Hardened Concrete Properties

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29 Νοε 2013 (πριν από 3 χρόνια και 6 μήνες)

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Hardened Concrete
Properties


Strength


compressive strength 2000
-
8000 psi


tensile strength 200
-
800 psi


flexural strength


compression >> tension since concrete is notch
sensitivite

Factors Affecting Strength


Curing conditions, humidity


temperature


w/c , (inversely related) Abram’s law


air content, (inversely related), short and long term


aggregate characteristics, roughness,grading,
minerological.


cement type, composition, fineness, type I vs. type III


cement content (directly related)


Strength porosity relationship


mixing water


Strength and Curing

time

in air entire time

moist cured entire time

in air after 3 days

in air after 7 days

Strength

28

100%

Factors Affecting Concrete Strength

Durability


Resistance to freezing and thawing


Cracking


Internal Problems


Rebar Corrosion

Resistance to freezing and thawing
-
Major
factors


Air Entrainment


w/c, low water
-
cement ratio/ water content


volume stability, stiff aggregates with low coefficient of
thermal expansion

Air Entrainment


Water gains 9% in volume upon freezing


nighttime freezing followed by daytime thawing,


approximately 40 cycles per year, average. max of 200
cycles per year.


fatigue loading of ice formation within pores


Air Entraining Admixture (AEA) Must provide:


Pore size


Pore spacing


Pore specific surface area


Internal voids

Durability
-
Cracking


path for harmful material to get into
concrete


sulfates
-

soils


cause severe expansion, and deterioration


chlorides
-
deicing salts


initiate corrosion


Excessive shrinkage

Durability
-
internal problems


Alkali
-
silica reaction


excessive sulfates

Rebar corrosion

NaCl

+

-

Mechanism of corrosion

Corrosion Protection


Proper cover of at least 2”


lower w/c


denser concrete


avoid using chlorides

Review


Strength


Durability


Water Tightness


water cement ratio


Permeability


Volume Stability


shrinkage deformation with no load applied


creep deformation under sustained loading

Load Induced Volume Changes


Instantaneous, 1D




E


Secant modulus

Tangent modulus

c
.
concrete
'
f
E
5
1
33


ft
cubic
/
lbs
,
concrete
of
weight
unit


psi
,
strength
e
compressiv
'
f
c

Load Induced Volume Changes


Time dependant


Creep deformation

Deformation

Time

Creep in Concrete

Creep in Concrete

water

Creep

Consequences of creep


Loss in pre
-
stress


possibility of excessive deflection


stressing of non load bearing members

Economy


Cement Content


50
-
60$/ton


Aggregates


5
-
6 $/ton


minimum cement required at the minimum
water cement ratio, with the maximum
strength and durability