EARLY USE OF “CEMENTITIOUS” MATERIAL
n Egyptians used gypsum mortars
to build Pyramid of Cheops (3000 BC).
Greeks and Romans interground calcined limestone and the volcanic ash “pozzolan” found near Pozzouli,
Smeaton (British Engineer) rebuilt Eddystone Lighthouse in England using a mix of lime and clay pozzolans.
James Parker (England) took out patent on hydraulic cement.
n 1824: Joseph Aspdin patented work on cement and named it “Portland”
n Resembled stone from
Ready-Mixed industry started up in the United States.
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.
Paste hardens through chemical process…HYDRATION.
n Other: cementitious materials and admixtures, which can
n 2 GROUPS: COARSE AND FINE.
n PROVIDE THE BULK AND STABILITY
OF VOLUME TO CONCRETE.
BY ITSELF WILL NOT BE STRONG AND WILL SHRINK AND CRACK.
GENERAL COMPOSITION OF CONCRETE
n 10% CEMENTITIOUS MATERIALS
n 15% WATER
n 5-6% AIR
n 1800# COARSE AGG.
1200# FINE AGG.
n 600# CEMENTITIOUS
WATER (35 gal.)
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.
be strong and durable for intended use.
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.
Quality affected by quality of cementitious material and “Water / Cement Ratio”
n H2O / Cement Ratio:
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
of cementitious materials.
water content without adversely affecting WORKABILITY.
FRESH CONCRETE CHARACTERISTICS
n Rate of slump loss
n Setting Time
n Air Entrainment
n Ability to handle and place fresh
concrete without segregation.
FA / CA Ratio Greatly Affects:
Too much CA = harsh & difficult to handle
n Too much FA = higher water demand…reduces strength and increases chance
have ratio that results in lowest void content…”Densest mixture”
n Add more sand for finishability.
Measure of Workability
MISCONCEPTION: “Slump is a measure of water in the mixture.”
n Various admixtures can increase the slump while maintaining
a low H2O / Cement Ratio.
Slabs / Slabs on grade: 3-5 inch
Basement walls / sections with heavy reinforcement: > 6 inch
Rate of Slump Loss
reacts with water causing concrete to lose slump.
Rate of slump loss greater in higher temp.
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.
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
After placement and strike-off, heavier materials settle and water rises to surface.
n “Bleed Water” helps prevent plastic
shrinkage cracking on surface.
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.
Finishing will “trap” bleed water at surface...This will spall off with time.
n Important to properly time finishing
operations and scheduling form removal.
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
Characteristics of cementitious materials and amount of water.
n More water = Slower setting time
n Higher tempreature = Quicker setting time.
n Retarding admixes will slow set
n Accelerating admixes
will speed up set time
n Necessary to remove big AIR VOIDS.
n Internal vibrators commonly used.
n Overconsolidation causes
segregation and excessive bleeding.
Underconsolidation causes internal or surface voids…”Bugholes”
n Billions of tiny air bubbles form by mixing.
n Gives concrete ability to withstand freeze /
water turns to ice, it expands 9%.
If concrete saturated, the resulting pressure created by freezing and thawing will cause concrete to crack or
cause surface scaling.
relieve this internal pressure.
Recommended 5-8% depending on size of CA and exposure conditions.
CURING OF CONCRETE
n Maintaining adequate temperature and moisture so concrete will achieve its
potential strength and durable properties.
needed for continued hydration.
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.
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.
n Concrete is strong in compressive
strength is about 10-15% of compressive strength.
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.
Aggregates have great contribution.
measured at 28 days.
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
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
STRENGTH FACTORS FOR CONCRETE
/ Cement Ratio
size and quantity of aggregates and admixtures.
the same set of cementitious materials…
Increased mixing water = lower strengh.
Non-Air-Entrained > Air-Entrained
n However, reducing the mixing water in the air-entrained concrete will provide strength
nominal aggregate size results in higher strength, especially with a low water to cement ratio.
