Self-consolidating concrete (SCC) is a revolutionary building material that is widely regarded as one of the most significant technological advancements in the concrete industry in years. Its inherent fresh properties and ability to flow under its own weight opens the door to countless opportunities that would not be possible with conventional concrete.
In spite of this, there has been a significant lag between the development of the technology and full scale acceptance and use. While there is always a period of adjustment prior to the adoption of any new innovation, the cast-in-place concrete construction industry as a whole has traditionally been slower than most to embrace new methodologies. For example, a large percentage of precast concrete is manufactured today using SCC technology.
An increasing number of specifiers and contractors alike are starting to see the real benefits offered by SCC, but there is still a reticence to fully accept this technology by a large segment of the cast-in-place industry. The main reasons for this are the consistency of the product, cost of the material, and concerns with formwork pressure.
SCC can best be described as “…highly flowable, non-segregating concrete that can spread into place, fill the formwork, and encapsulate the reinforcement without any mechanical consolidation” (ACI 237-07 Self-Consolidating Concrete). Its origin dates back to the late 1980s in Japan in response to a lack of labour skilled in the proper techniques of concrete consolidation.
SCC is essentially an extension of traditional concrete technology, offering similar or improved engineering properties while providing enhanced plastic performance. In order to achieve its fresh performance characteristics (including filling ability, passing ability and stability) and flow-ability, a much larger cementitious component (Portland cement and supplementary cementing materials) is required to generate increased paste content. In addition, the water to cementitious materials ratio (w/cm) of these mixtures tends to be low, necessitating a greater dosage of polycarboxylate-based high range water reducing admixtures (HRWRA) to achieve fluidity or workability. In some instances, inert mineral fillers, such as limestone powder, and viscosity modifying admixtures (VMA) are used to increase the stability of the mix or resistance to segregation.
It is important to first understand that SCC is not a ‘one size fits all’ proposition. SCC mixture constituents and proportions will vary from one mix to another depending on the project requirements, type of application and placement technique. There is a range of plastic performance levels that exist with SCC so it is therefore essential to determine the appropriate targets for the project and design accordingly. For example, SCC used for a relatively open pour with minimal reinforcement can be designed with flow-ability on the lower end of the spectrum. Conversely, if placing concrete in a complex shape with intricate formwork, then flow-ability becomes critical to the design to ensure complete filling. Communicating the project requirements and logistics of placement to the ready-mixed producer well in advance is crucial to developing and proportioning the mix appropriately.
The true value of SCC is relative to what practices or methods are currently being used. Therefore, this would vary from project to project. However, the following is a general list of the benefits that can be realized when using SCC:
- Ease of placement and consolidation into heavily congested or tight formwork
- Improved surface finish, specifically for architectural elements
- Reduction of remedial patching work
- Cost savings due to reduction in labour crew needed to place and consolidate concrete
- Reduction in noise levels due to elimination of mechanical vibration
- Improved safety as a result of reduced labour, equipment and noise levels during concreting operations
- Speed of construction, as SCC can be placed at higher rates than conventional concrete.
It should be noted that there is an increasing trend in the design community to push the envelope by incorporating features into structures that have intricate shapes and complex geometry. In many cases, the construction of these elements would not be achievable using conventional concrete and traditional placement techniques. The true benefit in these cases is that the available technology allows the designer the freedom to create and innovate.
One of the main obstacles to more widespread use and acceptance of this technology across the industry is the producer’s ability to consistently provide high quality SCC. This concrete is a very different animal to tame, as it is highly sensitive to material variations and requires an increased level of quality control. A slight increase in aggregate moisture content, for example, can transform a robust SCC mix into one with less stability that is prone to segregation. A higher level of understanding in regards to material selection and proportioning is needed than for conventional concrete. Thus, a greater commitment is required from the producer to increase resources in areas such as quality control, training, mixture development, equipment and batching/delivery logistics.
The higher per unit volume cost of SCC compared to conventional concrete is often cited as an argument against its use. While the cost of SCC will vary from market to market, it is generally significantly more than conventional concrete due to the increased material component (cementitious and admixtures) and level of attention required. However, when compared to other high-performance mixtures, such as high strength concrete, the gap closes significantly. In these cases it is important to consider the total in-place cost and not solely that of the concrete. It often becomes difficult to justify the use of SCC in lower compressive strength applications due to the significant cost premium. One approach to dealing with this is to replace a portion of the cementitious component with inert mineral fillers, a practice widely used in Europe. This acts to effectively reduce the upfront cost of the SCC while not sacrificing the paste content.
The design codes and standards currently dictate that, due to the fluidity of these mixtures, formwork used in SCC applications must be designed to withstand full hydrostatic pressure. This approach leads to increased formwork costs and creates another roadblock to more widespread use of the product. Presently, there are a number of research groups around the world investigating the effects of SCC on formwork pressure. Early indications are that the current design requirements are over-conservative, good news for proponents of this technology.
Robert Quattrociocchi is building sciences manager in EllisDon’s construction sciences – building sciences group.
