1. Concrete Technology

2. What is Hydraulic Cement Concrete?
A homogeneous mixture of aggregates held together by a hardened hydraulic cement paste
While concrete looks simple, it is really a highly complex material
What can be said about this sample of concrete?

3. Hydraulic cement concrete consists of two primary components:
Cement paste
Portland cement (Hydraulic)
Water
Air
Admixtures
Aggregates
Coarse (and intermediate)
Fine

4. Inlets & Manholes Concrete Pavement Box Culvert Ready Mix Cementitious4
Inlets &
Manholes
Concrete
Pavement
Box Culvert
Ready
Mix
Cementitious
Aggregates
Admixtures
Water
Concrete
Block
Autoclave
Air entrained
Concrete (ACC)
Flowable
fill
Pipe
SCC

5. Strength & Weakness
compressive flexural tensile shear torsion
Unreinforced concrete can provide high compressive strength, but relatively low tensile, flexural, shear and torsional strengths 5

6. Cement Hydration

7. Cement Hydration
Hydration is a chemical reaction between cement and water.
Hydration produces three things:
calcium silicate hydrate (CSH) gel that hold aggregates in place and gives concrete its strength (glue)
heat
calcium hydroxide that fills the remaining voids, does not contribute much to strength

8. Cement Hydration C3S + C2S + C3A + H2O CS Hydrates (CSH)
Glue
Glue
CS Hydrates (CSH)
+ CA Hydrates (CAH)
+ Ca(OH)2 (CH)
+ heat
CSH gel is the glue that holds concrete together.
Cement
and Water

9. These fingers start to lock together...
…and grab onto sand and gravel particles as well

10. Eventually, these crystals get so intertwined that the concrete stiffens...
…this is what we call “setting.”

11. As the water and cement are used up, the crystals form a hard, dense structure
…water that isn’t used in the chemical reaction evaporates, leaving spaces and channels in the concrete

12. Cement Hydration Hydrated Cement Paste
Depending on the time of hydration and the Portland cement composition, several crystalline pictures can be observed in hydrated cement paste. A typical one contains CSH, calcium hydroxide and ettringite as shown in this picture (photo courtesy of Mr. Jim Margeson, NRC-IRC). 

13. Heat of Hydration Stage1: initial hydrolysis Stage 2: dormant period
Stage 3: accelerated hydration determines rate of hardening and final set
Stage 4: determines the rate of early strength gain
Stage 5: slow formation of hydration products establishing the rate of later strength

14. Water and W/C Ratio

15. Why Water is Needed in Concrete
Hydration:
The chemical reaction between cement and water which creates strength by forming interlocking crystals of CSH gel that holds aggregates in place
Workability:
Can have 2 to 3 times more water in the mix than needed for hydration (known as water of convenience)

16. Rules of Thumb for Adding Water to Concrete
Adding 1 gal / yd3 of water
increases slump 1” (25 mm )
decreases compressive strength by about 5%
wastes the effect of 24 lbs/yd3 of cement
increases shrinkage by 10%
increases permeability by up to 50%
decreases freeze-thaw durability by 20%
decreases resistance to deicing salts and lowers quality in many other ways

17. 100 pounds of water is 12 gallons /yd3
Drying Shrinkage
More Cement
More Water
100 pounds of water is 12 gallons /yd3
Leads to more:
Shrinkage
Curling
Cracking
Don't chase the W/C ratio to obtain strength. Use reasonable combinations of Cement, Water, and Water reducer to control drying shrinkage.

18. Abram’s Water / Cement Ratio Rule
Water / Cementitious Ratio
Abram’s Water / Cement Ratio Rule
As the water to cement ratio increases, the strength of a concrete mix decreases.
In other words: If everything else is constant and you add more water, you get less strength. 18

19. Water / Cementitious Ratio
It’s a calculation:
w/c ~ lbs. of water / lbs. of cement
w/cm ~ lbs. of water / lbs. of cementitious
Often when w/c is discussed its really w/cm that is intended as the reference =
Water cement ratio
45 lbs of water
0.45 expressed as decimal
100 lbs of cement
Water needs to be drinkable or meet ASTM 1602

20. Water / Cementitious Ratio
The strength & durability of concrete is
greatly influenced by the w/c ratio!

21. Proportioning

22. Volume of materials totals to 27ft3
Proportioning
Volume of materials totals to 27ft3
Simply showing what a mix design looks like and must ALWAYS add to 27 ft3 22 22

23. Proportioning
The goal is to have 27.0 cubic feet of materials blended together to form a homogeneous substance with the following properties:
Strength
Durability
Watertightness
Appearance
Economy
Dimensional Stability

