1. Concrete
Man made stone

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

4. Water 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. Aggregates cheap fillers hard material provide for volume stability
reduce volume changes
provide abrasion resistance

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

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

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

9. Slump Test
slump cone
rod
concrete

10. Slump test
Ruler
Slump

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

12. Factors 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 lbs./cu. yard
w/c = 292/685 = 0.43

13. water 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) = lbs.
total water =
new w/c = / 685 = >> 0.43

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

15. Factors Affecting Slump- Water 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-paste content
Low paste content
Harsh mix
High paste content
Rich mix

17. ball bearing effect-start
starting height

18. ball bearing effect-end
slump

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

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

21. effect of aggregate size1” 1” 1”
Consider a single aggregate the size of 1”x1”x1”

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

23. Break it up further

24. Compute the surface area
0.5 in
0.25 in

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

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

27. Effect of aggregate size

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

29. Temperature
fresh concrete
aggregates
paste

30. Bleeding

31. Water accumulation on surface
Examine the concrete surface

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

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

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

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

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

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

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

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

40. Plastic Shrinkage Cracking-Remedies
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. Curing
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 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 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 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.