Concrete for the Small Pour in Cold and Hot Weather

Why this matters

Most shops that pour concrete are not concrete contractors. A fence crew sets post footings, an HVAC shop pours a pad, a handyman replaces a cracked sidewalk flag or patches a step. These pours are a few bags, done between other work, and they fail in ways the crew never sees: a patch that scales in its first winter because it froze its first night, a pad with a web of surface cracks because it baked in the July sun and wind before anyone covered it.

Small pours are actually more exposed to weather than big ones, because a thin slab or a narrow footing has little mass to hold its own heat and a lot of surface to lose water from. This card is the weather side of the small pour for the trade that does it occasionally. The full practice belongs to the concrete trade's own cards: 'Cold-Weather Concrete' and 'Hot Weather Concreting per ACI 305'.

What weather does to fresh concrete

Concrete hardens by hydration, a chemical reaction between cement and water. It does not dry hard; it reacts hard, and the reaction needs both water and warmth for days after the pour.

• Cold slows the reaction. Strength gain slows sharply as the concrete cools, and in a cold slab it can nearly stall. The concrete is not ruined by being cold, it is just not getting stronger.
• Freezing early damages it. Fresh concrete is full of water. If that water freezes before the concrete has gained enough strength to resist the expansion of ice, the internal structure is damaged and the surface is often permanently weakened. The cold-weather standard, ACI 306, uses 500 psi compressive strength as the point the concrete must reach before it is allowed to freeze. A small pour reaches that in days in mild conditions, longer in the cold.
• Heat speeds it. Hot concrete sets fast, which shortens the time the crew has to place and finish it, and strength gained very fast tends to be lower in the long run.
• Sun, wind and dry air pull water out of the surface. If the surface loses water faster than the mix brings it up (the bleed water), the surface shrinks while the concrete beneath is still plastic, and it cracks. These are plastic shrinkage cracks, short, often parallel, appearing within hours of finishing. Then, for days after, a surface that dries out stops hydrating and ends up weaker and dustier than the concrete below it.

So cold weather is about keeping the concrete warm enough, long enough, and never letting it freeze early. Hot weather is about keeping it wet and slowing its water loss. Both are about the days after the pour, not the hour of it.

Never on frozen ground

The first rule of cold-weather pouring is about what the concrete sits on. Concrete placed on a frozen base loses its heat into the ground from below, freezes from the bottom, and then sits on a base that softens and settles when it thaws. Never pour on frozen ground, frozen gravel, or a base with ice or snow in it. Thaw the base first, with an insulating blanket left over it overnight or a ground-thaw blanket, and check it with a probe before the mix arrives. How frozen ground behaves under a footing or a pad is covered in Frozen Ground and What It Does to Digging and Setting.

The cold-weather line and why thin pours have less room

ACI 306 treats it as cold weather when the air temperature has fallen to, or is expected to fall below, 40 F during the protection period. That is the trigger to plan protection, not a temperature below which pouring is forbidden.

For thin sections, under 12 inches in their least dimension, which covers every sidewalk flag, pad and step patch, ACI 306 calls for a minimum concrete temperature of 55 F as placed and maintained during protection, a higher figure than it sets for massive sections, because a thin section loses heat fastest. (Use the edition of ACI 306 your job or local practice follows.) In practice, for a small pour from bags, that means warm mixing water within the bag's instructions, materials stored warm, the pour early in the day so the concrete has the warmest hours, and insulated blankets on before the temperature falls, not after.

There is no antifreeze for concrete. Accelerating admixtures and high-early mixes make concrete gain strength faster, which shortens how long it is vulnerable; they do not make it safe to freeze.

Worked example: one sidewalk flag, two seasons

A handyman shop replaces one 4 by 4 foot sidewalk flag, 4 inches thick, from bagged concrete. The volume is 4 feet times 4 feet times 4/12 foot, which is 5.33 cubic feet. An 80-pound bag of a common mix yields about 0.60 cubic feet, so 5.33 divided by 0.60 is 8.9 bags; with an allowance for waste and base irregularity the crew brings 10 bags, which yields 6.0 cubic feet, about an eighth more than the slab needs. The same job lands on the board twice in one year.

In November

The forecast for pour day is a high of 46 F, a low of 29 F overnight; day two, high 42 F, low 27 F; day three, high 50 F, low 35 F. Every night of the three falls below the 40 F line, two of them below freezing, so this is a cold-weather pour from start to finish.