24. Proportioning Wet Cast Concrete Proportion to ACI 211
“Conventional” slump concrete
Water/Cement Ratio
Usually high
typically (Precast)
Relatively light vibration
Admixtures common & enhance properties
May require air entrainment for resistance to freeze/thaw damage
Conventional = High slump w/ no supers 24

25. Proportioning Dry Cast Concrete Absolute Volume Method
Zero slump concrete
Water/Cement Ratio
Is very low
Typically 0.33 to 0.45
Large form vibrators consolidate the mix
May or may not include additives
Historically, resists freeze/thaw damage without entrained air 25

26. Durability of Concrete

27. Durability of Concrete
Freeze/Thaw Attack
Alkali-Aggregate Reaction
Chemical Attack
Sulfate attack from sources external to concrete
Physical salt attack

28. Freeze/Thaw Attack

29. Freeze/Thaw Attack Water Expands about 9% when frozen
Air-entraining agent binds cement grains around air

30. Freeze/Thaw Attack
Aggregates with a total smaller pore size result in a lower resistance to freezing & thawing

31. Compressive Strength Vs % Air
Water content was reduced with increased air content to maintain a constant slump

32. Freeze/Thaw Attack Air Content Entrapped air Entrained air 32 32

33. Entrapped Air Air trapped during mixing and placement process
Large, non-uniform voids, generally undesirable
Reduced through proper vibration & consolidation 33

34. Entrained Air (Wet cast)
Additive creates a uniform network of small spherical voids/bubbles
Voids provide relief reservoirs and prevent freeze/thaw damage
Required for all products exposed to freeze/thaw conditions
It’s the quality of air that gives durability
Quality Harden Air
Spacing Factor < 0.008
Specific Surface > 600
Model Chord length 34

35. How do you measure? Wet cast
ASTM C231 Test Method for Air Content of Freshly Mixed Concrete by the pressure method
ASTM C173 Air Content of Freshly Mixed Concrete by Volumetric Method.

36. How do you measure? Dry cast ASTM C457 Standard Test Method for
Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete.

37. Alkali–Aggregate Reaction

38. Alkali-Aggregate Reaction
ACR (Alkali-Carbonate Reaction)
A reaction between an alkali source and certain calcium magnesium carbonate rocks (dolomites)
ASR (Alkali-Silica Reaction)
Reaction between an alkali and certain forms of reactive silica that can originate from some types of siliceous aggregate – far more widespread than ACR

39. ASR - How it works
Cement paste and reactive siliceous aggregate form a gel around the aggregate particle
Gel reaction product & moisture creates expansion
Expansion usually creates three cracks 39

40. ASR - How it works
Potential for ASR is widespread throughout the United States and the world due to the presence of reactive aggregates. 40

41. Reaction product (gel)
Cement paste
In ASR, aggregate will crack…DEF the aggregate will not
Reactive Aggregate
Reaction product (gel) 41

42. Pics showing ASR 42 42

43. Preventative Measures for ASR
Use of non-reactive aggregate
Use of low-alkali cement
Limited alkali content of concrete
Use of supplementary cementing materials (typically Class F or slag)
Use of suitable chemical admixtures 43

44. Chemical Attack

45. Chemical Attack
Sulfate attack from sources external to concrete (Not to be confused with hydrogen sulfide from sanitary sewers)
Physical salt attack

46. How does Sulfate Attack Work in Concrete?
Sulfate salts, when present in solution,
react with the hydration products
Sodium Sulfate attacks the CH & CAH
Yields a semi-hydrated form of gypsum and ettringite
Can lead to softening of the paste, loss of strength & Increase in porosity
Calcium Sulfate attacks the CAH
Yields ettringite
Increases the solids volume causing expansion and cracking
Magnesium Sulfate attacks CH, CAH, & CSH
Yields gypsum, ettringite, and brucite (Mg(OH)2)
Destroys CSH
Particularly devastating
Other sulfate related processes can damage concrete without expansion
Evidence of sulfate attack verified by petrographic and chemical analyses

47. Sulfate Attack Recommendations for protection
Use concrete that retards the ingress of the external sulfate
Low w/c ratio
Low permeability
Sulfate resistant cement (low C3A<5% Type V; <8% Type II)
Slag cement and or Class F Fly ash

48. Sulfate Attack

49. Physical Salt Attack
Deterioration occurs by physical action of salts from groundwater containing sodium sulfate, sodium carbonate, & sodium chloride
Damage typically occurs at exposed surfaces of moist concrete that is in contact with soils containing salts.
The distress in surface scaling is similar in appearance to freezing-and-thawing damage

50. How to make Durable Concrete?
Low w/cm ratio
Ample amount of cementitious
Quality Aggregates
Correct admixtures
Proper mixing & placing
Adequate Curing

51. QUESTIONS?