By: Stephen J. Worsfold, P.E. Contributing Author
Self-consolidating concrete (SCC) is rapidly creating a niche in ready-mix delivered, cast-in-place (CIP) concrete construction. As contractors are exposed to this new and exciting technology, they are finding more uses for this concrete, which can provide brilliant aesthetic finishes. As with any new technology, there is a learning curve which must be tackled for first time users of SCC.
The keys to success for SCC in CIP construction lie in the planning, preparation and attention to detail when executing the work. Obviously, any concrete
construction requires planning, preparation and attention to detail. However, because SCC does such a good job of providing aesthetically pleasing finishes, it is particularly vulnerable to the many field conditions that can affect the finished product.
The first element of planning to be considered is the tuning of the SCC mix design to suit the application. SCC can be made of an infinite combination of readily available materials, but each SCC mix is different. Depending on the aggregate selection in the mix, one SCC mix may have much better flow characteristics than another. That is, an SCC mix created with round gravels will typically flow much better than a mix designed with crushed stone. Therefore, if the mix is expected to flow great distances, a gravel design may be more appropriate. If project specifications or locally available materials preclude that option, a crushed-stone design can still provide good results but a viscosity modifying admixture (VMA) may be required in the mix. The VMA will help inhibit the tendency for the mix to segregate (dynamic segregation) during its travel.
There is one potential drawback to relying on a VMA. The more VMA in a mix, the more cohesive (like honey) it becomes and therefore slower moving during the pour. If the mix is poured too slowly, there may not be enough energy in the SCC to provide the desired high-quality finish.
True to form
Another extremely important, yet commonly misunderstood, element of planning is the proper formwork design for the work. Many opinions have been rendered on this topic, all of which have been correct and simultaneously incorrect. In other words, this is the trickiest choice the contractor has to make when deciding to use SCC.
Some authors state that forms should be designed to accommodate the full liquid head of the concrete in the forms. This is certainly not a necessity and may be much too costly and conservative.
Some authors state that successful results can be had with much weaker form systems with strengths rated around 1,000 psf. This has been field-verified with very good results. This too, however, may not be the correct choice for the job. The contractor has the unenviable task of trying to determine the most cost-effective choice for the individual project. There is no “one-size-fits-all” equation that works. With no SCC experience or guidance, the contractor runs a risk of making the wrong choice.
The contractor must look at many variables. The faster he is able to pour the work, the greater value he gets from the premium paid for the SCC. With short-elevation pours, a weaker form system will certainly be adequate, because little head pressure develops. These short pours can be completed very quickly, resulting in high productivity with reduced man-hours (keep in mind that additional man-hours can be saved by eliminating the need to run vibrators). Yet even with tall pours, weaker forms may be sufficient if the pour is long enough (in distance, not time) such that concrete volumes in cubic yards per hour can be placed at a very rapid pace yet the vertical pour rate in feet per hour does not exceed the rated form strength.
The decision becomes more difficult when considering tall pours that are short in length (for example, 24 ft tall, 30 ft long). With weaker forms, the contractor is limited to a stiflingly slow pour rate. The problem here is twofold. First, slow pour rates cut into the value returned from the premium paid for the SCC. The contractor must hope that man-hour reductions during the pour and elimination of patching and rubbing costs after the pour still offset the SCC premium.
Second, slow pour rates reduce the potential for the SCC to do the job it was asked to do. SCC likes to be poured fast with plenty of energy. The more energy imparted to it during placement, the greater the likelihood it will produce the high-grade finish it is known for. If it is poured too slowly, the potential for bug holes and lift lines increases. For purpose of clarification, lift lines are not cold joints. They are merely visual distinctions between subsequent lifts of concrete that can be seen in the finished product. Taken to the extreme, some SCC pours have been placed so slowly that small vibrators have been needed to ensure that aesthetic problems, particularly lift lines, are prevented. It should be noted that properly designed SCC mixes can be vibrated without introducing segregation of the mix. Even though this may seem an oxymoron with a self-consolidating product, the SCC is still a wise choice, because it provides an architectural-grade, higher-quality, more-durable product than a nonself-consolidating concrete vibrated in place.
Another element the contractor may look at to determine what form strength is appropriate for his job is the number of SCC pours needed. If only one pour will be made with SCC, then perhaps a less costly, weaker form with a slower pour rate is appropriate. However, if the contractor is required to make many consecutive pours, or if rapid production is paramount, then conventional wisdom dictates that the strongest form system the contractor can justify purchasing is the better choice on many fronts.
There are no unusual steps that a contractor must go through when preparing for an SCC pour that don’t also apply to a conventional concrete pour. The multitude of items that a contractor needs to address to ensure a successful outcome is the same whether conventional concrete or SCC is used. There are, however, a few items that require special attention.
