Zooming in on the numerous benefits of no fines concrete and how it should be used to achieve the desired results.
No fines concrete (NFC) consists of coarse aggregate and cement paste. In its hardened state, aggregate particles are covered by a thin layer of cement paste and are in point-to-point contact with each other. According to Bryan Perrie, MD of The Concrete Institute, at each point, the paste forms a small fillet; these fillets hold the particles together and give strength to the concrete.
“NFC therefore has large interconnected voids and a much lower density than conventional concrete,” he explains. “The structure of NFC makes it ideal for use as a drainage layer under reservoir and basement floors. It can also serve as an insulating layer and damp-proofing material. It is important to note, however, that NFC is not suitable for drainage purposes where the water is soft or aggressive to concrete.”
To make NFC, Bryan suggests utilising the following materials:
1. Common cement that complies with SANS 50197-1. Masonry cements are not suitable.
2. Water that is suitable for making ordinary concrete.
3. Clean, single-sized concrete stones should be used for aggregates. The use of flaky aggregates should be avoided. The most commonly used aggregate is 19mm crushed stone but smaller stones can also be used. Mixes made with smaller stones are easier to handle and place but consume much more cement.
“For most applications, mix proportions range from 400 to 600 litres of aggregate (standard builder’s wheelbarrow holds 65 litres of material) per 100kg of cement,” he continues. “The water content of the mix is critical. If the paste is too dry, it will not coat the aggregate properly, if it is too wet, it will run off the aggregate particles and possibly block the voids at the bottom of the pour. Experience has shown that the water content should be between 36 and 44 litres of water per 100kg of cement.”
A cubic metre of compacted NFC requires roughly 1,05 cubic metres of stone, measured in its loose state. In addition, cement content should be between 180kg and 269kg depending on the mix ratio.
Bryan goes on to say that NFC should be machine mixed as hand mixing can be difficult and laborious. If hand mixing cannot be avoided, it is best to mix the cement/water paste in a container prior to mixing the paste with the stone. “When mixing the paste, mix the cement into the water rather than the other way around,” he continues.
“NFC must be placed and compacted as soon as possible after mixing as it tends to dry out rapidly because of its open structure. It must also be thoroughly wet cured for at least a week unless it is plastered, screeded or covered before that time.”
As NFC has a rough surface texture for plastering, normal plaster mixes are used and its surface must be dry when applying the plaster. When used for underfloor drainage, roof insulation and domestic floors, NFC must be screeded within 72 hours of placing, with particular attention paid to wet curing the screed.
“NFC has negligible flexural or tensile strength,” concludes Bryan. “Compressive strength is usually between 4 and 10MPa at 28 days for mixes in the range previously mentioned. Higher strengths may be obtained by including 100kg of fine sand per 100kg of cement, which will increase the size of the fillets and, in turn, its strength. However, it will reduce voidage and thereby increases the density.”
For more information visit www.theconcreteinstitute.org.za