Concrete is helpful in reinforcing fence posts, providing stability against the strain of wire tension and the weight of gates. The holes for the fence posts must be deep and wide enough to allow you to pour concrete in. Once the concrete dries, it acts as a solid support beneath the ground. Pouring concrete for fence posts is more difficult in cold weather, as freezing temperatures can damage the concrete as it sets up. You must take extra precautions to ensure that the concrete stays warm enough to set and harden without freezing.
Dig the hole for your fence post. If you're digging the hole for a corner post, dig it 20 inches square and 3 1/2 feet deep. If you're digging the hole for a bracing post, dig it 20 inches square and 2 feet deep. Dig holes for other posts at least 12 inches across and 2 feet deep.
Mix the fast-setting concrete mix according to package instructions, using hot water when mixing. For best results, keep the concrete mix near a heater or in a warm environment until it is ready to mix.
Place the fence post in the center of the hole and level it. Pour the warm concrete into the hole around the post. Fill the hole with at least 1 to 2 feet of concrete according to its depth.
Check the concrete at regular intervals to determine when it has set up enough to become stiff. Place a layer of plastic over the top of the concrete, then cover it with a concrete blanket. As the concrete sets, it will give off a small amount of heat; the plastic and blanket will prevent this heat from escaping into the air so that the water within the concrete doesn't freeze.
Remove the blanket and plastic once the concrete has set fully. Fill in the remainder of the hole with dirt, packing it around the fence post and concrete.