How to Mix Bagged Concrete

What you need

• Bagged concrete mix. Use a mix intended for your job, such as a general-purpose mix or a fast-setting post mix.
• Clean water in a bucket or hose, and something to measure it with.
• A wheelbarrow, mixing tub or a rented drum mixer.
• A hoe or a mixing paddle, and a square shovel.
• Gloves, eye protection, a dust mask and long sleeves.
• Forms, base and tools ready before you start.

Prepare the site before you open the first bag. Concrete does not wait, and the mix starts to set once water is added. Check the bag for the quantity of water it requires and how long the working time is.

How many bags will I need?

A 4 × 4 ft slab at 4 in thick needs 5.33 ft³, which is 10 bags of 80 lb mix with 10% waste. Each 80 lb bag yields about 0.60 ft³. Use the bag calculator to work out the number and the pallet calculator if you are buying many. Mix only what you can place within the working time, normally a bag or two at a time by hand, or more in a drum mixer.

Step-by-step mixing in a wheelbarrow or tub

1. Pour the dry mix into the barrow. Make a hollow in the middle.
2. Measure about three-quarters of the water recommended on the bag and pour it into the hollow.
3. Draw the dry mix into the water with the hoe. Chop and fold until all the dry mix is wet.
4. Add the rest of the water slowly, in small amounts, until the mix is uniform and holds its shape without being soupy.
5. Mix for a few minutes. Dry lumps in the middle are a common error.
6. Check the consistency. It should be like thick oatmeal or stiff peanut butter, and should slump a little but not flow like batter.
7. Place it right away.

Mixing in a drum mixer

A drum mixer turns the work into a machine. Add most of the water first, then the dry mix, then the remaining water as the drum turns. Mix for about three to five minutes or as the product instructions say. Do not overfill the drum. Clean the drum between batches so hardened concrete does not build up.

How much water?

The bag gives the amount, usually in quarts or pints per 80 lb bag. Follow it. Too much water makes concrete easy to pour but weaker, more likely to crack and prone to dusting and scaling. Too little makes it hard to place and may leave dry pockets. If the mix is too stiff, add water in very small amounts and mix again. If it gets too wet, add a little more dry mix rather than leaving it as it is.

Working time and temperature

• Standard mixes remain workable for around an hour in moderate weather. Fast-setting mixes set in minutes, so mix one bag at a time.
• Hot weather shortens working time. Use cool water and shade the mix.
• Cold weather slows setting. Do not pour on frozen ground or when freezing is expected before the concrete has hardened.
• Do not re-temper concrete that has begun to set by adding water.

Placing, consolidating and finishing

Wet the forms and base lightly if they are dry. Shovel the concrete into place and use a rake or shovel to work it into corners. Tamp or rod it to remove air pockets. Screed it level, float it and finish as in the slab guide. See how to pour a concrete slab. For a post hole, add the mix in lifts and keep the post plumb while it sets. See concrete for fence posts.

Safety

Wet concrete is alkaline and can cause chemical burns if it stays on the skin. Wear gloves and rubber boots and rinse with water at once if it gets on you. Dry cement dust is irritating to the eyes and lungs, and many mixes contain crystalline silica, so wear a dust mask when opening bags and mixing, and wet the mix rather than shaking it. Lift with your legs, not your back. An 80 lb bag is a heavy load.

Cleaning up

Wash tools before the concrete sets. Do not wash residue into storm drains. Let the leftover mix harden in a safe place and dispose of it as local rules allow. Store unused bags dry, off the floor and out of the weather, since moisture will harden the mix inside the bag.

Troubleshooting a batch

Choosing a bag for the job

• General slab or footing: standard concrete mix.
• Post setting: a fast-setting mix, mixed in the hole or according to its label.
• Thin repair: a patching or sand mix.
• Crack control: a mix with fibers.
• Laying block or brick: mortar, not concrete.

A routine for a pour with bags

1. Set up the work area with the bags, water and tools near the forms.
2. Divide the job into batches and assign a person to each task: mixing, hauling, placing and finishing.
3. Mix a batch while the previous batch is being placed.
4. Place each batch against the previous one before the first has begun to set.
5. Stop mixing when you have enough, and finish the surface before it stiffens.
6. Clean up and start curing at once.

Frequently asked questions