Saturday, April 30, 2011

Sand Equivalents

According to Caltrans' test method 217-Sand Equivalent, "The sand equivalent test provides a measure of the relative proportions of detrimental fine dust or clay-like material in soil or fine aggregates."  Fine dust (#200s and smaller) and clay material in aggregates can be detrimental to the performance of hot mix asphalt for a variety of reasons.  The clay-like material can absorb water, making it expand and giving the pavement mixture the potential for cracking and moisture damage.  Fine materials can lead to stripping and stability issues such as rutting and shoving because it may make the mix too fluid.  Think about when you're at the beach walking through the sand.  Sand is all small particles, usually #50-#200 size and the particles move to the side as you walk through it.  This is the same as shoving and rutting, although an extreme example.  The more smaller particles there are in the mix, the less stable the mix will be. 

Basically the test separates these detrimental fines from the non-detrimental (sand) particles.  The test results are a number but it is a percentage of good material in your sample.  So if your sample has a 90 sand equivalent that means that 90% of the sample was good material and 10% was detrimental.  If you have a 40 sand equivalent that means that 40% of the sample was good material and 60% was detrimental.  Many materials such as concrete sands will have high SEs, in the 80s or 90s but it is very rare to see a 100 SE.  On the flip side it is not uncommon to see a 1/4" x Dust product have an SE in the 40s or 50s because they usually have a considerable percentage of the sample passing the #200 sieve.  But overall it really depends on your aggregate source and what minerals of rock you are processing into your finished product.  Commonly you will see sand and gravel deposits with lower SEs because sedentary rocks can be cemented and formed with clay as bonds.  Hard rock deposits on the other hand are often formed metomorphically from volcanic activity and rarely have clay seams.  As such, their SEs are usually higher than sand and gravel deposits but don't take that as a fact that covers all deposit.  Every rock deposit is different from another and that rule of thumb may not hold true for all locations.  The best way for you to really know what is normal for your deposit is to test, and test often so that you have hundreds of tests to show what is normal across various areas of your deposit.

Like all test methods, the Sand Equivalent method has it's flaws.  Clay particles float so it is easy to know that this portion of the results is correct but the test also measures the content of "fine dust" in combination with the clays and this is the part of the test that I sometimes question.  The test flushes the clay and fine particles out of the rest of the sample by pushing a tube to the bottom of the sample and having liquid float these particles to the top.  It is true that these small particles will float up to the top and separate from the rest of the sample but what if you have particles that are flat and elongated?  If you have a parachute all wadded up, the chances are that it won't be lifted into the air.  That is like a standard round or cube shaped particle.  If you spread that parachute out, it still weighs the same but now the wind can lift it into the air.  This is like the flat and elongate particles.  So if you have lightweight, flat and elongated particles, say maybe your #100s or even #50s, these particles will rise to the top with the -#200 and clay particles giving you an abnormally low SE, even though those #50 and #100 particles won't act detrimentally to your product.  So be aware of your deposit and how your rock breaks down in the crushing process.  The specifications won't change for you if you have these special types of deposits but if you are aware of the problem you can combat it with either making your products with less #200s than other companies or by changing your crushing process to break the products in a different way. 

For instance, I've read multiple studies that say that if you use an impactor crusher instead of a cone crusher you will be able to force your products into a more cubical shape instead of a flat and elongated shape.  There are other possible negative impacts from this process though such as higher #200s and lower stabilities but like I said, every deposit is different and you won't really know what will work for your plant until you try it.    

1 comment:

  1. Very informative!!!! Great read for me, I have need this post a long time, Finally i have got it, Keep sharing like this...