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Publication Number: FHWA-HRT-05-057
Date: October 2006

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Appendix C-Proposed Revisions To The AASHTO Standard Specifications For Highway Bridges

PROPOSED CHANGE TO AASHTO Standard Specifications Division I

(X) Revision or ( ) Addition 3.3.6

Item No. 1

Revise 3.3.6 as follows:

3.3.6 The following weights are to be used in computing the dead load:

#/cu. ft

Concrete, fc' ≤ 5,000 psi

Normal weight, plain

Normal weight, reinforced

Concrete, 5,000 psi < fc' # 15,000 psi

Normal weight, plain.............................................. 140 + 0.001 fc'

Normal weight, reinforced ..................................... 145 + 0.001 fc'

Item No. 2

Add the following to 3.1:

3.1 Notations

fc' = specified compressive strength of concrete, psi (Article 3.3.6)

Other Affected Articles

8.7

Background

Variation of concrete unit weight  with compressive strength

Variation of concrete unit weight with compressive strength

Analysis of data from the FHWA Showcase projects and other sources indicates a trend that the unit weight of concrete increases as concrete compressive strength increases.(1,2,3) Unit weights range from about 140 to 155 lb/ft3 (2.24 to 2.48 Mg/m3) with an average increase of about 1 lb/ft3 for every 1000 psi increase in compressive strength (2.3 kg/m3 for every 1 MPa).

Anticipated Effect on Bridges

More precise calculation of dead load.

References

  1. High Performance Concrete, Compact Disc, Federal Highway Administration, Version 3.0, February 2003.
  2. Burg, R. G. and Ost, B. W., "Engineering Properties of Commercially Available High-Strength Concretes," PCA Research and Development Bulletin RD104T, Portland Cement Association, Skokie, IL, 1992.
  3. Burg, R. G. and Fiorato, A. E., "High-Strength Concrete in Massive Foundation Elements," PCA Research and Development Bulletin RD117, Portland Cement Association, Skokie, IL, 1999.

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division I

(X) Revision or ( ) Addition 8.3.3

Revise 8.3.3 as follows:

8.3.3 Designs shall not use a yield strength, fy, in excess of 60,000 psi, except as permitted elsewhere.

Other Affected Articles

9.20.3.4

Background

This change is made to permit the use of other yield strengths for particular design situations.

Anticipated Effect on Bridges

More economical bridges.

References

None

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division I

(X) Revision or ( ) Addition 8.5

Revise 8.5 as follows:

8.5.3 The coefficient of thermal expansion should be determined by laboratory tests on the specific mix to be used.

In the absence of more precise data, the thermal coefficient of expansion may be taken as:

  • For normal weight concrete 0.000006 per deg F
  • For lightweight concrete: 0.000005 per deg F

8.5.4 Shrinkage coefficients for normal weight and lightweight aggregate shall be determined for the type of aggregate and intended application.

(Deleted text is indicated by strikethrough. Inserted text is underlined.)

Other Affected Articles

None

Background

The thermal coefficient of expansion of concrete depends primarily on the types and proportions of aggregates used in the concrete and ranges from 3 to 8 x 10-6/F.(1,2) More precise calculations of thermal movements can be made when the actual value for a particular aggregate and concrete mix is utilized. The proposed change is consistent with the AASHTO LRFD Bridge Design Specifications.

Shrinkage is affected by aggregate characteristics, relative humidity, concrete mix proportions, curing procedure, volume to surface area ratio of the member, drying period, and presence of reinforcement. Shrinkage ranges from almost zero for members in water to 0.0008 for thin sections made with high shrinkage aggregates. The use of a single value for the shrinkage of all concrete is misleading and can lead to large errors in the calculation of length changes. Consequently, calculations for shrinkage should be based on the actual materials and intended application.

Anticipated Effect on Bridges

More precise calculation of length changes and movements caused by temperature changes and shrinkage.

References

  1. Kosmatka, S. H., Kirkhoff, B., and Panarese, W. C., Design and Control of Concrete Mixtures, Fourteenth Edition, Portland Cement Association, Skokie, IL, 2002, 358 pp.
  2. ACI Committee 209, "Prediction of Creep, Shrinkage, and Temperature Effects in Concrete Structures, (ACI 209R-92)," American Concrete Institute, Farmington Hills, MI, 1992, 47 pp.

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division I

(X) Revision or ( ) Addition 8.7

Item No. 1

Revise 8.7 as follows:

8.7 Modulus of Elasticity and Poisson's Ratio

8.7.1 The modulus of elasticity, Ec, for concrete may be taken as K1 in psi for values of wc between 90 and 155 pounds per cubic foot and fc' less than 15,000 psi where,

K1 = correction factor for type of aggregate to be taken as 1.0 unless determined by physical tests

wc = unit weight of concrete (lb per cu ft)

When the measured unit weight of concrete is unknown, wc for normal weight concrete may be taken as 140 + 0.001 fc'

For normal weight concrete (wc = 145 pcf) and fc' less than or equal to 5,000 psi, Ec may be considered as.

Item No. 2

In 8.1.2 Notations, add the following:

K1 = correction factor for type of aggregate to be taken as 1.0 unless determined by physical tests (Article 8.7.1)

Other Affected Articles

3.3.6

All articles that include Ec. (Changes are not needed.)

Background

Using a significant amount of test data, NCHRP Project 18-07 entitled "Prestress Losses in Pretensioned High-Strength Concrete Bridge Girders," has identified that the accuracy of the existing equation for predicting modulus of elasticity can be improved by the proposed modifications.(1) A more accurate prediction of modulus of elasticity is needed in calculating prestress losses and camber of high-strength concrete girders as the values are larger than for conventional strength concrete girders.

Anticipated Effect on Bridges

More accurate prediction of prestress losses and camber.

References

  1. Tadros, M. K., Al-Omaishi, N., Seguirant, S. P, and Gallt, J. G., "Prestress Losses in High Strength Concrete Bridge Girders," NCHRP 18-07 Final Report, August 2002.

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division I

(X) Revision or ( ) Addition 8.15.2.1.1

Revise 8.15.2.1.1 as follows:

8.15.2.1.1 Flexure

Modulus of rupture, fr, from tests, or, if data are not available:

  • For normal weight concrete-x

    When used to calculate the cracking moment of a member in 8.13.3 or for any other calculation where a lower bound value is appropriate............................................. 7.5 The square root of f prime subscript c

    When used to calculate the cracking moment of a member in 8.17.1 or for any case where an upper bound value is appropriate........................................................ 11.7 The square root of f prime subscript c

  • For sand-lightweight concrete............................................................... 6.3 The square root of f prime subscript c
  • For all-lightweight concrete................................................................... 5.5 The square root of f prime subscript c

When physical tests are used to determine modulus of rupture, the tests shall be performed in accordance with AASHTO T 97 "Standard Method of Test for Flexural Strength of Concrete (Using Simple Beam with Third-Point Loading.)" and shall be performed on concrete using the same proportions and materials as specified for the structure.

