The current AASHTO Standard Specification for Concrete Made by Volumetric Batching and Continuous Mixing (M 241) contains a table that lists the overdesign criteria for concrete compressive strengths. The table was developed before today’s high-strength concretes existed. The ACI Building Code Requirements for Structural Concrete (ACI 318-02) and commentary (ACI 318R-02) has recently revised its equivalent requirements to limit the previous values to concrete compressive strengths less than or equal to 5000 psi (34 MPa). For compressive strengths greater than 5000 psi (34 MPa), the new ACI requirements, when stated in the AASHTO style, are as follows:
where:
f’c = specified compressive strength
s = standard deviation
M = modification factor depending on number of tests as follows:
These requirements are based on a probability of 1 in 100 that the average strength of three consecutive tests will not fall below the specified strength and no individual est result will be less than 0.90 f’c. Because of the above changes, the AASHTO acceptance criteria for concretes with compressive strengths greater than 5000 psi (34 MPa) also needs to be revised to state that no individual strength test shall be more than 0.10 f’c below the specified strength.
AASHTO Specification M 241, as well as the AASHTO LRFD Construction Specifications, currently define a strength test as the average strength of two cylinders. For concrete compressive strengths greater than 5000 psi (34 MPa), a strength test should be based on the average of three cylinders to improve the reliability. Also, when 4×8-in. (100×200-mm) cylinders are used, the strength should be based on three cylinders because of the higher variability with the smaller cylinders.
The AASHTO Standard Method of Test for Making and Curing Concrete Test Specimens in the Field (T 23) should be revised to be consistent with recent revisions in the equivalent ASTM Method (C 31). For HPC with a specified strength of 6000 psi (40 MPa) or greater, the initial on-site curing temperature should be between 68 and 78°F (20 and 26°C) compared to the current range of 60 to 80°F (16 to 27°C).
Editor’s Note
This article is the fourth in a series that addresses specification changes that are needed to facilitate the implementation of HPC. The proposed revisions are based on work performed as part of FHWA Project No. DTFH61-00-C-00009.