Concrete Technology Module
Semester 2 – 2018‐2019 AD
Semester 2 outline
references
Main Reference
Advanced concrete technology by Zongjin Li
Other references
Concrete technology by Dr. Moaid Nory
Concrete Technology ‐2dn Ed by A.M. NEVILLE
CONCRETE
TECHNOLOGY 2
Mix Design of
Concrete
Reference: Advanced Concrete Technology - Zongjun Li
1
Mix Design
Mix design can be defined as the processes of selecting
suitable ingredients and determining their relative
quantities, with the purpose of producing an economical
concrete that has certain minimum properties, notably
workability, strength, and durability.
Mix design of concrete is frequently done by
trial and error. Hence, mix design of concrete is
an art, not a science.
Advanced Concrete Technology - Zongjun Li
2
Principal requirements for concrete
• Quality (strength and durability)
• Workability
• Economy
Advanced Concrete Technology - Zongjun Li
3
Weight method
&
absolute volume method
In the weight method, the unit weight of fresh concrete is
known from previous experience for the commonly used
raw materials and is used to calculate the weight of the
last unknown component of concrete, usually the sand.
If the weights of cement, water, coarse aggregate, and admixtures have been
determined, then the weight of sand can be obtained from above equation. The
unit weight of wet concrete usually ranges from 2300 to 2400 kg/m3.
Advanced Concrete Technology - Zongjun Li
4
Absolute volume method
when the weights of cement, water, coarse aggregate, and
admixture have been determined, their corresponding volumes
can be calculated, with their densities known.
Since the weight of each ingredient is easier to measure than the volume, the
design proportion of concrete is usually expressed as a weight ratio. Hence, the
proportion obtained in the volume method has to be converted to weight units
by multiplying the volume with the density of the material.
Advanced Concrete Technology - Zongjun Li
5
Factors to be considered
•
•
•
•
•
•
Maximum water/cement ratio
Minimum cement content
Projected strength
Projected workability
Maximum size of aggregate
Air content.
The projected strength is usually specified by the structural designer. Normally,
the strength at 28 days is used as the design index for structural purposes, but
other considerations may dictate the strength at other ages, e.g., formwork
demolding time.
Advanced Concrete Technology - Zongjun Li
6
Water/cement ratio
Although the w/c ratio can be estimated from
Abram’s law, based on projected strength of
concrete, in concrete design, the w/c ratio
required to produce a given mean compressive
strength is usually determined from previously
established relations for mixes made from similar
ingredients, or by carrying out tests using trial
mixes made with the actual ingredients to be
used in the construction, including admixtures.
𝑓𝑐 =
𝐵1.5(𝑤/𝑐)
where, A is compressive
strength in Mpa (usually 96).
B is a constant that depends
mostly
on
the
cement
properties (usually 4).
𝑓𝑐 =
Advanced Concrete Technology - Zongjun Li
𝐴
96
41.5(𝑤/𝑐)
7
Water/cement ratio
Back to example 1
Advanced Concrete Technology - Zongjun Li
8
Water/cement ratio
Advanced Concrete Technology - Zongjun Li
9
Durability
Severe exposure conditions require a stringent control
of the w/c ratio because it is the fundamental factor
determining the permeability and diffusivity of the
cement paste and, to a large extent, of the resulting
concrete.
The adequate cover to embedded reinforcing steel is
essential.
If the w/c ratio is determined due to durability
requirements, the cement content can be reduced by
the use of a larger-size aggregate.
Advanced Concrete Technology - Zongjun Li
10
Back to example 1
Advanced Concrete Technology - Zongjun Li
13
Advanced Concrete Technology - Zongjun Li
14
Advanced Concrete Technology - Zongjun Li
15
Workability
Advanced Concrete Technology - Zongjun Li
16
Workability
Back to example 1
Advanced Concrete Technology - Zongjun Li
17
Workability
After choosing the workability, the water content of the mix (mass
of water per unit volume of concrete) can be estimated by
considering the workability requirement.
ACI 211.1-81 gives the water content for various maximum sizes of
aggregate and slump value (as an index of workability), with and
without air entrainment (see Table 3-8).
The values apply for well-shaped coarse aggregates and, although
the water requirement is influenced by the texture and shape of the
aggregate, the values given are sufficiently accurate for a first
estimate.
Advanced Concrete Technology - Zongjun Li
18
Advanced Concrete Technology - Zongjun Li
Back to example 1
19
Cement type and content
The choice of the types of cement depends on the
required hydration rate
• strength development
• chemical attack
• thermal considerations
It is needed to choose a cement with a high rate of heat of hydration developed
for cold-weather concreting, and with a low rate of heat of hydration for mass
concreting, as well as for concreting in hot weather.
