Testing Concrete Quality Control – Check the Need for Additional Testing!

It has been observed that consulting engineers issue structural specifications with concrete testing requirements that may not be appropriate, especially when the concrete is supplied from a concrete batching plant that holds a Certificate of Audit confirming that its concrete production complies with NZS 3104:2021.

Examples of additional concrete testing requirements in structural engineering specifications that may not be appropriate for small in situ concrete pours or precast concrete manufacture include:

• Requirements to make five standard cylinders for the first 25 m³ of concrete placed.
• Requirements to test the cylinders; which consists of breaking one at 7 days, breaking one at 14 days, breaking two at 28 days; with a spare cylinder held, and if required by the Engineer, tested at 56 days.

These testing requirements exceed the testing specified in NZS 3104:2021 and can significantly increase testing costs with no improvement in monitoring quality assurance for concrete plants that perform quality control tests in accordance with NZS 3104 and hold a Certificate of Audit. At worst, when these onerous testing requirements are applied to precast concrete manufacture, they could result in 5 cylinders per precast item, meaning 5 cylinders for every 2 or 3 m³ of concrete.

The following sections set out guidance on how quality assurance for concrete production is typically performed in New Zealand for concrete production specified to comply with NZS 3104.

QUALITY CONTROL OF CONCRETE – AS REQUIRED BY NZS 3104
Control tests for concrete production and criteria for demonstrating compliance are specified in NZS 3104:2021, which has specific provisions for the following:

• Frequency of sampling of concrete during production.
• Target strength of different grades that is typically between 5-8 MPa above specified strength grade and results in approximately 0.5% risk of failure when analysing multiple test series.
• Controlling variability of test series to show good production control with coefficient of variation limited to typically less than 10%.
• Controlling within test variability of cylinders tested at 28 days to ensure reliable mean strengths are reported.
• Specifying early strength testing at 7 days and allowing longer term testing at 56-days when appropriate, typically for concretes with slower strength gain properties.

The standard strength testing procedures for concrete that is specified to be produced in accordance with NZS 3104 are therefore as follows:

• Plant testing during concrete production at a minimum test frequency of one test for every 75 cubic metres of concrete produced.
• Each test to typically consist of one cylinder tested at 7 days and two cylinders tested at 28 days for normal concrete. If dealing with some concretes that contain Supplementary Cementitious Materials (SCMs), such as fly ash, 56-day testing would replace 28-day testing.
• During the initial phase of a specific project some extra site testing might be undertaken to confirm concrete properties.

SITE TESTING OF CONCRETE
Specifying site testing of concrete should be done with care as this requires considerably more resources and generally provides little extra value. The commentary clause C2.15.8.1 in NZS 3104:2021 provide reasons why this form of testing is often not appropriate for compliance testing of concrete production.

In some cases, the integrity and quality of any additional testing may be compromised since technicians struggle to undertake multiple tests on sites remote from the testing laboratory or production plant.

TESTING FREQUENCY
Testing frequency is specified in NZS 3104 and NZS 3109 as being typically one test for every 75 cubic metres of concrete produced. This testing frequency means that at least one test is taken from each tanker load of Portland cement or for every truck and trailer of fine or coarse aggregate. Any significant change in raw materials is therefore captured within the testing data.

Designers and specifiers should be aware that each c. 5 m³ truck load of concrete is subject to quality control procedures regardless of whether it is tested or not. This is done by analysing batch records where all materials are accurately weighed and recorded. The final water/cement ratio of a concrete mix can be accurately used to predict the compressive strength of concrete and this also helps confirm that untested loads are consistent and comparable with tested loads.

NUMBER OF TEST SPECIMENS
Three cylinders for the test frequency outlined previously are usually sufficient for confirming strength properties:

• One cylinder provides an early estimation of strength performance (7 days).
• Two cylinders used to provide the mean strength at 28 days (three cylinders for new concrete batching plants or when the coefficient of variation of test results exceeds 4%).

Providing extra cylinders for additional strength testing is unnecessary since 14-day and 56-day strength can be accurately predicted based on 7-day and 28-day strengths. When dealing with concretes containing slower reacting materials such as fly ash, NZS 3104:2021 allows 56-day testing but this is usually done by substituting cylinder strength tests from 28 to 56 days. It would be unlikely that concrete supplied for precast concrete would require more than the standard 7-day or 28-day testing due to the rapid strength development of precast concrete mixes.

CONCLUSIONS
Quality assurance of concrete production is undertaken by a number of control tests as outlined in NZS 3104. Both batch and test records of concrete production are used to evaluate that output complies with the project specification. Specifiers who require additional testing requirements to those specified in NZS 3104 will incur significant additional costs to the project without any benefit to monitoring quality assurance if the batching plant already routinely performs quality control tests.

It is more important to specify that concrete supplied for a project, irrespective of whether it is poured at a construction site or poured at a precast manufacturing facility, is produced in accordance with NZS 3104 by a concrete batching plant with a Certificate of Audit.