PoolQ We had a spa added to an existing pool. The shotcrete was too liquid and sloughed off to the bottom. The shotcrete contractor scooped the sloughed material from the bottom with his hands and put it back on the wall. There are fissures and holes in the wall. There was also reinforcing steel close to the surface of the wall. The cold joint at the existing pool wall wasn’t prepared. They added a shotcrete seat to the existing pool over the old Marcite with no removal or roughening up of the surface. They then refused to water cure it. There wasn’t enough reinforcing steel and formwork from the pool company, so the shotcrete contractor had to stop and add more steel from steel I had lying around. So, the shotcrete sat in the truck for quite a while before shooting. We are concerned about the quality of the pool.
You are correct in suspecting quality issues with your pool. These are the specific issues that lead to poor quality, that can affect the serviceability and durability of your pool.
- Shotcrete placement requires high velocity and impact for compaction of the concrete. Hand-applying “sloughed-off” concrete would not provide proper compaction needed for producing monolithic concrete sections. The resultant fissures and voids in your pool reflect the lack of proper velocity and compaction.
- Proper preparation of the substrate is essential for good bond and creating a concrete section that acts monolithically. The surface needs to have any materials that would interfere with the bond removed, be roughened, cleaned, and brought to a saturated surface-dry condition before shotcrete placement. This article from Shotcrete magazine gives more details on how and why surface preparation is important (shotcrete.org/wp-content/uploads/2020/01/2014Spr_TechnicalTip.pdf).
- Concrete cover over the reinforcing bar is critical for maintaining corrosion protection of the embedded steel, and thus providing long-term durability. Low cover will often result in premature corrosion and subsequent spalling of the concrete cover, reducing the serviceability and life of the pool concrete.
- Curing is important for all concrete, and especially for the relatively cement-rich concrete we use for wet-mix shotcrete. Curing essentially provides additional water to hydrate the cement in the concrete, and produces stronger, less permeable concrete. Not curing concrete yields concrete that is weaker, more permeable, and ultimately less suitable for creating a watertight pool shell.
- You haven’t indicated the actual time concrete sat for “a while.” Industry standards are that concrete should be placed within 90 minutes of the introduction of water to the mixture unless special precautions are taken. Water is usually added at the ready mix plant. If concrete sits too long it can start to lose workability. At the point of losing workability, some contractors will add additional water on site over and above the concrete mixture design requirements, but this “retempering” produces concrete that is weaker and more permeable than the original design mixture.
Based on your input, you have many good reasons to ask the contractor to provide full remediation of these quality issues.
