We are currently working on a project for a new engineering building in Surrey, BC for Simon Fraser University. A key part of the building’s architectural expression is an undulating façade element that recalls the appearance of printed electronic circuit boards. In order to ensure long term durability and reduce maintenance concerns, we have chosen to express these elements using precast concrete cladding.
Typical to any precast panel installation, the fabricator will provide a sample of the final mix ratios and casting process to showcase the range of finish expected on the completed façade. Recently our fabricator provided us with three 500 x 500mm samples of the formliner to be used on the façade of the SFU SE3P project in Surrey.
Each of the samples utilized the same mix and were cast on polyurethane rubber liners. The only difference in the casting process was the time spent vibrating the cast in order to achieve proper concrete consolidation. On a simple panel, this process is fairly straightforward and often as an industry we take the ‘tried and true’ methodology for achieving predictable results.
However, with this project’s undulating formliner, the team was anxious to make sure the formliner as designed did not have any adverse impacts on the consolidation of the concrete mix. We looked at three options which ranged from under-vibrating to over-vibrating the concrete to determine which methodology would best meet our performance requirements. Critical to our approach is coming at this exercise from first principles, allowing us to consider each result objectively.
Sample 1: Under Vibration
The under-vibrated sample displayed noticeable bugholes on the finish surface. While the colour across the sample was consistent, the bugholes are undesirable because their appearance mars the finished concrete with unanticipated shadows along the face. In typical concrete installations, this usually isn’t noticeable. However, in this application, the bugholes may appear directly at the peak of the soft ridges moving vertically across the panel, destroying the integrity and continuity of the shadows.
Sample 2: Standard Vibration
The standard vibrated sample was the most consistent of the three. The finish was mostly absent of the bugholes found in the under-vibrated sample and had good colour consistency across the face. Any bugholes that did appear on the sample were usually very small.
Sample 3: Over Vibration
The defining feature of the over-vibrated sample was the exposure of the concrete aggregate. The aggregate for the façade panels is a standard grey gravel as white aggregate would have had to be shipped from a far distance. Over-vibrating the cast caused the aggregate to settle at the bottom of the pan. When the form was removed, small portions of the aggregate showed through. Because of the aggregate colour and its contrast with the white cement, the material takes on a bluish hue more akin to stone than concrete.
As concrete weathers, the outermost layer of cement tends to erode, exposing some of the aggregate and ultimately leading to a development of a somewhat patchy patina. Due to its uniform exposure of aggregate, we believe this over-vibrated version will weather more evenly than either of the previous samples.
The decision was to use the over-vibration method and this will be the process used for this project. It was not what we expected at the start of the process. Given the impact of the pre-cast façade on the overall composition of the building, this process was critical in giving the design team and client the information needed to ensure the design intent was met.