Other Affected Articles

8.13.3 (No change required)

8.17.1 (No change required)

Background

The value of 7.5 The square root of f prime subscript c has been traditionally used for modulus of rupture (MOR) for concrete. Data show that this value greatly underestimates the MOR at higher compressive strengths. ACI 363 indicates that a value for MOR of 11.7 The square root of f prime subscript c could be used for concrete compressive strengths from 3,000 to 12,000.(1) However, examination of other data shows that most MOR values fall between these two limits.(2-10) Thus, the AASHTO value of 7.5 The square root of f prime subscript c appears to be an appropriate lower bound while the ACI 363 value of 11.7 The square root of f prime subscript c appears to be an appropriate upper bound. Since the cracking moment is directly proportional to the MOR, use of these values yields lower and upper limits of the cracking moment. It is appropriate to use the lower bound value of the MOR when considering service load stresses, serviceability (whether a member is cracked and possible crack widths) or deflections, where lower values of a cracking moment yield more critical quantities. However, the upper bound value is more appropriate for determining minimum amounts of reinforcement. The purpose of the minimum reinforcement in article 8.17.1 is to ensure that the nominal moment capacity of member is at least 20 percent greater than the cracking moment. If the cracking moment is too close to the nominal moment capacity, the beam could fail in a brittle manner. Since the data show that the actual MOR could be as much as 50 percent greater than the lower bound MOR, the actual cracking moment could be as much as 50 percent greater than that calculated using the lower bound MOR. This effectively removes the 20 percent margin of safety. Using the upper bound value of MOR alleviates this problem.

The existing provisions of the code allow the use of measured values for the modulus of rupture in design calculations. The properties of higher strength concretes are particularly sensitive to the constitutive materials. In many specifications, structural concrete is specified by a class or type with given proportions. The actual constitutive materials are often specified in broad terms rather then being specified as specific materials. Thus, a given type or class of concrete could be made with any one of a number of different aggregates, cements or admixtures. However, a concrete made with crushed limestone will have different properties from a concrete with the same proportions but using a gravel aggregate; especially at higher compressive strengths. If test results are to be used in design, it is imperative that tests be made using concrete with the same mix proportions, and the same materials specified for the structure.

Anticipated Effect on Bridges

This change provides a more reasonable estimate of the cracking moment, Mcr. The cracking moment is used to assess minimum reinforcement requirements for beams (8.17.1) and to determine effective moment of inertia for deflection calculations (8.13.3).

References

  1. ACI Committee 363, "State-of-the-Art Report on High-Strength Concrete (ACI 363R-92)," American Concrete Institute, Farmington Hills, MI, 1992, 55 pp.
  2. Price, W. H., "Factors Influencing Concrete Strength," Journal of the American Concrete Institute, Vol. 47, February 1951, pp. 417-432.
  3. Walker, S. and Bloem, D. L., "Effect of Aggregate Size on Properties of Concrete," Journal of the American Concrete Institute, Vol. 57, No. 3, September 1960, pp. 283-98.
  4. Zia, P., Leming, M. L., Ahmad, S., Schemmel, J. J., Elliot, R. P., and Naaman, A. E., Mechanical Behavior of High Performance Concrete, Vol. 1 - Summary Report, SHRP Report C-361, Strategic Highway Research Program, National Research Council, Washington, D. C., 1993.
  5. Zia, P., Leming, M. L., Ahmad, S., Schemmel, J. J., and Elliot, R. P., Mechanical Behavior of High Performance Concrete, Vol. 2, - Production of High Performance Concrete SHRP Report C-362, Strategic Highway Research Program, National Research Council, Washington, D. C., 1993.
  6. Zia, P., Ahmad, S., Leming, M. L., Schemmel, J. J., and Elliot, R. P., Mechanical Behavior of High Performance Concrete, Vol. 3 - Very Early Strength Concrete, SHRP Report C-363, Strategic Highway Research Program, National Research Council, Washington, D. C., 1993.
  7. Zia, P., Ahmad, S., Leming, M. L., Schemmel, J. J., and Elliot, R. P., Mechanical Behavior of High Performance Concrete, Vol. 4 - High Early Strength Concrete, SHRP Report C-364, Strategic Highway Research Program, National Research Council, Washington, D. C., 1993.
  8. Zia, P., Ahmad, S., Leming, M. L., Schemmel, J. J., and Elliot, R. P., Mechanical Behavior of High Performance Concrete, Vol. 5 - Very High Strength Concrete, SHRP Report C-365, Strategic Highway Research Program, National Research Council, Washington, D. C., 1993.
  9. Khan, A. A., Cook, W. D., and Mitchell, D., "Tensile Strength of Low, Medium, and High-Strength Concretes at Early Ages," ACI Materials Journal, Vol. 93, No. 5, September-October 1996, pp. 487-493.
  10. High Performance Concrete, Compact Disc, Federal Highway Administration, Version 3.0, February 2003.

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division I

(X) Revision or ( ) Addition 8.19.1

Revise 8.19.1 Minimum Shear Reinforcement as follows:

8.19.1 Minimum Shear Reinforcement

8.19.1.1 A minimum area of shear reinforcement shall be provided in all flexural members, except slabs and footings, where:

(a) For design by Strength Design, factored shear force Vu exceeds one-half the shear strength provided by concrete ΦVc.

(b) For design by Service Load Design, design shear stress v exceeds one-half the permissible shear stress carried by concrete vc.

8.19.1.2 Where shear reinforcement is required by Article 8.19.1.1, or by analysis, the area provided shall not be less than:

A subscript v equals the product of 50 times b subscript w times s divided by f subscript y (8.64A)

or,

A subscript v equals the square root of f prime subscript c times the quotient of b subscript w times s divided by f subscript y (8-64B)

where bw and s are in inches and the square foot of f prime subscript c and fy are in psi.

Other Affected Articles

9.20.3.3

Background

Tests of high-strength reinforced concrete beams have indicated the need to increase the minimum area of shear reinforcement to prevent shear failures when inclined cracking occurs.(1) These tests indicated a reduction in the reserve shear strength after cracking as fc' increases in beams reinforced with the minimum amount of reinforcement required by the existing equation. The proposed equation requires a gradual increase in the minimum amount of shear reinforcement as concrete strength increases. A comparison of different equations for minimum shear reinforcement is given in the following figure.

The proposed equation is consistent with the current LRFD equation up to a concrete compressive strength of 10,000 psi (70 MPa), which is the current upper limit of the LRFD Specifications. The proposed equation is more conservative than the equation in ACI 318-02.(2)

A comparison of different equations for minimum shear reinforcement.

Comparison of minimum shear reinforcement requirements.

Anticipated Effect on Bridges

More shear reinforcement will be required in the midspan regions of high-strength reinforced concrete beams.

References

  1. Roller, J. J. and Russell, H. G., "Shear Strength of High-Strength Concrete Beams with Web Reinforcement," ACI Structural Journal, Vol. 87, No. 2, March-April 1990, pp. 191-198.
  2. ACI Committee 318, Building Code Requirements for Structural Concrete (318-02) and Commentary (318R-02), American Concrete Institute, Farmington Hills, MI, 2002.

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division I

(X) Revision or ( ) Addition 9.1.2

In 9.1.2 Notations, revise the definition of fc' as follows:

fc' = specified compressive strength of concrete for use in design

Other Affected Articles

None

Background

Since the compressive strength of high-strength concrete is frequently specified at ages other than 28 days, the definition of fc' needs to allow other ages to be utilized.(1) For design purposes of Chapter 9, the age is not important.

Anticipated Effect on Bridges

Use of ages other than 28 days allows for more economical construction.

References

  1. High Performance Concrete, Compact Disc, Federal Highway Administration, Version 3.0, February 2003.

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division I

(X) Revision or ( ) Addition 9.2

Revise 9.2 Concrete as follows:

9.2 Concrete

The specified compressive strength, fc' and fci' where appropriate, of concrete for each part of the structure shall be shown on the plans. The requirements for fc' and fci' shall be based on tests of cylinders made and tested in accordance with Division II, Section 8, "Concrete Structures."

(Deleted text is indicated by strikethrough. Inserted text is underlined.)

Other Affected Articles

None

Background

For prestressed concrete, it is equally important to specify fci' as well as fc'.

