The AASHTO Standard Specifications for Highway Bridges – Division II, AASHTO LRFD Bridge Design Specifications, and the AASHTO LRFD Bridge Construction Specifications include a table that defines classes of concrete for use in highway structures. The table, which is similar in all three documents, has prescriptive requirements for minimum cement content, maximum water-cement ratio, air content range, size of coarse aggregate, and specified compressive strength. For many high performance concrete (HPC) applications, performance based specifications are more applicable.
The transition to performance based specifications can be facilitated by the introduction of two new classes of concrete as shown in the table. 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 strength are specified.
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 ratio 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.
The introduction of these two classes of concrete allows provisions to be developed that are only applicable to the HPC concretes while retaining many of the existing provisions for conventional concrete. For Class P(HPC), a total cementitious materials content of up to 1000 lb/cu yd (593 kg/cu m) needs to be allowed instead of the existing limit of 800 lb/cu yd (475 kg/cu m). The higher cementitious materials content is needed to achieve the higher strength concretes. For both Class P(HPC) and Class A(HPC), the use of fly ash pozzolans, calcined natural pozzolans, ground granulated blast-furnace slag, and silica fume as mineral admixtures needs to be allowed since these are essential ingredients for HPC.
Editor’s Note
This article is the first 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.