1. Concrete

2. zis a mixture of portland cement, sand, gravel, and water. zFine aggregates are sand. It should be clean of clay, silt, etc. zCoarse aggregates are gravel, rocks, etc., anything over ¼” inch in diameter.

3. Concrete zWater should be clean. Contaminated water might have impurities in it that could damage the concrete curing process. zA cubic yard of concrete weighs from 150-160 pounds

4. Components of Concrete 1.Portland Cement 2.Sand - fine Aggregate 3.Gravel of crushed rock – coarse aggregate 4.Water 5.Admixtures - when necessary

5. Characteristics zThe cement and water form a paste that hardens and bonds the aggregates together. zMortar does not contain coarse aggregates and is used to join bricks or blocks. zConcrete is often looked upon as “man made rock”.

6. Characteristics zAbout ¾ of all concrete used in construction is delivered by trucks to the construction site as ready-mixed concrete. zAlthough many people call concrete “cement”, cement is the powdered material and concrete is the mixture of all ingredients.

7. zConcrete is a versatile construction material, adaptable to a wide variety of agricultural and residential uses. zConcrete has strength, durability, versatility, and it’s economical. zIt can be placed or molded into virtually any shape and reproduce any surface texture. Characteristics

8. zConcrete is the most widely used construction material in the world. zIn the United States almost twice as much concrete is used as all other construction materials combined. Info

9. Mixing Concrete  Quality of concrete is determined by: 1.Slump 2.Uniformity 3.Workability

10. Slump zSlump is the consistency or wetness of the mixture. Stronger concrete requires less slump. zIf concrete has too much slump ( is too wet), adding aggregates such as sand and gravel will correct the problem.

11. Uniformity and Workability zUniformity is thoroughly mixing all ingredients so that the “glue” which is wet cement will be around all sand and gravel particles. (aggregates) zWorkability refers to the ease with which concrete can be placed and shaped

12. MIXING RATIO zThe mixing ratio is expressed as a 3 digit number. Example: 1-2-3 zThe first number is parts of cement zSecond= is parts of fine aggregates(sand) zThird= parts of large aggregates (gravel) zParts may be shovels full, pounds, cubic feet, etc.

13. MIXING RATIO zLess sand, gravel, and water are used for the same amount of cement to make concrete stronger. zHand mixing may be done in a wheelbarrow or mixing box (mortar box) by using a shovel or hoe.

14. Hand Mixing Procedure 1.Add Sand first 2.Then cement, mix thoroughly 3.Add and mix gravel 4.Add water and mix well

15. Machine Mixing Procedure zUses a concrete or mortar mixer z1 st - add 10% of water needed to prevent ingredients from sticking zThen add gravel, then cement, then sand, and then add remainder of water needed. zMix all ingredients 2 minutes

16. Placing and Finishing Concrete zForm – wood or metal structure that confines concrete to the desired shape zTrenches – dug in ground for building footings; soil walls are the forms. zIf wood form is used, 2” lumber should be used to prevent the form from warping. yExample: 2” x 4”, 2” x 6”, 2” x 8”

17. Curing Time zIn warm weather, forms should stay in place 2-3 days and twice that long if weather is cold. zConcrete should be poured or placed as evenly as possible in the forms to prevent having to move it.

18. FINISHING CONCRETE zFinishing is bringing the surface of concrete to the proper grade and smoothness zScreeding is using a straightedge such as a 2” x 4” board to level concrete with top of forms. zA Bull Float is used to impact the concrete at the surface and to make it easier to finish.

19. Finishing Texture zThe texture of the finishing tool will determine smoothness of surface zFor gritty surface such as floors, driveways, or walkways; use a wood float, stiff broom or burlap zFor very smooth surface, a steel finishing trowel should be used.

20. Reinforcement zReinforcement is needed b/c of low tensile strength and high compressive strength. This means it has low stretch and high compaction. zReinforcing helps equalize and prevent cracking or breaking. zClean steel rods or bars called rebar and clean welded wire are used for reinfor.

21. Estimating Materials Needed zConcrete is measured in cubic yards. z Cu. Yds.= Length x Width x Thickness 27 z27 is the number of cubic ft in a cubic yd zL, W, and T must be in Feet zThickness is usually in inches and must be changed to a fraction of a foot. zExample: 3”=1/4 ft; 4”=1/3 ft; 6”=1/2 ft

22. Example zHow many yd of concrete are needed to pour a driveway 100 ft long, 8 ft wide, and 3” thick? z 100’ x 8’ x ¼’ = 200 = 7.41 cu. yds 27 27

23. Example zIf concrete is purchased in only whole yards and 10% should be allowed for waste and uneven ground level, how much concrete should be ordered? z7.41 x 10% =.741 z7.41 +.741 = 8.15 zAlways round up to a whole yard, so 9 cubic yards should be ordered.