Anticipated Effect on Bridges

None

References

None

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division I

(X) Revision or ( ) Addition 9.15

Revise 9.15 as follows:

9.15 Allowable Stresses

The design of precast prestressed members ordinarily shall be based on fc' ≥ 10,000 psi. An increase above 10,000 psi is permissible where, in the Engineer's judgment, it is reasonable to expect that this strength will be obtained consistently. Still higher concrete strengths may be considered on an individual area basis. In such cases, the Engineer shall verify that the controls over materials and fabrication procedures will provide the required strengths. The provisions of this Section are equally applicable to prestressed concrete structures and to components designed with lower concrete strengths.

(Deleted text is indicated by strikethrough. Inserted text is underlined.)

Other Affected Articles

None

Background

Ready-mixed concretes with actual compressive strengths in excess of 10,000 psi (70 MPa) have been available for many years. These strengths can now be achieved in precast, prestressed concrete. In three FHWA-sponsored HPC projects, strengths in excess of 10,000 psi (70 MPa) were specified and successfully achieved.(1)

Anticipated Effect on Bridges

Allow longer span lengths, wider girder spacings, or shallower sections through the use of higher strength concrete.

References

  1. High Performance Concrete, Compact Disc, Federal Highway Administration, Version 3.0, February 2003.

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division I

(X) Revision or ( ) Addition 9.15.2.3

Revise 9.15.2.3 as follows:

9.15.2.3 Cracking Stress

Modulus of rupture, fr, from tests, or, if data are not available:

  1. For normal weight concrete-

    When used to calculate the cracking moment of a member in any calculation where a lower bound value is appropriate................................................................................... 7.5 The square root of f prime subscript c

    When used to calculate the cracking moment of a member in 9.18.2 or for any case where an upper bound value is appropriate...................................................................... 11.7 The square root of f prime subscript c

  2. For sand-lightweight concrete 6.3 The square root of f prime subscript c
  3. For all-lightweight concrete 5.5 The square root of f prime subscript c

When physical tests are used to determine modulus of rupture, the tests shall be performed in accordance with AASHTO T 97 "Standard Method of Test for Flexural Strength of Concrete (Using Simple Beam with Third-Point Loading.)" and shall be performed on concrete using the same proportions and materials as specified for the structure.

(Deleted text is indicated by strikethrough. Inserted text is underlined.)

Other Affected Articles

9.18.2 (No change required)

Background

The value of 7.5 The square root of f prime subscript c has been traditionally used for modulus of rupture (MOR) for concrete. Data show that this value greatly underestimates the MOR at higher compressive strengths. ACI 363 indicates that a value for MOR of 11.7 The square root of f prime subscript c could be used for concrete compressive strengths from 3,000 to 12,000 psi.(1) However, examination of other data shows that most MOR values fall between these two limits.(2-10) Thus, the AASHTO value of 7.5 The square root of f prime subscript cappears to be an appropriate lower bound while the ACI 363 value of 11.7 The square root of f prime subscript c appears to be an appropriate upper bound. Since the cracking moment is directly proportional to the MOR, use of these values yields lower and upper limits of the cracking moment. It is appropriate to use the lower bound value of the MOR when considering service load stresses, serviceability (whether a member is cracked and possible crack widths) or deflections, where lower values of a cracking moment yield more critical quantities. However, the upper bound value is more appropriate for determining minimum amounts of reinforcement. The purpose of the minimum reinforcement in article 9.18.2 is to ensure that the nominal moment capacity of member is at least 20 percent greater than the cracking moment. If the cracking moment is too close to the nominal moment capacity, the beam could fail in a brittle manner. Since the data show that the actual MOR could be as much as 50 percent greater than the lower bound MOR, the actual cracking moment could be as much as 50 percent greater than that calculated using the lower bound MOR. This effectively removes the 20 percent margin of safety. Using the upper bound value of MOR alleviates this problem.

The existing provisions of the code allow the use of measured values for the modulus of rupture in design calculations. The properties of higher strength concretes are particularly sensitive to the constitutive materials. In many specifications, structural concrete is specified by a class or type with given proportions. The actual constitutive materials are often specified in broad terms rather then being specified as specific materials. Thus, a given type or class of concrete could be made with any one of a number of different aggregates, cements or admixtures. However, a concrete made with crushed limestone will have different properties from a concrete with the same proportions but using a gravel aggregate; especially at higher compressive strengths. If test results are to be used in design, it is imperative that tests be made using concrete with the same mix proportions and the same materials specified for the structure.

Anticipated Effect on Bridges

This change provides a more reasonable estimate of the cracking moment, Mcr. The cracking moment is used to assess minimum reinforcement requirements for beams (9.18.2).

References

  1. ACI Committee 363, "State-of-the-Art Report on High-Strength Concrete (ACI 363R-92)," American Concrete Institute, Farmington Hills, MI, 1992, 55 pp.
  2. Price, W. H., "Factors Influencing Concrete Strength," Journal of the American Concrete Institute, Vol. 47, February 1951, pp. 417-432.
  3. Walker, S. and Bloem, D. L., "Effect of Aggregate Size on Properties of Concrete," Journal of the American Concrete Institute, Vol. 57, No. 3, September 1960, pp 283-98.
  4. Zia, P., Leming, M. L., Ahmad, S., Schemmel, J. J., Elliot, R. P., and Naaman, A. E., Mechanical Behavior of High Performance Concrete, Vol. 1 - Summary Report, SHRP Report C-361, Strategic Highway Research Program, National Research Council, Washington, D. C., 1993.
  5. Zia, P., Leming, M. L., Ahmad, S., Schemmel, J. J., and Elliot, R. P., Mechanical Behavior of High Performance Concrete, Vol. 2, - Production of High Performance Concrete SHRP Report C-362, Strategic Highway Research Program, National Research Council, Washington, D. C., 1993.
  6. Zia, P., Ahmad, S., Leming, M. L., Schemmel, J. J., and Elliot, R. P., Mechanical Behavior of High Performance Concrete, Vol. 3 - Very Early Strength Concrete, SHRP Report C-363, Strategic Highway Research Program, National Research Council, Washington, D. C., 1993.
  7. Zia, P., Ahmad, S., Leming, M. L., Schemmel, J. J., and Elliot, R. P., Mechanical Behavior of High Performance Concrete, Vol. 4 - High Early Strength Concrete, SHRP Report C-364, Strategic Highway Research Program, National Research Council, Washington, D. C., 1993.
  8. Zia, P., Ahmad, S., Leming, M. L., Schemmel, J. J., and Elliot, R. P., Mechanical Behavior of High Performance Concrete, Vol. 5 - Very High Strength Concrete, SHRP Report C-365, Strategic Highway Research Program, National Research Council, Washington, D. C., 1993.
  9. Khan, A. A., Cook, W. D., and Mitchell, D., "Tensile Strength of Low, Medium, and High-Strength Concretes at Early Ages," ACI Materials Journal, Vol. 93, No. 5, September-October 1996, pp. 487-493.
  10. High Performance Concrete, Compact Disc, Federal Highway Administration, Version 3.0, February 2003.

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division I

(X) Revision or ( ) Addition 9.16

Item No. 1

Replace the existing 9.16 Loss of Prestress with the proposed revisions to 5.9.5 Loss of Prestress of the AASHTO LRFD Bridge Design Specifications incorporating appropriate changes in article numbers, notations, and format.

Item No. 2

Revise 9.1.2 Notations for the notation used in 9.16.

(Deleted text is indicated by strikethrough. Inserted text is underlined.)

Other Affected Articles

Service load analysis and deflection. No change needed.

