New forming technologies and materials help save time and money
November 19, 2014 by ROSS MONSOUR
Forms and forming technology have been the basis for advancements in concrete construction and the design of buildings for decades. New techniques and materials have allowed for greater flexibility in design and construction, resulting in overall time and cost savings at all levels on the job site.
More specifically, innovations in forms and form materials have allowed for flexibility in design such as architecturally finished panels that leave a final interior finish after removal. Techniques for moving forms from section to section, known as flying forms, have sped up construction times and resulted in significant savings.
These changes in materials and techniques have also created a greater need for consistency. As such, there are standards in place that address the structural requirements for forming systems. In Canada, standards for concrete formwork are specified under Part 4 of the National Building Code.
The main standard for concrete formwork is CAN/CSA–S269.3M “Concrete Formwork”. This standard dictates the rules and requirements for the design, fabrication, erection and use of concrete formwork that provides temporary lateral support or containment of freshly placed concrete for buildings or other structures. This standard is applicable to alterations or repairs to existing buildings. Falsework and scaffolding are governed by other standards.
New materials and systems are continually being developed, which in turn creates demand for better performance from supporting systems. This is particularly evident in the use of self-consolidating concrete (SCC). This product has been around for more than 20 years and has gained popularity in the last decade due to the versatility of usage. SCC has revolutionized and opened the door to a range of concrete applications that include architectural finishes after removal of the forms, repairs in areas that were unreachable in the past, bonding to existing surfaces, and placing methods that save time and money. The characteristic of SCC that makes it a unique product is the highly fluid nature of the concrete. The simplest way to describe its appearance is a soup that can be placed minimizing the need for vibration as it is a highly flowable mix that does not separate the aggregate from the paste at this viscosity.
The challenge this fluidity creates is the need for additional requirements for the formwork. Concerns with leakage become a consideration but the main structural issue is the formwork pressure that has to be addressed. Standard formwork design is predicated on a full head of hydrostatic fluid pressure of plastic concrete being placed on the job site. Unless there is field data to support a different analysis traditional design criteria must be used. As SCC is highly flowable, unlike any other concrete, the design of formwork ends up with either short walls or extremely strong formwork. This results in overdesign and added cost to the job, which in turn reduces some of the initial benefits of using SCC.
Field experience over the years indicates that the thixotrophy (the structural buildup of concrete at rest) might be better modeled based on field trials. Many prediction models have been developed in the past few years and a field trial in Sweden two years ago shed new light on the design parameters.
EllisDon Construction Science’s group, a pioneer in using SCC, has joined with other leading researchers to conduct a field study of the pressures and attempt to measure them and provide new data into their prediction models. This study was conducted from August 25 to 28 at a Toronto site. The four supporting research groups were: 1) EllisDon Construction Sciences Group and John Gardener from the University of Ottawa; 2) Kamal Khayat from Missouri S&T; 3) Ahmed Omran from the University of Sherbrooke; and 4) David Lange from the University of Illinois. Each group will be analyzing methods of formwork pressure prediction. Group 1 will be using a method using slump loss measurement; Group 2 & 3 will be using the portable vane method and Group 4 using pressure decay from a column method.
The field test will look at three levels of thixotrophy: low, medium and high. The concrete will be placed into eight identical columns at various casting rates. Two columns per day will be placed over the four-day demonstration. The casting rates have an impact on the thixotrophy of the concrete as the flowability is a function of the pressure build-up. Formwork pressure will be continuously monitored using cells mounted at various heights along the forms. The fresh properties of the concrete will be measured as part of the various prediction models and the results will be compared to the actual pressures. The objective will be to establish consistent prediction models that can be used to modify codes and standards, and result in more cost efficient construction on site.
Several of the photographs in this article give an overview of what the constructed test site looks like. The results are being analyzed and will be the subject of many forthcoming technical papers. One such presentation will take place at the Concrete Canada show on Dec. 3 in Toronto. The project will be presented by Robert Quattrociocchi, R&D engineer with EllisDon Construction. Supporting partners in this research project were BASF, CBM, Gilbert Steel, Amherst Pumping, Oxford Building Supplies, Meva Formwork Systems and Premform.
Construction research is an ongoing development. Experience on the job site is translated into design and pre-planning models, making the Canadian building industry one of the most cost effective and efficient in the world. The ability to adapt advanced thinking to new materials and systems will continue to keep EllisDon Construction and other Canadian general contractors at the forefront of development.