On the watch
When preparing for an SCC pour, the contractor needs to coordinate well with the ready-mix supplier delivering the SCC. The contractor must have an accurate idea of how many cubic yards per hour are needed to attain the desired vertical pour rate for his forming system. This information must be passed to the supplier so that an even, steady delivery of concrete is provided. It is very beneficial with SCC to have a continuous supply of concrete so that there are no pauses in the pour itself. Delays in the pour increase the potential for unsightly lift lines in the finished product. In some cases, if the forming system is strong enough to provide very rapid placement rates, it may be good practice to have all trucks on site prior to starting the pour itself.
Also, the contractor must stress with the supplier the importance of good quality control at the plant. Modified batching sequences and additional mixing time at the plant are common to ensure consistent SCC spreads (“spread” tests are taken in lieu of slump tests) and to reduce the potential for the formation of “dough balls” in the trucks. If dough balls become a problem, a supplier may have to hold back some initial mix water while revs are placed on the truck to smash the dough balls. Then the remaining mix water with additional revs can be added and the truck sent to the jobsite. If this prep time is not taken at the plant, inconsistencies between trucks may slow the pour, resulting in the potential for aesthetic problems. The contractor also must tell the supplier the desired SCC spread for the pour.
Typically, the contractor should target a spread as high as possible within the SCC’s ability to resist dynamic segregation for the work performed. As stated earlier, SCC pours are not all the same. Some are short with little reinforcement; others are long and heavily reinforced, requiring an ultrastable SCC mix. The contractor and the supplier need to discuss the performance requirements for the SCC so that the supplier can tune the mix accordingly.
Forming materials and preparation is the next area of focus for the contractor. With SCC, form selection is pretty simple: The smoother the forms, the better the finish. Metal, MDO plywood and fiberglass forms have all been used with excellent results. The forms should be as clean as possible and oiled adequately and uniformly prior to the pour. Inconsistency or excessive form oil may lead to marbling of the finished surface. Also, the form joints should be tight. If an excessive amount of bleed water from the concrete is allowed to seep through loose-fitting forms, the concrete’s water-to-cement ratio may change just slightly at the location of the gap. This could cause the color of the concrete to become noticeably darker at that point. The bottom line is this: SCC is almost too good at providing smooth surfaces; it will exactly mirror the quality and condition of the forms it is placed against.
Tighten it up
Assembling the forms also is a critical step and should not be taken casually when using SCC. Quality of craftsmanship and attention to detail are essential. With conventional concrete, if a small blowout or gap in the forms occurs during the pour, a small amount of concrete may be lost. Usually the pour can be halted, the problem rectified in one way or another and the pour resumed without much further delay.
In contrast, SCC is unforgiving. If a small gap occurs while pouring SCC, a large amount of flowing concrete may be lost before the gap can be plugged. However, if a small blowout occurs while pouring SCC, the chances of saving the pour are next to none. Usually, the contractor is forced to flush out the concrete that has already been placed and start all over again, losing valuable time and resources.
The last key to success for using SCC in CIP construction is paying close attention to the details during the pour. After all of the planning and preparation, the contractor should observe how the SCC is reacting while it is being placed. Typically the looser the SCC spread, the better and quicker it will flow in the forms. If the mix is traveling lethargically in the forms, try increasing the spread or rate of placement of the mix.
If the mix is dynamically segregating as it travels to the limits of the forms, a VMA additive may be needed. Another solution is to vary the point of placement during the pour to reduce the total distance traveled by the SCC. If subsequent lifts of mix are “rolling over” previous lifts rather than integrating seamlessly, then the spread may be too stiff. Or, possibly, the pour is progressing too slowly or there was a delay in concrete delivery. If so, additional energy must be imparted in the lower lift to eliminate the potential lift-line. This can be done by rodding with a piece of wood or even using short bursts with a small concrete vibrator.
Generally speaking, most of the potential aesthetic problems can be avoided with fast and continuous placement of SCC.
Although the learning curve for using SCC in cast-in-place construction can be steep at times, one thing is certain: SCC can provide finishes in the field that are very difficult to achieve with conventional concrete. Contractors who are progressive and forward-thinking, who embrace this technology and conquer the learning curve, will most certainly have an advantage over their competitors. By using SCC, they will be able to sell to their customers the concept of a value-added product that is not only aesthetically pleasing, but stronger, longer-lasting and more durable.
About The Author: Worsfold is a mixtures control engineer with the Illinois Department of Transportation’s Bureau of Materials, District 4, Peoria, Ill.