Background

The proposed revisions are the results of an extensive theoretical and experimental research on NCHRP Project 18-07.(1,2) Seven full-scale bridge girders were instrumented in four states, Nebraska, New Hampshire, Texas, and Washington. The research included material properties of a number of high-strength concrete mixes including those used in the bridge projects. The results of experiments from previous research were also examined. It was found that concrete creep and shrinkage were dependent on concrete strength in addition to the parameters previously recognized. Also, apparent neglect (or implicit inclusion) of elastic elongation gain of steel stress due to external load application causes confusion among designers who wish to use computer software for design and wish to invoke the higher transformed section properties to optimize their design. In the proposed prestress loss estimates, no elastic shortening loss or elastic gain need be considered if transformed section properties are used. Only long-term loss due to creep, shrinkage and relaxation are estimated and introduced to the cross section as a "negative" prestress force to be applied in addition to the initial prestress just before release and the external gravity loads, to calculate concrete stress at service. It was found that the great majority of the loss takes place before application of the deck weight in composite construction. This is reflected in the coefficients established in the approximate loss method with the estimated loss assumed to be fully applied to the precast section properties.

Anticipated Effect on Bridges

More accurate prediction of prestress losses and camber. Extension of design provisions to concrete strengths greater than 10,000 psi (70 MPa).

References

  1. Al-Omaishi, N., "Prestress Losses in High Strength Pretensioned Concrete Bridge Girders," Ph. D. Dissertation, University of Nebraska-Lincoln, December 2001, 265 pp.
  2. Tadros, M. K., Al-Omaishi, N., Seguirant, S. P., and Gallt, J. G., "Prestress Losses in High Strength Concrete Bridge Girders," NCHRP 18-07 Final Report, August 2002.

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division I

(X) Revision or ( ) Addition 9.20.3.3

Revise 9.20.3.3 as follows:

9.20.3.3 The minimum area of web reinforcement shall not be less than- A subscript v equals the product of the square root of f prime subscript c times the quotient of b prime times s divided by f subscript s y (9-31)

where b' and s are in inches and The square root of f prime subscript c and fsy are in psi.

Other Affected Articles

8.19.1

Background

Tests of high-strength reinforced concrete beams have indicated the need to increase the minimum area of shear reinforcement to prevent shear failures when inclined cracking occurs.(1) These tests indicated a reduction in the reserve shear strength after cracking as fc' increases in beams reinforced with the minimum amount of reinforcement required by the existing equation. The proposed equation requires a gradual increase in the minimum amount of shear reinforcement as concrete strength increases. Although there are a limited number of tests of prestressed concrete beams with high-strength concrete, the equation for minimum web reinforcement should be revised for consistency with the proposed revision for minimum shear reinforcement for reinforced concrete beams and with the LRFD Specifications.

Anticipated Effect on Bridges

More shear reinforcement will be required in the midspan regions of high-strength prestressed concrete beams.

References

  1. Roller, J. J. and Russell, H. G., "Shear Strength of High-Strength Concrete Beams with Web Reinforcement," ACI Structural Journal, Vol. 87, No. 2, March-April 1990, pp. 191-198.

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division I

(X) Revision or ( ) Addition 9.20.3.4

Revise 9.20.3.4 as follows:

9.20.3.4 The design yield strength of web reinforcement, fsy, shall not exceed 75,000 psi. For design yield strengths in excess of 60,000 psi, fsy shall be the stress corresponding to a strain of 0.35 percent.

Other Affected Articles

8.3.3

Background

High strength steels are especially effective in high-strength prestressed concrete members with narrow web widths. The effectiveness of using higher yield strengths has been demonstrated in several research projects.(1-7) Measured shear strengths of beams have exceeded calculated design strengths even when measured yield strengths of reinforcement greater than 60 ksi were used in the calculations.

Anticipated Effect on Bridges

More economical bridge girders.

References

  1. Griezic, A., Cook, W. D., and Mitchell, D., "Tests to Determine Performance of Deformed Welded Wire Fabric Stirrups," ACI Structural Journal, Vol. 91, No. 2, March-April 1994, pp. 211-220.
  2. Shahawy, M. A. and Batchelor, B. deV., "Shear Behavior of Full-Scale Prestressed Concrete Girders: Comparison Between AASHTO Specifications and LRFD Code," PCI Journal, Vol. 41, No. 3, May-June 1996, pp. 48-62.
  3. "Reader Comments on Shear Behavior of Full-Scale Prestressed Concrete Girders: Comparison Between AASHTO Specifications and LRFD Code," PCI Journal, Vol. 42, No. 3, May-June 1997, pp. 72-93.
  4. Ma, Z. and Tadros, M. K., "Simplified Method For Shear Design Based on AASHTO Load and Resistance Factor Design Specifications," Paper No. 99-0266, Transportation Research Record 1688, Transportation Research Board, Washington, D.C., November 1999, pp. 10-20.
  5. Ma, Z., Tadros, M. K., and Baishya, M., "Shear Behavior of Pretensioned High Strength Concrete Bridge I-Girders," ACI Structural Journal, Vol. 97, No. 1, January-February 2000, pp. 185-192.
  6. Tadros, M. K. and Yehia, S., "Shear of Design of High Strength Concrete NU I-Girders," Final Report for Nebraska Department of Roads, December 2001.
  7. Bruce, R. N., Russell, H. G., and Roller, J. J., "Fatigue and Shear Behavior of HPC Bulb-Tee Girders," Louisiana Transportation Research Center, Baton Rouge, LA, to be published.

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division I

( ) Revision or (X) Addition 9.23

Revise 9.23 Concrete Strength at Stress Transfer as follows:

Unless otherwise specified, stress shall not be transferred to concrete until the compressive strength of the concrete as indicated by test cylinders, cured in accordance with Division II, Article 8.5.7.5, is at least 4,000 psi for pretensioned members (other than piles) and 3,500 psi for post-tensioned members and pretensioned piles.

Other Affected Articles

Division II Article 8.5.7.5

Background

Since high-strength concrete generates more heat of hydration than conventional strength concrete, it is important that test cylinders be cured at the same temperature as the member.(1,2) Curing conditions for test cylinders are defined in proposed additions to Division II.

Anticipated Effect on Bridges

Provides a more realistic measure of the compressive strength of the concrete in the member.

References

  1. High Performance Concrete, Compact Disc, Federal Highway Administration, Version 3.0, February 2003.
  2. Meyers, J. J. and Carrasquillo R. L., "Production and Quality Control of High Performance Concrete in Texas Bridge Structures," Center for Transportation Research, The University of Texas at Austin, Research Report 580/589-1, 2000, 553 pp.

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division II

(X) Revision or (X) Addition 8.2

Revise 8.2.2 Normal Weight Concrete and add two new classes of high performance concrete to Table 8.2:

8.2.2 Normal Weight Concrete

Ten classes of normal weight concrete are provided for in these specifications as listed in Table 8.2.

Class of Concrete Minimum Cement Content Maximum Water/Cementitious MaterialsRatio Air Content Range Size of Coarse Aggregate Per AASHTO M 43 Specified Compressive Strength

Pounds per CY

Lbs per Lb

Percent

Square Openings

(lb/in.2)

P(HPC)

-c

0.40

As specified in the contract

£ 3/4 in

> 6,000 as specified in the contract

A(HPC)

-c

0.45

As specified in the contract

-c

4,000

c Minimum cementitious materials content and coarse aggregate size to be selected to meet other performance criteria specified in the contract.

