Conventional concrete has been used as a building material since Joseph Aspdin invented portland cement in 1824. It works well for many applications since it’s easy to produce and deliver to the jobsite. But concrete is not an easy material to work with. It requires labor and heavy equipment to move, place, and finish. With many contractors struggling to find help in a tight job market, a concrete solution that would reduce the amount of labor needed would certainly be welcome.
Self-consolidating concrete (SCC) came about, in part, to address this problem. It has high passing ability around congested rebar and it’s also self-leveling, thus reducing the labor involved with pumping and placing. SCC is also, though, expensive to produce because it requires a specialized mix design with high powder content and small coarse aggregates. And it doesn’t entirely solve the labor problem, because its sensitive mix requires extra quality control by the producer to minimize batch variability.
Recently, a new concrete category, control flow concrete, has emerged to bridge the gap between conventional concrete and SCC. It offers many of the desirable characteristics of each type of concrete, without the negatives. Like SCC, control flow concrete flows and places easily, without losing stability, which prevents segregated aggregate. It can be placed with minimal vibration, which helps to cut labor costs. And it doesn’t require extensive quality control to produce, so it eliminates some of the costs that come with SCC. Furthermore, control flow concrete is manufactured using conventional mix designs, with lower powder contents and larger coarse aggregates, further reducing material costs.
Choosing the right concrete
Before selecting the best type of concrete for your project, you should understand the flow characteristics of each, and the placing requirements of the project.
|Concrete Type||Slump / Slump flow||Leveling characteristics||Water-to-cement ratio||Admixtures used|
|Conventional concrete||6 to 8 inches||Requires vibration for consolidation||0.42||High-range water reducing admixture (superplasticizer)*|
|Control flow concrete||16-25" slump||Requires minimal vibration for consolidation||0.42||High-range water reducing admixture (superplasticizer) + proprietary admixture**|
|Self-consolidating concrete||Greater than 25" slump flow||Self-leveling and consolidating||0.40||Superplasticizer + viscosity-modifying admixture***|
GCP Applied Technologies recommends these admixtures:
* ADVA 198, polycarboxlate-based high-range water-reducing admixture
** ADVA 198 + CONCERA CP1028 or CONCERA SA8080
*** ADVA 405 high efficiency polycarboxylate-based superplasticizer intended for the production of SCC
Creating control flow concrete
Good slump flow retention is necessary for the transport and placement of concrete. Superplasticizers can be used to disperse the cement particles and reduce the yield stress of the concrete, although high doses of traditional superplasticizers can lead to aggregate segregation. GCP Applied Technologies has introduced a new line of admixtures (CONCERA) that produce segregation-resistant, high slump-flow concrete using conventional mix designs. Using these admixtures allows for good slump flow, without having to use high doses of superplasticizers. This means that control flow concrete can be produced relatively inexpensively compared to SCC.
Properties of control flow concrete
Control flow concrete has compressive strength that is equivalent to conventional concrete. The flow properties come from the chemistry of the admixture and not from water, so compressive strength is not negatively affected.
Static yield stress is the measure of the force required to start motion in concrete that is at rest. The less effort required to move concrete, the lower the strain on laborers and equipment at the job site. As one would expect, conventional concrete has the highest static yield stress, meaning it requires the most force to start moving. As shown in Figure 1, control flow concrete requires roughly two thirds less force and SCC moves even more easily, requiring a small fraction of the force to start it moving from a resting state.
Another important measurement is dynamic yield stress, which measures the effort required to stop concrete once it is moving. Concrete that will come to rest easily is more convenient to place. Recent tests have shown that it is easier to begin the flow of control flow concrete compared to conventional concrete, and that it stops flowing more easily than SCC. In Figure 1, the greater the value of dynamic yield stress the more easily the concrete stops moving.
Control flow concrete shows considerable promise for reducing the labor costs associated with traditional concrete without leaving concrete vulnerable to segregation. For many projects requiring highly flowable concrete, control flow concrete offers the best of both worlds: Concrete that’s easy to work with, but without high production and quality control costs. This new class of concrete is now available.