Concrete

1. Concrete

2. Major Topics • History • Uses • Materials Used To Make Concrete • Cement • Aggregate • Water • Admixture

3. Major Topics con’t • Testing • Slump Test • Compressive Strength Test • Air Content Test • Strength • Placing

4. Major Topics con’t • Transporting • Curing • Finishing • Reinforced Concrete • Pre-cast Concrete • Pre-Stressed Concrete

5. Concrete History Facts The History of Concrete: Textual Noteworthy: The Hoover Dam, outside Las Vegas, Nevada, was built in 1936. 3 ¼ million cubic yards of concrete were used to construct it.

6. Concrete Resources Concrete Admixtures - The Concrete Network

7. Uses • Foundations and Driveways • Architectural Details • CMU (Concrete Masonry Units) • Concrete Roofing (Arches & Domes) • Columns, Piers, Caissons • Walls and Beams • Bridges

8. Materials Used to Make Concrete • Portland Cement – 5 types • Should conform to ASTM C150 • Type 1 – standard; widely used; columns, floor slabs, beams • Type 2 – has a lower heat of hydration; used in massive pours; e.g. Dam construction • Type 3 – high early strength; suitable for cold weather • Type 4 – termed low heat; used in massive pours to diminish cracking • Type 5 – sulfate resistant; used in sewage treatment plants & concrete drainage structures

9. Air-Entraining Portland Cement • Produces billions of tiny bubbles • Greatly reduce segregation of mix • Less water needed to produce a “workable” mix • Has a better resistance to freezing and thawing • Classified as Type 1A, 2A, 3A

10. Aggregate • 2 classes • Fine – sand; < 3/8 “ large • Coarse – gravel or crushed stone • Grading should conform to ASTM C33 • Sieve analysis test (ASTM C136) and analyses for organic impurities (ASTM C40) often done • Represent 60-80% of the concrete volume

11. 5 Aggregate Types • Natural – sand and gravel • By-Product – blast-furnace slag or cinders • Lightweight – materials heated and forced to expand by the gas in them • Vermiculite – a type of mica that will greatly expand • Perlite – a type of volcanic rock which expands

12. The Critical Role of Water in Mix • Hydration – chemical reaction caused by mixing the water with cement • Too much – prevents proper setting • Laitance (bleeding) – white scum or light streaks on the surface of concrete which are very susceptible to failure • Too little – prevents complete “chemical reaction” from occurring

13. Proportioning of Mix • 1: 2: 4 – concrete consisting of : • 1 volume of cement • 2 volumes of fine aggregate • 4 volumes of coarse aggregate • Emphasis now on “Water-Cement” ratio methods of proportioning

14. Typical Design Mix (Yield: 1 cu.yd. of 3,000 psi of Concrete) *** • 517 lb. of cement (5 ½ sacks) • 1,300 lb. of sand • 1, 800 lb. of gravel • 34 gal. of water (6.2 gal. per sack) *** Data from Architectural Graphics Standards, 2000

15. Admixtures • Materials added into the standard concrete mixture for the purpose of controlling, modifying, or impacting some particular property of the concrete mix. • Properties affected may include: • Retarding or accelerating the time of set • Accelerating of early strength

16. Admixtures con’t • Increase in durability to exposure to the elements • Reduction in permeability to liquids • Improvement of workability • Reduction of heat of hydration • Antibacterial properties of cement • Coloring of concrete • Modification in rate of bleeding

17. Testing of Concrete May Include • Slump Test [ASTM C143] • Determines the consistency and workability • Compressive (Cylinder) Strength [ASTM C192] • Determines the “compressive unit strength” of trial batches • Air Content

18. Slump Test **Concrete sample is placed into a 12” sheet metal cone using 3 equal volumes. **Each layer is tamped 25 times with a bullet-nosed 5/8” by 24” rod. **Last layer is leveled off with the top of the cone. **Cone is removed **The vertical distance from the top of the metal cone to the concrete is measured