Other Affected Articles

8.4.3

Background

With high performance concrete, it is desirable that the specifications be performance based. The introduction of two new classes of concrete is a move in this direction. Class P(HPC) is intended for use in prestressed concrete members with a specified concrete compressive strength greater than 6000 psi (41 MPa). Class A (HPC) is intended for use in cast-in-place construction where performance criteria in addition to concrete compressive strengths are specified. Other criteria might include shrinkage, chloride permeability, freeze-thaw resistance, deicer scaling resistance, abrasion resistance, or heat of hydration.(1,2)

The proposed change to the heading of the third column will affect all classes of concrete listed in the existing table and makes the table more consistent with the state-of-the-art of concrete technology.

The requirement to measure concrete strength at 28 days has been deleted because later ages are more relevant for high-strength concrete. The designer should specify the age based on the anticipated strength development of the concrete and the intended application.

For both classes of concrete, a minimum cement content is not included since this should be selected by the producer based on the specified performance criteria. A maximum water/cementitious materials has been retained to be consistent with the existing water/cement ratios for Class P and Class A concretes. For Class P(HPC) concrete, a maximum size of coarse aggregate is specified since it is difficult to achieve the higher concrete compressive strengths with aggregates larger than 3/4 in (19 mm). For Class A(HPC) concrete, the maximum aggregate size should be selected by the producer based on the specified performance criteria.

Anticipated Effect on Bridges

Encourage the use of high performance concrete with higher strength, lower permeability, or other performance criteria.

References

  1. Goodspeed, C. H., Vanikar, S., and Cook, R., "High Performance Concrete Defined for Highway Structures," Concrete International, Vol. 18, No. 2, February 1996, pp. 62-67.
  2. High Performance Concrete, Compact Disc, Federal Highway Administration, Version 3.0, February 2003.

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division II

(X) Revision or (X) Addition 8.3.1

Revise 8.3.1 Cements as follows:

8.3.1 Cements

Portland Cements shall conform to the requirements of AASHTO M 85 (ASTM C 150) and Blended Hydraulic Cements shall conform to the requirements of AASHTO M 240 (ASTM C 595) or ASTM C 1157. For Type IP Portland-pozzolan cement, the pozzolan constituent shall not exceed 20 percent of the weight of the blend and the loss on ignition of the pozzolan shall not exceed 5 percent.

Except for Class P(HPC) and Class A(HPC) or when otherwise specified, only Type I, II, or III Portland Cement, Types IA, IIA, IIIA Air Entrained Portland Cement or Types IP or IS Blended Hydraulic Cements shall be used. Types IA, IIA, and IIIA cements may be used only in concrete where air entrainment is required.

Low-alkali cements conforming to the requirements of AASHTO M 85 for low-alkali cement shall be used when specified or when ordered by the Engineer as a condition of use for aggregates of limited alkali-silica reactivity.

Unless otherwise permitted, the product of only one mill of any one brand and type of cement shall be used for like elements of a structure that are exposed to view, except when cements must be blended for reduction of any excessive air-entrainment where air entraining cement is used.

For Class P(HPC) and Class A(HPC), trial batches using all intended constituent materials shall be made prior to concrete placement to ensure that cement and admixtures are compatible. Changes in the mill, brand, or type of cement shall not be permitted without additional trial batches.

Other Affected Articles

None

Background

ASTM C 1157 is a standard performance specification for blended cements and should be included.(1)

Restricting the cements to Types I, II, III, IA, IIA, IIIA, IP, or IS may prevent innovation and selection to enhance the performance of HPC.

Interactions between cementitious material and chemical admixtures can cause incompatibility leading to premature stiffening, extended setting time, or inadequate air-void system. HPC may be very sensitive to the brand, type, and mill of origin of the cement. Studies have shown that changing the brand of cement can cause large differences in the hardened properties of HPC.(2)

Anticipated Effect on Bridges

More choices, improved properties, and less problems in the field.

References

  1. ASTM C 1157 Standard Performance Specification for Blended Hydraulic Cement.
  2. ACI 363 Committee, "State-of-the-Art Report on High-Strength Concrete (ACI 363R-92)," American Concrete Institute, Farmington Hills, MI, 1992, 55 pp.

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division II

( ) Revision or (X) Addition 8.3.5

Add a new article 8.3.5 and renumber subsequent articles.

8.3.5 Combined Aggregates

Blends of fine and coarse aggregates shall conform to the requirements of AASHTO M XX1

Other Affected Articles

Material specifications M 6, M 43, and M 80

Background

A new specification on combined aggregates has been proposed and needs to be referenced. Combined aggregates enable the use of less water, cementitious materials, and paste leading to improved properties in the freshly mixed and hardened concrete.

Anticipated Effect on Bridges

Improved concrete properties.

References

None

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division II

(X) Revision or ( ) Addition 8.3.7

Replace the first paragraph of 8.3.7 Mineral Admixtures as follows:

8.3.7 Mineral Admixtures

Mineral admixtures in concrete shall conform to the following requirements:

  • Fly ash pozzolans and calcined natural pozzolans - AASHTO M 295 (ASTM C 618)
  • Ground granulated blast-furnace slag - AASHTO M 302 (ASTM C 989)
  • Silica fume - AASHTO M 307 (ASTM C 1240)

(Deleted text is indicated by strikethrough. Inserted text is underlined.)

Other Affected Articles

8.4.4

Background

Slag and silica fume are widely used in HPC and need to be referenced.

Anticipated Effect on Bridges

More choices of materials.

References

None

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division II

(X) Revision or ( ) Addition 8.4.1

Item No. 1

Revise 8.4.1.1 Responsibility and Criteria as follows:

8.4.1.1 Responsibility and Criteria

The contractor shall design and be responsible for the performance of all concrete mixes used in structures. The mix proportions selected shall produce concrete that is sufficiently workable and finishable for all uses intended and shall conform to the requirements in Table 8.2 and all other requirements of this Section.

For normal weight concrete the absolute volume method, such as described in American Concrete Institute Publication 211.1, shall be used in selecting mix proportions. For Class P(HPC) with fly ash, the method given in American Concrete Institute Guide 211.4 shall be permitted. For structural lightweight concrete, the mix proportions shall be selected on the basis of trial mixes with the cement factor rather than the water/cement ratio being determined by the specified strength using methods such as those described in American Concrete Institute Publication 211.2.

The mix design shall be based on the specified properties. When strength is specified, select an average concrete strength sufficiently above the specified strength so that, considering the expected variability of the concrete and test procedures, no more than 1 in 10 strength tests will be expected to fall below the specified strength. Mix designs shall be modified during the course of the work when necessary to ensure compliance with the specified fresh and hardened concrete properties. For Class P(HPC) and Class A(HPC), such modifications shall only be permitted after trial batches to demonstrate that the modified mix design will result in concrete that complies with the specified concrete properties.

Item No. 2

Revise 8.4.1.2 Trial Batch Tests as follows:

8.4.1.2 Trial Batch Tests

For classes A, A(AE), P, P(HPC), and A(HPC) concrete, for lightweight concrete, and for other classes of concrete when specified or ordered by the Engineer, satisfactory performance of the proposed mix design shall be verified by laboratory tests on trial batches. The results of such tests shall be furnished to the Engineer by the contractor or the manufacturer of the precast elements at the time the proposed mix design is submitted.

If materials and a mix design identical to those proposed for use have been used on other work within the previous year, certified copies of concrete test results from this work which indicate full compliance with these specifications may be substituted for such laboratory tests.

The average values obtained from trial batches for the specified properties, such as strength shall exceed design values by a certain amount based on variability. For compressive strength, the required average strength used as a basis for selection of concrete proportions shall be determined in accordance with AASHTO M 241.

