracking. Each field condition was evaluated to identify independent concrete performance parameters that represent an acceptable durability or strength characteristic for defining HPC.
Climatic Conditions: Temperature affects concrete by thermal expansion and contraction on heating and cooling, and by freezing water that induce internal stresses. Structural designs normally consider thermally induced expansion and contraction and,
|Field Conditions||Related Characteristics|
|Climate: Temperature fluctuations
Freezing and thawing resistance
|Exposure: Applied salts/salt water,
|Loads: Traffic, wind, earthquake||Abrasion resistance
Modulus of elasticity
therefore, thermal expansion and contraction are not typically considered in specifying a mixture.
Mixture ingredients, their proportions, mixing sequence, curing condition and concrete density control the ability of concrete in a saturated condition to resist deterioration when subjected to freezing and thawing. Important characteristics include the air-void system, soundness of the aggregate, and concrete maturity. Although these concrete characteristics can be measured independently, it is the combined effect of these characteristics that results in overall long term performance . It is the combined effect that must be represented in a long term HPC definition.
Exposure Conditions: The application of road salts results in a pore water solution high in chloride ions. Over time these solutions promote corrosion of reinforcing steel. The corrosive reaction is expansive and causes tensile stress in the concrete. When the tensile stresses exceed concrete tensile strength the concrete begins to spall. Steel corrosion occurs in concrete when the acid-soluble chloride content minus the background chloride reaches 72 Kg/m3 (1.2 lb/yd3), when pore water exists, and when oxygen is present [6,7]. The presence of all three is required for corrosion to occur. Concrete with low permeability slows the corrosion process by reducing the rate of chloride ion diffusion into the concrete. Reducing the presence of this one corrosion ingredient is often sufficient to adequately delay the onset of corrosion. Thus, it can represent resistance to corrosion.
Aside from causing steel corrosion the repeated application of deicing chemicals has the potential to cause scaling, pitting, spalling, and flaking of concrete surfaces. The exact cause of these problems is not completely understood. However, when deicing chemicals are used to melt ice, the following process occurs: the ice melts, the concrete thaws, the melt water is absorbed, the surface concrete becomes more fully saturated, the melt water is diluted; and if the concrete surface freezes again it undergoes a freezing and thawing cycle it would not have experienced had it remained frozen. This cycle can repeat and deteriorate concrete lacking adequate freezing and thawing resistance in one winter, whereas the same concrete when not exposed may not show any frost damage. Furthermore, endothermic nature of melting ice with salt is detrimental to concrete. The melting absorbs energy that causes the temperature of the concrete to drop rapidly just below the ice surface. This may result in damage from the effects of rapid freezing and differential thermal strains. Curing history, water-cementitious ratio, air content, moisture content, characteristics of the freezing and thawing cycle, and salt concentration may affect concrete scaling resistance. Again, it is their combined effect that represents performance against scaling and should all be represented in a definition scaling parameter.
Care must be taken to investigate the effect of aggressive chemicals when field conditions warrant. Highway structures can be exposed to a wide range of aggressive chemicals that deteriorate concrete. The diversity of chemical attack makes it difficult to represent concrete resistance to aggressive chemicals by a single durability performance parameter. Thus, it is considered the responsibility of the designer to address the potential effects of ambient project conditions. The need to be aware of aggressive chemicals is footnoted in Table 1.
Loading Conditions: Concrete durability and strength parameters are not necessarily independent; an increase in a durability parameter can result in an increase in strength and vice versa. Loading conditions may not warrant the strength developed in concrete proportioned to meet durability criteria. Structural designers may specify concrete performance in terms of limiting volume change (i.e., creep and shrinkage) and achieving a minimum modulus of elasticity. These characteristics along with strength are generally sufficient to represent the mechanical concrete properties used in structural design. Other characteristics, such as modulus of rupture, can generally be estimated using these primary characteristics. Other parameters can be calculated or may need to be experimentally determined.
The action of vehicular traffic or solids suspended in flowing water abrade concrete surfaces. Surface wear is normally not a controlling factor in deck and roadway performance. However, in areas where the use of studded tires is permitted, abrasion can be significant. In these situations the ability of the concrete to resist abrasion is an important performance parameter.
