There are three types of specification methods for concrete:

1. Prescribed Concrete

2. Normal Concrete

3. Special Concrete

 

The choice of concrete specification is the prerogative of the design consultant. The detailed production requirements of the three concretes are contained in NZS 3104. 

 

PRESCRIBED CONCRETE

The prescribed concrete permitted by NZS 3104 and NZS 3109 Concrete Construction ranges from 17.5 MPa to 25 MPa. This concrete is not tested for strength but by checking that the materials used are batched correctly. The specifier’s control over these mixes is by way of checking cement content of the hardened concrete by chemical analysis, however, the accuracy of the test is low with an accuracy of ± 15%. Typical use of prescribed concrete would be on small remote projects outside the operating areas for ready mixed concrete plants, and where concrete strength is not required over 25 MPa.

 

NORMAL CONCRETE

Specifications calling for the use of normal concrete 17.5 to 50 MPa in accordance with NZS 3104 and NZS 3109 should be used where the structural designer’s primary concern is the compressive strength of the concrete. The structural designer has the following to specify:

• Concrete strength at 28 days

• Maximum nominal aggregate size

• Workability

• Method of Placement

• Additional requirements for Special Concrete (see below)

Based on this information the concrete producer will design, produce and take responsibility for the concrete. The quality assurance that concrete will be produced in accordance with NZS 3104 lies in requirements for the concrete producer to have an independent audit of the plant’s production capability and viability of production on a statistical basis. This function is performed by Concrete NZ’s Plant Audit Scheme.

 

SPECIAL CONCRETE

This concrete will have performance requirements that may be outside the strength range 17.5 to 50 MPa or have special features not necessarily measured by compressive strength such as shrinkage, tensile strength, chloride durability etc. concrete requiring specialist skills of the concrete producer. The plant must have a current audit certificate for Normal concrete. The structural designer must now specify the special features required, together with a test method or other means that the concrete supplier can demonstrate specification compliance. The specifier also needs to get some prior assurance from concrete producer that these special mixes will perform at the time of the project, this can cover:

• Producers records in producing similar compliant concrete

• Need for test trials prior to production

• Acceptable production variation in special parameter

• Required test frequency for compression and for the special parameter initially for production and reduction in testing rate once compliance is established

 

As Published by Concrete NZ Plant Audit Scheme

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