Other Affected Articles

AASHTO M 241

Background

Item No. 1

ACI Guide 211.4 describes selecting proportions for high-strength concrete with portland cement and fly ash.(1) In HPC, type, size, and shape of aggregate become important.

Properties other than strength are also important in bridge structures.

Any modification to the mixture proportions and ingredients must be tested using trial batches.

Item No. 2

Properties other than strength are also included. Overstrength requirements are updated for all strength levels including high-strength concrete by referring to AASHTO M 241.(2,3) Revisions to AASHTO M 241 are also proposed.

Anticipated Effect on Bridges

More durable structures. Inclusion of high-strength concrete.

References

  1. ACI Committee 211, "Guide for Selecting Proportions for High-Strength Concrete with Portland Cement and Fly Ash (ACI 211.4)" American Concrete Institute, Farmington Hills, MI, 1993, 13 pp.
  2. Cagley, J. R. "Changing from ACI 318-99 to ACI 318-02," Concrete International, American Concrete Institute, June 2001.
  3. AASHTO M 241 Standard Specification for Concrete Made by Volumetric Batching and Continuous Mixing.

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division II

( ) Revision or (X) Addition 8.4.3

Revise 8.4.3 Cement Content as follows:

The minimum cement content shall be as listed in Table 8.2 or otherwise specified. For Class P(HPC), the total cementitious materials content shall be specified not to exceed 1,000 pounds per cubic yard of concrete. For other classes of concrete, the maximum cement or cement plus mineral admixture content shall not exceed 800 pounds per cubic yard of concrete. The actual cement content used shall be within these limits and shall be sufficient to produce concrete of the required strength, consistency, and performance.

Other Affected Articles

8.2

Background

Many high-strength concretes require a cementitious materials content in excess of 800 lb/yd3 (475 kg/m3).(1) A higher limit is, therefore, appropriate.

Anticipated Effect on Bridges

Facilitate the use of high-strength and high performance concrete.

References

  1. High Performance Concrete, Compact Disc, Federal Highway Administration, Version 3.0, February 2003.

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division II

(X) Revision or ( ) Addition 8.4.4

Revise 8.4.4 Mineral Admixtures as follows:

8.4.4 Mineral Admixtures

Mineral admixtures shall be used in the amounts specified. For all classes of concrete except P(HPC) and A(HPC), when Types I, II, IV or V (AASHTO M 85) cements are used and mineral admixtures are neither specified nor prohibited, the Contractor will be permitted to replace up to 25 percent of the required Portland cement with fly ash or other pozzolan conforming to AASHTO M 295, up to 50 percent of the required Portland cement with slag conforming to AASHTO M 302, or up to 10 percent of the required Portland cement with silica fume conforming to AASHTO M 307. When any combination of fly ash, slag, and silica fume are used, the Contractor will be permitted to replace up to 50 percent of the required Portland cement. However, no more than 25 percent shall be fly ash and no more than 10 percent shall be silica fume. The weight of the mineral admixture used shall be equal to or greater than the weight of the Portland cement replaced. In calculating the water -cementitious materials ratio of the mix, the weight of cementitious materials shall be considered to be the sum of the weights of the Portland cement and the mineral admixtures.

For Class P(HPC) and Class A(HPC) concrete, mineral admixtures (pozzolans or slag) shall be permitted to be used as cementitious material with portland cement in blended cements or as a separate addition at the mixer. The amount of mineral admixture shall be determined by trial batches. The water-cementitious materials ratio shall be the ratio of the weight of water to the total cementitious materials, including the mineral admixtures. The properties of the freshly mixed and hardened concrete shall comply with specified values.

Other Affected Articles

8.3.7

Background

Mineral admixtures are widely used in HPC today. These include fly ash, ground granulated blast-furnace slag, and silica fume. The use of these materials results in a concrete with a finer pore structure and, therefore, lower permeability. The proposed replacement percentages are based on those in ACI 318 for concrete exposed to deicing chemicals.(1)

Trial batches are required with HPC to ensure that the specified properties are achieved.

Anticipated Effect on Bridges

Improved concrete for more durable structures.

References

  1. ACI Committee 318, Building Code Requirements for Structural Concrete (318-02) and Commentary (318R-02), American Concrete Institute, Farmington Hills, MI, 2002, 443 pp.

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division II

(X) Revision or ( ) Addition 8.5.7.1

Revise 8.5.7.1 Tests as follows:

8.5.7.1 Tests

A strength test shall consist of the average strength of at least two 6x12-in. or at least three 4x8-in. compressive strength test cylinders fabricated from material taken from a single randomly selected batch of concrete, except that, if any cylinder should show evidence of improper sampling, molding, or testing, said cylinder shall be discarded and the strength test shall consist of the strength of the remaining cylinder(s). A minimum of three cylinders shall be fabricated for each strength test when the specified strength exceeds 5,000 psi .

(Deleted text is indicated by strikethrough. Inserted text is underlined.)

Other Affected Articles

AASHTO M 241

Background

4x8-in cylinders are commonly used for testing high-strength concrete and may exhibit higher variability.(1) For high-strength concrete, strength is more critical, and at least three cylinders are recommended for any size.(2)

Anticipated Effect on Bridges

Improved quality of concrete and more valid measurements of compressive strength.

References

  1. Ozyildirim, C., "4 x 8 inch Concrete Cylinders versus 6 x 12 inch Cylinders," VHTRC 84‑R44, Virginia Transportation Research Council, Charlottesville, VA, May 1984, 25 pp.
  2. ACI Committee 363, "Guide to Quality Control and Testing of High-Strength Concrete (ACI 363.2R-98)," American Concrete Institute, Farmington Hills, MI, 1998, 18 pp.

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division II

(X) Revision or ( ) Addition 8.5.7.3

Revise 8.5.7.3 For Acceptance of Concrete as follows:

8.5.7.3 For Acceptance of Concrete

For determining compliance of concrete with a specified strength, test cylinders shall be cured under controlled conditions as described in Article 9.3 of AASHTO T 23 and tested at the specified age. Samples for acceptance tests for each class of concrete shall be taken not less than once a day nor less than once for each 150 cubic yards of concrete or once for each major placement.

Except for Class P(HPC) and Class A(HPC) concrete, any concrete represented by a test that indicates a strength that is less than the specified compressive strength at the specified age by more than 500 psi will be rejected and shall be removed and replaced with acceptable concrete. Such rejection shall prevail unless either:

  1. The Contractor, at his or her expense, obtains and submits evidence of a type acceptable to the Engineer that the strength and quality of the rejected concrete is acceptable. If such evidence consists of cores taken from the work, the cores shall be obtained and tested in accordance with the standard methods of AASHTO T 24 (ASTM C 42) or,
  2. The Engineer determines that said concrete is located where it will not create an intolerable detrimental effect on the structure and the Contractor agrees to a reduced payment to compensate the Department for loss of durability and other lost benefits.

For Class P(HPC) and Class A(HPC) concrete, any concrete represented by a test that indicates a strength that is less than the specified compressive strength at the specified age will be rejected and shall be removed and replaced with acceptable concrete.

Other Affected Articles

None

Background

Test ages other than 28 days are frequently specified for high-strength concrete.(1) The elimination of 28 days in this provision allows the use of other test ages.

A goal of HPC is to provide concrete that meets the specification for the intended application. Accepting concrete that does not meet the specified compressive strength is not an acceptable practice for HPC. A reduced payment cannot compensate for a loss of durability and possible reduced service life.

Anticipated Effect on Bridges

Improved quality of concrete.