In the latter case, it may be necessary to use a lower w/c ratio to ensure a
satisfactory strength at early ages.
Advanced Concrete Technology - Zongjun Li
20
Cement type and content
Because cement is more expensive than aggregate, it is
desirable to reduce the cement content as much as
possible, provided it can satisfy the strength, durability,
and workability requirements.
Moreover, low to moderate cement content confers the
technical advantage of a lower hydration heat as well as
cracking potential in the case of mass concrete, where
the heat of hydration needs to be controlled, and in the
case of structural concrete where shrinkage cracks should
be minimized.
The cement content has to meet the minimum requirement by specification from the
durability considerations . Advanced Concrete Technology - Zongjun Li
21
Major aggregate properties and
aggregate content
Usually, the maximum size of aggregate is determined first, as it
has a significant influence on concrete properties.
In reinforced concrete, the maximum size of an aggregate is
governed by the geometry of the member and the spacing of the
reinforcement.
The maximum aggregate size has to be smaller than
to 1/3 of the depth of slabs
 to 1/5 of the smallest size of the cross section of a member
 3/4 of the net spacing distance of reinforcement.
The improvement in the properties of concrete with an increase in the size of aggregate
does not extend beyond about 40 mm.
For high-strength concrete, the maximum aggregate size is limited to 20 mm.
Advanced Concrete Technology - Zongjun Li
22
Major aggregate properties and
aggregate content
Another important parameter of an aggregate is its grading. In
all cases, dense-graded or well-graded aggregate is preferred,
and uniformity has to be achieved.
In the case of a coarse aggregate, uniformity can be obtained
relatively easier by the use of separate stockpiles for each size
fraction.
Advanced Concrete Technology - Zongjun Li
23
The parameters for aggregate content in a concrete mix design that need to be decided
include the total aggregate-to-binder ratio and the fine aggregate-to-coarse aggregate
ratio.
Table 3-11 provides the dry bulk volume of coarse aggregate per unit volume of
concrete, which is expressed as a function of both fineness modules of the fine
aggregate and the maximum size of aggregate. The mass of the coarse aggregate can
then be calculated from the product of the dry bulk volume and the density (or unit
weight) of the dry coarse aggregate.
Back to example 1
Advanced Concrete Technology - Zongjun Li
24
Fine aggregate content
The fine aggregate content per unit volume of concrete can be then
estimated using either the mass method or the volume method.
In the former, the sum of the masses of cement, coarse aggregate, and
water is subtracted from the mass of a unit volume of concrete, which is
often known from previous experience with the given materials. However, in
the absence of such information, Table 3-12 can be used as a first estimate;
adjustment is made after trial mixes.
Advanced Concrete Technology - Zongjun Li
25
Precise Density of Concrete
Advanced Concrete Technology - Zongjun Li
26
Fine aggregate content
W = mixing water requirement, kg/m3
C = cement content, kg/m3
γ = specific gravity of cement (generally 3.10 for Portland cement)
A = air content, %
Advanced Concrete Technology - Zongjun Li
26
PROCEDURES FOR CONCRETE MIX
DESIGN
• Sieve analysis results and fineness modulus of fine and coarse
aggregate
• Dry-rodded density (unit weight) of coarse aggregate
• Bulk specific gravity of each raw material
• Absorption capacity or moisture content of the aggregates
• Variation of the approximate mixing water requirement with
slump, air content, and grading of the available aggregates
• Relationships between strength and water/cement ratio for
available combinations of cement and aggregate
• Job specifications, if any, e.g., maximum water/cement ratio,
minimum air content, minimum slump, maximum size of
aggregate, and strength at early ages (normally, 28-day
compressive strength is specified)
Advanced Concrete Technology - Zongjun Li
27
Concrete mix design steps









Step 1: Choice of slump
Step 2: Choice of maximum size of aggregate
Step 3: Estimation of mixing water and the air content
Step 4: Selection of water/cement ratio
Step 5: Calculation of cement content
Step 6: Estimation of coarse aggregate content
Step 7: Estimation of fine aggregate content
Step 8: Adjustment of amount of free water
Step 9: Trial mixes
Advanced Concrete Technology - Zongjun Li
28
Adjustment of the design
Due to so many assumptions underlying the foregoing theoretical
calculations, the mix proportions for the actual concrete have to
be checked and adjusted by means of laboratory trials consisting
of small batches (e.g., 0.02 m3 or 50 kg of concrete).
