Printed from: https://conservationhandbooks.com/concrete/
Health and hygiene
Fresh cement is caustic and rapidly damages unprotected skin. It may also cause dermatitis. Use a gauntlet style of PVC glove, and avoid gloves with cotton cuffs.
Care of tools and mixer
Concrete sets rapidly and extra water hastens the process. Do not leave tools to ‘soak’ in water. Keep a bucket of water and a brush handy, and wash tools completely once an hour and before every break.
After emptying a mixer, put the water for the next batch in immediately, and leave the mixer running. A shovel of aggregate will help keep the drum clean. On hot days wash the mixer out before lunch. At the end of the day, start to clean out the mixer well before work finishes. Use plenty of water and aggregate to clean the drum. The blades may need knocking occasionally with a hammer to remove encrustations. Wash off the mixer with water and a brush. Check the oil and fuel. Leave in the emptying position and remove the cleaning aggregate or it will be set by morning!
Strength of mix
Most concrete for small jobs is made of 20mm ‘all-in’ aggregate and Portland cement. If the all-in aggregate (often called gravel) is too ‘sharp’ (ie not enough fine material) then add some sand.
Use tap or spring water, but not stream or sea water which contain impurities that will interfere with setting. Use as little water as possible while achieving a workable mix. Only a fraction of the water in the mix is used in the chemical reaction that hardens the concrete. Because the moisture content of the aggregate varies, the exact proportion cannot be specified. If making several batches for one job, use a consistent proportion of water or the strength of the concrete will vary.
The smaller the proportion of cement in the mix, the drier it can be made. For 4:1 – 6:1, a well mixed batch should flow when tipped from a mixer, and form a hump about 150mm high in a filled wheelbarrow. Don’t overfill the barrow, as movement will flatten the hump!
Buried concrete takes water from the ground and needs little or no water in the mix if full strength is not needed immediately. For example, posts and pipes can be concreted in using a dry mix.
Cement has a short life. Buy what you need just before you use it. Keep bags of cement off the ground and concrete floors by resting them on pallets or planks. Keep rain off the bags. Upturned wheelbarrows make good ‘roofs’ for single bags on site.
Shuttering and formwork
Shuttering and formwork must be strong, water-tight and simple to dismantle. There are cheap ‘shuttering grades’ of plywood available, and less commonly, shuttering nails with a special head for easy removal.
Paint the wood with a shuttering oil to prevent the concrete sticking.
Pouring, vibrating and finishing
Pour the concrete gently to avoid putting stress on the shuttering. Use a vibrating poker to remove the air from the mix. If you don’t have a poker, use a stick and work hard! Then hammer the sides of the shuttering with a club hammer to further shake down the concrete.
If there are poor patches or gaps when the shuttering is removed, render them up with 4:1 sharp sand to Portland cement.
Listed below are various finishes suitable for small jobs:
Use a board across the setting concrete to level and consolidate the top. Use either as final finish, which gives good grip for the feet, or as preparation for any of the following methods.
After tamping, a steel float can be used to give a very smooth finish. Requires skill to use effectively.
After tamping, use a wood float to give a good roughish finish. This looks effective and is much easier to achieve than a finish with a steel float, especially on dryish mixes.
When the concrete is just ‘going off’, brush with a hard broom to leave stones exposed. This finish can also be made using a hosepipe.
Curing and protecting
In the summer, cover work with damp hessian, and keep damp for as long as possible. Rain should be kept off wet concrete, but once ‘gone off’, rain is good for it. In the winter, protect from frost by covering with boards, hessian, straw or any other available material.
Concrete takes about 48 hours to be strong enough for shuttering to be removed, two to three weeks to gain reasonable strength, and about 100 years to reach maximum