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Revised standard for water and aggregate for concrete

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NZS 3121:2015 Water and Aggregate for Concrete replaces an outdated 30-year standard.

This new standard is aligned with modern concrete and was made possible by the Ready Mixed Concrete Association with assistance from the AAQA and Standards NZ.

A close liaison between the ready mixed concrete plant engineer and the aggregate supplier is implicit throughout the new Standard, which covers three new areas. These are:

The identification of alkali aggregate reactivity with reference to CCANZ TR 3 Alkali Silica Reaction Minimising the Risk of Damage to Concrete Guidance Notes and Recommended Practice. If an aggregate is potentially reactive, the aggregate producer has to declare this to the concrete producer. The most common source of reactive aggregate is from the volcanic plateau in the North Island.

The chloride content of aggregate now has to be tested by the aggregate producer at a frequency agreed to meet limits set by the concrete producer. These chloride limits are based on the maximum limits for concrete in NZS 3109 Concrete Construction and NZS 3101 Concrete Structures Standard.

The third new area is in regard to the use of recycled coarse aggregate with reference to CCANZ TR 14 Best Practice Guide for the Use of Recycled Aggregates in New Concrete. Recycled aggregate can contain undesirable constituents that require more frequent testing. Recycled aggregate from fresh concrete of known constituents with known properties is preferable to the use of recycled demolition concrete. The new Standard provides a list of recycled aggregate reportable properties that may be requested by the plant engineer.

Sand use

In the past, the use of natural rounded river-run aggregate and sand was common in ready mixed concrete operations around the world. Protection of natural sand resources means that crushed aggregate and sand are used in most concrete plants these days.

Grading tests, monitoring the variability over time of aggregate and sand remain as compulsory tests, but with a greater reliance on quarried aggregates and risks associated with marginal aggregates, controlling the ‘cleanness’ of sand, particularly clay content, has become very important.

Assessment methodologies

Different aggregates require different assessment methodologies and control limits. In addition to mandatory control testing meeting standard limits, the plant engineer can choose a minimum number of additional tests with site specific control criteria in conjunction with the aggregate supplier.

For coarse aggregate, Australian tests for wet and dry strength variation and weak particles in coarse aggregate have been introduced and typical compliant values are given.

For quality of fines tests, the extent to which detrimental fine dust or clay like material may be present in the final sand can be determined by these tests: sand equivalent; clay and silt content; weighted clay index; clay index; and petro-graphic analysis.

To purchase a copy of NZS 3121:2015 Water and Aggregate for Concrete, visit the Standards NZ website: www.standards.co.nz

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