Nathan Tregger

Influence of Micro- and Nanoclays on Fresh State of Concrete

The processing of cement and concrete in its fresh state has become a crucial part of todays construction industry. Concretes such as self-consolidating concrete, shotcrete, and extruded cement-based materials have found wide application in the national infrastructure. These engineered materials rely on their rheology during the first few hours after casting to achieve superior performance. This paper discusses the use of micro- and nanoclays to tailor the balance between a concretes ability to consolidate under minimal energy before processing and to achieve shape stability afterwards. A concrete has been developed for use in slipform paving to mitigate durability issues resulting from improper consolidation. This concrete, termed “slipform self-consolidating concrete,” is one that consolidates under minimal energy yet retains shape stability after slip-forming, thus eliminating internal vibration and associated durability problems. The finished product is a pavement with smooth surfaces and straight edges without the use of formwork. In this investigation, a multi-scale approach is used to demonstrate the effects of micro- and nanoclays on the cement microstructure, the concretes early-age strength properties, and the slipform processing itself. Results show a clear relationship between all levels of analysis, and specifically they show how clays can increase shape stability with only a minimal loss in flowability.

Improving the slipform process via material manipulation

Nearly thirty-thousand miles of the U.S. Interstate Highway system has been constructed of concrete, particularly in areas of heavy anticipated traffic volumes [1]. Concrete is usually chosen over asphalt because of superior durability, visibility, traction, cost-efficiency and smoothness [2]. Another advantage of concrete pavements is that they can be rapidly constructed utilizing slipform paving techniques. The slipform paving process has made highway construction faster and more cost-effective since its development during the late 1940s. As a result, it is used extensively by the worldwide paving industry. Different from fixed-form paving, slipform paving brings together concrete placing, casting, consolidation, and finishing into one unique process. In the slipform paving process, a stiff concrete mixture with a slump less than 51 mm is placed in front of a paver. As the paver moves forward, the mixture is spread, leveled, consolidated (by internal vibrators and surface vibrators), and then extruded. After extrusion, the fresh concrete slab can hold its shape for further surface finishing and curing until the concrete sets. Because of the low consistency of the mixture, a great deal of vibration is required to remove entrapped air and consolidate the concrete. However, internal vibrators can cause over-consolidation if the vibration frequency is too high, or if the paving machine moves too slow. As a result, a significant reduction in the air content occurs in addition to aggregate segregation [3]. Both of these problems can lead to vibrator trails and even more seriously, longitudinal cracks.

Flow-induced fiber orientation in SCSFRC: monitoring and prediction

The dispersion and the orientation of fibers in concrete can be governed through a suitably balanced set of fresh state properties and a carefully designed casting procedure, if proved effective. This would allow one to achieve a mechanical performance of the fiber-reinforced cementitious composite which is optimal to the foreseen structural application, even keeping the fiber content at relatively low values (e.g. maximum 1% by volume) and aligning them with the direction of the principal tensile stress within the structural element when in service. Modeling the casting of fresh concrete, through suitable numerical tools, in order to anticipate the direction of flow lines, along which fibers may orient, and optimize the whole process to the foreseen structural application is of the foremost importance. Monitoring fiber dispersion related issues through suitable non destructive methods would also be crucial for reliable, time and cost-effective quality control. In this paper a pioneer study has been performed in the above said framework. The results are really encouraging and pave the way towards a holistic approach to the design of self-consolidating fiber-reinforced concrete structures.

Predicting Dynamic Segregation of Self-Consolidating Concrete from the Slump-Flow Test

Two key characteristics of self-consolidating concrete are flowability and segregation resistance. Quality control of flowability is typically predicted by the final diameter of the slump-flow test. In this paper, experimental results demonstrate that dynamic segregation of a self-consolidating concrete mix can also be predicted from the slump-flow test by measuring the time it takes for the flow to reach its final diameter. For a constant final diameter and aggregate content, increasing the time to final diameter led to a more stable mix. Two sets of slump-flow and segregation data were obtained for flow diameters of 65 and 70 cm, both with constant water-to-binder and aggregate-to-binder ratios. Dynamic segregation was determined by comparing the aggregate content in three regions: Within the diameter of the slump cone, between the diameter of the cone and a diameter of 50 cm, and between a diameter of 50 cm and the final diameter. In addition, the rapid penetration test was used to compare dynamic and static segregation characteristics.