Replacing a Beach-House Foundation - Continued
The main additive was the fly ash, a waste product that comes from the dust collectors in coal-burning power plants. A fine powder consisting mostly of glass particles, fly ash can be substituted for some of the cement in a concrete mix. Our local concrete plant has been doing this for years because fly ash is cheaper than cement and has properties that make for a better finished product. As the owner of a "green" building company, I like fly ash because using it reduces the amount of cement I need; cement production is a major source of the carbon dioxide that contributes to global warming. Also, fly ash typically ends up in landfills, so using it for concrete diverts it from the waste stream.
Thus, to minimize shrinkage, we replaced 35 percent of the cement with fly ash. And to reduce cracking, we used Degussa's Polyheed 997, a water-reducing admixture (less water means less cracking), and Pozzolith 300R, a water-reducing plasticizer (Degussa Admixtures, 800/628-9990, www.degussa.com). With these additives, we could limit the amount of water in the mix yet still have concrete flowable enough to pump and place.
As an extra precaution, we added fiber to reduce cracking still further and used a corrosion-inhibiting admixture — another Degussa product — called Rheocrete CNI.
Pouring the Foundation
Since there wouldn't be room to bring in a drill rig until the pile of excavated soil was used to backfill the grid, we planned to pour the house foundation separately from the deck piers. A couple of days before the pour, the structural engineer and the city inspector inspected the rebar and forms. On the day of the pour, a special inspector from an independent testing company showed up to observe; he verified that we followed proper procedures and saw to it that test cylinders were made from the concrete mix.
It took eight trucks all morning to deliver the 77 yards of concrete needed to pour the grid and house columns. The design strength of the mix was 3,000 psi, but when the cylinders were tested at 28 days, we found that the concrete was already at 6,000 psi and would likely continue to gain strength for years.
Access was poor, so the crew used a pump to place concrete for the grid.
Access was poor, so the crew used a pump to place concrete for the columns.
Pouring the columns tight to the bottom of the column caps wouldn't have been practical, because when we vibrated them, the concrete would consolidate and separate from the caps. So we poured the columns an inch or so short, then grouted them tight to the bottom of the brackets with high-strength grout once the concrete had set.
The threaded rod coming down from this column cap is attached to a large nut and washer that will be embedded in the concrete. The Sonotube was lifted to within an inch or two of the beam before the concrete was poured, and the gap was later packed with grout.
Because the column received a stuccolike waterproof coating, it's not possible to see the joint between the concrete and the grout.
After about a week of cure time, the foundation was strong enough to support the cottage, so the house mover came back and removed the cribbing. We all breathed a sigh of relief once the cottage was firmly bolted to the new foundation. With the cribbing out of the way, we were able to bury the grid with the excavated material and begin working on the individual piers that would support the decks.
The grid and columns are both visible.
Once the foundation was backfilled, only the columns could be seen.
Friction Piers for the Deck
It had always been part of the plan to rebuild the rear deck on new concrete piers. The old piers and piles weren't worth saving, and with no obstructions overhead it would be simple to get a drill rig into position.
But end-bearing piers — what you get when you bury a Sonotube and fill it with concrete — weren't practical in this situation. With the high water table, we couldn't pour an end-bearing pier in such iffy soil without first inspecting the bottom of the hole. And even if the hole were dry, it would need to be large enough for someone to climb inside and inspect the bottom. In addition, the low bearing capacity of the soil would require large-diameter end-bearing piers to provide the necessary support.
We decided to use friction piers instead. A friction pier is "gripped" by the surrounding soil, so its bearing capacity depends on how much of it is in the ground — the deeper the pier, the more it will carry. The soils engineer determined that the friction capacity of the soil was 300 psf. Given the local wind and seismic conditions, and knowing we wanted to use 16-inch piers, the structural engineer designed them to go 12 to 15 feet below grade.
Drilled cased piers. In firmer soil, we could have drilled the holes, dropped in the steel, and gone from there. But with loose soil and a high water table, we had to drill and case the piers. Casing is a steel pipe that prevents a hole from caving in. It's big enough for the drill bit to fit through and is pushed into the ground as the hole is being drilled; it's pulled back out as the concrete is placed. Removing the casing allows the concrete to "key" into the soil, thereby creating the necessary friction. If the casing is not removed, the pier will have less load-bearing capacity.
At each pier location, the drilling contractor drilled several feet down, retracted the bit, and used the rig to push a 4-foot casing most of the way into the hole. He tack-welded a second section of casing onto the first, reinserted the bit, and drilled deeper before pushing both casings farther into the ground. This continued until the holes were 12 to 15 feet deep and contained multiple sections of casing, with the top one projecting a few inches above grade. While the casing kept the hole from caving in, it didn't prevent ground water from coming halfway up inside.
Here, the drilling contractor prepares to drill the first pier for the rear deck structure. The metal cylinders in the left foreground were used to case the hole.
Pouring the piers. As with the cottage columns, we had rebar cages for the piers fabricated and hot-dip galvanized off site. On the day of the pour, the drilling contractor redrilled the bottom of each hole to clean it out, placed the rebar cage, and began filling the hole with concrete. Because the holes were half filled with water, he put a tremie pipe on the concrete pump hose, inserted it into the hole, and pumped from the bottom up, making sure to keep the nozzle below the surface of the liquid concrete.
Before pouring a pier, the drilling contractor redrilled the bottom of the hole to make sure it hadn't filled up with silt.
Like the columns, the piers were heavily reinforced with galvanized rebar cages.
Since concrete is denser than water, it forced the water and silt up and out of the hole without diluting or mixing with the concrete. As each hole was being filled, the drilling contractor vibrated the concrete and pulled out the casing, one section at a time. When the process was complete, the rebar cages projected several feet into the air and were encased in concrete up to grade.
The crew pumped from the bottom up to prevent concrete from mixing with the water in the hole.
The casing was pulled out as the concrete was poured. A welder cuts off a section that had just come out of the ground.
The silt pouring out of the top indicates that this hole was nearly filled with concrete.
Finishing up above grade. It's impractical to pour the above-grade and below-grade portions of a friction pier in a single operation. While the lower end requires no form because it's surrounded by soil, the upper end requires a Sonotube, but there's no way to brace it into position while the drilling crew is working on nearby piers. So we had to pour the piers in two separate operations.
After the lower sections had set, our crew braced the deck beams into position and tacked stainless steel column caps onto them. Then all the drilling contractor had to do was apply a bonding agent, Rezi-Weld 1000 (W.R. Meadows, 800/342-5976, www.wrmeadows.com), to the top of the pier, flex the rebar off to one side, slide a Sonotube over it, and finish the pier by pumping the Sonotube full of concrete. That way, there was no possibility the piers would not line up with the beams or that the mounting brackets would be set at the wrong height.
Once the piers were up to grade, members of the crew braced the deck support beams into position and installed the column caps.
Then they formed up to the beam with Sonotubes and poured the rest of the way.
Once the concrete was set, we stripped the Sonotubes and grouted up to the column caps before bolting them permanently in place. As an added defense against the elements and to give the exposed portions of the foundation a uniform appearance, we coated the columns, piers, and grout with Super Blockade (Merlex Stucco, 714/637-1700, www.merlex.com), a cementitious waterproof coating. After that, finishing the cottage was a regular remodeling job.
Jeff Morosois an owner of Moroso Construction Inc. in Pacifica, Calif.