DETERIORATION RESISTANCE -- PERFORMANCE GRADES
Eight parameters were identified as sufficient to represent HPC long-term performance, (see Table 1). To use the definition as a basis for specifying concrete, relationships must be established between the performance parameter and resistance to exposure conditions. To accomplish this it is necessary to identify desired performance grades for the definition parameters and their relationship to project field conditions. Each parameter grade must represent a measure of performance when subjected to a field condition. Using grades to represent performance, an engineer can specify a mixture to yield a desired concrete service life. Each parameter can be independently specified by grade, for example: a mixture for a bridge deck subjected to high usage of deicing salts, high frequency of freezing and thawing cycles, and narrow beam spacing may be specified by a high Grade resist freezing and thawing distress, a medium to high grade to resist scaling, abrasion, and chloride attack and a low grade for strength and elasticity.
Performance is represented by test variables such as the percentage of dynamic modulus of elasticity remaining after 300 prescribed cycles of freezing and thawing or a range of compressive strengths. Grades start at low performance levels and small enough increments are defined to allow engineers to begin specifying higher quality concrete incrementally. The strength grades start at a performance level that is easily attainable and spans to a superior grade. The definition is intended to cover all grades of concrete that can be readily used by the highway i/ndustry.
STANDARD TEST PROCEDURES
Standard test methods are identified to ascertain performance for the eight definition parameters. Test procedures and specimen preparation not specified in the standard test procedures are given in Table 2. To achieve uniformity in evaluating performance the following specimen and curing procedures are specified for each test:
|Performance Characteristic2||Standard Test Method||FHWA HPC Performance Grade3|
(x = relative dynamic modulus of elasticity after 300 cycles)
|AASHTO T 161
ASTM C 666Proc. A
|60% ≤ x ≤ 80%||80% ≤ x|
(x = visual rating of the surface after 50 cycles)
|ASTM C 672||x = 4,5||x = 2,3||x = 0,1|
(x = avg. depth of wear in mm)
|ASTM C 944||2.0 > x ≥ 1.0||1.0 > x ≥ 0.5||0.5 > x|
(x = coulombs)
|AASHTO T 277
ASTM C 1202
|3000 ≥ x > 2000||2000 ≥ x > 800||800 ≥ x|
(x = compressive strength)
|AASHTO T 22
|41 ≤ x < 55 MPa
(6 ≤ x < 8 ksi)
|55 ≤ x < 69 MPa
(8 ≤ x < 10 ksi)
|69 ≤ x < 97 MPa
(10 ≤ x < 14 ksi)
|x ≥ 97 MPa
(x ≥ 14 ksi)
(x = modulus of elasticity)
|ASTM C 469||24 ≤ x < 40 GPa
(4 ≤ x < 6 x 106 psi)
|40 ≤ x 50 GPa
(6 ≤ x < 7.5 x 106 psi)
|x ≥ 50 GPA
(x ≥ 7.5 x 106 psi)
(x = microstrain)
|ASTM C 157||800 > x ≥ 600||600 > x ≥ 400||400 > x|
(x = microstrain/pressure unit)
|ASTM C 512||
1This table does not represent a comprehensive list of all characteristics that good concrete should exhibit. It does list characteristics that can quantifiably be divided into different performance groups. Other characteristics should be checked. For example, HPC aggregates should be tested for detrimental alkali silica reactivity according to ASTM C227, cured at 38°C, and tested at 23°C and should yield less than 0.05% mean expansion at 3 months and less than 0.10% expansion at 6 months (based on SHRP C-342, p. 83). Due consideration should also be paid to (but not necessarily limited to) acidic environments and sulfate attack.
2All tests to be performed on concrete samples moist or submersion cured for 56 days. See Table 3 for additional test information.
3A given high performance concrete mix design is specified by a grade for each desired performance characteristic.. For example, a concrete may perform at Grade 4 in strength and elasticity, Grade 3 in shrinkage and scaling resistance, and Grade 2 in all other categories.
4Based on SHRP C/FR-91-103, p. 3.52.
