Concrete mix design is a crucial process in construction that involves determining the composition of concrete to meet specific performance requirements. It involves selecting appropriate materials, proportions, and water-cement ratio to achieve desired strength, durability, workability, and other ...
Concrete Mix Design
Concrete mix design is used to select the correct proportions
of cement, water and fine & coarse aggregate to produce
concrete with specified properties.
The properties usually specified are:
• Workability of fresh concrete
• Compressive strength at a specified age
• Durability (by specifying a minimum cement content and /
or the maximum free-water/cement ratio)
The workability depends mainly on the free-water content
and the fineness & shape of the aggregate
The compressive strength depends mainly on the cement
strength and the free-water/cement ratio. There are several
cement strength classes, the most common are 42.5 and 52.5.
, 2
Free-Water / Cement Ratio
mass of free water
w/c ratio
mass of cement
Example:
Mass of water mixed at the plant: 145 kg
Mass of cement: 340 kg
w/c = = 0.43
Adding 40 kg (litres) of water to the same batch gives:
Total water content = 145 kg + 40 kg = 185 kg
New w/c = = 0.54 >> 0.43
Care must be taken to measure the amount of water
correctly
Moisture content in aggregate has to be taken into account
(1000 kg aggregate with 4% moisture content adds 40kg
water to the mixture).
, 3
Strength of Cement – Distribution of Results
One of the most obvious parameters affecting the strength of
concrete is the strength of the cement used in the mixture.
There are variations of the strength of cement, even within a
strength class. To make sure the finished concrete is not weaker
than the specified strength we have to take these variations into
account.
Statistics:
x = individual result
n = number of results
m = the mean of the n results m
x
n
The standard deviation s is a measure of the distribution of the
data. The larger s, the wider is the distribution of the data.
s
x m 2
n 1
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