Association announces key projects for 2006

28 February 2006

New Zealand Ready Mixed Concrete Association announces key projects for 2006

The New Zealand Ready Mixed Concrete Association (NZRMCA) has identified its three key projects for 2006 to ensure quality in concrete production and construction.

The following projects illustrate a range of industry concerns held by NZRMCA:

1.) Promoting Self Compacting Concrete as the cost-effective product of choice for New Zealand architects, engineers and contractors.

2.) Promoting the correct curing of concrete to ensure maximum strength in construction.

3.) Promoting the safe application of ready mixed concrete to ensure minimal environmental effects are placed on the New Zealand environment.

NZRMCA incorporates an active concern for both the production of concrete and its end use in the construction industry, and takes a responsible and active interest in overall concrete industry affairs.

ENDS

28 February 2006
Technical Committee NZRMCA

Ready Mixed Concrete – the future

The objectives of the New Zealand Ready Mixed Concrete Association (NZRMCA) incorporate an active concern for both the production of concrete and its end use in the construction industry. To deal with this dual responsibility and ensure quality in concrete production, the Association has a Plant Auditing Scheme. The NZRMCA also operates a Technical Committee that focuses on the dissemination of technical information for the future needs of the industry, the promotion of research and the resolution of current technical issues.

Three projects from the Technical Committee’s work are summarised below.

1. Self Compacting Concrete

Current activities relate to ensuring the satisfactory growth of self compacting concrete (SCC). SCC has significant advantages of use as it removes the labour involved with compaction of concrete. Excellent surface finishes can also be achieved with SCC, which remove costs associated with repairing potential defects. The objectives of this project are to develop an information package for architects, engineers and contractors that will allow for the specification and practical application of SCC.

2. Concrete Slabs

The concrete slab project, in contrast, revisited the techniques to promote appropriate construction and curing applications. To highlight the effects of curing, a series of concrete slabs were constructed.

Three types of concretes were used with specified characteristic strength of 30 MPa. Standard concrete test cylinders were made at the time of delivery. The concrete in the slabs was compacted/cured in four different ways:
a) Vibrated / cured
b) Vibrated / non-cured
c) Non-vibrated / cured
d) Non-vibrated / non-cured

The concrete slabs were cored at 28 days, 56 days, 180 days and 360 days. The minimum recommended core diameter for a concrete using a maximum aggregate size of 20mm is 80mm (4 x max. aggregate size).

Correct coring requirements are set out in the CCANZ information bulletin IB72 Coring of Concrete, which can be downloaded from www.cca.org.nz.

The results found:

- As expected, cores generally have strengths less than the equivalent standard test cylinder, which has been prepared fully compacted and in an optimum curing regime in a situation that cannot usually be replicated during construction. For concretes tested, the average reduction was of 15% strength.

- Lack of surface curing in this trial resulted in a strength reduction of approximately 5%. However, other trials dealing with surface wear have shown that surface curing, measured in terms of permeability of the slab surface, show a 15 fold improvement, ie. significant improvements in durability and surface strength.

- Failure to compact and cure the slab efficiently contributed to an overall 10% drop in strength. Therefore, it needs to be recognised that due to the practicalities of construction, concrete will not achieve the potential strengths obtained from the standard prepared and cured cylinders.

Compaction and curing of the concrete is essential to maximising the strength of the in-situ placed concrete towards that of the standard test cylinder result. However, the placed concrete will give a lower value.

3. Environmental Matters

The NZRMCA has responded to environmental concerns relating to the washing out of concrete trucks at the production plant. Many plants now have facilities to deal with washing out and often washwater is recycled into the mixing process.

While washwater arising from onsite activities is not the direct responsibility of the ready mixed concrete supplier, the NZRMCA has prepared the leaflet Management of Onsite Concrete Washwater, which describes methods of safe washwater disposal. It is important to realise that it is the pH of water that has been in contact with the concrete which constitutes an environmental hazard.

Ironically, the high pH water makes an excellent fertiliser (except where an acid soil is required), meaning that disposing of washwater onto the garden is safer than washing it into surface water drains. Filtering water to remove solids like sand and cement does not reduce the pH level of the water, neither does “washing down” to dilute the water pH, as many hundreds of litres of fresh water are needed to reduce the pH.

These projects illustrate the range of industry concerns held by the NZRMCA. While not examined here, the NZRMCA’s commitment to producing quality concrete is reflected in the independent auditing of its plants to the standards of NZS 3104 Concrete Production via a Plant Audit Scheme, the details of which are available on www.cca.org.nz/RMCA/rmca. The NZ Ready Mixed Concrete Association takes a responsible and active interest in overall concrete industry affairs.

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