Lecture 4 - Fundamentals

1. Lecture 4 - Fundamentals January 22, 2003 CVEN 444

2. Lecture Goals Loading (continued) Concrete Mixing and Proportioning Concrete Properties Steel Reinforcement

3. Earthquake Loads

4. Earthquake Loads

5. Earthquake Loads

6. Earthquake Loads

7. Earthquake Loads

8. Earthquake Map

9. Roof Loads Ponding of rainwater Roof must be able to support all rainwater that could accumulate in an area if primary drains were blocked. Ponding Failure: ? Rain water ponds in area of maximum deflection ? increases deflection ? allows more accumulation of water ? cycle continues�? potential failure

10. Roof Loads Roof loads are in addition to snow loads Minimum loads for workers and construction materials during erection and repair

11. Construction Loads Construction materials Weight of formwork supporting weight of fresh concrete

12. Concrete Mixing and Proportioning Concrete: Composite material composed of portland cement, fine aggregate (sand), coarse aggregate (gravel/stone), and water; with or without other additives. Hydration: Chemical process in which the cement powder reacts with water and then sets and hardens into a solid mass, bonding the aggregates together

13. Concrete Mixing and Proportioning Heat of Hydration: Heat is released during the hydration process. In large concrete masses heat is dissipated slowly temperature rises and volume expansion later cooling causes contraction. Use special measures to control cracking.

14. Concrete Mixing and Proportioning 1. Proportioning: Goal is to achieve mix with Adequate strength Proper workability for placement Low cost Low Cost: Minimize amount of cement Good gradation of aggregates (decreases voids and cement paste required)

15. Concrete Mixing and Proportioning Water-Cement Ratio (W/C) Increased W/C: Improves plasticity and fluidity of the mix. Increased W/C: Results in decreased strength due to larger volume of voids in cement paste due to free water.

16. Concrete Mixing and Proportioning Water-Cement Ratio (W/C) (cont..) Complete hydration of cement requires W/C ~ 0.25. Need water to wet aggregate surfaces, provide mobility of water during hydration and to provide workability. Typical W/C = 0.40-0.60

17. Concrete Mixing and Proportioning Water/Concrete table

18. Concrete Mixing and Proportioning Proportions have been given by volume or weight of cement to sand to gravel (ie. 1:2:4) with W/C specified separately Now customary to specify per 94 lb. Bag of cement: wt. Of water, sand & gravel Batch quantity: wt. per cubic yard of each component

19. Concrete Mixing and Proportioning 2. Aggregates 70-75% of volume of hardened concrete Remainder = hardened cement paste, uncombined water, air voids More densely packed aggregate give better strength weather resistance (durability) Economical

20. Concrete Mixing and Proportioning 2. Aggregates Fine aggregate: sand (passes through a No. 4 sieve; 4 openings per inch) Coarse aggregate: gravel Good gradation: 2-3 size groups of sand Several size groups of gravel

21. Concrete Mixing and Proportioning Maximum size of coarse aggregate in RC structures: Must fit into forms and between reinforcing bars:(318-99, 3.3.2) 1/5 narrowest form dimension 1/3 depth of slab 3/4 minimum distance between reinforcement bars

22. Concrete Mixing and Proportioning Aggregate Strength Strong aggregates: quartzite, felsite Weak aggregates: sandstone, marble Intermediate strength: limestone, granite

23. Concrete Mixing and Proportioning Quality Workability Economical

24. Concrete Mixing and Proportioning Quality of concrete is measured by its strength and durability. The principal factors affecting the strength of concrete , assuming a sound aggregates, W/C ratio, and the extent to which hydration has progressed. Durability of concrete is the ability of the concrete to resist disintegration due to freezing and thawing and chemical attack.

25. Concrete Mixing and Proportioning Workability of concrete may be defined as a composite characteristic indicative of the ease with which the mass of plastic material may deposited in its final place without segregation during placement, and its ability to conform to fine forming detail.

26. Concrete Mixing and Proportioning Economical takes into account effective use of materials, effective operation, and ease of handling. The cost of producing good quality concrete is an important consideration in the overall cost of the construction project.

27. Concrete Mixing and Proportioning The influence of ingredients on properties of concrete.

28. Concrete Mixing and Proportioning 3. Workability Workability measured by slump test

29. Concrete Mixing and Proportioning

30. Concrete Mixing and Proportioning 4. Admixtures Applications: Improve workability Accelerate or retard setting and hardening Aid in curing Improve durability

31. Concrete Mixing and Proportioning 4. Admixtures Air-Entrainment: Add air voids with bubbles Help with freeze/thaw cycles, workability, etc. Decreases density: reduces strength, but also decreases W/C Superplasticizers: increase workability by chemically releasing water from fine aggregates.

