Fundamentals of Concrete Concrete 101

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Fundamentals of ConcreteConcrete 101

• Joseph E. Diedrich
• Technical Service Manager
• Southdown, Inc.

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Concrete 101

• Concrete is basically a mixture of two
  components
• Paste
• Aggregates
• Paste (Portland cement, water, and air)
• Aggregates (sand, gravel, crushed stone)

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Concrete 101

• Portland cement meets AASHTO M-85 or ASTM C-150
• Type I, II, III, IV, V
• Blended Portland Cement meets AASHTO M-240 or
  ASTM C-595
• Type I(pm), I (sm), I (p), etc...

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Concrete 101

• Aggregate are divided into two groups
• Fine - AASHTO M-6 or ASTM C-33
• Coarse - AASHTO M-80 or ASTM C-33
• Fine aggregate consist of natural or manufactured
  sand with particle size up to 3/8 inch
• Coarse aggregate particles are retained on the 4
  sieve and range up to 6 inches

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Concrete 101

• Cement Water Air Fine Aggregate
  Coarse Aggregate

Cement paste constitutes about 20 to 40 volume
of concrete
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Concrete 101

• Cement is usually between 7 and 15 based on
  absolute volume
• Water is between 14 and 21 based on absolute
  volume
• Air volume ranges up to 8 based on top size of
  coarse aggregate
• Aggregates, fine coarse complete the mix

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Air Entrained Concrete

• Why the difference between line 1 and line 2?
• Line 1, small size CA, increased H2O and cement
• Line 2, larger CA, less H20, air, cement

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Non-Air Entrained Concrete
Cement Water Air Fine Aggregate Coarse
Aggregate

• Similar to Air Entrained Concrete slide
• See how aggregate size impacts water demand and
  amount of paste in the mix

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Concrete Ingredients

• Quality of concrete depends on quality of paste
  and quality of aggregates
• each particle of aggregate is completely coated
  with paste
• All spaces between aggregate is filled with paste

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Quality of Concrete

• Is determined by
• W/C Ratio - Advantage of reducing water
• Increased compressive and flexural strength
• lower permeability
• increased resistance to weathering
• reduced shrinkage cracking tendencies
• The less water used, the better quality of
  concrete, provided it can be consolidated
  properly.

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Admixtures

• Plastic and hardened properties of concrete may
  be
• changed by adding admixtures

• Admixtures are commonly used to

• increase set time
• decrease set time
• increase workability
• adjust other concrete properties

• reduce water demand
• entrain air
• inhibit corrosion

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Curing

• Critical to durable concrete
• Increases strength
• Decreases permeability
• Increases durability

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Curing Concrete

• How concrete is cured
• Ponding or Immersion
• Spraying or Fogging
• Wet Coverings
• Impervious Paper
• Plastic Sheets
• Membrane-Forming Curing Compounds

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General Curing Requirements

• Time 3-7 days
• Temperature 50-100 F
• Moisture Saturated at all times

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Cold Weather Concreting

• Cold weather exists when, for more than 3
  consecutive days, the following conditions are
  present
• Average daily temperature is less than 40 F
• Air temperature is not greater than 50 F for more
  than one-half of any 24-hour period

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The Main Objectives

• Prevent concrete from freezing at an early age
  (first 24-48 hours)
• Assure concrete reaches strength for loading or
  form removal
• Maintain normal curing conditions
• Limit rapid temperature changes

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Concrete Placement Temperatures

• Dont exceed placing temperature target by more
  than 20 F
• Higher temp requires more water, increases rate
  of slump loss, and increases thermal cracking

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Preparation Before Placement

• Temperature of surfaces in contact with the
  concrete should be above freezing (35 F)
• Embedments should have temp above 10 F
• All snow, ice, and frost must be removed
• Never place on a frozen subgrade. Cover subgrade
  with insulated blankets a few days prior to
  concrete placement.

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Insulated Curing Box
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Other Considerations

• Discuss all aspects of cold weather concreting at
  the pre-construction meeting
• Watch weather in the spring and fall, must
  provide protection during first 24-48 hours after
  placement

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Be Prepared!!!

• Monitor the weather forecast
• Monitor the temperature of the concrete
• Have all materials available before placing
  concrete
• Anticipate the worst case and be prepared for it
• Schedule the placement during the warmest part of
  the day

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Remember!!!

• If you need a coat to be comfortable outside, so
  does the concrete!!!

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What is Hot Weather Concreting?

• When concrete is mixed, transported, and place
  under conditions of high ambient temperature, low
  humidity, solar radiation, or wind, an
  understanding of the effects there environmental
  factors have on concrete properties and
  construction operations is required (ACI 305).

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General

• Initial curing of concrete test specimens for
  checking adequacy of mix for strength or basis of
  acceptance are cured initially at 60 to 80F
  (16 to 27C), ASTM C31
• 24hr cure at 100F, 28 day may be 10 to 15
  percent lower than cured ASTM C31
• drying at early age, strengths will be reduced
  even further

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Definition of Hot Weather

• High ambient temperature
• High concrete temperature
• Low relative humidity
• Wind velocity
• Solar radiation
• Hot weather is any combination of the above which
  impair quality of fresh or hardened concrete
  and/or cause detrimental results

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Hot Weather

• Hot weather concrete problems generally occur
  during the summer season
• Thermal shrinkage is more severe in the spring
  and fall (temperature differential)
• When air temperatures are identical, a calm,
  humid day is less severe than a dry, windy, sunny
  day

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Potential Problems in Hot Weather - Plastic
Concrete

• Increased water demand
• Increased rate of slump loss, tendency to add
  water at the jobsite
• increased rate of setting, resulting in difficult
  handling, finishing, and risk of cold joint
• Increase for plastic shrinkage
• Increased difficulty in controlling entrained air
  content.

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Potential Problems in Hot Weather - Hardened
Concrete

• Decreased 28 day and later strengths, higher
  water demand, concrete temperatures
• Increase for drying shrinkage and thermal
  cracking
• Decreased durability from cracking
• Increased permeability
• Increased potential for reinforcing steel
  corrosion due to increased cracking

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Avoiding Potential Problems

• Use concrete materials and proportions with
  satisfactory records in hot weather
• Use cool concrete
• Use a concrete consistency that permits rapid
  placement and effective consolidation
• Transport, place, consolidate, and finish with
  least delay
• Protect concrete against moisture loss at all
  times, during placement and curing period

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Temp, Slump, Water Demand
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Questions?

• Remember
• Use Common Sense
• Temperature effects on your body have very
  similar effects on concrete
• If you are not sure, ASK!!!