FUNDAMENTALS OF
QUALITY CONCRETE
EARLY USE OF “CEMENTITIOUS” MATERIAL
n Egyptians used gypsum mortars
to build Pyramid of Cheops (3000 BC).
n
Greeks and Romans interground calcined limestone and the volcanic ash “pozzolan” found near Pozzouli,
Italy.
n 1756-John
Smeaton (British Engineer) rebuilt Eddystone Lighthouse in England using a mix of lime and clay pozzolans.
DEVELOPMENT OF
“PORTLAND
CEMENT”
n 1796:
James Parker (England) took out patent on hydraulic cement.
n 1824: Joseph Aspdin patented work on cement and named it “Portland”
cement.
n Resembled stone from
Portland, England.
n 1920’s:
Ready-Mixed industry started up in the United States.
n
Ready mixed concrete consumes 75% of the supply of Portland Cement.
What Is this stuff called “CONCRETE?”
n Mixture of paste and aggregates.
n Paste: primarily cement and water.
n Aggregates bound together by paste
to form homogenous mass when hardens.
n
Paste hardens through chemical process…HYDRATION.
n Other: cementitious materials and admixtures, which can
enhance concrete.
AGGREGATES
n 2 GROUPS: COARSE AND FINE.
n PROVIDE THE BULK AND STABILITY
OF VOLUME TO CONCRETE.
n CEMENT
BY ITSELF WILL NOT BE STRONG AND WILL SHRINK AND CRACK.
GENERAL COMPOSITION OF CONCRETE
n VOLUME:
n 70%
AGGREGATES
n 10% CEMENTITIOUS MATERIALS
n 15% WATER
n 5-6% AIR
n WEIGHT
(Typical Yd.)
n 1800# COARSE AGG.
n
1200# FINE AGG.
n 600# CEMENTITIOUS
n 300#
WATER (35 gal.)
AIR???
n Improves resistance to freeze/thaw cycles.
n Air-Entrained: 4-7% of volume.
n Non-air Entrained: 1-3% of volume.
n Considered part of the paste.
n Protects paste from freeze/thaw if it makes
up approx. 18% of the volume of paste and has good distribution of air bubbles.
AGGREGATE IMPORTANCE
n Should
be strong and durable for intended use.
n
Quality of concrete is dependant on the quality of the aggregates and the paste, and the bond between the two.
n Each particle should be coated
w/ paste and all spaces, other than entrained air, should be minimized.
PASTE???
n
Quality affected by quality of cementitious material and “Water / Cement Ratio”
n H2O / Cement Ratio:
Water (lbs)
Cement (lbs)
n Higher
the water content, the weaker the paste.
Water to Cement Ratio (cont)
n Low H2O/Cement Ratio:
n Increases strength
n Reduces permeability
n Improves bond w/ aggregates and reinforcement steel.
n Reduces potential for volume changes…shrinkage.
Ideal Goals for Water / Cement
n Adjust mix for variety of factors affecting the amount of
water needed.
n Properties
of cementitious materials.
n Properties
of admixtures.
n Properties
of aggregates.
n Minimize
water content without adversely affecting WORKABILITY.
FRESH CONCRETE CHARACTERISTICS
n Workability
n Rate of slump loss
n Segregation
n Bleeding
n Setting Time
n Consolidation
n Air Entrainment
WORKABILITY
n Ability to handle and place fresh
concrete without segregation.
n
FA / CA Ratio Greatly Affects:
n
Too much CA = harsh & difficult to handle
n Too much FA = higher water demand…reduces strength and increases chance
for cracking.
n Should
have ratio that results in lowest void content…”Densest mixture”
n Add more sand for finishability.
“Slump”…A
Measure of Workability
n
MISCONCEPTION: “Slump is a measure of water in the mixture.”
n Various admixtures can increase the slump while maintaining
a low H2O / Cement Ratio.
n Slip-form:
1-2 inch
n Elevated
Slabs / Slabs on grade: 3-5 inch
n
Basement walls / sections with heavy reinforcement: > 6 inch
Rate of Slump Loss
n Cement
reacts with water causing concrete to lose slump.
n
Rate of slump loss greater in higher temp.
n
Retarders and water-reducers can be added to offset the rate of slump loss.
n ASTM C 94 permits a “one-time” addition
of water at the jobsite provided it does not exceed the mix design.
