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Always use a concrete
mix suitable for its intended application. Tables I and II give details of mixes
appropriate for the more usual types of work undertaken in the home and garden.
The following general rules apply to all concrete, whether site mixed or readymixed. |
|
| 1. |
Concrete must be sufficiently workable to allow full compaction after
placing. Air voids left in the concrete reduce its strength and durability. |
| 2. |
As workability starts to decrease shortly after mixing, only the quantity
of concrete that can be compacted within one hour should be produced at any one time. This
is particularly important in hot dry weather. |
| 3. |
The addition of extra water to either increase workability unnecessarily
or to restore workability to concrete left unused for extended periods weakens the cement
‘‘glue’’ and reduces the overall performance of the concrete. |
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|
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Table I |
| Application |
Proportions by Volume
Cement : All-in Aggregate |
| Foundations |
1 : 7 |
| Internal Floors |
1 : 6 |
| All outside paths, slabs, patios, driveways, roads |
1 : 5 |
|
|
| Note re Aggregate - 60% coarse aggregate and 40% sand by volume generally
gives acceptable results. All aggregate should be washed. |
|
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Readymix concrete
producers employ a standard mix designation which should be used when ordering. The
initial figures in Table II indicate the strength grade and the latter figures indicate
the maximum size of aggregate.
|
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Table II |
| Application |
Mix Designation |
x |
| Foundations |
20N20 |
Medium Workability |
| Internal Floors |
25N20 |
Medium Workability |
| All outside paths, slabs, patios around the house |
25N20 |
Medium Workability
Minimum cement
content 240kg/m3 |
| Driveways, Roads |
30N20 |
Medium Workability
Minimum cement
content 300kg/m3 |
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