Larkspur Tilt-Up Concrete Panels

Tilt-up gets sold on the picture: a crane, a panel swinging up, a wall that appeared in an afternoon. Fair enough, it's a good picture. But whether that afternoon goes well was decided weeks earlier, six inches below where anybody was looking.

Panels are cast face down on a temporary or permanent floor slab. Every dip, soft spot, and settled corner in that slab shows up in a finished wall you'll be looking at for decades. On a Larkspur site, whether that's a shop lot toward Sage Port or a bench cut in the hills above Perry Park, the pad is carved into a slope and half the ground under it is engineered fill placed last month. Getting that floor right is most of the job.

The casting slab is a mold, a work deck, and a crane pad

It has to be three things at once, and they pull in different directions. As a mold it needs to be flat and smooth, because whatever texture the slab has becomes the outside face of your building. As a work deck it carries crews, rebar, embeds, and forms for weeks. As a crane pad it has to survive an outrigger dropping serious load on a small footprint.

That last role is the one people forget. A slab poured perfectly for casting and never designed for wheel and outrigger loads can crack under the crane, which is a bad day if that slab is also your finished floor. We design for the heaviest thing that will ever sit on it, then decide whether the casting surface is temporary or permanent. Both are fine. Guessing is not.

Subgrade work that happens before the casting slab exists

• Topsoil, roots, and organics stripped out to competent material across the whole pad
• Cut and fill halves identified, with the fill placed and compacted in tested lifts
• Aggregate base spread and compacted to a uniform depth, not thicker where the dirt was soft
• Surface drainage cut around the pad so hillside runoff never crosses it
• Subdrain installed where the uphill cut face will seep after a melt
• Proof roll walked with the engineer before anybody sets a screed

Under the casting slab, and why the panels care

Water under a pad cut into a slope

Every hillside pad has an uphill cut face, and that face will seep. Not always, not immediately, but after a big melt or a wet week it delivers water sideways into the base course under your slab. Base course is the most permeable thing on the site, so it accepts that water happily and holds it. Then the subgrade softens under one part of the pad and nowhere else.

The fix is unglamorous and cheap compared to the alternative: a subdrain along the cut, a swale that carries surface water around rather than across, and a base section that's continuous so nothing dams up inside it. We'd rather spend a day on that than watch a panel casting bed go soft in April.

Weather, altitude, and the window between pour and lift

Panels have to reach a specified strength before anyone picks them up, and that number comes from the engineer running the lift stresses, not from a calendar. Cold slows strength gain, so a November casting means blankets, heated enclosures where they fit, and cylinders broken on a schedule so the decision to lift is made off data. We've stood down a crane over a low break. Nobody enjoys it. Everybody prefers it to a cracked panel.

Summer at 6,720 feet brings the other problem: fast surface drying, thin dry air, and afternoon storms that arrive with about ten minutes of warning. We cast early, cure aggressively, and keep protection staged. The comfortable stretch for a tilt-up program up here runs late spring through early fall, and if crane day matters to your schedule, book it around that window rather than against it.