The Condenser Pad That Froze the First Night
Why this matters
A condenser pad is the smallest concrete job most HVAC shops ever do, and that is the problem. Nobody thinks of five bags of mix as a pour that needs a weather plan, so it gets the weather plan of whatever the install day happens to be. When the pad fails, it fails slowly and in public: a flaking, crumbling slab under brand-new equipment, noticed by the customer a few weeks later and blamed on the install. This case works through what the pad could have failed from, how freeze damage was told apart from the cracking every slab gets, and what the redo was.
The pad
A two-person crew replaced a condenser on a mid-November afternoon. The old unit had sat on a sunken, tilted pad, so they broke it out and poured a new one in place: a 36 by 36 inch pad, 4 inches thick, on a compacted gravel base, in a simple wood form. That is 3 by 3 feet by a third of a foot, or 3 cubic feet. At about 0.6 cubic feet of yield per 80-pound bag of general-purpose mix, it took 5 bags, mixed in a wheelbarrow with water from the outside hose.
They poured at about 2:00 p.m. with the air at 41 F, finished it by 3:15, pulled the form the next morning and set the new condenser on it at 9:00 a.m. Nothing covered the pad overnight. The forecast low was 27 F; the low that night was 26 F at about 6 a.m., under a clear sky.
Four weeks later the homeowner called. The top of the pad was flaking off in thin layers, two corners had broken away where the condenser's feet sat near the edge, and the edges crumbled when she pushed at them with a shoe.
Five candidates
The lead went out with the job notes, a steel nail, a hammer and the homeowner's photos from the weeks since.
Too much water in the mix. Bag mix overwatered for easy placing makes weak concrete everywhere, and a weak pad can dust and wear at the surface. The crew had measured water with a marked bucket to the bag's instructions, and the evidence on the pad pointed elsewhere: when the lead struck the middle of the pad with the hammer, it rang hard, and the nail would barely scratch the concrete a half inch down at a broken corner. Overwatered concrete is soft through its depth. This was soft only at the surfaces. Not the main cause.
Bleed water worked into the surface during finishing. Troweling while bleed water is still on top, or sprinkling water to ease the finish, makes a weak top layer that can scale. It is a real cause of scaling and it was worth asking about. But the vertical sides of the pad, which never saw a trowel and were behind a form until morning, were as soft and crumbly as the top. Finishing does not reach a formed edge. Not the cause, though it could not be ruled out as a small contributor to the top.
Deicing salt. Salt on young concrete through freeze-thaw cycles is a classic scaling cause. The homeowner had used none anywhere near the unit, and the first photos of the flaking predate any snow. Removed.
Loading too early. The condenser was set on the pad at about 19 hours old. Its feet sit on four small spots, and two broken corners were under them. That damage is real, and it is part of the story. But the flaking covered the whole top, including the open center the unit's feet never touched. Early load explains the corners, not the surface.
Freezing in the first night. This one had evidence on its side from the first morning. The homeowner's photo at 7:15 a.m. on the day after the pour, sent at the time only to show the pad was finished, shows a sparkle of frost across the surface and faint feathered patterns in it. Those feathered marks are what ice crystals leave when they form in the surface of fresh concrete. The weather record put the air at 26 F at 6 a.m., and a bare horizontal slab under a clear sky runs colder than the air.
How freeze damage differs from shrinkage cracking
Every concrete slab cracks a little, and the first thing a homeowner sees is often a crack, so a shop has to be able to say which kind it is looking at. The two common shrinkage cracks look nothing like what this pad did.
| What you see | Plastic shrinkage cracking | Early freezing |
|---|---|---|
| When it appears | Within hours of finishing, while the concrete is still plastic | Visible as surface damage over days to weeks, worsening with each thaw and refreeze |
| What it looks like | Short, fairly straight cracks, often roughly parallel, in a surface that is otherwise hard | No crack pattern to speak of; the surface flakes, scales or turns powdery, and edges and corners crumble |
| What causes it | Water leaving the surface faster than bleed water replaces it: wind, low humidity, warm concrete under dry air | Water in the paste freezing before the paste is strong enough to resist ice forming in it |
| Scratch test with a nail | Concrete between cracks resists the nail | Surface paste scrapes away easily, often to a quarter inch or more |
| Where it is worst | Across the finished face | The top and the edges, the surfaces that lose heat fastest |
Drying shrinkage cracking is a third kind, appearing weeks or months later as a few straight cracks right through a slab whose surface stays sound; a pad this size usually has none, because it is short enough to shrink without cracking. Nothing on this pad matched either shrinkage pattern. It matched freezing in every row.
