With the widespread acceptance by specifiers and contractors of self-consolidating concrete (SCC) for construction projects in most Common Market countries, European scientists have turned their attention to analyzing how to reduce production costs. Specifically, what should be the best way to reduce the longer than normal mixing times associated with SCC?
In their presentation, “The Effect of Mixing Energy on Fresh Properties of SCC,” at the 2005 SCC Conference presented by Northwestern University's Center for Advanced Cement-Based Materials, Dirk Lowke and Peter Schiessl of the Technical University of Munich, Centre for Building Materials (CBM), focussed on producer concerns about extended mixing durations when batching SCC.
The authors began their paper by suggesting “if mixing energy is insufficient, the [workability] properties will not be achieved....The necessary mixing duration depends mainly on the mixer design, as well as the mixture proportion. Due to the low water contents relative to the powder contents and high additive dosages, more energy is required for the production of self-compacting concrete to distribute the raw materials evenly. Mixing times of 240 [seconds] are not rare in a ready mixed concrete plant. This limits the concrete output in the plant significantly, compared with common vibrated concrete and is, therefore, a substantial cost factor.”
From their initial investigations, the authors discovered that several alternative approaches could reduce SCC mixing time. Their research was directed at the influence of mixing energy on the initial consistency, as well as the time-dependent development of the fresh concrete properties of SCC. Based on these results, the mixing procedure could be optimized and the mixing time significantly reduced.
Their research focused on the basic engineering mechanics and mechanics of mixing. The authors reported, “Particle movements during mixing can be divided, in principle, into convective and dispersive transport. Convective transport is a forced, directed movement of larger portions of the mix, e.g. by the mixing tool. Dispersive transport is the random movement of individual particles due to collisions between the particles.”
Their research identified three phases that can be distinguished by the progression of the slump flow measure and power.
Phase 1 - Dispersing
“Water was added at the t0 (starting time) within 10 seconds. Fluid bonds between the particles are formed by adding and dispersing of water. Due to the surface tension of the water and the capillary pressure inside the fluid bond, the inter-particle forces increase. Therefore, the first phase is initially characterised by a significant increase in power required at the mixing tool.
“With the progress of dispersion of water and superplasticizer, a transition takes place from a grain bulk to a suspension. As soon as the particles are in a liquid environment, the capillary forces [disappear]. This is shown by the decrease in required power, which then follows. During this first phase, flowability clearly increases with increasing distribution of the raw materials.”
Phase 2 - Optimum
“The power at the mixing tool reduces asymptotically during the mixing process. As soon as a plateau is reached, a further homogenizing of the raw materials and a complete dispersion of the superplasticizer can be assumed. At this time, the flowability reaches its maximum.”
Phase 3 - Overmixing
“The further mixing energy leads to a decrease of the flowability of the concrete. Due to the convective and dispersive transport, further position changes between the particles take place. However, a significant improvement of the mixing quality is impossible. At the same time, further collisions between particles, as well as between particles and the mixing tool, continue to occur, so that the agglomerates are further disintegrated. It can also lead to a further enrichment of the finest particles due to abrasion of the coarse aggregates. Eventually, it leads to a decrease of flowability.”