19. Compressive Strength Test • Comply with ASTM C39 • Basic steps: • # of samples taken vary (no less than 3) • 3 layers of concrete placed in a cardboard cylinder 6” in diameter and 12” high. • Each layer is rodded 25 times with a 5/8” steel rod • Samples are cured under controlled conditions • Test ages vary but usually done after 7, 14, and 28 days • Sample removed from cardboard and placed in testing apparatus which exerts force by compressing the sample until it fails (breaks)

20. Strength of Concrete: • Stated as the minimum compressive strength at 28 days of age • Design strength: • Typical residential 2,500 – 4,000 psi • Pre- or Post tensioned typically 5,000 – 7,000 psi • 10,000 – 12,000 psi used in columns for high- rise buildings

21. Placing Concrete • Temperature • Optimum temperature for curing is 73 degrees F; may have problems curing if temperature below 40 degrees F • Forms • Wood and metal commonly used (reused) • Clean and sufficiently braced to withstand the forces of the concrete being placed • Concrete weighs 135 – 165 pcf; if lightweight then 85 – 115 pcf; often in estimating the figure 150 pcf is used

22. Placing Concrete con’t • Free falling distance should not exceed 4-5 feet due to the threat of “segregation” of aggregates occurring

23. Transporting Concrete • Method selected depends on quantity, job layout, and equipment available • Chutes • Wheelbarrows/Buggies • Buckets • Conveyors • Pumps

24. Curing • Proper curing is essential to obtain design strength • Key factor: the longer the water is retained in the mix – the longer the reaction occurs – better strength

25. Evaporation of Water Reduced by: • Cover with: • Wet burlap or mats • Waterproof paper • Plastic sheeting • Spray with curing compound • Leave concrete in forms longer

26. Joints • 3 types: • Isolation (expansion) – allow movement between slab and fixed parts of building • Contraction (control) – induce cracking at pre-selected locations • Construction – provide stopping places between pours • Materials used: • Rubber/plastic • Vinyl, neoprene, polyurethane foams • Metal/wood/cork strips

27. Finishing • Screeds – used to level the concrete placed in the forms • Consolidation – may be accomplished by hand tamping and rodding or using mechanical vibration • Floating – done while mix still in plastic state; provides a smooth surface

28. Finishing con’t • Final stage may include: • Incorporation of materials for toppings (adjust the “look”) • Non-slip finish – use broom to “rough-up” the surface • Patterns – accomplished by pressing form patterns into surface

29. Reinforced Concrete • Concrete has good compression strength but little tensile strength • Steel excels in tensile strength and also expands and contracts at rates similar to concrete • Steel and concrete compliment each other as a unit

30. Reinforcing Steel [Rebar] • Manufactured as round rods with raised deformations for adhesion and resistance to slip in the concrete • Sizes available from #3 to #18 –the size is the diameter in eighths of an inch • Galvanized and epoxy coatings often used in corrosive environments (parking structures & bridge decks – where deicing agents used)

31. Reinforcing Bar • Placement, size, spacing, and number of bars used vary according to the specific project • Markings on bars include: • Symbol of producing mill • Bar size • Type steel used • Grades (yield & ultimate strength – grades of 40, 50, 60, & 75 common)

32. Welded Wire Reinforcing • Also may be used as a reinforcement in concrete • 2 sets of wires are welded at intersections to forms squares/rectangles of a wire mesh

33. Pre-Cast Concrete • Individual concrete members of various types cast in separate forms before placement (may be at job site or another location) • Walls and partitions are often made of pre-cast units

34. Pre-Stressed Concrete • Concrete which is subjected to compressive stresses by inducing tensile stresses in the reinforcement • Attributes: • Concrete strength is usually 5,000 psi at 28 days and at least 3,000 psi at the time of pre-stressing. • Use hardrock aggregate or light weight concrete • Low slump controlled mix is required to reduce shrinkage

35. Advantages of Pre-Stressed Concrete • Smaller dimensions of members for the same loading conditions, which may increase clearances (longer spans) or reduce story heights • Smaller deflections • Crack-free members