5Based on SHRP S-360.
6Based on SHRP C/FR-91-103.
7Based on PCA Engineering Properties of Commercially Available High-Strength Concretes.
8Based on SHRP C/FR-91-103, p. 3.25.
9Based on SHRP C/FR-91-103, p. 3.30.
10Based on SHRP C/FR-91-103, p. 3.17.
Cylinders: 100 mm diameter x 200 mm long or (4 in by 8 in) or 150 mm diameter x 300 mm long or (6 in by 12 in)
Curing: Non-Steam Cured Products: Moist Cure specimens for 56 days or match cure and use a maturity meter.
Steam Cured Products: Cure specimens with the member or match cure
The standard tests, performance parameter variables, and respective grades are described below.
Resistance to Freezing and Thawing, ASTM C666, Procedure A, or AASHTO T 161: The test procedure is to be continued for 300 cycles or until the relative dynamic modulus of elasticity drops below 60%. Two HPC grades of resistance to freezing and thawing are delineated by the percentage of dynamic modulus of elasticity after 300 cycles. Grade 1 is defined as 60% to 80% remaining of the original dynamic modulus of elasticity and Grade 2 is defined as greater than 80% of the original dynamic modulus of elasticity.
Scaling test, ASTM C 672: The ASTM C 672 test must be done for 50 cycles. Scaling performance is evaluated after 50 cycles by visually inspecting specimens as prescribed by C 672. Grade 1 is defined by a visual inspection rating of 4 or 5, Grade 2 by a rating of 2 or 3, and Grade 3 by 0 or 1.
Abrasion, ASTM C 944: Test areas shall receive a light trowel finish. Specimens shall be field cured for 56 days and air dried for 2 hours before testing. The tests shall be done on three different cylinders or at three different areas on the surface of a concrete structure. Each abrasion test shall be done using a 196 N force for three 2 minutes periods for a total of 6 minutes of abrasion testing; a wear depth is then measured. The grades are inversely proportional to wear; a low performance grade is assigned to the higher measurements of wear and a high grade is assigned to the lower measurements of wear.
Creep and Shrinkage, ASTM C 512 and ASTM C 157: Creep and shrinkage specimens shall be moist cured for 28 days, and then tests shall be performed for 180 days. Creep test loading and air storage of shrinkage specimens shall be begun at 28 day age. Grades of performance are as shown in Table 1.
|Performance Characteristic||Standard Test Method||Notes1|
|Freeze/Thaw Durability||AASHTO T 161
ASTM C 666 Proc. A
|1. Test specimen 76.2 x 76.2 x 279.4 mm (3 x 3 x 11 in) cast or cut from 152.4 x 304.8 mm (6 x 12 in) cylinder |
2. Acoustically measure dynamic modulus until 300 cycles.
|Scaling Resistance||ASTM C 672||1. Test specimen to have a surface area of 46,451 mm2 (72 in2).|
2. Perform visual inspection after 50 cycles
|Abrasion||1. Concrete shall be tested at 3 different locations.|
2. At each location, 98 Newtons, for three, 2 minute, abrasion periods shall be applied for a total of 6 minutes of abrasion time per location.
3. The depth of abrasion shall be determined per ASTM C 799 Procedure B.
|Chloride Permeability||AASHTO T 277
ASTM C 1202
|1. Test per standard test method.|
|Strength||AASHTO T 22
|1. Molds shall be rigid metal or one time use rigid plastic.|
2. Cylinders shall be 100 mm dia. x200 mm long (3.9 x 7.8 in) or 150 mm dia. x300 mm long (5.9 x 11.2 in).
3. Ends shall be capped with high strength capping compound, ground parallel, or placed onto neoprene pads per AASHTO specifications for Concretes
4. Use of neoprene pads on early age testing of concrete exceeding 70 Mpa at 56 days should use neoprene pads on the 56 day tests
5. The 56 day strength is recommend
|Elasticity||ASTM C 469||1. Test per standard test method.|
|Shrinkage||ASTM C 157||1. Use 76.2 x 76.2 x 285 mm (3 x 3 11 1/4 in) specimens |
