Q: We are urging an engineer to specify self-consolidating concrete (SCC) on a project that includes several tall, vertical forms. The engineer recognizes the benefits of SCC regarding consolidation and final surface appearance. The contractor also wants to use SCC because it can speed up the pouring process.
But the engineer is concerned that formwork may not be strong enough to withstand the lateral pressure SCC can exert in its fluid state. He is requiring the contractor to prove that the formwork will withstand any potential additional pressure caused by using SCC.
Are there any guidelines or is there any research on this topic? Is there any advice for selecting ingredients in our SCC mix designs that can help reduce formwork pressure?
A. Recognizing that some SCC can be almost liquid-like in nature, American Concrete Institute (ACI) Committee 347, Formwork for Concrete, recommends a very conservative approach to formwork design. The committee suggests that formworks are designed with the assumption that full liquid pressure head will be achieved unless trial study measurements can prove otherwise.
The ACI requirement for engineering documentation of formwork pressure has generated some concern from the concrete community because it creates some difficulties for SCC acceptance. Much of the research on formwork was conducted before the introduction of SCC, in an era when contractors often poured in rather short lifts.
To comply with the recommendations by using formwork calculations found in the documents, contractors would have to build extremely strong forms. Because there are no standard methods for studying formwork pressure in the field or in the laboratory, engineers are reluctant to vary too far from the ACI approach.
Fortunately, there is a good deal of current research on this topic. The National Ready Mixed Concrete Association's (NRMCA) Web site on the subject (www.selfconsolidatingconcrete.org) lists three new form pressure computer models that are undergoing field tests and peer review. These models are gaining acceptance from design engineers regarding formwork sizing.
The Web site posts other information on SCC's effects on formwork. NRMCA advises that with conventional concrete, there are many factors governing the pressure exerted by SCC on formwork.
These include the stiffening behavior of undisturbed SCC, the casting rate, the height of the formwork, temperature, vibration, and susceptibility to disturbance of the concrete. The Web site also includes some field studies performed by University of Illinois researchers on a recent project in Peoria, Ill.
Concrete producers can play a significant role in reducing formwork pressure in their SCC mix designs. Recent studies have shown that not all SCC exerts the same lateral pressure on forms, given the same height and strength requirements. In fact, some mix designs never achieve full hydrostatic levels.
Researchers have documented that some SCC mixes change faster from a fluid state to hardened state. They have shown that mixture composition is a key factor in influencing formwork pressure.
Funded by a grant from the Ready Mixed Concrete Research Foundation, the Strategic Development Council of ACI, and the National Science Foundations, researchers at the Center for Advanced Cement-Based Materials at Northwestern University conducted a study to see how ingredients in a concrete mix design may affect stiffening in SCC.
Their focus was on the structural build-up of the concrete. The term does not refer to a construction practice. Rather, they analyzed the manner in which the cement particles grow into interlocking crystals.
They found that each SCC mix requires a different time period in which this microstructure grows. SCC mixes with a faster structural build-up will exert less lateral form pressure.
The researchers tested 23 cement paste mixtures to determine if there was a protocol suitable for evaluating a mix's rate of structural build-up. They found the rate of structural build-up was measurable using their approach.
Using this protocol, researchers were able to make three observations producers can incorporate into the SCC mix designs:
- To increase the rate of structural build-up (stiffening) at a targeted fluidity level (spread), producers should use high-range water-reducing admixtures, rather than increasing a concrete mix's water content.
- The rate of structural build-up is reduced when fly ash is incorporated in the mix.
- The overall rate of structural buildup may increase by adding silica fume to the mix. But the effect only occurs in the later stages of the rebuilding process.
References: “Rheological Method to Evaluate Structural Buildup in Self-Consolidating Concrete Cement Pastes,” R. Ferron, A. Gregori, Z. Sun, and S. Shah, ACI Materials Journal, May-June 2007.
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