DURABILITY OF CONCRETE
and water tightness
n Drying shrinkage
n Heat of Hydration
n ASR (Alkali Silica Reation)
(Alkali Carbonate Reaction)
n Physical Salt Attack
of reinforcing steel
PERMEABILITY & WATER TIGHTNESS
Permeability: the resistance to permeation of water under pressure through concrete.
n In general, a lower H2O / Cement
ratio will reduce permeability.
Cementitious materials such as fly ash, slag, and silica fume significantly improve the microstructure in concrete,
thus improving watertightness and permeability.
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).
Air “bubbles” should be small in size and spaced close enough to relieve
pressure from migrating water during freezing.
“Technically” spacing factor should not exceed .008 inches.
should be 3500-4000 psi to withstand internal pressures from freezing.
n Plastic Shrinkage cracks occur on the surface of plastic
concrete if it dries before the concrete sets.
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 Drying shrinkage is the reduction in concrete volume after it sets
in moisture and/or temperature
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.
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.
Siliceous (rocky) sand is harder and more resistance than softer limestone sand.
HEAT OF HYDRATION
n Hydration reaction of cement and water
n Can be an advantage in cool weather…concrete can be maintained at an adequate temperature with
insulating blankets or formwork.
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)
of hydration can be reduced by:
Using fly ash or slag
n Using Type II cement (not common)
n Sulfate attack is a reaction between compounds in cement and sulfate in water or the ground the concrete is in contact
n Causes internal expansion and cracking
n Occurs over a long period:
5 to 30 years
How to minimize:
n Use of pozzolans (fly ash) and slag
water / cement ratio
Type II or Type V cement with lower C3A
Silica Reaction (ASR): reaction between alkali in cement and reactive silica aggregate.
alkali-silica gel that will absorb water or moisture so that the volume of the gel increases and creates cracks.
n Use non-reactive aggregates
n Low alkali cement combined with pozzolans
water / cement ratio
ALKALI AGGREGATE REACTION (cont.)
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.
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.
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.
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
of lime with carbon dioxide in the atmosphere.
Efflorescence appears as whitish
deposits on the concrete surface.
All surfaces undergo carbonation which does not harm concrete by itself.
is highly alkaline (high pH)
Helps protect reinforcing steel and embedded metal from corrosion.
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
high alkaline (pH) of concrete provides a passive and non-corrosive oxide layer on steel.
ions from admixes, deicing salts, or marine exposure destroy this protective layer.
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:
Reducing permeability of concrete
n Using non-chloride
Using corrosion inhibitor admixes
n Increase cover
of concrete to the rebar
Use epoxy-coated rebar
PROPORTIONING CONCRETE MIXTURES
(can be found in ACI 211 document)
BASIC INFORMATION NEEDED FOR “DESIGNING”
n Type of structure
n Cementitious materials
n Strength Requirement
n Requirements of Fresh Concrete
Slump, Set time, Air Content
n Prescriptive limitations:
/ Cement Ratio
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
BASICS OF WATER
n Mixing Water is ALL ADDED WATER and the free moisture on the aggegates.
n Higher slump = more mixing water
Larger nominal size CA = less mixing water
fractions is less in mixture
Air-entrained concrete = less mixing water
of sand…manufactured (more angular) sand requires more water than natural sand.
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:
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%.
Air content decreases with larger nominal maximum size of CA…< volume
Effects of entrained air on mixing water requirements are more significant in
lean or high water-cement mixtures.
ACI recommends lower air content for moderate exposure than severe exposure.
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.
CA feasible for construction should be used.
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!
shrinkage and temperature rise
n Quantity depends on:
Nominal max. aggregate size
n Fineness modulus (FM) of fine
n Lower FM of sand, more CA used
CA and increase FA
n Stiffer mix = more CA
n Use smaller nominal maximum sized aggregate.
n Increases cementitious content of mixture
Create With Concrete…
A Solid Investment