2. Shrinkage measurements are to start 28 days after moist curing and be taken for a drying period of 180 days.
|Creep||ASTM C 512||1. Use 152 x 305 mm (6 x 12 in) specimens |
2. Cure specimens at 73° F and 50% RH after 7 days until loading at 28 days.
3. Creep measurements to be taken for a creep loading period of 180 days.
Strength, AASHTO T 22: Strength test specimens must be cast in metal or rigid plastic molds. Compression specimens shall have the ends capped, ground parallel, or be tested using neoprene pads per AASHTO or ASTM specifications. The diversity of strength needs and the variation of strengths used in practice necessitates a wide range of strength grades starting at 41 MPa (6 ksi) for Grade 1 to greater than 97 Mpa (14 ksi) for Grade 4. Bridge engineers currently specifying strengths less than grade 1 can begin the transition to a higher durability and strength concrete by specifying minimum HPC performance grades. The highest level is specified to define the state of the art in highway concrete usage.
Static Modulus of Elasticity, ASTM C 469: Standard test procedures shall be followed. Grades range from a low of 28 Gpa (4 x 106 psi) for grade 1 to greater than 50 GPa (7.5 x 106 psi) for grade 3.
Chloride Penetration, AASHTO T277, ASTM C 1202: Chloride penetration specimens shall be moist cured for 56 days. Grades are as shown in Table 1.
TEST PERFORMANCE AND FIELD CONDITION SEVERITY
Grades of performance are defined for each of the eight parameters used for the HPC definition. Field condition severity has been estimated for the full range of potential field conditions occurring in the United States (see Table 3).
Freeze and Thawing: A field freezing and thawing cycle is defined as a decrease in temperature to -2.2 C (28 F) or below followed by a thaw . This field condition is recorded throughout the United States by the Geological Society and is reported by the number of occurrences per year and shown on a national map . A relationship between the deteriorating effect of a field cycle and a laboratory cycle, per AASHTO T 161, is estimated. The currently recommended relationship is as follows: when fewer than 3 field cycles occur per year no consideration is required, between 3 and 50 field cycles per year, the use of Grade 1 is recommended, and above 50 field cycles, Grade 2. This relationship is recommended as a lower bound for specifying HPC.
Scaling: Data are not available to substantiate a strong recommendation between performance grade and field severity. The relationship given should be taken as a suggestion until further research is available.
|Recommended HPC Grade for Given Exposure Condition|
|Exposure Condition||N/A2||Grade 1||Grade 2||Grade 3||Grade 4|
|Freeze/Thaw Durability Exposure
(x = F/T cycles per year)1
|x < 3||3 ≤ x < 50||50 ≤ x|
|Scaling Resistance Applied Salt3
(x = tons/lane-mile-year)
|x < 5.0||5.0 ≤ x|
(x = average daily traffic, studded tires allowed)
|no studs/chains||x ≤ 50,000||50,000 ≤ x < 100,000||100,000 ≤ x|
|Chloride Permeability Applied Salt3
(x = tons/lane-mile-year)
|x < 1||1.0 ≤ x < 3.0||3.0 ≤ x < 6.0||6.0 ≤ x|
1F/T stands for "freeze/thaw". A freeze/thaw cycle is defined as an event where saturated concrete is subjected to an ambient temperature which drops below -2.2°C (28°F) followed by a rise in temperature above freezing.
2N/A stands for "not applicable" and indicates a situation in which specification of an HPC performance grade is unnecessary.
3As defined in SHRP S-360.
Abrasion: Normal surface abrasion from rubber tires typically does not warrant an abrasion resistance consideration assuming well cured concrete of appropriate strength; the use of studded tires does. Thus, a Grade 1 is recommended for less than a 50,000 average daily traffic count, Grade 2 for greater than 50,000 and less than 100,000, and Grade 3 for greater than 100,000 when steel studded tires are permitted. Similar estimates can be made by local engineers if the use of car chains is prevalent. Recommendations for other abrasion conditions such as stream flow laden with abrasive materials are the responsibility of the project engineer.
