Innovative concrete used to achieve special finish

The Challenge

Opened in late 2018 the open auditorium of DCU’s U Building forms the center piece of this state-of-the-art student facility. The design of the auditorium required that all primary structural elements be exposed with a ‘special finish’ to EN 13670:2009 Execution of concrete structures specified for the concrete elements. This special finish placed numerous requirements on the concrete beyond what would be typically expected including: 

A light colour 

Consistency of colour 

Minimal blemishes 

“DCU SU” embossed lettering 

The Solution

An architectural concrete was required to achieve the special finish and the specific design team requirements for the concrete used in the central auditorium.  

The design of the architectural concrete mix required a slightly different approach to that of conventional concrete mixes, with particular attention placed on the mix proportions of the cements and aggregatesA relatively high cement content was required with the proportion of sand in the mix not being twice that of the cement content.

In addition, a single sized 20mm coarse aggregate was usedDesigning the mix in this manner resulted in a gap graded mix and allowed for the finer materials in the mix to migrate to the formwork surface following placement and compaction, producing a high-quality finish with minimal blemishes. 

To aid in achieving uniformity of colour and finish Roadstone tested the consistency of each load of concrete prior to dispatch to site and maintained close communication with site engineers for the duration of the project.  

Completed in separate pours over a six-month period, over 20 loads of architectural concrete were supplied to DCU Hub. As well as meeting the demands of an architectural finish the concrete also met structural and durability requirements. 

Takeaways

Sample panels were poured to assess the performance of the architectural concrete mix and the workmanship required to achieve high-quality finish prior to pours commencing. This allowed for an assessment of a variety of criteria and their impact on the quality of the concrete finish. 

This aided the concrete contractor, site engineers and Roadstone concrete technologists in understanding the impact of workmanship and the concrete mix on the concrete finish. The criteria included: 

Type of formwork 

Grout-tight formwork joints 

The release agent and its application 

The method and rate of placement of the concrete 

The suitability and consistency of the concrete mix 

The compaction plan 

The most suitable time at which to strike the formwork

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