Sand Quality Control – silt content testing and washing
So what is it that I (Peter) am doing whilst volunteering with the International Non-Governmental Organisation (INGO) Bridges to Prosperity (www.bridgestoprosperity.org)? My role is focused on training the growing team to implement systems to ensure consistent quality, safety and construction protocols are followed on all our bridge projects. I plan to write a few short blog notes on some of the engineering aspects of my role for anyone who is interested in knowing a bit more. The first of these is on a really interesting topic – sand!
Along with cement and aggregates (stones), sand is a key component of a concrete mix and the quality of the sand has an effect on the final strength and durability. From a recent test on one of our bridge sites in Bolivia it was found that the target strength of concrete was reduced by 50% due to contamination of silt in the sand. Considering the inconsistencies of hand mixing concrete rather than using concrete mixers, we need to be careful to be sure to produce a mix that meets our strength requirements. With this in mind I have introduced a simple test for silt content in sand that is now being used on our sites.
The test is simply done using some saltwater in a measuring cylinder, to which you add a sample of sand and shake it up to suspend all the silt in the water. The sand quickly falls to the bottom of the measuring cylinder and then you wait for 3 hours whilst the silt settles. The silt forms a visible layer on top of the sand. The ratio of silt to sand can then be measured with the graduations on the cylinder and a percentage calculated. If silt content is found to be too high (greater than 6%) then we have implemented sand washing using water pumped from the river and the sand retested. This has been successful in reducing the silt content within the sand at several sites from 10-12% to an acceptable 1-3%. Check out this link for more detailed instructions of the silt content in sand testing: http://www.civilology.com/silt-content-test-sand-fine-aggregate/
This has ensured the quality of the sand used in our concrete mixes and has the potential to save significant cost whilst purchasing sand. In Rwanda there are three sources of sand available to us:
1) Kiyumbi sand – this is a high-quality product that which is often transported long distances from certain areas of Rwanda. It is therefore expensive (20,000 RWF per m3).
2) River sand – is about half the price of Kiyumbi sand but when tested we have found that it often has a high silt content (8-12%).
3) Sand found locally on site – this is our cheapest option as it only requires labour to collect the sand from the river rather than purchasing transported sand from a supplier. At some of the bridge sites there is suitable sand in the river, but quality is unknown, so testing is necessary.
By implementing testing and then washing where necessary we can avoid the high cost of Kiyumbi sand and purchase river sand or use sand directly from the river on site. The cost of washing sand is minimal as labour is inexpensive and we already have several water pumps available – therefore the overall saving achieved by using river sand is around 40%.
I believe in the work we do at Bridges to Prosperity and I’m excited to fundraise for our organisation to provide safe access to isolated communities around the world. Check out my fundraising page at https://www.kindlink.com/fundraising/bridges-to-prosperity-uk-charitable-trust/petercroshwaite to see what it’s all about, and join me in unlocking opportunity around the globe!
All views expressed are those of author’s and not that of Bridges to Prosperity.