Chloride Penetration: Coulombs measured in the rapid chloride permeability test are currently used herein to estimate performance grades relative to steel corrosion. Grade 1 is defined between 2000 and 3000 coulombs, Grade 2 between 800 and 2000, Grade 3 less than 800 coulombs. These recommendations use a corrosion model to predict service life based on chloride content in the concrete using the following assumptions [5, 10]:
- 30-year life span;
- 2 in of cover with a standard deviation of 0.3 in; and
- a range of applied quantities of deicing salt.
Mechanical Properties: Grades of performance are designated in Table 1. Material and structural designers can select and specify appropriate grades for a project.
The HPC definition presented in this paper identifies a set of concrete performance characteristics sufficient to estimate long-term concrete durability and strength for highway structures. Standard laboratory tests, specimen preparation procedures, and grades of performance are suggested for each definition parameter. Relationships between performance and severity of field conditions are estimated to assist designers in selecting the grade of HPC that should be used for a particular project. Because there is a lack of information correlating field condition severity and laboratory performance, these relationships serve only as suggestions. Thus, this definition is a guide, and identifies areas in which additional research is needed.
Bridge engineers and other concrete designers are encouraged to begin using the definition as a tool in expanding their understanding and confidence in using higher performing concrete. It is anticipated that research and experience gained from the FHWA HPC demonstration projects and other sources will result in continued updates to these tables. Note that specified relationships between laboratory performance data and resistance to field conditions are only suggestions. Information gained from local experience should receive careful consideration.
In transportation it has always been the goal to use concrete with characteristics at appropriate levels to ensure satisfactory performance for the intended service life. Through success has often been achieved, attention seems to focus on the cases where success has not been achieved. When concrete does not perform as desired either the specifications were inadequate or they were not followed. Modern QC/QA procedures should greatly increase the likelihood that specifications are met when followed. At a recent HPC workshop  it was suggested that if the concrete used was produced to strictly comply with relevant code requirements it should be a high-performance concrete. The intent of high-performance concrete, as defined here, is not to produce a high-cost product, but simply to provide the means for producing concrete that will perform satisfactorily with only reasonable maintenance costs for the intended service life .
1. Strategic Highway Research Program (SHRP), "SHRP Products Catalog," 1992
2. Burg, R.G., Ost, B.W., "Engineering Properties of Commercially Available High-Strength Concretes," PCA Research and Development Bulletin, RD 104T
3. Zia, P., Leming, M.L., Ahmad, S.H., "High Performance Concretes A State-of-the-Art Report," SHRP-C/FR-91-103
4. Mather, B., "Concrete in Transportation: Desired Performance and Specifications," TRB No 1382 Materials and Construction
5. Mather, B., "How to Make Concrete That Will Be Immune To The Effects of Freezing and Thawing," in Paul Klieger Symposium of Performance of Concrete, David Whiting, Editor, ACI SP 126, pg 1-10, American Concrete Institute, Detroit, MI
6. Weyers, R.E., Prowell, B.D., Sprinkel, M.M., Vorster, M., "Concrete Bridge Protection, Repair, and Rehabilitation Relative to Reinforcement Corrosion," SHRP-S-360.
7. Cady, P.D., Weyers, R.E., "Deterioration Rates of Concrete Bridge Decks," Journal of Transportation Engineering, 110 No 1 (1984): pg 35-44.
8. Russel, J., "Freeze-and-Thaw Frequencies in the United States," America Geophysical Union Transactions 24 1943 pg 125.
9. Visher, S.S., "Climatic Maps of Geological Interest," Geological Society of America Bulletin 56, July-Dec. 1945 Pg 730.
10. Goodspeed, C.H., et al., "HPC Service Life Using Corrosion Model," CORROSION/95, The NACE Annual Conference and Corrosion Show, March 1995.
11. Personal communication from Bryant Mather referring to the workshop sponsored in Bangkok, Thailand by NSF 20-22 November 1994, Moderated by Professor Paul Zia.
12. Forster, S.W., "High-Performance Concrete-Stretching the Paradigm", Concrete International, October 1994, Vol. 16, Number 10. This paper was originally published in the February 1996 issue of Concrete International, a publication of the American Concrete Institute.