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

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Appendix E—Proposed Revisions To The AASHTO LRFD Bridge Construction Specifications

PROPOSED CHANGE TO AASHTO LRFD Bridge Construction Specifications

(X) Revision or (X) Addition 8.2

Item No. 1

In 8.2.2 Normal-Density Concrete, revise the first paragraph and add two new classes of high performance concrete to Table 8.2.2-1:

8.2.2 Normal-Density Concrete

Eight Ten classes of normal-density concrete are provided for in these specifications as listed in Table 8.2.2-1, except that for concrete on or over saltwater or exposed to deicing chemicals, the maximum water/cementitious materials ratio shall be 0.45.

Class of Concrete Min. Cement Content Max. Water/Cementitious Materials Ratio Air Content Range Size of Coarse Aggregate Per AASHTO M 43 (ASTM D 448) Size Number Specified Compressive Strength

kg/m3

kg per kg

%

Nominal Size

Square Openings

 

MPa

P(HPC)

a

0.40

As specified in the contract

£ 19 mm

67

> 41 as specified in the contract

A(HPC)

a

0.45

As specified in the contract

a

a

28

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

Item No. 2

Add a commentary as follows:

C8.2.2

Class P(HPC) concrete is used for prestressed concrete when strengths in excess of 41 MPa are specified and should always be used for specified concrete strengths greater than 69 MPa.

Class A(HPC) concrete is used for cast-in-place substructures and superstructures when low permeability or other performance characteristics are specified.

Other Affected Articles

8.4.3 and AASHTO LRFD Bridge Design Specifications Table C5.4.2.1-1

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 41 MPa (6000 psi). 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.

Square Openings has been changed to Nominal Size because the listed quantities are aggregate sizes.

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. Maximum water-cementitious materials ratios have been included. The value of 0.40 for Class P(HPC) is less than the value of 0.49 for Class P whereas the value of 0.45 for Class A(HPC) is the same as that for Class A(AE). 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 19 mm (3/4 in). 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 LRFD Bridge Construction Specifications

(X) Revision or (X) Addition 8.3.1

Item No. 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 95M) or ASTM C 1157. For Type IP portland-pozzolan cement, the pozzolan constituent shall not exceed 20 percent of the mass 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 in the contract documents, 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 (ASTM C 150) for low-alkali cement shall be used when specified in the contract documents 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.

Item No. 2

Add a commentary as follows:

C8.3.1

ASTM C 1157 is a performance specification that does not require restrictions on the composition of the cement or its constituents. It can be used to accept cements not conforming to AASHTO M 85 (ASTM C 150) and AASHTO M 240 (ASTM C 595M).

The low alkali requirement of AASHTO M 85 (ASTM C 150) does not provide protection against alkali-silica reactivity in all cases. A better approach is provided in AASHTO M 6 and M 80.

Other Affected Articles

AASHTO M 6 and M 80 with proposed Supplementary Requirement

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 materials 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 LRFD Bridge Construction Specifications

( ) Revision or (X) Addition 8.3.5

Item No. 1

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

Item No. 2

Add a commentary as follows:

C8.3.5

The use of a combined aggregate grading can result in the use of less water, cementitious materials, and paste, and lead to improved fresh and hardened concrete properties.

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 LRFD Bridge Construction Specifications

(X) Revision or ( ) Addition 8.3.7

Item No. 1

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)

Item No. 2

Add a commentary as follows:

C8.3.7

Pozzolans (fly ash, silica fume) and slag are used in the production of Class P(HPC) and Class A(HPC) concretes especially to extend the service life.

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 LRFD Bridge Construction Specifications

(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 8.2.2-1 and all other requirements of this Section.

For normal-density 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 low-density 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 one in ten 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

Add a commentary as follows:

C8.4.1.1

For Class P(HPC) with fly ash, the method given in ACI Guide 211.4 is permitted.

In Class P(HPC) and Class A(HPC) concretes, properties other than compressive strength are also important, and the mix design should be based on specified properties rather than only compressive strength.

Item No. 3

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 low-density concrete, and for other classes of concrete when specified in the contract documents 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 that 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.

Item No. 4

Add a commentary as follows:

C8.4.1.2

In Class P(HPC) and Class A(HPC) concretes, properties other than compressive strength are also important. However, if only compressive strength is specified, AASHTO M 241 provides the method to determine the required average strength.