SEGREGATION
n
Separation of CA from mortar during placement and consolidation.
n Results in “Honeycombing.”
n Possible Causes of Segregation:
n High Slump (improper mix design)
n Segregation during discharge from truck
n Over-vibration
BLEEDING
n
After placement and strike-off, heavier materials settle and water rises to surface.
n “Bleed Water” helps prevent plastic
shrinkage cracking on surface.
n
Excessive bleeding reduces H2O/Cement ratio at the surface and could reduce the wear resistance and durability.
n Final finishing should be scheduled
after bleeding has completed.
n
Finishing will “trap” bleed water at surface...This will spall off with time.
SETTING TIME
n Important to properly time finishing
operations and scheduling form removal.
n
Concrete sets due to hydration…process is controlled so concrete doesn’t set too fast.
n Initial Set:
point where concrete can no longer be vibrated and made to flow.
n Final Set: point where person can walk on.
FACTORS AFFECTING SET TIME
n
Characteristics of cementitious materials and amount of water.
n More water = Slower setting time
n Tempreature:
n Higher tempreature = Quicker setting time.
n Admixtures:
n Retarding admixes will slow set
time
n Accelerating admixes
will speed up set time
CONSOLIDATION
n Necessary to remove big AIR VOIDS.
n Internal vibrators commonly used.
n Overconsolidation causes
segregation and excessive bleeding.
n
Underconsolidation causes internal or surface voids…”Bugholes”
AIR CONTENT
n Billions of tiny air bubbles form by mixing.
n Gives concrete ability to withstand freeze /
thaw cycles.
n When
water turns to ice, it expands 9%.
n
If concrete saturated, the resulting pressure created by freezing and thawing will cause concrete to crack or
cause surface scaling.
n Bubbles
relieve this internal pressure.
n
Recommended 5-8% depending on size of CA and exposure conditions.
HARDENED CONCRETE
CHARACTERISTICS
n CURING
n STRENGTH
n DURABILITY
CURING OF CONCRETE
n Maintaining adequate temperature and moisture so concrete will achieve its
potential strength and durable properties.
n Moisture
needed for continued hydration.
n
Temperature of newly placed concrete determines its rate of strength gain and ultimate strength.
Effects of CURING
n Concrete kept “in air” will have about 55% of
the strength of 28-day moist cured concrete.
n
Higher temperature will result in higher early-strength, but lower ultimate strength.
n NEW CONCRETE SHOULD NOT BE ALLOWED
TO FREEZE UNTIL IT GAINS A STRENGTH OF AT LEAST 500 psi.
CONCRETE STRENGTH
n Concrete is strong in compressive
strength.
n Tensile
strength is about 10-15% of compressive strength.
n
Measure with cylinders, mortar cubes, and beams (flexural strength.)
n Modulus of Elasticity: measure of stiffness.
n Higher modulus of elasticity will deform
and deflect less under loads.
n
Aggregates have great contribution.
MEASURING STRENGTH
n Usually
measured at 28 days.
n Achieves
close to 90% of strength in 28 days, depending on the contents of the design.
n Tests taken at 7 days will develop about 75% of the 28 day
strength.
n Mixes with
Pozzolans (fly ash, slag, silica fume) will reach a higher ultimate strength at a slower pace.
DOES STRENGTH = DURABILITY?
n STRENGTH IS USUALLY SPECIFIED
TO ENSURE A LEVEL OF DURABILITY DUE TO THE FACT THAT STRENGTH IS MORE EASILY MEASURED.
n IT SHOULD NOT BE ASSUMED THAT A HIGHER STRENGTH CONCRETE WILL BECOME MORE
DURABLE!
STRENGTH FACTORS FOR CONCRETE
n
Cement type
n Water
/ Cement Ratio
n Maximum
size and quantity of aggregates and admixtures.
n
PROPER CURING!!!
STRENGTH
SCENARIOS
n Assuming
the same set of cementitious materials…
n
Increased mixing water = lower strengh.
n
Non-Air-Entrained > Air-Entrained
n However, reducing the mixing water in the air-entrained concrete will provide strength
gain.
n Smaller
nominal aggregate size results in higher strength, especially with a low water to cement ratio.
