Concrete Mix, Definition, Usage and Material Storage
Below are details of concrete mix for different applications, how to estimate quantities and what these quantities mean. The various mixes are quoted as volume ratios, however the individual components are normally sold by weight, so a little calculation work is required. Cement in concrete and mortar creates its strength by chemical reaction, it is not like wallpaper glue which ‘drys out’, it ‘goes off’. In fact, if the concrete or mortar should ‘dry out’ before the chemical reaction is substantially complete, it will fail. Likewise if it is subjected to a frost, it can also fail.
Concrete/Mortar will take months to reach it’s full strength however after about 24 hours it should be fairly hard and after 4 or 5 days, the strength should be enough to stand upto normal punishment.
The various components:
For normal ‘around the house’ types jobs, Ordinary (or Portland) cement is required. This is normally available in 50kg bags, although smaller 25kg bags are increasingly becoming available to take account of the UK manual handling regulations. Try not to buy more cement than you need as it cannot be stored for very long. Some special cements are available (i.e. quick drying) which may be required for special situation.
Lime is sometimes used in cement and reduces the amount of water drawn into the bricks, thus preventing the cement from drying out too quick and shrinking. The main types of lime are:
• hydraulic – which sets by combining with water.
• Non-hydraulic – this hardens by drying out.
• Semi-hydraulic – which is a half-and-half, it mainly needs to dry out but has some hydraulic properties.
Each type is available as either hydrated lime and quicklime – hydrated is more convenient to use as it has been processed ready for use. Hydrated hydraulic lime must be used straight from the bag. Hydrated semi- or non-hydraulic are better to use after soaking in water for 24 hours – stir it into water and leave it overnight – the lime sinks to the bottom and excess water can then be poured off. Mix the lime with the sand before adding the cement.
Two types of sand are available but are not interchangeable in all applications, whichever type is being used, always pass it though a sieve before use to remove any small stones etc.
Soft sand (or Builders sand):
a smooth sand, non-gritty, loamy and with cohesive properties, can be used for:
• Bricklaying mortar
• For bedding paving slabs
• For rendering walls
this has a gritty feel and is similar to that used to condition soils and potting composts, can be used for:
• For rendering floors and walls
Aggregate (also referred to as ballast): is used for concrete and consists of various sized particles from fine grains of sand to small stones. Aggregate is normally graded by size of the largest stones; 10mm Aggregate will have been sieved though a 10mm mesh to remove all larger stones.
General building (above ground)
sand : cement = 5:1
sand : lime : cement = 5:1:1
General building (below ground)
sand : cement = 3:1
sand : lime : cement = 6:1:1
sand : cement = 8:1
sand : lime : cement = 9:2:1
Foundations, drives, floor slab, other heavy duty
ballast : cement = 5 : 1 (20mm Ballast)
Foot paths and thin sections
ballast : cement = 3.25 : 1 (10mm Ballast)
Paving less than 50mm (2 inch), bedding for slabs
sharp sand : cement = 3 : 1
1 cubic yard = 21.6 cwt
1 cubic metre = 1.4 tonne
1 cubic yard = 10.8cwt
1 cubic metre = 0.7 tonne
1 cubic yard = 25 cwt
1 cubic metre = 1.7 tonne
The weight of Aggregate depends upon the proportion of solid stone and can vary between 20 to 27 cwt per cubic yard. For practical purposes, the sand volume/weight conversions can be used.
Cement and lime
keep undercover and dry. Cement will tend to absorb water and turn hard if left for any long periods in high humidity.
Sand and Ballast
small quanties can be stored in heavy duty plastic bags. Larger quantities should, ideally, be stored on a solid base. If it has to be stored on earth, place a lining of some sort on the earth before the sand/ballast is deposited – this will prevent the sand being absorbed into the earth and the earth into the sand. Try to keep the pile covered when not being used, this will ensure that local pets will not contaminate the top side as well as preventing any rain from washing some of it away – but do not let the pile get too dry as this may cause problems when it is being used.