The Cathedral of Our Lady of the Angels. Los Angeles, California
Rafael Moneo
Form a group of 3-4 students for mix design lab next week
Write all your all names on a piece of paper and turn in at the end of class
Possible Group Mix Times in Structures Lab
Tuesday
Wednesday
Thursday
8 – 10 am
Friday
Saturday
8 – 10 am
8 – 10 am
10 - noon
10 - noon
noon - 2 pm
2 - 4 pm
4 - 6 pm
4 - 6 pm
4 - 6 pm
6 - 8 pm
6 – 8 pm
6 - 8 pm
Group 1
4000 psi
no AE
no fiber
Group 2
5000 psi
no AE
no fiber
Group 3
6000 psi
no AE
no fiber
Group 4
4000 psi
no AE
yes fiber
Group 5
5000 psi
no AE
yes fiber
4 - 6 pm
• Stiff
0-2”
– massive sections, little reinforcement
• Medium
2-5”
– columns, beams, retaining walls
• Fluid
5-7” (sometimes even 10”)
– heavily reinforced section, flowable concrete
Different Slumps
Taipei 101
Pumping Test of HPC
Steel Rebar Congestion
High Slump Achieved
with Superplasticizers
Mix Design: Target Strength = 10,000 psi
w/c = 0.31
Achieved Strength = 12,000 psi
with slag & silica fume and superplasticizers
½” diameter aggregate maximum
Result of Poor Concrete Placement due to Inadequate Workability of the Mix
Concrete Mix Design
• Design a mix that provides the required strength with
minimum cement amounts, the required durability and
acceptable workability/consolidation (slump)
• Factors: Intended Use, Exposure, Size & Shape of
Members, Strength
• Minimize cement content
– Use highest w/c ratio permitted
– Use largest size aggregate with good gradation
– Do not use too much fine aggregate
– Use admixtures – fly ash, superplasticizers, etc.
• Higher slump mixes require more water and therefore
more cement (to keep w/c ratio constant) than lower
slump mixes for the same f’c strength provided
• Mixes with rounded aggregate will have a higher slump
than with angular aggregate for the same w/c and air
How do I design a concrete mix?
Produce a trial batch w/ the ACI 211 Design Procedure:
1.
2.
3.
4.
Water and air entrainment determines slump
w/c determines strength (durability requirements also)
Compute cement from 1 & 2.
Use as much coarse aggregate as you can (compute
from empirical tables).
5. Fine aggregate fills the rest of the space (compute by
subtracting weights of other ingredients
from estimated total weight)
6. Adjust weights of aggregate and water
added based on each aggregate’s
moisture content and absorption capacity
Get Concrete ‘Cookies”
Mix Design I
Concrete for an interior column in a three-story building
The desired properties are:
• Slump = 3” – 4”
• 28-day compressive strength, f’c = 4000 psi
• No air-entrainment required (interior, not in contact with soil)
•
Coarse aggregate:
– Nominal maximum size = 1” (max. aggregate size is 1-1/2”)
– Dry-rodded bulk density = 105 lb/ft3
– Absorption capacity (AC) = 1.0%
– Moisture content (MC) = 0.5%
•
Fine aggregate:
– Fineness modulus (FM) = 2.70
– Absorption capacity (AC) = 0.7%
– Moisture content (MC) = 1.5%
Table 1
Table 2
Table 3
Table 4
Level of Moisture in Aggregate
•
Oven-dry (OD). All moisture is removed. All pores connected to
the surface are empty and the aggregate is fully absorbent.
•
Air dry (AD). All surface moisture is removed, but internal pores
connected to the surface are partially filled with water. The
aggregate is somewhat absorbent.
•
Saturated surface dry (SSD). All pores connected to the surface
are filled with water, but the surface is dry. The aggregate is
neither absorbent nor does it contribute water to the concrete mix.
•
Wet. All internal pores connected to the surface are filled with
water and there is excess moisture on the surface. The
aggregate contributes water to the concrete mix.
Adjustments for Aggregate Moisture & Adsorption
Aggregate weights. Aggregate quantities are calculated based on
oven dry (OD) unit weights, but aggregate is typically batched based
on weight as-is the lab or field. The as-is lab or field aggregate may
have a moisture content – have water in its pores or on its surface/
water. This moisture in the aggregate will increase its weight.
Without correction for the moisture content, the batched aggregate
volumes will be incorrect.
