Transcript Basic Principles of Concrete Design, Construction, and

Concrete
One Definition of Portland Cement Concrete…
 Portland cement concrete (PCC) is a heterogeneous system of
solid, discrete, gradiently sized, inorganic mineral aggregates,
usually plutonic or sedimentary-calcareous in origin, embedded
in a matrix compounded of synthesized polybasic alkaline and
alkaloidal silicates held in aqueous solution and co-precipitate
dispersion with other amphoteric oxides, this matrix being
originally capable of progressive dissolution, hydration, reprecipitation, gelation and solidification through a continuous
and co-existent series of crystalline, amorphous, colloidal and
cryptocrystalline states and ultimately subject to thermoallotriomorphic alteration, the system when first conjoined being
plastic during which stage it is impressed to a predetermined
form into which it finally consolidates, thus providing a structure
relatively impermeable and with useful capacity to transmit
tensile, compressive, and shear stresses.
(source unknown)
A Real Definition of PCC…
 A mixture of:
 Portland Cement
 Fine Aggregate
 Coarse Aggregate
 Water
 Air
 Cement and water
combine, changing from a
moist, plastic consistency
to a strong, durable rocklike construction material
by means of a chemical
reaction called “hydration”
Further Defined…
 Concrete exists in three
states
 Plastic
 Curing
 Hardened
Mix Design
 Combination of materials to provide the most
economical mixture to meet the performance
characteristics suitable for the application
 Developed in laboratory - produced in a batch
plant
 Mix proportions will typically vary over a range
for a given job
 Required strength and exposure conditions
 Mix consistency must be ensured to guarantee
concrete performance
Mixture Design Concepts
 Cement content
 Sacks/yd3 or lbs/yd3
 To a point, increasing cement content
increases strength and durability
 Too much cement is uneconomical and
potentially detrimental
 Amount of water
 Proper selection of aggregate and grading
 Admixtures?
Water-to-Cement Ratio
 The ratio of water-to-cement, or w/c, is the single
most important parameter with regards to concrete
quality
 Theoretically, about 0.22 to 0.25 is required for
complete hydration
 Practically, the useful limit is around 0.33
 Reducing the water for a given amount of cement
will move the cement particles closer together,
which in turn densifies the hydrated cement paste
 This increases strength and reduces permeability
 It also makes the concrete more difficult to work
 In combination, the w/c and degree of hydration
control many of the properties of the hardened
concrete
Voids in Hydrated Cement
 Concrete strength, durability, and volume
stability is greatly influenced by voids in
the hydrated cement paste
 Two types of voids are formed in hydrated
cement paste
 Gel pores
 Capillary pores
 Concrete also commonly contains
entrained air and entrapped air
Voids in Hydrated Cement
Paste
 Gel Pores
 Space between layers in C-S-H with thickness
between 0.5 and 2.5 nm
 Includes interlayer spaces, micropores, and small
isolated capillary pores
 Can contribute 28% of paste porosity
 Little impact on strength and permeability
 Can influence shrinkage and creep
Voids in Hydrated Cement Paste
 Capillary Voids
 Depend on initial separation of cement
particles, which is controlled by the w/c
 It is estimated that 1 cm3 of anhydrous portland cement
requires 2 cm3 of space to accommodate the hydration
products
 Space not taken up by cement or hydration products is
capillary porosity
 On the order of 10 to 50 nm, although larger
for higher w/c (3 to 5 mm)
 Larger voids affect strength and permeability,
whereas smaller voids impact shrinkage
w/c = 0.5
Source: Mindess, Young, and Darwin, 2004
Source: Mindess, Young, and Darwin, 2004
Source: Mindess, Young, and
Darwin, 2004
High Permeability
(Capillary Pores Interconnected)
Capillary Pores
C-S-H
Framework
Neville
Low-Permeability Capillary Pores
Segmented and Only Partially
Connected
Capillary Pores
C-S-H
Framework
Dimensional Range of Solids
and Voids in Hydrated Cement
Paste
Source: Mehta and Monteiro, 1993
Source: Mindess, Young, and Darwin, 2004
Source: Mindess, Young, and Darwin, 2004
Source: Mindess, Young, and Darwin, 2004
Interfacial Transition Zone
 Zone between the aggregate and bulk paste
 Has a major impact on the strength and permeability of the
concrete
 The interfacial zone is 10 to 50 mm in thickness
 Generally weaker than either the paste or aggregate due to
locally high w/c and the “wall effect” (packing problems) – in
some cases predominately large crystals of calcium hydroxide
and ettringite are oriented perpendicular to aggregate surface
 Greater porosity and few unhydrated cement grains
 Microcracking commonly exists in transition zone
 Results in shear-bond failure and interconnected
macroporosity, which influences permeability
 Modification of transition zone is key to improving concrete
Entrained Air
 Provides the path for
water to migrate from
larger voids to smaller
voids
 Water in smallest
capillary/gel pores
won’t freeze
 For adequate
protection
 6-8% air by volume
 Entrained air spacing
factor = 0.2mm
Entrained Air Measurement
 Proper air entrainment is one of
the most critical aspects of
producing durable concrete
 Air entrainment affects
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Strength
Freeze-Thaw durability
Permeability
Scaling Resistance
Workability
 Air content must be measured
accurately at the job site
Air-Void System
ASTM C 231 and
C 173
Stereo Microscope ASTM C 457
Curing Concrete
 Extremely important
 Concrete will not achieve its potential strength
unless it is properly cured
 Concrete will crack if not properly cured
 Curing should be started immediately after
final set
 Curing includes providing both moisture
and temperature
Curing
 Concrete must not dry out, especially at a
young age
 Preferably water is applied after the concrete
has set
 Steam curing applies both heat and
moisture, accelerating hydration
 Often, waterproof barriers are used to hold
mix water in…not as good as wet curing
Durability
 Concrete is inherently durable, having a
history of exceptional long-term performance
 In some instances, the structure’s service life
has been adversely affected by the concrete’s
inability to maintain its integrity in the
environment in which it was placed
 These distress manifestations are
categorized as materials-related distress
(MRD)
What is Materials-Related
Distress?
 MRD is commonly associated with the
“durability” of the concrete
 Durability is not an intrinsic material property
 “Durability” cannot be measured
 Concrete that is durable in one application may
rapidly deteriorate if placed in another application
 It is not related to loading, although loading can
exacerbate the distress
Common Types of MRD
 Physical Mechanisms
 Freeze-thaw Deterioration of Hardened Cement
Paste
 Deicer Scaling/Deterioration
 Freeze-Thaw Deterioration of Aggregate
 Chemical Mechanisms
 Alkali-Aggregate Reactivity
 Alkali-Silica and Alkali-Carbonate Reactivity
 Sulfate Attack
 External and Internal Sulfate Attack
 Corrosion of Embedded Steel
Important Considerations
 The concrete constituents, proportions,
and construction all influence MRD
 Water is needed for deleterious expansion
to occur
 Severe environments (e.g. freezing and
thawing, deicer applications, high sulfate
soils, etc.) make it worse
 Strength does not equal durability
Summary
 Concrete is an immensely complex material that
will perform to its potential only if treated
properly during the entire construction phase
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Mix design and proportioning
Transporting
Placing and consolidating
Finishing and curing
 As billions are spent annually on concrete
construction, the most sophisticated testing is
used to ensure quality
ASTM C 143-00 Standard Test Method for Slump
of Hydraulic Cement Concrete