SELF COMPACTING CONCRETE, CHECKS AND TIPS
Updated: Mar 25, 2021
I still remember with a bit of nostalgia when SCC “took its first steps” in the market. It was still like a little baby to me: immature, fickle, inconstant but charming and exciting. I could write pages and pages about all the problems I faced in order to set a proper industial production; no matter how I tested the mixes in laboratory, I still had a lot to adjust when it came about production. But I don’t want to bore you or to become too nostalgic. The aim of this post is just to share with you, dear readers, what I discovered being the most important checks to perform on site (and why this is my conclusion), and at the same time highlight some useful tips in order to avoid problems which can arise when SCC is chosen, especially for the first time. If you need more detail on SCC please read my post "Self Compacting Concrete".
Checks on site
Basically there are two types of checks (in my opinion): checks according to the standards, which in Europe are issued by UNI and visual checks. Several checks are listed in the standards, each with a precise goal. Sure, when one have to fine-tune a mix, it is better not to skip anyone of these, but when production is ongoing it is not practical and often not even useful to test everything. All in all, to me, three standard tests are absolutely vital: slump flow, J-ring and V-funnel.
Slump flow: Everybody who had ever seen SCC knows that the slump flow test aims to assess the ”fluidity” of the mix; to some extent it's like the slump test for all the other types of concrete; it helps us determine the ”workability” of the specific mix. Talking about workability in connection with SCC is simplistic, it is actually SELFWORKABLE but on different degrees according to the specific use...and that’s why a test is needed and super important; any given ”workability degree” is related to a specific body of the building. Repeating the test several times during the casting helps to verify the consistency of the batches. The slup flow test opens up to some visual checks too, but I will come to this in a while.
J-ring: It’s a test similar to the slump flow test but the presence of the J-ring helps to evaluate the SCC ability to flow in presence of steel reinforcement. 5 cm. max difference on the flow value without j-ring is accettable.
Keeping this value monitored along the casting helps to assess the consistency of the batches and eventually to address the concrete where the reinforcement is lesser dense.
V-funnel: The test is aimed to assess the SCC ability to flow through narrowing of the cross section and to evaluete the tendency to segregation. Sometimes along the production something can change unexpectedly and although the slump flow test and the J-ring test continue to provide consistent data, it’s possible to witness an undesired tendency of the ”gravel” to collapse.
It’s wise enough to repete the V-funnel test three or more times along the casting according to the dimension of the body.
Some visual checks helps to ”feel” the behavior of SCC. When performing the slump flow test (please lift up the cone slowly!!!) check the speed of the flow. A too fast flow indicates that the mix will easily segregate, so the V-funnel test should be the next step, while a too slow flow is a sign of possible difficulties of rehology in presence of steel reinfocement (J-ring to be checked immediately). When the flow is completed, check the circumference in order to verify the homogenity of the mix: all the componenets must be on the edge and not only the paste or even worst the exeeding water (segregation is ongoing). Try even the ”Moses test”....you don’t have to open the red sea in two, but just the circle of concrete; if it closes back fully your SCC is working good!
Self Compacting Concrete - Quick Tips
Never costipate SCC! That’s trivial….yes it is! BUT, what about walking onto it, finishing the surface, hammer the formwork?
All this “operations” costipate the concrete, or it’s better to say that “breaks the balance” between ”transporter (the paste) and transported (the gravel)” with the natural consequence of inner segragtion, superficial bleeding, shrinkage and sometimes honeycombs.
Prefer the concrete pump to cast scc because its action adds boost to its flow without creating segregation phenomena. When a slab is to be casted NEVER pump all the concrete in one single point, rather pump SCC radially in order to avoid excess of transport of the “gravel”. Use the concrete pump also for vertical casting and prefer wher possible the ”reverse casting” from the bottom to the top, which increases the capability of SCC to expell the excess of air. Specific flanges are designed to this purpose.
All formworks set in place have to be thoroughly checked in order to find holes or discontinuity where SCC could flow with the risk of a dramatic break down of the formwork during the casting itself; if these cavities are not too big, polyurethane foam could be used successfully.
The props must be increased due to the higher pushing force (in comparison with a superfluid concrete) especially at the corners, the most vulnerable part of the body.
Curing (read my post "Concrete Curing") is even more important with SCC than regular concrete. Due to its limited bleeding (which is good) water evaporation on the surface is faster and can lead to early shrinkage.