SELF COMPACTING CONCRETE
Updated: Mar 11, 2021
Time is money. That’s what they say…indeed, that’s absolutely true especially when it comes to construction. It’s not easy to quantify the economic weight of every single choice but for sure when one has the chance to try different ways to achieve the same goal it is easier to take a good decision that is based not only on the economic result “here and now” but on the long term and on the overall economy of the work done, rememebering that quality is not a cost but a money-saving issue.
Setting in place concrete is a time-consumig operation because it requires good skills and a certain number of workers (about this please take a look to my post “Concrete compaction”), and, despite the efforts, sometimes the results are not satisfactory enough.
A way to reduce problems related to the quality of men’s work is…avoiding men’s work! Automating casting operations could be a solution, and here’s where ”Self Compacting Concrete (SCC)” comes in.
This kind of concrete is mixed with the same ingredients of almost every other concrete, namely cement, water, fillers, sands and gravels. Just mix design and the admixture used are very different.
We are not going to discuss the details about how SCC is made, but only remember that the quantity of ”powders” (cement, fillers and sands) is particularly increased and the gravel used is only the one with diameter up to 20 mm. The peculiarity of SCC is that the fresh concrete is going to compact itself by means of gravity and movement. It tends to literally flow as a fluid eliminating excess air along its path and adjusting in place under its own weight.
It is clear that there are pros and cons in using SCC, here’s some pros:
1) drastic casting manpower reduction (up to ”no one at the casting”)
2) speeding up of construction operations (while casting the crew can work to other stuff)
3) no segregation phenomena
4) no blocking phenomena
5) complete wrapping of the reinforcement bars
6) greater adherence to the reinforcement bars
7) aesthetic quality of hardened concrete
8) less capillarity with equal w/c ratio and less presence of voids, therefore greater durability
So it looks like SCC is solving every problem. Well, it’s not exactly so. There are cons one should carefully take into consideration:
1) mansory slabs cannot be casted with SCC because the hollow blocks will be totally filled up with concrete
2) SCC’ pressure on the formworks is higher. Hence formworks, must be braced thoroughly especially at the corners
3) every void in the formork (especially at the joints) must be closed preferably with polyurethane foam
4) avoid to walk or finish the SCC’ surface. It is a sort of compaction that will send the gravel down and expose the surface to bleeding and shrinkage (of course NO COMPACTION is ever to be done)
5) never miss curing operations (read my post ”Concrete curing”) since they are even more effective with SCC and if skipped, shrinkage could be more dramatic (see my post ”Concrete shrinkage part 1” and "Concrete shrinkage part 2")
SCC can be used for fair-faced structures, slabs, foundations but not for flooring and ramps or sloping roofs. It can be casted with pumps and even pumped from bottom to top, it is an invaluable allied in renovations were hoops are required.
Details about how to check SCC’ properties and how to get better results from casting operations are discussed in my post ”Self compacting concrete checks and tips”.