References

  1. High Performance Concrete, Compact Disc, Federal Highway Administration, Version 3.0, February 2003.

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division II

( ) Revision or (X) Addition 8.5.7.5

8.5.7.5 Precast Concrete Cured by the Waterproof Cover Method, Steam or Radiant Heat Heat

When a precast concrete member is cured by the waterproof cover method, steam, or radiant heat, the compressive strength test cylinders made for any of the above purposes shall be cured under conditions similar to the member. Such concrete will shall be considered to be acceptable whenever a test indicates that the concrete has reached the specified compressive strength provided such strength is reached no later than the specified age for the compressive strength.

Test cylinders shall be cured by only one of the following methods:

(1) For concrete with specified design compressive strengths less than or equal to 6,000 psi, test cylinders shall be stored next to the member and under the same covers such that the cylinders are exposed to the same temperature conditions as the member.

(2) For all specified concrete strengths, test cylinders shall be match-cured in chambers in which the temperature of the chamber is correlated with the temperature in the member prior to release of the prestressing strands. Temperatures of the chamber and member shall be verified by use of temperature sensors in the chamber and member. Unless specified otherwise, temperature sensors in I-beams shall be located at the center of gravity of the bottom flange. For other members, the temperature sensors shall be located at the center of the thickest section. The location shall be specified on the drawings. After release of the prestressing strands, cylinders shall be stored in a similar temperature and humidity environment as the member.

Other Affected Articles

Division I, 9.23

Background

Research on several FHWA-State high performance concrete showcase projects has shown that the strength of quality control cylinders is affected by the curing temperatures that the cylinders experience.(1,2) A high initial curing temperature accelerates the strength gain at early ages but results in a slower strength gain at later ages. Consequently, a test cylinder that experiences a different temperature history from the member that it represents does not truly represent the strength of the concrete in the member either at an age corresponding to release of the strands or at later ages. This effect becomes more significant with high-strength concrete because of the higher cementitious materials content and higher heat of hydration.

Placing the test cylinders under the same covers as the member has proved to be an acceptable method for conventional strength concretes. However, for high-strength concretes, match curing is essential if realistic values of strength are to be measured.(3) The proposed changes allow the traditional method to be used for conventional strength concretes while requiring match curing for high-strength concretes and allowing match curing for conventional strength concretes.

Anticipated Effect on Bridges

Provides a more realistic measure of the compressive strength of concrete in the member.

References

  1. High Performance Concrete, Compact Disc, Federal Highway Administration, Version 3.0, February 2003.
  2. Meyers, J. J. and Carrasquillo R. L., "Production and Quality Control of High Performance Concrete in Texas Bridge Structures," Center for Transportation Research, The University of Texas at Austin, Research Report 580/589-1, 2000, 553 pp.
  3. Russell, H. G., "Consider Match Curing for High-Strength Precast," Concrete Products, Vol. 102, No. 7, July 1999, pp. 117-118.

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division II

(X) Revision or ( ) Addition 8.6.4.1

8.6.4.1 Protection During Cure

When there is a probability of air temperatures below 35°F during the cure period, the Contractor shall submit for approval by the Engineer prior to concrete placement, a cold weather concreting and curing plan detailing the methods and equipment which will be used to ensure that the required concrete temperatures are maintained. The concrete shall be maintained at a temperature of not less than 45°F for the first six days after-placement except that when pozzolans or slag are used, this period shall be as follows:

Percentage of Cement Replaced, by Weight, With Required Period of Controlled Temperature

Pozzolans

Slag

10%

25%

8 Days

11-15%

26-35%

9 Days

16-20%

36-50%

10 Days

The above requirement for an extended period of controlled temperature may be waived if a compressive strength of 65 percent of the specified design strength is achieved in 6 days using site-cured cylinders or the match-curing system or the maturity method.

When the percentage of cement replacement is larger than the values listed above or when combinations of materials are used as cement replacement, the required period of controlled temperature shall be at least 6 days and shall continue until a compressive strength of 65 percent of the specified design strength is achieved using site-cured cylinders or the match-curing system or the maturity method.

If external heating is employed, the heat shall be applied and withdrawn gradually and uniformly so that no part of the concrete surface is heated to more than 90°F or caused to change temperature by more than 20°F in 8 hours.

When requested by the Engineer, the Contractor shall provide and install two maximum-minimum type thermometers at each structure site. Such thermometers shall be installed as directed by the Engineer so as to monitor the temperature of the concrete and the surrounding air during the cure period.

Other Affected Articles

None

Background

The current provision only addresses pozzolans up to a cement replacement of 20 percent and needs to be more general. Instead of fixed periods of controlled temperature, the match-curing system or maturity method should be allowed. Both methods can be effective with HPC.

Anticipated Effect on Bridges

The changes allow for a wider range of cement replacements and optional methods to reduce the required period of controlled temperature. The latter will allow for faster bridge construction.

References

None

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division II

(X) Revision or ( ) Addition 8.6.6 and 8.6.7

Item No. 1

8.6.6 Concrete Exposed to Salt Water

Unless otherwise specifically provided, concrete for structures exposed to salt or brackish water shall comply with the requirements of Class A(HPC) concrete Class S for concrete placed under water and Class A for other work. Such concrete shall be mixed for a period of not less than 2 minutes and the water content of the mixture shall be carefully controlled and regulated so as to produce concrete of maximum impermeability. The concrete shall be thoroughly consolidated as necessary to produce maximum density and a complete lack of rock pockets. Unless otherwise indicated on the plans, the clear distance from the face of the concrete to the reinforcing steel shall be not less than 4 inches. No construction joints shall be formed between levels of extreme low water and extreme high water or the upper limit of wave action as determined by the Engineer. Between these levels the forms shall not be removed, or other means provided, to prevent salt water from coming in direct contact with the concrete for a period of not less than 30 days after placement. Except for the repair of any rock pockets and the plugging of form ties holes, the original surface as the concrete comes from the forms shall be left undisturbed. Special handling shall be provided for precast members to avoid even slight deformation cracks.

Item No. 2

8.6.7 Concrete Exposed to Sulfate Soils or ulfate Water

When the special provisions identify the area as containing sulfate soils or sulfate water, the concrete that will be in contact with such soil or water shall be Class A(HPC) and shall be mixed, placed, and protected from contact with soil or water as required for concrete exposed to salt water except that the protection period shall be not less than 72 hours.

Other Affected Articles

None

Background

HPC with low permeability are essential to provide the needed protection for concrete exposed to salt or sulfate solutions.(1) Class A(HPC) is intended for these applications.

Anticipated Effect on Bridges

Provide a lower permeability concrete.

References

  1. ACI Committee 222, "Corrosion of Metals in Concrete (ACI 222R-96)," American Concrete Institute, Farmington Hills, MI, 1996, 30 pp.

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division II

(X) Revision or ( ) Addition 8.11.1

All newly placed concrete shall be cured so as to prevent the loss of water by use of one or more of the methods specified herein. Except for Class A(HPC) concrete, curing shall commence immediately after the free water has left the surface and finishing operations are completed. For Class A(HPC) concrete, water curing shall commence immediately after finishing operations are complete. If the surface of the concrete begins to dry before the selected cure method can be applied, the surface of the concrete shall be kept moist by a fog spray applied so as not to damage the surface.

Curing by other than waterproof cover method with precast concrete or steam or radiant heat methods shall continue uninterrupted for 7 days except that when pozzolans in excess of 10 percent, by weight, of the Portland cement are used in the mix. When such pozzolans are used, the curing period shall be 10 days. For other than top slabs of structures serving as finished pavements and Class A(HPC) concrete, the above curing periods may be reduced and curing terminated when test cylinders cured under the same conditions as the structure indicate that concrete strengths of at least 70 percent of that specified have been reached.

