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Estimating
& Ordering Materials
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This
section gives guidelines to
answer the most commonly asked
questions,
such as:
-
How
much cement, stone and
sand do you need for each
cubic metre
of concrete?
-
What
are the mix proportions
for the correct
workability and required
strength? How does this
translate into how much
cement, stone
and sand to order?
Concrete
Strength |
|
Remember
that we use the term
"cement' to
cover all branded, mark-bearing
cements that comply
with SABS EN 197-1
/SANS 50197-1
strength class
32,5N. |
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Three
grades of concrete are generally
specified for general building
work:
-
Low
strength concrete (15 MPa): suitable
for unreinforced foundations
(single storey only), mass fill,
infill concrete in masonry
(only with 13,2 mm stone)
-
Medium-strength
concrete (25 MPa): suitable for unreinforced
slabs or floors, reinforced slabs
and foundations,
footpaths and domestic driveways
-
High-strength
concrete (30 MPa): suitable for
reinforced concrete
members, heavy duty floors and
precast items such as
concrete flagstones
For
smaller jobs and projects, concrete is
generally hatched by volume.
Where more sophisticated equipment is
available, hatching
by mass ensures greater accuracy.
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The
tables below give different
mix proportions for different
stone sizes.
The
mix proportions given are
conservative so that,
even where workmanship and
material quality is likely to be
poor, the concrete should
achieve
the desired strength. |
ALWAYS
use as little water as
possible. Use only enough to
achieve the required
workability and
finishability of the fresh
concrete. The addition of too
much water
will reduce the concrete's
strength. |
The
cement must also comply with other
physical and chemical requirements as
specified in SABS EN 197-1
/SANS
50197-1
Cement
-
Part
1: Composition, specifications and conformity
criteria for common cements.
The
figures given in Tables 1 and 2
for
the most commonly used stone
sizes (26,5
and
19 mm, as well as 13,2
mm)
are an initial estimate
of proportions and quantities of each
material required.
Volumes of stone and sand may
have to be adjusted on site to give
better workability and finishability,
depending on the quality, fineness,
etc of the aggregates.
NOTE
THAT the mix proportions are based on
stone and sand batched in a loose
state, i.e. poured into the batching
container without being compacted.
TABLE
1: Material for a two-bag batch of
concrete, using 26,5
or
19 mm stone.
|
TABLE
1 |
Cement
50
kg
bags |
Sand |
Stone |
Yield
m3 |
|
Litres |
Wheelbarrows |
Litres |
Wheelbarrows |
|
Low
strength, 15
MPa |
2 |
230 |
3½ |
230 |
3½ |
0.35 |
|
Medium
strength, 25
MPa |
2 |
160 |
2½ |
160 |
2½ |
0,26 |
|
High
strength, 30
MPa |
2 |
130 |
2 |
130 |
2 |
0,22 |
TABLE
2:
Material
for a two-bag batch of concrete, using 13,2
mm
stone
|
TABLE
2 |
Cement
50
kg
bags |
Sand |
Stone |
Yield
m3 |
|
Litres |
Wheelbarrows |
Litres |
Wheelbarrows |
|
Low
strength, 15
MPa |
2 |
230 |
3½ |
160 |
2½ |
0,30 |
|
Medium
strength, 25
MPa |
2 |
160 |
2½ |
130 |
2 |
0,24 |
|
High
strength, 30
MPa |
2 |
130 |
2 |
100 |
1½ |
0,20 |
|
Yield
is the volume of fresh
concrete produced by mixing
the dry
materials with enough water to
produce a workable mix (technically,
75 to 100 mm slump) for hand
compaction.
The
amounts specified do not
include allowance for wastage. |
Note:
Filled flush, a
wheelbarrow holds 65 litres.
The
above mix proportions refer to
average materials, and
an estimated 5% moisture in
the sand. In
practice, the ratio of sand to
stone may have to be adjusted
if the mix is too stony or too
sandy.
|
Too
stony: individual stones
protrude from the top surface of
the concrete. Reduce the stone
content by 5%, and add the
same volume (not percentage)
of sand. |
|
|
Too
sandy: scratch through the
mortar on the surface with
a nail; if the top mortar
layer is thicker than a few millimeters
above the stone, the mix is
too sandy. Increase the
stone content by 5%, and
reduce the amount of sand by
the volume of stone added.
In
each case, the total volume of
sand and stone
remains the same. |
Before
ordering cement, stone and sand,
calculate how much concrete
you need.
MEASURE:
the
depth or height, length and width of
each element for which you require
concrete in
metres
MULTIPLY:
Height
x length x width (measured in metres)
EQUALS:
Volume of concrete required in cubic
metres (m3)
USE:
the tables above to help you order the
required material. Or alternatively you
could use Tables 3 & 4 below to
assist you.
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TABLE
3: One cubic metre of
concrete, using 26,5
or
19 mm stone |
Cement
50kg
Bags |
Sand
M3 |
Stone
M3 |
TABLE
4: One cubic metre of
concrete, using 13,2 mm stone |
Cement
50kg
Bags |
Sand
M3 |
Stone
M3 |
|
Low
strength, 15
MPa |
5,8 |
0,65 |
0,65 |
Low
strength, 15
MPa |
6,7 |
0,65 |
0,54 |
|
Medium
strength, 25
MPa |
7,7 |
0,62 |
0,62 |
Medium
strength, 25
MPa |
8,4 |
0,62 |
0,54 |
|
High
strength, 30
MPa |
9,2 |
0,60 |
0,60 |
High
strength, 30
MPa |
10,0 |
0,60 |
0,50 |
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Dry
mix materials:
A
viable alternative for the
home handyman is to buy
materials from the hardware
store or
builder's merchants as dry
mixes.
Bags
of dry mixes for concrete,
mortar and plaster contain all
the materials needed for the
application in the correct
proportions. All you
need do is just
add water.
Now
you are ready to start
mixing... and placing,
compacting and finishing
the concrete -
all
of
which
are very important in ensuring
that you produce good quality
concrete. This
applies to concrete cast
in
place, as
well
as
to
concrete for precast
items such
as decorative
stepping stones,
concrete
sleepers,
birdbaths and garden
ornaments.
All
of this is covered in the next
section Working
The Concrete. |
Please
navigate in the especially provided
table at the top.
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