DURABILITY OF CONCRETE
n Permeability
and water tightness
n
Freeze-thaw resistance
n Shrinkage
n Plastic
shrinkage
n Drying shrinkage
n
Abrasion resistance
n Heat of Hydration
n Sulfate
Attack
n
Alkali-Aggregate Reation
n ASR (Alkali Silica Reation)
n ACR
(Alkali Carbonate Reaction)
n
Thermal Movement
n Physical Salt Attack
n Carbonation
n Corrosion
of reinforcing steel
PERMEABILITY & WATER TIGHTNESS
n
Permeability: the resistance to permeation of water under pressure through concrete.
n In general, a lower H2O / Cement
ratio will reduce permeability.
n
Cementitious materials such as fly ash, slag, and silica fume significantly improve the microstructure in concrete,
thus improving watertightness and permeability.
FREEZE-THAW RESITANCE
n Scaling: freeze thaw damage on the surface of concrete due to insufficient
air entrainment or inappropriate finishing practices.
n Can also occur in properly air entrained concrete
with non-durable aggregate…(D-Cracking).
n
Air “bubbles” should be small in size and spaced close enough to relieve
pressure from migrating water during freezing.
n
“Technically” spacing factor should not exceed .008 inches.
n Strength
should be 3500-4000 psi to withstand internal pressures from freezing.
SHRINKAGE
n Plastic Shrinkage cracks occur on the surface of plastic
concrete if it dries before the concrete sets.
n
Mixes with extended setting time and low rates of bleeding are susceptible.
n Factors that increase the rate of evaporation:
n High wind velocity
n Low humidity (“sucks” water from surface)
n Temperature
SHRINKAGE (cont.)
n Drying shrinkage is the reduction in concrete volume after it sets
and hardens.
n Changes
in moisture and/or temperature
n
Restraint of contraction due to drying shrinkage
n Increases with:
n Smaller aggregate and increased paste content
n Increased cement and water content
n Proper jointing allows drying
shrinkage cracks to be controlled and appear neat.
ABRASION RESISTANCE
n Pavements,
warehouse floors, and hydraulic structures are subject to abrasion.
n Closely related to compressive strength
n Also…harder aggregate = greater resistance
n In pavements, the sand type and
fraction in concrete will determine abrasion resistance.
n
Siliceous (rocky) sand is harder and more resistance than softer limestone sand.
HEAT OF HYDRATION
n Hydration reaction of cement and water
generates heat.
n Can be an advantage in cool weather…concrete can be maintained at an adequate temperature with
insulating blankets or formwork.
n
In thick sections this can create a problem:
n Thermal cracking
is a threat when there is a large difference in the interior and surface temperature (generally 35 degrees F)
n Heat
of hydration can be reduced by:
n
Using fly ash or slag
n Using Type II cement (not common)
SULFATE ATTACK
n Sulfate attack is a reaction between compounds in cement and sulfate in water or the ground the concrete is in contact
with.
n Causes internal expansion and cracking
n Occurs over a long period:
5 to 30 years
n
How to minimize:
n Use of pozzolans (fly ash) and slag
n Low
water / cement ratio
n
Type II or Type V cement with lower C3A
ALKALI-AGGREGATE REACTION
n Alkali
Silica Reaction (ASR): reaction between alkali in cement and reactive silica aggregate.
n Creates
alkali-silica gel that will absorb water or moisture so that the volume of the gel increases and creates cracks.
n To
minimize:
n Use non-reactive aggregates
n Low alkali cement combined with pozzolans
n Low
water / cement ratio
ALKALI AGGREGATE REACTION (cont.)
n
Alkali Carbonate Reaction (ACR): chemical reaction between alkali in cement and reactive dolomite aggregate.
n Low alkali cement is the only recognized
option to minimize.
THERMAL MOVEMENT
n Long sections of flatwork may
expand and “blow up” if no accommodation is made for thermal movement.
n Generally due to the high thermal coefficient
of expansion of aggregate.
n Construction
joints with expansion fill material should be used to accommodate for expansion.