Other Affected Articles

AASHTO M 241

Background

Item Nos. 1 and 2

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 Nos. 3 and 4

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 LRFD Bridge Construction Specifications

( ) Revision or (X) Addition 8.4.3

Item No. 1

Revise 8.4.3 Cement Content as follows:

The minimum cement content shall be as listed in Table 8.2.1-1 or otherwise specified in the contract documents. For Class P(HPC), the total cementitious materials content shall be specified not to exceed 593 kg/m3 of concrete. For other classes of concrete, the maximum cement or cement plus mineral admixture content shall not exceed 475 kg/m3 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.

Item No. 2

Add a commentary as follows:

C8.4.3

Many high-strength concretes require a cementitious materials content greater than the traditional AASHTO limit of 475 kg/m3. However, when cementitious materials contents in excess of 593 kg/m3 are required in high-strength concrete, optimization of other constituent materials or alternative constituent materials should be considered.

Other Affected Articles

8.2

Background

The current maximum cement content of 363 kg/m3 (611 lb/yd3) appears to be an error since table 8.2.2-1 lists minimum cement contents as high as 390 kg/m3 (657 lb/yd3). It is also inconsistent with 475 kg/m3 (800 lb/yd3) specified in the LRFD Bridge Design Specifications, 5.4.2.1.

Many high-strength concretes require a cementitious materials content in excess of 475 kg/m3 (800 lb/yd3).(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 LRFD Bridge Construction Specifications

(X) Revision or ( ) Addition 8.4.4

Item No. 1

Revise 8.4.4 Mineral Admixtures as follows:

8.4.4 Mineral Admixtures

Mineral admixtures shall be used in the amounts specified in the contract documents. For all classes of concrete except P(HPC) and A(HPC), when Types I, II, IV, or V AASHTO M 85 (ASTM C 150) cements are used and mineral admixtures are neither specified in the contract documents 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 mass of the mineral admixture used shall be equal to or greater than the mass of the portland cement replaced. In calculating the water-cementitious materials ratio of the mix, the mass of the cementitious materials shall be considered to be the sum of the mass 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 materials 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 mass 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.

Item No. 2

Add a commentary as follows:

C8.4.4

Mineral admixtures are widely used in concrete in the percentages given. For Class P(HPC) and Class A(HPC) concretes, different percentages may be used if trial batches substantiate that such amounts provide the specified properties.

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 LRFD Bridge Construction Specifications

(X) Revision or ( ) Addition 8.5.7.1

Item No. 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 150x300-mm or at least three 100x200-mm 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 34 MPa.

Item No. 2

Add a commentary as follows:

The use of 100x200-mm cylinders for measuring concrete compressive strengths is increasing. Test results using the smaller size cylinder have a higher variability compared to 150x300 mm cylinders. This can be offset by requiring three cylinders of the smaller size compared to two for the larger size. Since measurement of compressive strength is more critical for high-strength concrete, three cylinders are required for both cylinder sizes.

Other Affected Articles

AASHTO M 241

Background

100x200-mm 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 LRFD Bridge Construction Specifications

(X) Revision or ( ) Addition 8.5.7.3

Item No. 1

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 AASHTO T 23 (ASTM C 31), Article 9.3, 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 100 m3 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 3.5 MPa will be rejected and shall be removed and replaced with acceptable concrete. Such rejection shall prevail unless either:

  • The Contractor, at the Contractor's 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
  • 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 Owner 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.

Item No. 2

Add a commentary as follows:

C8.5.7.3

The concrete age when the specified strength is to be achieved must be shown on the project drawings.

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 LRFD Bridge Construction Specifications

( ) Revision or (X) Addition 8.5.7.5

Item No. 1

Revise 8.5.7.5 Steam and Radiant Heat-Cured Concrete as follows:

8.5.7.5 Precast Concrete Cured by the Waterproof Cover Method, Steam or Radiant 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 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 41 MPa, 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.

Item No. 2

Add a commentary as follows:

C8.5.7.5

For specified concrete compressive strengths greater than 41 MPa, test cylinders should 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. Temperature sensors for the match curing system should be placed at the most critical locations for strength development at release of the prestressing force and for design. The Engineer should determine the critical locations for temperature sensors in each type of member and show the locations on the drawings.

After release of the prestressing strands, cylinders should be stored in a similar temperature and humidity environment as the member.

Other Affected Articles

None

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 LRFD Bridge Construction Specifications

(X) Revision or ( ) Addition 8.6.4.1

Item No. 1

Revise 8.6.4.1 Protection During Cure as follows:

8.6.4.1 Protection During Cure

When there is a probability of air temperatures below 2°C 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 7°C for the first 6 days after placement, except that when pozzolans or slag are used, this period shall be as shown in Table 8.6.4.1-1:

Table 8.6.4.1-1 Pozzolan Cement and Temperature Control Period

Percentage of Cement Replaced, by Mass, 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 requirement in Table 8.6.4.1-1 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 32°C or caused to change temperature by more than 11°C 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.