Amount of mixing water. If the as-is aggregate is anything but
saturated surface dry (SSD) it will:
Take away (absorb) water (if oven dry (OD) or air dry (AD)) from the
added mix water and away from the cement paste.
or
Add water (if surface water is present) to the mix water and the
cement paste.
This causes a change in the amount of water available in the mix and
the amount of water added to the mix must be adjusted accordingly.
Adjusting for Aggregate Moisture & Adsorption
• The actual quantities of aggregate to be weighed out during batching
of a concrete mix must allow for the specific amount of moisture
present in the aggregate.
• Generally, the aggregates will be moist and their dry weights should
be increased by the amount of water they contain, both absorbed
and surface. This is the moisture content of the aggregate.
• The mixing water added to the batch must be reduced by an amount
equal to the free moisture contributed by the aggregate (i.e.,
moisture content of the mix minus absorption capacity of aggregate,
or the what is available from the surface moisture).
• Remember that water absorbed into the aggregate does not become
part of the available mixing water.
• If the MC is less than the AC, more mix water needs to be provided
because some of the mix water will be absorbed by the aggregate.
• If the MC is more than the AC, then surface water is included in the
mix already from the aggregate and less mix water needs to be
added.
Trial Batching Steps
• Use Table 1 to determine the amount of water per cubic
yard req’d to obtain the desired slump
• Use Table 2 to determine the w/c ratio req’d to obtain the
desired strength
• Calculate the amount of cement
• Use Table 3 to determine the amount of coarse aggregate
• Calculate the amount of fine aggregate using the estimated
weight of concrete from Table 4
Fine Weight = Total Weight – Coarse Weight – Cement Weight – Water Weight
• Adjust the amount of coarse and fine aggregates and
amount of mix water based on moisture in/on aggregate
(MC) and the adsorption capacity (AC) of aggregate
Mix Design II
Concrete for an exterior slab of a parking deck
The desired properties are:
• Slump = 1” – 2”
• 28-day compressive strength, f’c = 3500 psi
• Air-entrainment required
•
Coarse aggregate:
– Nominal maximum size = 3/4”
– Dry-rodded bulk density = 100 lb/ft3
– Absorption capacity (AC) = 1.5%
– Moisture content (MC) = 1.8%
•
Fine aggregate:
– Fineness modulus (FM) = 2.60
– Absorption capacity (AC) = 1.0%
– Moisture content (MC) = 2.0%
On-Going Research Area
Steel Rebar Diameters and
Areas
Summary
Adjusting for aggregate moisture
•
•
•
•
•
•
•
•
The aggregate quantities are based on OD conditions. The actual
quantities of aggregate to be weighed out during batching of a concrete
mix must allow for the specific amount of moisture in the aggregate.
Generally, the aggregates will be moist and their dry weights should be
increased by the amount of water they contain, both absorbed and surface.
The mixing water added to the batch must be reduced by an amount equal
to the free moisture contributed by the aggregate (i.e., moisture content of
the mix minus absorption capacity of aggregate or the what is reported as
the surface moisture).
Thus, if the total moisture content of the sand is 5.20% and the sand
absorption is reported to be 0.7 %, while the comparable values for coarse
aggregate are 2.0 % and 0.5 %, you would adjust the batch weights as:
For aggregates, Batch weight = Design weight * (1 + decimal equivalent of
total moisture content)
For water, Batch weight = Design water – coarse aggregate free moisture
– fine aggregate free moisture
Remember that water absorbed into the aggregate does not become part
of the available mixing water, and only the surface water of the aggregate
must be considered in the last step.
Coarse aggregate = 1,836 x 1.020 = 1,873 lbs
Fine aggregate = 1,243 x 1.052 = 1,308 lbs
Water = 315 - (1,836 x .015) - (1,243 x .045) = 315 - 84 = 231 lbs
WaterCementitious
Materials Ratio
Aggregates
Water Content
Cementing Materials Content
Cementing Materials Content
• The grading characteristics and nature of
aggregate particles have an important
influence on proportioning concrete
mixtures because they affect the
workability of the fresh concrete
Bleeding and its control
• Creates problems: • causes
– lack of fines
– poor pumpability
– too much water
– delays in finishing
content
– high w/c at the top
• Remedies
– poor bond
– more fines
between two
layers
– adjust grading
– entrained air
– reduce water content