Fresh concrete should be tested for slump, cohesiveness,
finishing properties, and air content, as well as for unit weight.
The specimens of hardened concrete cured under standard
conditions should be tested for strength at specified ages.
Advanced Concrete Technology - Zongjun Li
29
Adjustment recommendations
If any property cannot meet the design requirement,
adjustments to the mix proportions have to be
conducted.
If the correct slump is not achieved, the estimated water content
is increased (or decreased) by 6 kg/m3 for every 25mm increase
(or decrease) in slump.
If the desired air content is not achieved, the dosage of the airentraining admixture should be adjusted to produce the specified
air content. The water content is then increased (or decreased) by 3
kg/m3 for each 1% decrease (or increase) in air content.
Advanced Concrete Technology - Zongjun Li
30
Adjustment recommendations
If the estimated density (unit weight) of fresh concrete by the
mass method is not achieved and is of importance, the mix
proportions should be adjusted, with allowance being made for a
change in air content.
If the projected strength cannot be met, w/b should be reduced
at a rate of 0.05 for every 5 MPa.
Mix proportion adjustments
After several trials, when a mixture satisfying the desired criteria of workability
and strength is obtained, the mix proportions of the laboratory-size trial batch
can be fixed and scaled up for producing large amounts of field batches.
Advanced Concrete Technology - Zongjun Li
31
Example
Concrete is required for a column that will be moderately exposed to freezing and
thawing. The cross section of the column is 300 × 300 mm. The smallest spacing
between reinforcing steel is 30 mm. The specified compressive strength of
concrete at 28 days is 40 MPa with a slump of 80 to 100 mm. The properties of
materials are as follows:
(a) Cement used is type I Portland cement with a specific gravity of 3.15.
(b) The available coarse aggregate has a maximum size of 20 mm, a dry-rodded
unit weight of 1600 kg/m3, a bulk specific gravity (SSD) of 2.68, absorption
capacity of 0.5%, and moisture content (OD) of 0.25%.
(c) The fine aggregate has a bulk specific gravity (SSD) of 2.65, absorption capacity
of 1.3%, a moisture content (SSD) of 3%, and a fineness modulus of 2.60. The
aggregates conform to the ASTM C33-84 requirements for grading.
Advanced Concrete Technology - Zongjun Li
32
Step 1: Choice of slump.
The slump is given and consistent with Table 3-7.
Step 2: Maximum aggregate size.
The maximum aggregate size is 20 mm, which meets the limitations of 1/5 of the
minimum dimension between forms and 3/4 of the minimum clear space.
Step 3: Estimation of mixing water and air content.
The concrete will be exposed to freezing and thawing; therefore, it must be air
entrained. From Table 3-8, the recommended mixing water amount is 180 kg/m3, and
the air content recommended for moderate exposure is 5.0%.
Step 4: Water/cement ratio (w/c).
According to both Table 3-1 and Table 3-3, the estimate of the required w/c ratio to
give a 28-day compressive strength of 40 MPa is 0.35.
Step 5: Calculation of cement content.
Based on the steps 3 and 4, the required cement content is 180/0.35 = 514 kg/m3.
Advanced Concrete Technology - Zongjun Li
33
. Step 6: Estimation of coarse aggregate content
From Table 3-11, for fineness modulus of the fine aggregate of 2.60, the volume of dryrodded coarse aggregate per unit volume of concrete is 0.64. Therefore, there will be
0.64m3 coarse aggregate in per volume concrete. And, the O D weight of the coarse
aggregate is 0.64 × 1600 = 1024 kg. The SSD weight is 1024 × 1.005 = 1029 kg.
Step 7: Estimation of fine aggregate content.
The fine aggregate content can estimated by either the weight method or the volume
method.
Volume method.
Based on the known weights and specific gravity of water, cement, and coarse
aggregate, the air volume, the volumes per m3 occupied by the different constituents
can be obtained as follows: (Note: SG=Density of Substance/Density of Water)
Advanced Concrete Technology - Zongjun Li
34
Step 8: Adjustment for moisture in the aggregate .
Since the aggregates will be neither SSD nor OD in the field, it is necessary to adjust
the aggregate weights for the amount of water contained in the aggregate. Since
absorbed water does not become part of the mix water, only surface water needs to
be considered. For the given moisture contents, the adjusted aggregate weights
become
Advanced Concrete Technology - Zongjun Li
35