In the late 1980s, concrete professionals conceptualized and developed a revolutionary new type of concrete known today as self-compacting concrete (SCC). SCC mixes provide many benefits for concrete operators and contractors, but getting the mix design right can be challenging. According to the National Precast Concrete Association, high-range water reducers are essential to maintaining the consolidation and flowability of SCC mixes. So what makes superplasticizers essential for this unique type of mix design?
SCC Mix Design Qualities
There are three main requirements for a good SCC mix design: flowability, passability, and stability. Self-compacting concrete is popular for intricate and complex form work or molds because the concrete can fill every crevice without gaps or voids. This is why SCC mixes require higher rates of flowability than other types of concrete mixes.
In addition to being the preferred mix for formwork, self-compacting concrete is used for projects that involve densely packed reinforcements, which means the concrete has to flow through the rebar and fill the space. Finally, SCC mixes are promoted as not needing vibration, which means the mix has to be stable and not prone to segregation. These characteristics must be present for a concrete mix to be considered self-compacting. So how do you go about designing concrete like this?
Start with the Right Materials
When developing an SCC mix, choosing the right type and amount of aggregate and cement is vital.
If the aggregate doesn’t have the proper grading, it can affect the cohesiveness of the mix and prevent it from flowing through dense rebar. The NPCA recommends extensive testing of aggregate grading and working with a mixed design professional to ensure you get it right. They also recommend using low-carbon fly ash to reduce the risk of air content fluctuations. Once you have chosen the aggregates and cementitious materials, it is important to remember not to add too much water. Additional water in the mix will increase flowability, but also increases the risk of segregation and lowers the overall strength of the concrete mix. To improve the flow without additional water, adding a high-range water reducer or a superplasticizer is the best choice.
Choose the Right Superplasticizer
Like the other materials in the mix, it is important to choose the superplasticizer right for the project and not overdose on it. Be sure to speak with an admixture professional to ensure the proper dosage for your mix. Additionally, you want to choose the right admixture. SCC mixes require high-range water reducers because they provide the highest degree of slump decrease. Generally, the NPCA recommends using a polycarboxylate superplasticizer, such as Fritz-Pak Supercizer PCE, for these mixes as they provide the highest degree of water reduction and slump decrease in the industry. When working with forms in warm weather, you can use a superplasticizer with a slight delay set chemical to reduce the risk of cold joints and improve durability.
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