PHYSICAL SALT ATTACK
n Deterioration of hardened concrete caused by expansive forces associated
with salt crystals in pores within the concrete.
n
Usually occurs near the evaporative surface.
n Physical salt attack can be minimized by using low permeability concrete.
n Limit water / cement ratio to .45
n Use pozzolanic materials or slag
n PROPER CURING
CARBONATION
n
Carbonation: reaction
of lime with carbon dioxide in the atmosphere.
n
Efflorescence appears as whitish
deposits on the concrete surface.
n
All surfaces undergo carbonation which does not harm concrete by itself.
n Concrete
is highly alkaline (high pH)
n
Helps protect reinforcing steel and embedded metal from corrosion.
n Carbonation
reduces alkalinity in concrete, thereby allowing steel to corrode.
n Be sure to provide adequate cover of
concrete to the reinforcing steel locations
CORROSION OF REINFORCING STEEL
n The
high alkaline (pH) of concrete provides a passive and non-corrosive oxide layer on steel.
n Chloride
ions from admixes, deicing salts, or marine exposure destroy this protective layer.
n Rust
formed during corrosion has a larger volume than original steel and will cause spalling of the concrete around the steel.
n Corrosion can be minimized by:
n
Reducing permeability of concrete
n Using non-chloride
admixes
n
Using corrosion inhibitor admixes
n Increase cover
of concrete to the rebar
n
Use epoxy-coated rebar
PROPORTIONING CONCRETE MIXTURES
(can be found in ACI 211 document)
BASIC INFORMATION NEEDED FOR “DESIGNING”
n Type of structure
n
Characteristics of:
n Cementitious materials
n Aggregates
n Water
n Air
entrainment
n
Admixtures
n Strength Requirement
n Requirements of Fresh Concrete
n
Slump, Set time, Air Content
n Prescriptive limitations:
n Water
/ Cement Ratio
n
Minimum cement content
SEQUENCE FOR PROPORTIONING
►
Water requirement and air content
► Amount and type of cementitious materials
►
Coarse aggregate content
►
Sand to “fill up” the cubic yard
Follow up design with trial batches and necessary adjustments to meet
requirements.
BASICS OF WATER
n Mixing Water is ALL ADDED WATER and the free moisture on the aggegates.
n Higher slump = more mixing water
n
Larger nominal size CA = less mixing water
n Mortar
fractions is less in mixture
n
Air-entrained concrete = less mixing water
n Shape
of sand…manufactured (more angular) sand requires more water than natural sand.
n Shape
and texture of CA will have modest effects on water demand.
n Cement content:
n Lean mixtures need more water and as cement content is increased a decrease in water content is observed.
n At a point around 650 lbs/yd adding more cement increases water requirement, and water reducers should be used to efficiently
use this additional cement for strength properties.
n Type of cementitious material
affects demand of water:
n
Fly ash reduces water demand due to its rounded particle shape.
n Silica fume increases water demand due to its extremely fine particle shape…almost always used with a high range
water reducing admixture to keep mixing water content low.
BASICS OF AIR CONTENT
n Entrapped air in non-air entrained concrete
ranges from 1-3%.
n
Air content decreases with larger nominal maximum size of CA…< volume
of mortar
n
Effects of entrained air on mixing water requirements are more significant in
lean or high water-cement mixtures.
n
ACI recommends lower air content for moderate exposure than severe exposure.
n Moderate
Exposure: freezing temperatures expected, but concrete will not be saturated for
extended period of time to freezing, and will not be exposed to deicing salts.
n Severe Exposure: concrete exposed to deicing salts or where concrete will be in contact with water and will potentially
be saturated prior to freezing.
COARSE
AGGREGATE CONTENT
n Largest
CA feasible for construction should be used.
n
Spacing of rebar, minimum dimension of structure, and form spacing control size of CA to be used.
n Increased size increases amount of CA in
mix, therefore decreases amount of paste!
n Reduces
shrinkage and temperature rise
COARSE
AGGREGATE CONTENT
n Quantity depends on:
n
Nominal max. aggregate size
n Fineness modulus (FM) of fine
aggregate
n Lower FM of sand, more CA used
n Pumping?
n Decrease
CA and increase FA
n
Slip Form?
n Stiffer mix = more CA
n High
Strength?
n Use smaller nominal maximum sized aggregate.
n Increases cementitious content of mixture
Create With Concrete…
A Solid Investment