Properties of Concrete II

1. Properties of Concrete II UAA School of Engineering CE 334 - Properties of Materials Lecture # 15 Properties of Concrete II

2. Concrete Strength & Behavior • The compressive strength of concrete is high and is taken as an important index of its general quality. • The tensile strength of concrete is roughly 10% of the compressive strength and is often neglected by structural engineers. • The strength is influenced by the age, size, shape of the specimen and loading rate. • Nonlinear stress-strain relationship. Properties of Concrete II

3. Typical Stress-Strain Curves Properties of Concrete II

4. Concrete Behavior Properties of Concrete II

5. Concrete Strength & Modulus Elasticity Significant strength properties: • 28-day compressive strength, f ’c Determined by compression cylinder test. • Tensile strength, fct < fct< (psi) (normal weight conc) • Modulus of Elasticity, E (psi) Properties of Concrete II

6. Factors Affecting Strength • Water/cement (w/c) ratio • Age (3) Curing conditions (4) Other Factors Affecting Strength Properties of Concrete II

7. Factors Affecting Strength: (1) Water/Cement Ratio • Optimum is generally 0.45 < w/c < 0.55 • The lower w/c ratio, the higher strength is • Need sufficient water in mix for hydration to occur. • Excesswater must be drained off after hydration, leaving capillary voids that reduce strength • Strength increases with decreasing w/c • Approximate 28 day Strength = 24000/18w/c (psi) (Reference value for your Lab) Properties of Concrete II

8. Factors Affecting Strength: (2) Age • Concrete strength continues to increase with time. • Designvalues are generally based on the strength at 28days. 90%+ strength can be achieved in 28 days. Properties of Concrete II

9. Factors Affecting Strength: (3) Curing Conditions • Time, Moisture, Temperature (TMT) • Concrete hardens by hydration which is chemical surface reaction between the cement and water. • Hydration releases heat which can drive off moisture, hence concrete must be kept moist until hydration is complete. • The reaction takes time. • Increased temperature increases the reaction rate. • Allowing the mix to freeze during hydration will destroy the strength and durability of the concrete. Properties of Concrete II

10. Factors Affecting Strength: (4) Other Factors Affecting Strength • Character of Cement. • Since hydration is a surface reaction, the finer the particles, the more surface area and the more complete the reaction, resulting in increased strength. • Moisture content at the time of testing. • A moist specimen develops large pore pressures which cause premature failure of compression specimens. • Cement Content and Aggregate Condition. • Angular aggregates require more paste to be workable • More cement content requires more water. Properties of Concrete II

11. Start Design Procedure in Making Concrete If slump is not given, decide the slump, using Table 1 Decide the maximum size of aggregate; choose the maximum possible size using the following guidelines: Maximum size > 1/5 narrower dimension between forms > 1/3 depth of slab > 1/4 of clear spacing between reinforcing bars Decide the amount of water and air (Table 2) Select the W/C ratio (Table 3) Calculate cement content = C/W x wt of water Choose the amount of coarse aggregate (Table 4) Calculate the amount of fine aggregate using the estimated weight of fresh concrete (Table 5) and the known weights of water, cement, and coarse aggregate. Adjust for moisture content in coarse and fine aggregate Trial mix End Properties of Concrete II

12. Flow Chart for Selection and Documentation of Concrete Proportions Properties of Concrete II

13. Important Mix Design Considerations (1) Strength (2) Workability (3) Durability (4) Economy (5) Volume changes Properties of Concrete II

14. Mix Design Considerations: (1) Strength • The lower w/c ratio, the higher strength is • Character of Cement • Cement Content and Aggregate Condition Properties of Concrete II

15. Mix Design Considerations: (2) Workability - Properties of Fresh Concrete • Generally measured by the slump test • Increases with increased water content • Increases with increased air content • Decreases with increase in coarse aggregate • Decreases with increase in angularity of aggregate. Properties of Concrete II

16. Mix Design Considerations: (3) Durability • Freeze-Thaw Durability • Can be increased with proper entrained air content. • Use an air entraining admixture • Abrasion Durability • Affected by aggregate strength. • Durability of cement paste is typically lower than aggregate strength. Properties of Concrete II

17. (4) Economy Mix Design Considerations: (4) Economy • Minimize cement • Must have enough paste to coat aggregates • Maximize the use of aggregates Properties of Concrete II

18. (5) Volume Changes Mix Design Considerations: (5) Volume Changes • Shrinkage andcreep increase with cement content. • Chemical hydration generally reduces volume. • Moisture fluctuations cause expansion and contraction. This is more pronounced in concrete with high w/c ratios and/or highcement contents. Properties of Concrete II

19. Common Concrete Tests • Slump Test, ASTM C 143. Used to determine the workability of fresh concrete. Sometimes used as an indication of water content. • Air Test, ASTM C 231. Used to the determine the air content in fresh concrete. • Compression Test, ASTM C 39. Used to test the 7 and 28 day strength of 6” dia. by 12” long cast cylinders. Properties of Concrete II

20. Comprehensive Concrete Compression Test • Extensometer and chain kit for lateral strain measurement Concrete specimen • Extensometers for axial strain measurement • Software for test control, data collection and reporting Properties of Concrete II

21. Concrete Splitting Tensile Test • Platens for longitudinal loading of specimens • Safety chains • Software for test control, data collection and reporting Properties of Concrete II

22. Concrete Direct Tension Test • High strength, rapid setting epoxy to glue specimens to end caps • Chain kit to minimize bending moments during test • Spherically seated ends to minimize non-concentricity Properties of Concrete II

23. High Performance Concrete Testing Properties of Concrete II

24. Confined Concrete Behavior I Properties of Concrete II

25. Confined Concrete Behavior II Asp fyhAsp ds fyhAsp Confinement from spiral or circular hoop Forces acting on 1/2 spiral or circular hoop Confinement from square hoop Properties of Concrete II

26. Opened 90° hook on hoops Properties of Concrete II

27. Shear failure of bridge pier Properties of Concrete II

28. Column with spiral reinforcement Properties of Concrete II

29. Column with inadequate ties Properties of Concrete II

30. Column anchorage failure Properties of Concrete II

31. Properties of Concrete II

32. Properties of Concrete II

33. Properties of Concrete II

34. Topic 8 Concepts of Seismic Resistant Design

35. Suggestions for Lab • READ THE LAB HANDOUT IN ADVANCE. • Come prepared to get dirty. Fresh concrete can be hard on clothes and hands. • You will be cleaning all the equipment at the end of each group experience and may get “damp”. Be prepared for this. Boots might be a good idea. Properties of Concrete II

36. References • S. Mindness, Concrete, 1st ed, Prentice-Hall, Inc., 1981. • ACI 318: Building Code Requirements for Reinforced Concrete, American Concrete Institute (ACI), Chicago, Il, 1989. • ACI Manual of Concrete Practice, ACI, revised annually. • Specifications for Structural Concrete for Buildings: Field Reference Manual, ACI publication SP-15, ACI, 1989 Properties of Concrete II