the introduction of concrete and a overview

1. Concrete and its mix

2. constituentsconstituents
mixture of aggregate and paste
paste 30 to 40%
portland cement 7% to 15% by Vol.
water 14% to 21% by Vol.
Aggregates 60% to 70%
coarse aggregates
Fine aggregates
Admixtures

3. Portland CementPortland Cement
Dry powder of very fine particles
forms a paste when mixed with water
chemical reaction-Hydration
paste coats all the aggregates together
hardens and forms a solid mass

4. WaterWater
needed for two purposes:
chemical reaction with cement
workability
only 1/3 of the water is needed for chemical reaction
extra water remains in pores and holes
results in porosity
Good for preventing plastic shrinkage cracking and
workability
Bad for permeability, strength, durability.

5. AggregatesAggregates
cheap fillers
hard material
provide for volume stability
reduce volume changes
provide abrasion resistance

6. AdmixturesAdmixtures
chemical
set retarders
set accelerators
water reducing
air entraining
mineral
fly ash
silica fume
slags

7. Properties of fresh concreteProperties of fresh concrete
Workability
ease of placement
resistance to segregation
homogeneous mass
Consistency
ability to flow

8. Slump TestSlump Test
Inverted cone
fill it up with three layers
of equal volume
rod each layer 25 times
scrape off the surface
8”
4”
12”

9. Slump TestSlump Test
slump cone
rod
concrete

10. Slump testSlump test
Slump
Ruler

11. Slump test resultsSlump test results
stiff 0-2”
massive sections, little reinforcement
use vibration
medium 2-5”
columns, beams, retaining walls
Fluid 5-7”
heavily reinforced section, flowable concrete

12. Factors affecting slumpFactors affecting slump
water cement ratio
w/c = weight of water / weight of cement
example:
weight of water mixed at the plant 292 lbs.
weight of cement 685 lbs./cu. yard
w/c = 292/685 = 0.43

13. water cement ratiowater cement ratio
if you add 10 gallons of water per cubic yard at job
site, then:
extra water
10 gallons/cubic yard * (3.8 liters/gallon) * (2.2
lbs./kg) *( 1kg/liter) = 83.77 lbs.
total water 282 + 83.77 = 365.77
new w/c = 365.77 / 685 = 0.534 >> 0.43

14. Factors affecting slump-Factors affecting slump-
paste contentpaste content
constant water cement ratio
increase paste content
increase slump
NO GOOD
constant cement content
increase water content
increase slump
NO GOOD

15. Factors Affecting Slump-Factors Affecting Slump-
Water ContentWater Content
Add water at the constant cement content,
w/c increases, slump increases.
Add water at a constant water cement ratio,
have to increase cement as well, slump
increases.

16. Factors affecting slump-pasteFactors affecting slump-paste
contentcontent
Low paste content
Harsh mix
High paste content
Rich mix

17. ball bearing effect-startball bearing effect-start
starting height

18. ball bearing effect-endball bearing effect-end
slump

19. AdmixturesAdmixtures
set retarding admixtures
set accelerating admixtures
water reducing admixtures
superplasticizers
air entraining admixtures

20. Factors affecting slumpFactors affecting slump
Aggregates
grading the larger the particle size, the higher
the slump for a given paste content

21. effect of aggregate sizeeffect of aggregate size
1”
1”
1”
Consider a single aggregate the size of 1”x1”x1”

22. Compute the surface area asCompute the surface area as
you break up the particlesyou break up the particles
volume = 1 cubic in
surface area = 6 square inches volume = 1 cubic in
surface area = 1.5*8= 12 square inches
block surface area = 0.5*0.5*6=1.5
block surface area = 1*1*6= 6

23. Break it up furtherBreak it up further

24. Compute the surface areaCompute the surface area
0.5 in
0.25 in
surface area = 0.25*0.25*6*8*8=24

25. Larger particles, less surface area,Larger particles, less surface area,
thicker coating, easy sliding of particlesthicker coating, easy sliding of particles

26. Smaller particles, more surface area,Smaller particles, more surface area,
thinner coating, interlocking of particlesthinner coating, interlocking of particles

27. Effect of aggregate sizeEffect of aggregate size
size # of particles volume surface area
1" 1 1 cubic inch 6 square inches
.5" 8 1 cubic inch 12 square inches
0.25 64 1 cubic inch 24 square inches
0.125 512 1 cubic inch 48 square inches

28. Angularity and surface textureAngularity and surface texture
of aggregatesof aggregates
angular and rough
aggregate
smooth aggregate
river gravel

29. TemperatureTemperature
fresh concrete
aggregates paste

30. BleedingBleeding

31. Water accumulation on surfaceWater accumulation on surface
Examine the concrete surface

32. Interaction between bleeding and evaporationInteraction between bleeding and evaporation
surface water
Evaporation
Bleed water
Bleed water = evaporation

33. Too much evaporation leads to surface crackingToo much evaporation leads to surface cracking
no surface water
Evaporation
Bleed water < Evaporation
drying

34. Side diagram of surface contractionSide diagram of surface contraction
Wants to shrink
Does not want to shrink

35. Free Shrinkage,Free Shrinkage,
causes volume change, but no stressescauses volume change, but no stresses
before shrinkage After Shrinkage

36. Restrained Shrinkage- createsRestrained Shrinkage- creates
stresses, which may cause crackingstresses, which may cause cracking

37. Restrained shrinkage crackingRestrained shrinkage cracking
Parallel cracking perpendicular
to the direction of shrinkage

38. Bleeding and its controlBleeding and its control
Creates problems:
poor pumpability
delays in finishing
high w/c at the top
poor bond between
two layers
causes
lack of fines
too much water content
Remedies
more fines
adjust grading
entrained air
reduce water content

39. Causes of Plastic Shrinkage CrackingCauses of Plastic Shrinkage Cracking
water evaporates faster than it can reach the
top surface
drying while plastic
cracking

40. Plastic Shrinkage Cracking-Plastic Shrinkage Cracking-
RemediesRemedies
Control the wind velocity
reduce the concrete’s temperature
use ice as mixing water
increase the humidity at the surface
fogging
cover w/polyethylene
curing compound
Fiber reinforcement

41. CuringCuring
The time needed for the chemical reaction
of portland cement with water.
Glue is being made.
concrete after 14 days of curing has
completed only 40% of its potential.
70 % at 28 days.

42. Curing tipsCuring tips
ample water
do not let it dry
dry concrete = dead concrete, all reactions stop
can not revitalize concrete after it dries
keep temperature at a moderate level
concrete with flyash requires longer curing

43. Temperature effects on curingTemperature effects on curing
The higher the temperature the faster the curing
best temperature is room temperature
strongest concrete is made at temperature around
40 F.(not practical)
If concrete freezes during the first 24 hrs., it may
never be able to attain its original properties.

44. Temperature effects on curingTemperature effects on curing
real high temperatures above 120 F can cause
serious damage since cement may set too fast.
accelerated curing procedures produce strong
concrete, but durability might suffer.
autoclave curing.