32. Concrete Mixing and Proportioning 5. Types of Cement Type I: General Purpose Type II: Lower heat of hydration than Type I Type III: High Early Strength Higher heat of hydration quicker strength (7 days vs. 28 days for Type I)

33. Concrete Mixing and Proportioning 5. Types of Cement Type IV: Low Heat of Hydration Gradually heats up, less distortion (massive structures). Type V: Sulfate Resisting For footings, basements, sewers, etc. exposed to soils with sulfates.

34. Concrete Mixing and Proportioning

35. Concrete Mixing and Proportioning

36. Concrete Mixing and Proportioning

37. Concrete Properties 1. Uniaxial Stress versus Strain Behavior in Compression

38. Concrete Properties

39. Concrete Properties Compressive Strength, f�c Normally use 28-day strength for design strength Poisson�s Ratio, n n ~ 0.15 to 0.20 Usually use n = 0.17

40. Concrete Properties Modulus of Elasticity, Ec Corresponds to secant modulus at 0.45 f�c ACI 318-02 (Sec. 8.5.1): where w = unit weight (pcf) 90 pcf < wc <155 pcf For normal weight concrete (wc ? 145 pcf)

41. Concrete Properties In-Class Exercise: Compute Ec for f�c = 4500 psi for normal weight (145 pcf) concrete using both ACI equations:

42. Concrete Properties Concrete strain at max. compressive stress, ?o For typical ? curves in compression ?o varies between 0.0015-0.003 For normal strength concrete, ?o ~ 0.002

43. Concrete Properties Maximum useable strain, ?u ACI Code: ?u = 0.003 Used for flexural and axial compression

44. Concrete Properties

45. Concrete Properties

46. Concrete Properties 2. Tensile Strength Tensile strength ~ 8% to 15% of f�c Modulus of Rupture, fr For deflection calculations, use: Test:

47. Concrete Properties 2. Tensile Strength (cont.) Splitting Tensile Strength, fct Split Cylinder Test

48. Concrete Properties 2. Tensile Strength (cont.)

49. Concrete Properties 3. Shrinkage and Creep Shrinkage: Due to water loss to atmosphere (volume loss). Plastic shrinkage occurs while concrete is still �wet� (hot day, flat work, etc.) Drying shrinkage occurs after concrete has set Most shrinkage occurs in first few months (~80% within one year). Cycles of shrinking and swelling may occur as environment changes. Reinforcement restrains the development of shrinkage.

50. Concrete Properties

51. Concrete Properties Shrinkage is a function of W/C ratio (high water content reduces amount of aggregate which restrains shrinkage) Aggregate type & content (modulus of Elasticity) Volume/Surface Ratio

52. Concrete Properties Shrinkage is a function of Type of cement (finely ground�) Admixtures Relative humidity (largest for relative humidity of 40% or less). Typical magnitude of strain: (200 to 600) * 10-6 (200 to 600 microstrain)

53. Concrete Properties Creep Deformations (strains) under sustained loads. Like shrinkage, creep is not completely reversible.

54. Concrete Properties Magnitude of creep strain is a function of all the above that affect shrinkage, plus magnitude of stress age at loading

55. Concrete Properties Creep strain develops over time� Absorbed water layers tend to become thinner between gel particles that are transmitting compressive stresses Bonds form between gel particles in their deformed position.

56. Concrete Properties Tri-axial Compression Confined Cylinder Improved strength and ductility versus uniaxial compression Example: spiral reinforced where, F1 = longitudinal stress at failure F3 = lateral pressure

57. Concrete Properties Tri-axial Compression

58. Steel Reinforcement 1. General Standard Reinforcing Bar Markings

59. Steel Reinforcement 1. General Most common types for non-prestressed members: hot-rolled deformed bars welded wire fabric

60. Steel Reinforcement Areas, Weights, Dimensions

61. Steel Reinforcement 2. Types ASTM A615 - Standard Specification for Deformed and Plain-Billet Steel Bars Grade 60: fy = 60 ksi, #3 to #18 most common in buildings and bridges Grade 40: fy = 40 ksi, #3 to #6 most ductile Grade 75: fy = 75 ksi, #6 to #18

62. Steel Reinforcement 2. Types ASTM A616 - Rail-Steel Bars ASTM A617 - Axle-Steel Bars ASTM A706 - Low-Alloy-Steel Bars more ductile GR60 steel min. length of yield plateau = ?sh/?y = 5

63. Steel Reinforcement 3. Stress versus Strain Stress-Strain curve for various types of steel reinforcement bar.

64. Steel Reinforcement Es = Initial tangent modulus = 29,000 ksi (all grades) Note: GR40 has a longer yield plateau