Workmanship for visual in-situ concrete
Good workmanship is essential to achieve a quality concrete finish. The workforce should understand what is required and its importance.
All tools should be clean before use and all concrete handling equipment such as concrete skips, chutes and pumps should be wet before use. As concrete is placed air gets entrapped in the mix. Unless using self compacting concrete (SCC) this air should (for all concrete, not just visual concrete) be removed by compacting the concrete, usually with a vibrating poker. For visual concrete not using SCC, a high frequency constant amplitude electronic internal vibrating poker should be used. The poker should be the appropriate size for the element and be fitted with a rubber cone to prevent damage to the formwork.
For vertical elements the concrete should be placed into position in layers not more than 500mm deep along the full length of the section. If the layers are any deeper it restricts the air from escaping during compaction. The concrete should be placed where it is required and not moved by means of the vibrating poker. The poker should be immersed in the concrete sufficiently long to ensure full compaction and withdrawn slowly to avoid trapping air against the formwork. The next layer of concrete should be placed as soon as possible to avoid variations in colour between the concrete layers. The concrete should be placed at the rate of 2-3m/hour to avoid cold joints, pour planes and concrete splashes hardening on the upper form face. The concrete should be re-vibrated at the top of the pour after 1 to 2 hours to eliminate colour banding.
In horizontal elements the concrete should be placed in a systematic way. A continuous and uninterrupted supply of concrete will ensure that cold joints do not occur. For thin horizontal layers of concrete such as a floor slab the internal poker vibrator should be inserted at a slight angle and drawn horizontally through the concrete. This ensures better compaction than inserting the vibrator vertically. The position of construction joints should be agreed with the designer.
For more information, download The Concrete Centre's Visual Concrete publication.
CQ Special Issue: Visual Concrete Focus