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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

 

 

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