By Chris Olsen
CEO, Roading NZ
It’s always been very important for contractors to have confidence in the testing of roading aggregate so that the risk of any future pavement failures is minimal. This is true whether the failure occurs within the maintenance period or outside of it, as any failures ultimately reflect back on the contractor.
There are a number of tests used to determine the suitability of roading aggregate with the first test (a screening test) being the sand equivalent (SE) test as prescribed in NZS 4407. If the aggregate passes this test it can be used, if not other tests can be used to determine the aggregate’s suitability.
Recent work by Jason Lowe from Winstone Aggregates, Dr Doug Wilson from the University of Auckland and Professor Phillipa Black, also from the University of Auckland, analysed the SE test and found a number of concerns. These were reported at the recent Quarry Conference. Let me explain.
The SE test is a quick and simple test to make sure that the aggregate is clean and doesn’t have too many fine particles in it. Aggregate with too many clay particles will lack strength when wet and compromise the integrity of the pavement. It is therefore very important that the aggregate is tested to ensure that the fines within the aggregate are not “harmful” fines, i.e. clays.
The test to measure clay content is the clay index. According to M4, a clay index of less than or equal to three means the aggregate is suitable, while a SE of greater or equal to 40 means the aggregate is suitable. The study reported on at the Quarry Conference tested 197 aggregate samples from around New Zealand and found that while only 24 percent of samples passed the SE test 75 percent passed the clay index. If this is true then the SE test appears to be a poor screening test.
One of the possible reasons for this, according to Phillipa Black, lies in the geology of our rocks.
The SE test was developed in the USA where the continental rocks are much, much older than ours. Their Greywacke, for instance, has quite different properties and characteristics from ours. When Hveem developed the SE test in 1964 he found that all the fines in their samples took only 20 minutes to settle out. However, tests on New Zealand aggregate consistently show that all the fines in the SE test take around 40 minutes to fully settle out. Because the SE test says you must take the reading after 20 minutes we are cutting our test severely short and not recording the true amount of fines. This can give us a wrong reading.
This discrepancy could affect the performance of pavements in two ways. It could either allow a “boney” material to be used, which would need to be heavily compacted to create the fines required to bind the aggregate together. Alternatively, it could allow an aggregate with significant clay particles to be used because the clay particles passed the SE test.
It’s interesting to note that a paper was presented in 1980 to the sector outlining these issues with a statement from the chief engineer of the Californian Division of Transportation Laboratory (the organization that developed the SE test) encouraging New Zealand to review the 20 minutes settling period requirement if we found it inappropriate.
This is a significant issue for the sector so let’s hope that the current review of NZS 4407 fixes this problem once and for all by all parties working together.
Contractor Vol.34 No.8 September 2010
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