1. The base. When the old flag comes out the afternoon before, the gravel base under it is frozen about an inch deep at the edges. The crew lays an insulated blanket over the opening overnight. In the morning a probe goes into the base easily and it is not frozen. Had it still been frozen, the pour would have waited.
2. Timing. The pour starts at 10 a.m. with the air at about 41 F and rising toward the 46 F high. The bags have been in the shop overnight, not the truck. Mixing water is warm, within the range the bag's instructions allow. The slab is finished and tooled by about noon.
3. Protection. By 1:30 p.m., well before the temperature begins to fall, the slab is covered with plastic sheeting to hold moisture and an insulated concrete blanket over it, lapped past the edges onto the surrounding walk so the edges, which lose heat fastest, are covered.
4. Duration. The blankets stay on through all three nights. Nights one and two go below freezing, and the slab has not had long enough in cold conditions to be safely past early strength. Night three, at 35 F, is the first that does not freeze. The blanket comes off on the morning of day four. The length of protection is set from the cold-weather guidance and the forecast, not from how hard the surface feels; a slab can feel hard long before it is past the point where freezing would harm it.
5. The first winter. The customer is told in writing not to use de-icing salt on the new flag this winter; a new slab is most vulnerable to surface scaling from de-icers in its first season. Sand gives traction without the damage.

In July

The forecast for pour day is a high of 93 F, humidity around 30 percent in the afternoon, wind 10 mph, full sun on the walk from mid-morning on. Here the risk is water leaving the slab too fast.

1. Timing. The pour starts at 7 a.m. at about 74 F, so the placing and finishing happen before the sun is on the walk and before the afternoon wind.
2. The base. The gravel is dampened just before placing so it does not pull water out of the bottom of the slab, with no standing puddles left in it.
3. The mix. The crew adds only the water the bag specifies. Extra water makes a mix easier to work on a hot morning and makes a weaker, more crack-prone slab. Nobody sprinkles water on the surface to help finishing; water worked into the surface weakens it. If the surface is drying faster than it can be finished, a fine fog sprayed into the air above it, or a windbreak and shade, slows the loss.
4. Curing. As soon as the surface can take it without marking, the slab is covered with wet burlap under plastic, or coated with a curing compound, and kept damp for several days per the bag's curing instructions, commonly about a week for ordinary mixes. In 93 F sun and 10 mph wind, an uncovered 4-inch slab can lose surface water fast enough to crack within hours of finishing.
5. The crew. A July pour is also heat exposure for the people doing it; see related: Heat Illness Prevention (OSHA NEP).

The same 10 bags, the same flag, the same crew. In November the job is keeping heat in for three days. In July it is keeping water in for a week. In neither case is the hour of the pour the part that decides whether the slab holds.

Hazards in a small pour

• Wet concrete burns. Fresh concrete is strongly alkaline and causes chemical burns that may not hurt until the damage is done, especially when it soaks into clothing or gets inside a boot or a glove. Waterproof gloves, boots, eye protection, kneeboards rather than kneeling in it, and rinse any skin contact promptly with clean water.
• Dry mix dust. Opening and dumping bags raises dust that carries cement, which irritates eyes and lungs, and crystalline silica from the sand and stone. Mix outdoors and upwind, pour bags low into the mixer rather than from height, and use a respirator under a written 29 CFR 1910.134 program where the dust cannot be kept down.
• Breaking out the old slab. Cutting or breaking concrete releases respirable crystalline silica. Under the construction silica standard, 29 CFR 1926.1153, Table 1 sets the controls for common tasks, such as water delivered at the blade for a handheld saw; follow the row for the tool in use.
• Cold. A November pour means hours kneeling on cold ground with wet hands; see related: Cold Stress, Frostbite, and Hypothermia.

References

• ACI 306, Cold Weather Concreting, and ACI 305, Hot Weather Concreting (in the edition your local practice follows)
• Bagged concrete manufacturer instructions (water, yield, curing)
• 29 CFR 1926.1153, Respirable Crystalline Silica (construction); 29 CFR 1910.134, Respiratory Protection
• See related: Cold-Weather Concrete; Hot Weather Concreting per ACI 305
• See related: How to Protect a Small Concrete Pour From a Freeze; Frozen Ground and What It Does to Digging and Setting