When self-consolidating concrete (SCC), first billed as self-compacting concrete, was developed several years ago some thought it would quickly take a significant share of the concrete market and there were many claims made about its properties and what it could do. For instance, proponents said SCC could move as much as 120 feet from its point of placement. It could, but few would actually design mixes like that because they are expensive and not needed for most applications.
SCC is most used today by the precast concrete industry because they have good control of the variables. For cast-in-place projects, it's probably most used in placements with very congested structural reinforcement or on jobs to reduce labor costs, justifying the increased price of the concrete. The growth in the SCC industry has to do with a better understanding about where these mixes have their best application.
One of the challenges with SCC has always been achieving consistent results batch after batch, which is mainly due to the variability of mix ingredients—still a concern, but ready-mix producers and contractors have learned more about how to improve performance.
What is SCC?
SCC is nonsegregating highly flowable concrete—so much so that standard slump cone measurements don't work because the concrete can't support itself vertically. Instead, flowability is measured by spread. Testing personnel lift a slump cone full of concrete, let it spread out, and measure the diameter of the spread. SCC is usually defined as having a spread of 18 to 30 inches wide. Although this concrete is very fluid, its compressive strength is considerably more than normal mixes.
When normal mixes of concrete become too fluid, they segregate; large aggregates move to the bottom of a placement and water and cement paste move to the top. SCC mixes must maintain a tight balance; they must be fluid without segregating.
In the best of all possible worlds, coarse aggregates would be cubically crushed to provide the best shapes for concrete; long, slivery aggregates have the worst shape. Another problem for the concrete industry is that aggregates in the ½-inch range, needed for well-graded mixes, can be sold more profitably to other industries.
The best coarse aggregate for SCC is referred to as well graded, meaning that several different sizes of aggregate are brought together in measured amounts to produce minimal void spaces between particles. To accomplish this, quarries have to deliver several sizes to ready-mix producers, each with its own bin. But few ready-mix companies have the bin storage needed so most aggregate is gap graded and stored in single bins. The large spaces between aggregates are due to lack of intermediate sizes and blends. Also, stock piles of gap-graded aggregate do not have the same gradations in all locations of a pile, leading to more inconsistency between batches of concrete. All of these factors make it more difficult to produce SCC.
Admixtures make it happen
The two vital admixtures for constructing SCC mixes are polycarboxylate (PCE) high-range water reducers (superplasticizers) and viscosity-modifying admixtures (VMA).
Polycarboxylates are now the preferred superplasticizer, replacing both naphthalene- and melamine-derived products. They extend both the high-slump placing time and the maximum spread over previous admixtures. A complaint about all superplasticizers, especially polycarboxylates, is that under the right circumstances, they can cause entrained air to be produced or enhanced in concrete. However, Ondrej Masek, technical director for admixtures for Sika, Lyndhurst, N.J., says air entrainment levels are affected by many things and PCE-based admixtures will not cause significant changes when a proper air control system is applied.
“The key is good control of mix elements in order to have predictable results,” says Kevin Mahoney, a strategic account manager for BASF, Cleveland, adding that you can develop some entrained air with PCE admixtures but less if there is no air entraining admixture present in a mix. “Air control chemicals are added to PCE admixtures to help control the production of air. They cover what's regarded as normal PCE dosage rates between 6 to 12 ounces per 100 pounds of cementitious material. Further adjustment may be required when mixes are outside those parameters.”
VMAs have helped the industry achieve good results when material ingredients are less than perfect for a mix. Masek says VMAs make the difference—they are a crutch, but a good one. “Gap-graded aggregates and man-made sand need VMAs,” he adds. The new generations of VMAs hold mixes together without restricting the flow properties of SCC.
How much VMA is used for a mix becomes an issue, especially because the admixture is fairly expensive. Mahoney says it's best to use a minimum amount—the more VMA used, the more PCE is also required—overdosing with either admixture requires adding more of the other. There is also a movement toward larger sized coarse aggregate SCC, which VMAs make possible by improving the pumpability of mixes.
Tom Kowalewski, a quality control technician for Prairie Material, Bridgeview, Ill., says SCC mixes are touchy. Consistency between batches is the primary issue, once the mix design is worked out. Prairie Material supplied several thousand cubic yards of SCC for the Trump Tower building project in Chicago, taking care to ensure that every load arrived at the jobsite with the desired characteristics. Drivers constantly rotated their mixing drums so that admixtures remained evenly distributed and every load was tested and monitored by Prairie to be sure that everything was correct.
Although SCC is very fluid, its compressive strength is very high and sometimes ready-mix producers have to make adjustments to prevent mixes from developing strengths that are too high and too brittle.
High-flow concrete mixes
SCC isn't required for all applications where higher placing slump is required; high-flow concrete meets this need. The placing slump range for these mixes is typically 8 to 12 inches, equaling a maximum spread range up to 18 inches. A prime advantage to this option is that contractor's can use mixes they use every day. Larger aggregate top-size mixes aren't an issue either and VMAs typically aren't required. The principal change in the mix design is the addition of more PCE. Mahoney says there are fewer issues with regard to consistency between batches and on the jobsite as well.
A difference between working with SCC and high-flow mixes has to do with consolidation: high-flow mixes typically need to be vibrated either externally or internally when being placed in walls or around structural rebar. Paul Jaworski, sales manager for Wyco Tools, Racine, Wis., recommends vibrators in the range of 8000 vibrations per minute at slower speeds using smaller than normal heads. Vince Hunt, applications engineer for Wacker Neuson, Menomonee Falls, Wis., also recommends customers use high-speed, low-amplitude vibrators for shorter durations on high-flow concrete.
High-flow mixes don't require much vibration to achieve the right amount of consolidation and Jaworski recommends turning off the vibrator while placing the head in the concrete during placement, turning it on while lifting it to the top of the fresh concrete afterward to reduce the possibility of overvibrating.
Keith Wayne, president of Wayne Brothers, Kaanapolis, N.C., says they've had good luck using SCC mixes for vertical work where congested steel reinforcement is present. He adds that his company spends $10 to $17 more per cubic yard of concrete but they can minimize labor involvement to help make it more economical. He says his company doesn't use high-flow mixes very often but there are occasional applications. Typical uses include foundations, mat-slabs, and walls. When they do use them, he says concrete orders can get quite large. The cost for high-flow concrete in his area is $6 to $8 more per cubic yard but vibration labor adds to the cost.
Most aspects of construction require thought and planning today and the decision, or requirement, to use SCC is no different. Close coordination with your ready-mix producer and pumping contractor (if one is needed) is important as well. Do a trial batch to be sure that issues have all been resolved.