Concrete mix design
22.2 The UK method of ‘Design of normal concrete mixes’
(BRE 1997)
This method of mix design provides a good ex
ample of the process of making an initial estimate of the mix proportions. It has the advantage of being relatively straightforward and producing reasonable results with the materials most commonly available in the UK. It should be emphasised that it is not necessarily the ‘best’ method available worldwide, and that it may not give such good results with other materials.
The main part of the method is concerned with the design of mixes incorporating Portland cement, water and normaldensity coarse and fine aggregates only, and with characteristic cube strengths of up to about 70 MPa (since it is a UK method, all the strengths referred to are cube strengths). It encom
passes both crushed and uncrushed coarse aggregate.
The steps involved can be summarised as follows.
22.2.1 target mean Strength
As described in Chapter 2, the specified charac
teristic strength is a lower limit of strength to be used in structural design. As with all materials,
concrete has an inherent variability in strength, and an average cube compressive strength (or target mean strength) somewhat above the characteristic strength is therefore required. The difference be
tween the characteristic and target mean strength is called the margin; a 5% failure rate is normally chosen for concrete, and the margin should therefore be 1.64 times the standard deviation of the strength test results (Table 2.1).
This means that a knowledge of the standard deviation is required. For an existing concrete production facility this will be known from previous tests. Where limited or no data are available, this should be taken as 8 MPa for characteristic strengths above 20 MPa, and pro rata for strengths below this. When production is under way, this can be reduced if justified by sufficient test results (20 or more), but not to below 4 MPa for characteristic strengths above 20 MPa, and pro rata for strengths below this. The advantage of reducing the variabil
ity by good practice is clear.
22.2.2 free water:Cement ratio
For a particular cement and aggregate type, the concrete strength at a given age is assumed to be governed by the free water:cement ratio only.
The first step is to obtain a value of strength at a water:cement ratio of 0.5 from Fig. 22.2 for the relevant age/aggregate type/cement type combination (note: this figure has been produced from tabulated data in the method document). This value is then plotted on the vertical line in Fig. 22.3 to give a starting point for a line that is constructed parallel to the curves shown. The point of intersection of this line with the horizontal line of the required
20 30 40 50 60 70
0 10 20 30 40 50 60 70 80 90 100
Age (days)
Compressive strength (MPa)
42.5N cement 52.5R cement Uncrushed aggregate Crushed aggregate
42.5N cement 52.5R cement
Fig. 22.2 Compressive strength vs. age for concrete with a water:cement ratio of 0.5 (after BRE, 1997).
Concrete mix design
target mean strength then gives the required free water:cement ratio. The ranges of the axes in Fig. 22.3 indicate the limits of validity of the method.
22.2.3 free water Content
It is now assumed that, for a given coarse aggregate type and maximum size, the concrete consistence is governed by the free water content only. The con
sistence can be specified in terms of either slump or Vebe time (see Chapter 18), although slump is by
far the most commonly used. Figure 22.4 is a graph of data for slump, again produced from tabulated data in the method document, from which the free water content for the appropriate aggregate can be obtained.
22.2.4 Cement Content
This is a simple calculation from the values of the free water:cement ratio and free water content just calculated.
0 10 20 30 40 50 60 70 80 90
0.3 0.4 0.5 0.6 0.7 0.8 0.9
Free water:cement ratio
Compressive strength (MPa)
Starting line with data from Fig. 22.2
Fig. 22.3 Compressive strength vs. water:cement ratio of concrete (copyright BRE, reproduced with permission).
0 20 40 60 80 100 120 140
100 150 200 250 300
Free water content (kg/m3)
Slump (mm)
Crushed aggregate:
10 mm 20 mm 40 mm Uncrushed
aggregate:
10 mm 20 mm 40 mm
Fig. 22.4 Slump vs. free water content of concrete (after BRE, 1997).
22.2.5 total aggregate Content
An estimate of the density of the concrete is now required. This is obtained from Fig. 22.5, using known or assumed values of the relative density of the aggregates. A weighted mean value is used if the specific gravities of the coarse and fine aggregate are different. Subtraction of the free water content and cement content from this density gives the total aggregate content per m3.
22.2.6 fine and CoarSe aggregate Content
The estimated value of the proportion of fine aggre
gate in the total aggregate depends on the maximum size of the aggregate, the concrete consistence, the grading of fine aggregate (specifically the amount passing a 600micron sieve) and the free water:cement ratio. Fig. 22.6 shows the relevant graph for obtain
ing this proportion for a maximum aggregate size of 20 mm and slump in the range 60–180 mm.
Sufficient fine aggregate must be incorporated to produce a cohesive mix that is not prone to segre
gation, and Fig. 22.6 shows that increasing quan
tities are required with increasing water:cement ratio and if the aggregate itself is coarser. The mix design document also gives equivalent graphs for lower slump ranges and 10 and 40 mm coarse aggregate;
less fine aggregate is required for lower slumps, between 5 and 15% more fine aggregate is required with 10 mm aggregate, and between 5 and 10%
less with 40 mm aggregate.
The fine and coarse aggregate content is now calculated by simple arithmetic, and the amounts (in kg/m3) of free water, cement, coarse and fine
aggregates for the laboratory trial mix have now all been obtained.
It is important to note the simplifying assumptions used in the various stages. These make the method somewhat simpler than some other alternatives, but highlight the importance of trial mixes and sub
sequent refinements.
2100 2200 2300 2400 2500 2600 2700
100 120 140 160 180 200 220 240 260
Free water content (kg /m3) Wet density (kg/m3)
Relative density of aggregate (SSD basis)
2.9
2.4 2.5 2.6 2.7 2.8
Assume for:
crushed aggregates uncrushed aggregates
Fig. 22.5 Wet density of fully compacted concrete vs. free water content (copyright BRE, reproduced with permission).
% of fine aggregate passing 600-µm
sieve
Free water:cement ratio
Proportion of fine aggregate (%)
15
40
60 80 100
10 20 30 40 50 60 70 80
0.2 0.4 0.6 0.8
Fig. 22.6 Proportions of fine aggregate according to percentage passing 600mm sieve (for 60–180 mm slump and 20 mm max coarse aggregate size) (copyright BRE, reproduced with permission).
Concrete mix design