Section through the failed pad (not to scale)
top: scaled, soft to about 1/4 in
|
+--v------------------------------+
|:::::::::::::::::::::::::::::::::| <- frost side
|: :|
|: core: rings hard, resists :|
|: the nail :| <- sides soft too
|: :| (no trowel here)
+---------------------------------+
^ corners: broken, soft to 1/2 in or more
gravel base
The pattern is the confirmation. Damage concentrated where heat left fastest, top and edges, and the core that stayed warmest the longest was sound.
Why a small pad freezes when a driveway might not
Fresh concrete makes its own heat as the cement hydrates, and a large pour holds that heat in its mass. A small pad has very little mass and a lot of surface. This one had 3 cubic feet of concrete and 13 square feet of exposed surface counting the top and the four sides once the form came off, about 4.3 square feet per cubic foot. A 20 by 20 foot driveway slab 6 inches thick has 200 cubic feet and about 440 square feet of top and edges, roughly 2.2 square feet per cubic foot. The pad lost heat through about twice as much surface for each cubic foot of concrete, from a mix made with cold hose water on a 41 F afternoon, and then sat under a clear sky. It had little chance of reaching any useful strength before the surface froze.
The industry reference for this is ACI 306, the American Concrete Institute's cold weather concreting guide, which binds no one unless a specification or contract adopts it but is what most suppliers advise from. In substance it says fresh concrete should be protected from freezing until it reaches a compressive strength of about 500 psi, which is the point where the paste can resist a single freeze. It also recommends a minimum concrete temperature, as placed and while protected, for thin sections that is in the mid 50s F. This pad was poured near 45 F at best and was left bare in a night that went to 26 F.
What the supplier would have said
After the callback the lead called the yard that sold the mix and asked what they tell customers pouring small pads in November. The answer was the standard one for small cold-weather pours, covered in full in Concrete for the Small Pour in Cold and Hot Weather:
- Do not pour on frozen ground or into a frozen form, and do not pour if the forecast night will freeze unless you can protect it.
- Mix with warm water, not hot, and keep the bags out of the cold until use.
- Use a high-early-strength or cold-weather mix, or an accelerator. Where steel or anchors are cast in, an accelerator without chloride, since chloride accelerators corrode embedded steel.
- Cover with insulating blankets as soon as finishing allows, and leave them on for at least the first couple of days; tuck them down over the edges, which is where heat leaves.
- Do not load it the next morning.
For the condenser side, the supplier added a line HVAC shops already know: a precast or composite equipment pad on a compacted gravel base needs no cure at all.
The redo
- Isolate the condenser before anyone touches it. The tech switched off the unit's breaker, pulled the disconnect, locked the breaker, and proved the unit's line terminals dead with a meter he checked on a known live source before and after.
- Move the unit without stressing the line set. With two people, the condenser was lifted a few inches and set on blocks beside the pad, keeping the refrigerant lines in a gentle sweep so no brazed joint or service valve took the weight, and checking for any oil spot at the joints afterward.
- Break out the pad with dust and fragments controlled. Breaking concrete makes respirable crystalline silica, so the pad was kept wet while it was broken with a sledge and a pry bar, and the crew wore eye protection and respirators. Where power tools are used, OSHA's construction silica standard, 29 CFR 1926.1153, sets the controls for each tool in its Table 1, which binds the employer for its employees.
- Check the base. The gravel under the old pad had not heaved. It was releveled and compacted.
- Set a composite pad, leveled, and the condenser set back on it, its feet inside the pad's edges this time.
- Restore power last. The tech checked the line set and connections, confirmed the disconnect was fully seated and the unit's panels were on, removed his lock, restored the breaker, and ran the system through a full cycle, checking the operating readings against the install record.
The original pad took about 2 crew-hours. The redo took about 4.5, roughly 2.25 times the original, and the shop absorbed all of it.
The shop's rule now: a poured equipment pad needs an hourly that keeps the air above 40 F for 48 hours after the pour, or the crew brings insulating blankets and leaves them on for that time; otherwise the job gets a precast or composite pad. The steps for protecting a small pour that must go in are in How to Protect a Small Concrete Pour From a Freeze.
References
- ACI 306, Guide to Cold Weather Concreting, American Concrete Institute; binding only where a project specification or contract adopts it.
- 29 CFR 1926.1153, OSHA respirable crystalline silica in construction, including Table 1 task controls.
- Concrete mix manufacturer's bag directions for water, temperature and yield.
- See related: Concrete for the Small Pour in Cold and Hot Weather
- See related: How to Protect a Small Concrete Pour From a Freeze