Dene Cook, Technical Manager for Fletcher Building has been in the industry for more than 30 years and also has plenty of wisdom to share.
“Preparation is certainly the key point I would make when getting these pours right,” he says. “The formwork and the mixes are crucial.”
Dene suggests the following when undertaking insitu pours:
- The quality of the formwork dictates the quality of the finished surface. Defects in the formwork will reflect through to the final concrete finish.
- Concrete pressures can be considerable, the formwork needs to accommodate these pressures. Sometimes the forms are designed on the basis of an assumed fill rate. If this is the case then the information needs to be communicated to all.
- Form release agents can make a considerable difference to the final finished surface. Seek specialist advice. Oil based form oils tend to trap air and result in a greater number of bug holes. Bug holes are simply air pockets trapped against the formed surface.
- Plan the concrete pour so that delivery occurs at a rate consistent with the placing resources on the site.
- Consider back up equipment such as vibrators should breakdown occur.
- When filling forms avoid the concrete hitting reinforcement and forms on the way down. This invariably results in segregation. Use an elephant truck (flexible material that collapses flat when suspended), typically the truck diameter should be 6 times the maximum aggregate size.
- Place concrete as near as possible to final position. Do not move horizontally with vibrators. This can cause flow lines and discolouration.
- Take care placing concrete around box outs as air voids can occur underneath these. Place concrete from one side and provide air holes in the bottom boards (where practical).
- Place the concrete in uniform lifts of approximately 1.2m. Vibrate keeping vibrator vertical. Ensure the vibrator penetrates the lower lift by about 150mm. Observe the surface to determine the radius of effectiveness of the vibrator. Ensure that vibration overlaps.
- Do not let the vibrator touch the formwork as it will result in discolouration. Also avoid contact with the reinforcement.
- Ensure that forms are watertight to avoid grout loss and sections of boney concrete.