CLASSES OF CONCRETE USED IN CONSTRUCTION INDUSTRY

 CLASSES OF CONCRETE

 Definition of Classes of Concrete in AASHTO

AASHTO instructs that classes of concrete to be used in all part of structures shall be specified in contract documents. If not specified, the engineer shall designate the class of concrete to be used.

 Normal-Weight(-Density) Concrete

In AASHTO, ten classes of normal-weight (-density) concrete are specified as listed in Table 2.2-1, except that for concrete in or over saltwater or exposed to deicing chemicals. the maximum water/cement ratio shall be 45%.

At present, there are no specifications of classes of concrete in Myanmar. The classification of AASHTO shown below is as reference.

Notes:

a. As noted in AASHTO M 43 (ASTM D448), Table1-Standard Sizes of Processed Aggregate.

b. As specified in the contract documents.

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

With high performance concrete, it is desirable that the specifications be performance-based. Class P(HPC) is intended for use in prestressed concrete members with a specified concrete compressive strength more than 6.0 ksi (approx. 41.4 MPa) and should be always used for specified concrete strength more than 10.0 ksi (68.9 MPa). Class A(HPC) is intended for use in cast-in-place constructions which meet specified performance criteria in addition to concrete compressive strength Other criteria might include shrinkage, chloride permeability, freeze-thaw resistance, deicer scaling resistance, abrasion resistance, or heat of hydration.

For both Class P(HPC) and A(HPC), the minimum cement content of each class is not specified because it should be determined by a producer based on the specified performance criteria. The maximum water-cementitious materials ratio is specified. 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, the maximum size of coarse aggregate for Class P(HPC) concrete is specified since this class of concrete with aggregates larger than 0.75 in is difficult to achieve higher concrete compressive strength. The maximum aggregate size for Class A(HPC) concrete should be selected by a producer based on the specified performance criteria. Air content for Class A(HPC) and P(HPC) should be determined with trial tests but it is recommended that a minimum air content is two percent. The 28-day specified compression strength may not be appropriate for strength greater than 6.0 ksi (approx. 41.4 MPa).