In response to a blog concerning how ACI is defining concrete surface finishes, Chad Godwin, Godwin Forming Systems, provided these tips.
Concrete finishes are determined by the mix design, method of placement, and curing method. The formwork has to be accurate and you must use the proper materials but all the money spent there is wasted if the architect and GC aren't willing to pay for proper concrete placement.
Architectural concrete finishes can cost up to three times more than a Class C finish. When the bids hit they always accept the Class C but hold the concrete contractor to the Class A guidelines. It’s like paying for a hamburger and wanting the cheese for free.
I recommend the following when placing architectural concrete:
- To solve the color variation between trucks and get an accurate slump, have the trucks mix at the site for 1 minute per yard then sit with the barrel not turning for 30 to 45 seconds to allow the trapped air to escape. The driver will argue and tell you that it has been spinning en route, but it has not been spinning at full speed and has sloshed around more than mixed.
- When pumping you should make sure the driver keeps the concrete above the hopper so the paddles can’t bring in more air.
- My preferred mix design is a slump of 9 inches plus or minus 1 inch with a water/cement ratio of 0.40 maximum. The rest of the slump is produced with superplasticizer, not water. Water will just flow straight to the bottom and leave the rock behind.
- For 8-foot or higher walls, it is sometimes necessary to add a VMA (viscosity-modifying admixture). This prevents segregation by keeping the paste thicker which keeps the rock suspended.
- Concrete will also start to segregate after 20 feet of travel in the pump lines, so limit the length of the pump lines as much as possible.
- Keep the pump hose buried in the concrete about 12 inches using a long tremie pipe. This is the single most important thing if you can’t do anything else. Do not ever let the concrete fall more than 1 foot. This will cause immediate segregation of the rock and create air pockets. Think of pouring a Dr. Pepper in a glass. If you just dump it in, it will fizz up because the carbonation is trying to get to the top and trapping itself. Sometimes it is worth making the wall thicker just to have room to get a 4- to 6-inch hose to the bottom. You should not have to relocate the pump either. With this mix design the concrete will flow and the pump truck should let it flow by itself instead of constantly relocating the pump hose. The concrete will push the hose up letting the operator know to move it.
- Vibrating should mainly be done with the vibrator being pulled up not going down. The vibrator should be in the off position on the way down and then pulled out a rate of 1 foot per 2 seconds. Avoid a constant up and down motion. This is how the vibrator gets stuck between the rebar and form. Plus it will push the rock to the bottom and the paste out to the sides.
- To help seal the plywood edges, I recommend double-facing forms and using a 3 mm weather strip between the plywood joints. This keeps the paste from leaking out and leaving the sand behind which is what causes the dark spots at your form joints. I like to use ¾-inch BB plywood then stagger my joints with ½” HDO plywood.
- Lastly I prefer Olympic multi pour HDO plywood. It has a matte finish which is more forgiving and allows the air to dissipate. I have had problems with other form panels causing tiger stripping and dye coming off the film and staining the concrete.
Godwin Forming Systems