Basic Concrete Technology
Matt Dalkie
P. Eng., LEED AP BD+C
Barry Metzner
Basic Concrete Technology
What is Concrete?
Importance of water
Air in Concrete
Cracking
Green Concrete
Helix Dosing
Slump
2
What is Concrete?
3
What is Concrete?
Two components
Paste (cement, water and air)
Aggregates (sand and rock)
Hardening process by hydration
Chemical reaction with water
Does not harden by drying
Normal weight concrete
2200 to 2400 kg/m3
Lightweight concrete
As low as 240 kg/m3
High density concrete
Up to 6000kg/m3
4
Critical Influences on Concrete Quality
Environment
Temperature, humidity, wind, rain/snow
Water Content
Use of SCMs
Admixture use
Cracking tendencies
Shrinkage, restraint, moisture loss
5
Importance of Water
For a given set of materials, the strength of concrete depends solely
on the amount of water for a given amount of cement
6
Damaging Effects of Adding Water
Higher w/cm = lower strength
Reduced durability, higher permeability
Increased bleed water
Delayed finishing
Increased shrinkage and cracking
Increased likelihood of surface blemishes and discolouration
Scaling
Blisters
7
Air in Concrete
Entrapped Air
All concrete has it
Typically <2% of volume
Irregular shape/size, normally 1mm and larger
Does not contribute to freeze-thaw durability
Super P’s will often increase levels of entrapped air
Entrained Air
Microscopic spherical bubbles in paste normally 10 to 1000 micron
Typically 3 to 8% of volume
Intentionally created with admixtures
Affects various properties of concrete
Air entrained concrete typically 2 to 9% lower strength per percentage
point of air
8
Why Add Air?
9
What Affects Air Content?
Cementitious materials
Amount of material
Type of material, cement, ash
Aggregates
Changes in gradation of both coarse and fine aggs
Material finer than 160 micron
Water
Mixing, too much or not enough
Time
Temperature
Vibration
10
Cracking
Concrete Shrinks
Drying
Chemical
Thermal
11
A crack forms when tensile
stress exceeds tensile
strength
Unrestrained slab = No Cracks
Restrained slab = Cracks
Controlling Cracking
Jointing
Correct use of reinforcing steel
Design detailing
Incorrect application can cause restraint
Larger aggregate size
Well graded aggregate blend
12
Green Concrete
13
Concrete, Cement and CO2
Green Concrete
14
SCMs
Helix Dosage
• For a given structural design an appropriate Helix dosage can be
prescribed to remove most, if not all, of the rebar in a structural
element
• In the Canadian market, Helix dosage prescriptions are provided in
kg/m3
• Helix is supplied in 10 kg cartons
• The concrete supplier will generate an appropriate truck load quantity
of concrete to ensure a full number of boxes is consumed in each
truck
• This avoids partial boxes and potential dosing errors
Helix Dosage Example
Customer requires 34 m3 of concrete
Dosage prescription is 6.3 Kg/m3
34 m3 X 6.3 Kg/m3 = 214.2 Kg of Helix or 22 Boxes
3 T/L’s @ 6 Boxes/9.4 m3 truck = 6.38 Kg/m3
1 T/L @ 4 Boxes/6.0 m3 truck = 6.67 Kg/m3
Total 34.2 m3 delivered @ 6.43 Kg/m3
CSA 4.3.2.3.2 - Tolerances in slump or slump flow
• Tolerances for slump shall be within the following applicable ranges:
– (a) when the specified slump is less than 80 mm, the allowable variation shall be +/-
20 mm;
– (b) when the specified slump is 80 mm to 180 mm, the allowable variation shall be
+/- 30 mm; and
– (c) when the specified slump is greater than 180 mm, the allowable variation shall
be +/- 40 mm.
– Tolerance for slump flow shall be +/- 70 mm.
Slump Tolerances
• Slump tolerances exist to account for variability that the concrete
supplier faces in aggregate moisture content, environmental
conditions (temperature and humidity), and transit time.
• Concrete mixes are designed to achieve specified strengths @ 28 days
with the specified slump (i.e. conventional mixes target an 80 mm
delivered slump)
Helix and Slump
As a rule, the addition of any fibre to a concrete mixture will tend to
“stiffen” the mix
Even though Helix exhibits the highest degree of workability of any
steel fibre, it is not exempted from this rule
The conventional job site wisdom of “just give it a little drink” must be
avoided by any responsible contractor
“Give it a little drink?” - Source: NRMCA CIP 26
Helix and Slump Recommendations
Flat work – 110-120 mm Slump
ICF – 130-150 mm Slump to avoid material hangup on webs and rebar (if present)
Water reducers / Super Plasticizers are available to increase slumps up to 200 mm (and beyond for) conventional concrete mixes
Water reducers / Super Plasticizers are your cheapest form of insurance when it comes to concrete performance and strength
Contractor Specified Slump Recommendation
Have an open and honest discussion with your pump and placing
crews as to the slump they require and convey that requirement to
your concrete supplier.
Remember that one way or another, the pump and placers will get the
slump they desire – either with water reducers/super plasticizers or
with water
A workable mix reduces pump and placer time and as such also
reduces cost
Remember the there is a difference between low price and low cost.
(with or without fibres)
Questions…
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