Concrete mixes shall be proportioned to produce the strength, durability and workability required by the approved mix design. The Contractor shall submit his proposed mix designs to the Superintendent for approval at least six weeks before manufacture is due to commence. The Superintendent may direct the Contractor to undertake trial mixes and strength, durability and workability tests to prove that the proposed mixes are acceptable. Such trial mixes and tests shall be carried out prior to placement of concrete in the works and their costs shall be borne by the Contractor.

Unless otherwise specified or approved by the Superintendent, concrete shall have the following properties.

(i) Maximum Water/Cement Ratio by Weight280  0.6

(ii) Minimum Cement Content (kg per cubic metre of concrete)  280

(iii) Nominal Maximum Size of Aggregate (mm) (cast & prestressed)  20

(iv) Nominal Maximum Size of Aggregate (mm) (Centrifugally spun)  14

(v) Minimum characteristic compressive strength for cast concrete F’c at 28 days  25MPa

(vi) Minimum characteristic compressive strength for Centrifugally spun F’c at 28 days  50MPa

(vii) Minimum characteristic compressive strength for Prestressed at 28 days  30MPa

(viii) Minimum characteristic compressive strength for Prestressed at transfer  25MPa

The Contractor shall keep, at the mixing site, records showing for each batch of concrete produced, the time and date of water addition, the weight of cement, weight of each grade of aggregate, weight of added water, results of tests made to determine the water contained in the aggregate, the results of any strength tests and the location of concrete in the works. These records shall be made available to the Superintendent.

The proportions of aggregate and cement for any concrete shall be such as to produce a mix which will work readily into corners and angles of the forms and around tendons and reinforcement with the method of placing employed on the work, but without permitting the materials to segregate or water to collect on the surface.

The concrete materials, excluding water, shall be measured by weighing separately. However, a complete standard bag of cement may be assumed to weigh 20 kg.

Water shall be accurately measured by a calibrated tank or by an approved type of calibrated water meter attached to the mixer. Certification of water meter calibration shall be supplied to the Superintendent upon request.

Mixing shall be by an efficient type of power driven batch mixer operated at the speeds recommended by the manufacturer with particular regard to the use of low slump concrete. All concrete shall be mixed for a period of not less than 2 minutes after all materials including water are placed in the mixer.

No concrete that has reached its initial set (partially hardened) or that has left the mixer or agitator for more than 30 minutes shall be placed in the structure. Remixing shall not be permitted. All concrete shall be placed within the following elapsed times from the introduction of water or cement.

Concrete Temperature at Time of Placement    Maximum Elapsed Time

Up to 27℃      2 hours

27oC to 30℃    1.5 hours

30oC to 32℃    1 hour

Over 32℃       Only as approved by the Superintendent

Under no circumstances shall water or any other substance be added to the concrete between batching and placing without the approval of the Superintendent.

Ready-mixed concrete complying with AS 1379 - Ready Mixed Concrete and the requirements of this Specification, whether manufactured in a plant operated by the Contractor or approved Subcontractors, may be used.