NZS 3121:2015 Water and Aggregate for Concrete has recently been published and replaces a dated 30 year old standard.
During that time much has changed in terms of the natural materials used to manufacture concrete.
The Resource Management Act has also placed restrictions around the manufacture of concrete. Practices of yesteryear, particularly in respect of dumping highly alkaline waste concrete, carry heavy penalties today.
This has resulted in the routine recycling of wash-water for use as mix-water. To avoid affecting setting times, the fines content of agitated wash-water needs to be monitored and restricted to an upper limit SG of 1.07 by diluting with wash-water.
Close liaison between the ready mixed concrete plant engineer and the aggregate supplier is implicit throughout the Standard.
The Standard covers three new areas:
- 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 shall declare this to the concrete producer. The most common source of reactive aggregates is from the volcanic plateau in the North Island.
- The chloride content of aggregate is 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 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 may contain undesirable constituents requiring more frequent testing. Recycled aggregate from fresh concrete of known constituents with known properties is preferable to the use of recycled demolition concrete. The Standard gives a list of recycled aggregate reportable properties which may be requested by the plant engineer.
Thirty years ago the use of natural rounded river-run aggregate and sand was common in ready mixed concrete operations. Depleting natural resources and environmental restrictions mean that crushed aggregate and sand are used in most concrete plants today.
Grading tests, monitoring the variability over time of aggregate and sand remain as compulsory tests.
However, with the greater reliance on quarried aggregates, and the risks associated with the increased use of marginal aggregates, establishing and controlling cleanness of sand, particularly clay content, is important.
Different aggregates will require different assessment methodologies and consequently control limits. In addition to mandatory control testing meeting standard limits, the plant engineer may 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 choosing from the following tests:
- Sand Equivalent
- Clay and Silt Content
- Weighted Clay Index
- Clay Index
- Petro-Graphic Analysis
The ‘full revision’ of NZS 3121 was made possible by the New Zealand Ready Mixed Concrete Association (NZRMCA), with assistance from the Aggregate and Quarry Association of New Zealand (AQA) and Standards New Zealand.
To purchase a copy of NZS 3121:2015 Water and Aggregate for Concrete visit the Standards NZ website.
Taken from the February 2016 NZRMCA Newsletter
Image - PCA