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Technical Resource:
Formwork
Pressure
Mixture Factors that Influence Pressure
Many factors can affect the formwork pressure generated in the process of pouring SCC. This idea is no different than that for normal concrete. Simply, the faster the material is poured, the greater the maximum formwork pressure will be. As a general rule, formwork pressure is related to thixotropic characteristics of an SCC. SCC that is strongly thixotropic—that is, it quickly gels when the material comes to rest—will display a more rapid decay of pressure, and lower overall pressures will be observed during construction.
Other factors affecting formwork pressure include mixture temperature and the presence of set modifying admixtures. As would be expected, mixtures with retarding admixtures will experience a slower pressure drop and a higher maximum pressure whereas mixtures with accelerating admixtures will exhibit faster pressure drop and a lower maximum pressure. Figure 6 shows data collected regarding the influence of mixture temperature on formwork pressure decay. As might be expected, warmer mixtures exhibited faster pressure decay. Accelerating admixtures also lead to more rapid formwork pressure decay.
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Figure 6. Effect of Mixture Temperature on Pressure Drop |
A great deal of research has gone into the influence of thixotropic properties of SCC mixtures on formwork pressure. In general it has been found that mixtures developing cohesion at faster rates will exhibit lower maximum lateral pressures for similar casting rates and as a result can be poured at faster rates for a given strength of formwork.
Aggregate bridging is one mechanism that has been identified to explain the “self-support” of SCC that occurs soon after SCC material is at rest (and well before the point traditionally defined as “set”). It is thought that as the material fills its form the aggregate will line up and touch each other to form a skeleton in the fresh concrete. As the gelation and hydration processes occur, this bridging will get stronger and will lead to a drop in formwork pressure. This mechanism continues to be studied, but there have been results to show the affect of varying coarse aggregate fractions in SCC mixtures. It has been shown that for mixtures of similar proportions of binder but varying coarse aggregate to fine aggregate ratios that the mixtures with greater coarse aggregate contents will exhibit reduced initial pressure and a faster pressure drop.
As stated earlier, thixotropy of mixtures has a significant influence on formwork pressure. Binder proportioning is one factor that affects thixotropy of mixtures. Mixtures containing supplementary cementitious materials (SCM) exhibit greater thixotropy than mixtures with cement as the only binder material and ternary blends tend to show the greatest thixotropy due to increased solid concentration in the mixture. This increased solid concentration is due to the fact that cement replacements are done on a mass basis and SCM tend to have lower densities, resulting in a greater volume of binder material and increased packing density. Accelerators increase thixotropy while retarders reduce thixotropy. VMA has also been shown to increase thixotropy.
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