When deemed necessary by the Engineer during periods of hot weather, water shall be applied to concrete surfaces being cured by the liquid membrane method or by the forms-in-place method, until the Engineer determines that a cooling effect is no longer required. Such application of water will be paid for as extra work.

Other Affected Articles

8.11.4 and 8.13.4

Background

Changes to 8.11.1 are needed to make it consistent with changes to 8.11.4 and 8.13.4.(1,2,3)

Anticipated Effect on Bridges

Improved quality and durability of bridge decks.

References

  1. High Performance Concrete, Compact Disc, Federal Highway Administration, Version 3.0, February 2003.
  2. Meyers, J. J. and Carrasquillo R. L., "Production and Quality Control of High Performance Concrete in Texas Bridge Structures," Center for Transportation Research, The University of Texas at Austin, Research Report 580/589-1, 2000, 553 pp.
  3. HPC Bridge Views, Issue No. 15, May/June 2001.

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division II

( ) Revision or (X) Addition 8.11.3.5

Item No. 1

Add the following at the end of the second paragraph:

Steam curing or radiant heat curing shall be done under a suitable enclosure to contain the live steam or the heat. Steam shall be low pressure and saturated. Temperature recording devices shall be employed as necessary to verify that temperatures are uniform throughout the concrete and within the limits specified.

Item No. 2

Revise the third paragraph as follows:

The initial application of the steam or of the heat shall not occur prior to initial set of the concrete except to maintain the temperature within the curing chamber above the specified minimum temperature. The time of initial set may be determined by the Standard Method of Test for "Time of Setting of Concrete Mixtures by Penetration Resistance," AASHTO T 197 (ASTM C 403).

Item No. 3

Revise the fifth paragraph as follows:

Application of live steam shall not be directed on the concrete or on the forms so as to cause localized high temperatures. During the initial application of live steam or of radiant heat, the temperature within the concrete shall increase at an average rate not exceeding 40°F per hour until the curing temperature is reached. The maximum temperature within the concrete shall not exceed 160°F. The maximum temperature shall be held until the concrete has reached the desired strength. In discontinuing the steam application, the concrete temperature shall not decrease at a rate to exceed 40°F per hour until a temperature of 20°F above the temperature of the air to which the concrete will be exposed has been reached.

Item No. 4

Revise the last paragraph as follows:

For prestressed members, the transfer of the stressing force to the concrete shall be accomplished immediately after the steam curing or heat curing has been discontinued.

Other Affected Articles

8.2

Background

Item No. 1

Since high-strength concrete generates significantly more heat than conventional strength concrete, it is important that concrete temperatures be monitored rather than temperatures throughout the enclosure.(1)

Item No. 2

Since today's concretes may contain a wider variety of constituent materials than in the past, the current criteria of 2 to 4 hours or 4 to 6 hours may not be appropriate.(1) Measurement of time of set for the specific concrete is a more precise approach.

Item No. 3

Research has shown that delayed ettringite formation (DEF) can occur in concretes subjected to high temperatures during curing and subsequently exposed to moisture. A maximum temperature of about 160 °F (71 °C) is generally recognized as an upper limit below which DEF is unlikely to occur. The PCI Quality Control Manual contains a recommendation that maximum concrete temperature should be limited to 158 °F (70 °C) if a known potential for alkali-silica reaction or DEF exists. Otherwise, the maximum concrete temperature is 180 °F (82 °C).(2)

Item No. 4

The current provision allows the ambient temperature to fall as low as 60 °F (16 °C) before the strands are released. A large decrease in concrete and strand temperatures prior to release of the strands can result in vertical cracks in the member. This is more likely in deep members and high-strength concrete members. Immediate release of the strands after the steam or heat curing minimizes the likelihood of cracking.(3)

Anticipated Effect on Bridges

Improved quality of concrete in prestressed concrete girders and less cracking in bridge girders prior to transfer of the prestressing force.

References

  1. High Performance Concrete, Compact Disc, Federal Highway Administration, Version 3.0, February 2003.
  2. Manual for Quality Control for Plants and Production of Structural Precast Concrete Products, MNL-116-99, Precast/Prestressed Concrete Institute, Chicago, IL, 1999.
  3. Zia, P. and Caner, A., "Cracking in Large-Sized Long Span Prestressed Concrete AASHTO Girders," Center for Transportation Engineering Studies, North Carolina State University, October 1993, 87 pp.

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division II

( ) Revision or (X) Addition 8.11.4

Add the following paragraph at the end of 8.11.4 Bridge Decks:

When Class A(HPC) concrete is used in bridge decks, water cure shall be applied immediately after the finishing of any portion of the deck is complete and shall remain in place for a minimum period of 7 days irrespective of concrete strength. If conditions prevent immediate application of the water cure, an evaporation retardant shall be applied immediately after completion of finishing or fogging shall be used to maintain a high relative humidity above the concrete to prevent drying of the concrete surface. Following the water cure period, liquid membrane curing compound may be applied to extend the curing period.

Other Affected Articles

8.11.1

Background

High performance concrete tends to have very little bleed water, especially when a low water-cementitious materials ratio is used with mineral admixtures. As a result, the evaporation protection of the bleed water on the fresh concrete is lost. The most effective way to protect the concrete is by application of water cure as soon as screeding or tining of the concrete is complete and no later than 15 minutes after the concrete is placed in any portion of the deck.(1) If this is not possible, the next best alternative is to prevent or reduce moisture loss from the concrete until the water cure can be applied. In the water cure method, the concrete surface is kept continuously wet. The most appropriate method is to cover the deck with materials such as cotton mats, multiple layers of burlap, or other materials that do not discolor or damage the concrete surface and to keep these materials continuously and thoroughly wet. The water cure needs to continue for a minimum of 7 days irrespective of concrete strength. The use of a curing compound after the water cure extends the curing period while allowing the contractor to have access to the bridge deck.

Note that 8.11.1 requires 10 days curing when more than 10 percent pozzolans are used.

Anticipated Effect on Bridges

Improved quality and durability of bridge decks.

References

  1. HPC Bridge Views, Issue No. 15, May/June 2001.

(Submitted by: )

PROPOSED CHANGE TO AASHTO Standard Specifications Division II

(X) Revision or ( ) Addition 8.13.4

Unless otherwise permitted, precast members shall be cured by the water method, waterproof cover method, or the steam or radiant heat method. The use of insulated blankets is permitted with the waterproof cover method. When the waterproof cover method is used, the air temperature beneath the cover shall not be less than 50°F and live steam or radiant heat may be used to maintain the temperature above the minimum value. The maximum concrete temperature during the curing cycle shall not exceed 160°F. The waterproof cover shall remain in place until such time as the compressive strength of the concrete reaches the strength specified for detensioning or stripping.

Other Affected Articles

8.11.1

Background

High-strength concretes contain more cementitious material than used in conventional strength concrete.(1) Consequently, the heat generated during hydration is greater and sufficient heat can be generated to develop the compressive strength required for detensioning or stripping without the use of steam or radiant heating.(2) The new wording permits self-curing with or without insulated blankets by modifying the waterproof cover method. The revision also refers to concrete temperature rather than the enclosure temperature.

Anticipated Effect on Bridges

Reduce cost of girders since energy for heating is not required.

References

  1. High Performance Concrete, Compact Disc, Federal Highway Administration, Version 3.0, February 2003.
  2. Meyers, J. J. and Carrasquillo R. L., "Production and Quality Control of High Performance Concrete in Texas Bridge Structures," Center for Transportation Research, The University of Texas at Austin, Research Report 580/589-1, 2000, 553 pp.

(Submitted by: )

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FHWA-HRT-05-057

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United States Department of Transportation - Federal Highway Administration