Item No. 2

Add a commentary as follows:

C8.6.4.1

Addition of pozzolans or slag may result in slower development of properties. Therefore, longer curing periods may be needed. Thermal heating and cooling rates are limited to minimize the thermal strains.

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 LRFD Bridge Construction Specifications

(X) Revision or ( ) Addition 8.6.6 and 8.6.7

Item No. 1

Revise 8.6.6 Concrete Exposed to Saltwater as follows:

8.6.6 Concrete Exposed to Saltwater

Unless otherwise specified in the contract documents, 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 in the contract documents, the clear distance from the face of the concrete to the reinforcing steel shall be not less than 100 mm. 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 saltwater 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

Add a Commentary as follows:

C8.6.6

Penetration of harmful solutions accelerates the deterioration of concrete. The most widely experienced environmental distress is the corrosion of the reinforcing steel. Chloride solutions destroy the protective coating around the reinforcing steel initiating and accelerating the corrosion of the steel. Concrete should be prepared using the proper ingredients and proportions and cured for a period of time before exposure to the severe environment such that the penetration of the harmful solutions is minimized.

Item No. 3

Revise 8.6.7 Concrete Exposed to Sulfate Soils or Water as follows:

8.6.7 Concrete Exposed to Sulfate Soils or Sulfate Water

When the contract documents 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 saltwater except that the protection period shall be not less than 72 hours.

Item No 4

Add a Commentary as follows:

C8.6.7

Sulfate soils or water may contain high levels of sulfates of sodium, potassium, calcium, or magnesia. Penetration of sulfate solutions into concrete may result in chemical reactions that cause disintegration of concrete. Therefore, special precautions may be needed to minimize the intrusion of harmful sulfate solutions. Avoidance of construction joints that may facilitate the intrusion of sulfate solutions, proper material selection and proportioning, production of low permeability concrete, and avoidance of cracking through proper curing are needed.

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 LRFD Bridge Construction Specifications

(X) Revision or ( ) Addition 8.11.1

Revise 8.11.1 General as follows:

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 seven days except that when pozzolans in excess of 10 percent, by mass, 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 LRFD Bridge Construction Specifications

(X) Revision or (X) Addition 8.11.3.5

Revise 8.11.3.5 Steam or Radiant Heat Curing Method as follows:

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 enclosure concrete and within the limits specified in the contract documents.

Item No. 2

Revise the third paragraph as follows:

The initial application of the steam or of the heat shall be from 2 to 4 hours after the final placement of concrete to allow the initial set of the concrete to take place. If retarders are used, the waiting period before application of the steam or of the radiant heat shall be increased to between 4 and 6 hours after placement. 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)., and the time limits described above may then be waived.

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 22°C per hour until the curing temperature is reached. The maximum curing temperature within the concrete shall not exceed 71°C. 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 22°C per hour until a temperature 11°C 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.

Item No. 5

Add a commentary as follows:

C8.11.3.5

Since high-strength concrete generates more heat of hydration than conventional strength concretes, it is important that concrete temperatures be monitored rather than enclosure temperatures. It is also important that transfer of prestressing force to the concrete occur before the temperature of the concrete decreases. Otherwise, vertical cracking in the girders may result.

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 71 °C (160 °F) 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 70°C (158 °F) if a known potential for alkali-silica reaction or DEF exists. Otherwise, the maximum concrete temperature is 82 °C (180 °F).(2)

Item No. 4

The current provision allows the ambient temperature to fall as low as 16 °C (60°F) 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 LRFD Bridge Construction Specifications

( ) Revision or (X) Addition 8.11.4

Item No. 1

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 seven 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.

Item No. 2

C8.11.4

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. 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 seven 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.

Other Affected Articles

8.11.1

Background

See Item No. 2 and reference No. 1.

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 LRFD Bridge Construction Specifications

(X) Revision or ( ) Addition 8.13.4

Item No. 1

Revise 8.13.4 Curing as follows:

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 10°C 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 71°C. 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.

Item No. 2

Add a commentary as follows:

C8.13.4

Use of the waterproof cover method allows high-strength concretes to self cure without the addition of steam or radiant heat. The use of insulated blankets will depend on the external weather conditions.

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