24. Tips zWood or metal forms should be treated with oil and wet down with water before concrete is poured to make the forms easier to remove. Water will prevent swelling. zWater rising to the top of concrete poured in tall or deep forms is called bleeding. This problem can be corrected or reduced by pouring concrete slower and by using a stiffer or less watery mix.

25. Notable U.S. concrete projects: zthe Erie Canal zGrand Coulee Dam, which used nearly 10 million cubic yards of concrete, making it one of the largest portland cement concrete projects in history

26. Grand Coulee Dam

27. The Erie Canal

28. zDemand for concrete with higher strength and better quality, coupled with larger and faster mixer trucks, led to the emergence of the ready-mix concrete industry in the post-World War II period. zThe ready-mix concrete producer has made concrete an appropriate construction material for many agricultural applications. Concrete History

30. Properties of Concrete

31. zConcrete can withstand many acids, silage, milk, manure, fertilizers, water, fire, and abrasion. zConcrete can be finished to produce surfaces ranging from glass-smooth to coarsely textured, and it can be colored with pigments or painted. yBroom finish ySmoothed with a board Characteristics

32. zCompressive strengths of concrete generally range from 2000 to 5000 pounds per square inch (psi), but concrete can be made to withstand over 10,000 psi for special jobs. Strengths

33. Portland Cement zPortland cement was named for the Isle of Portland, a peninsula in the English Channel where it was first produced in the 1800's. zSince that time, a number of developments and improvements have been made in the production process and cement properties.

34. Three types of portland cement are used for agricultural applications: Type I Type II Type III

35. zType I cement is the general purpose and most common type. Unless an alternative is specified, Type I is usually used. Type I

36. zType II cement releases less heat during hardening. It is more suitable for projects involving large masses of concrete--heavy retaining walls, or deadmen for suspension bridges. Type II

37. zType III cement produces concrete that gains strength very rapidly. z It is very finely ground and sets rapidly, making it useful for cold weather jobs. Type III

38. Water zGood water is essential for quality concrete. zIt should be good enough to drink--free of trash, organic matter and excessive chemicals and/or minerals. zThe strength and other properties of concrete are highly dependent on the amount of water and the water-cement ratio.

39. Aggregates zAggregates occupy 60 to 80 percent of the volume of concrete. zSand, gravel and crushed stone are the primary aggregates used. zAll aggregates must be essentially free of silt and/or organic matter.

40. Admixtures zAdmixtures are ingredients other than portland cement, water, and aggregates. zAdmixtures are added to the concrete mixture immediately before or during mixing.

41. zare the most commonly used admixtures for agricultural concrete. zproduce microscopic air bubbles throughout the concrete. zEntrained air bubbles: y improve the durability of concrete exposed to moisture and freeze/thaw action. yImprove resistance to scaling from deicers and corrosive agents such as manure or silage. Air Entraining agents:

42. Retarding admixtures: zare used to slow the rate of concrete hardening. zThey are useful for concrete that is placed during hot weather.

43. Accelerating admixtures zsuch as calcium chloride, are used to increase the rate of hardening--usually during cold weather.

44. Proportions Determing the proper mix

45. zA properly proportioned concrete mix will provide: yWorkability of freshly mixed concrete. yDurability, strength, and uniform appearance of hardened concrete. yEconomy

46. zWorkability is the property that determines the ease with which freshly mixed concrete can be placed and finished without segregation. zWorkability is difficult to measure but redi-mix companies usually have experience in determining the proper mix. zTherefore, it is important to accurately describe what the concrete is to be used for, and how it will be placed. Workability

47. zIf acceptable materials are used, the properties of concrete, such as durability, freeze/thaw resistance, wear resistance, and strength depend on the cement mixture. zA mixture with a sufficiently low ratio of water to cement plus entrained air, if specified, is the most desirable. Durability

48. zThese properties--and thus the desired concrete quality--can only be fully achieved through proper placement and finishing, followed by prompt and effective curing.

49. zProportioning should minimize the amount of cement required without sacrificing quality. zQuality depends on the amount of cement and the water-cement ratio. z Hold the water content to a minimum to reduce the cement requirement. Economy

50. zUse: ythe stiffest practical mixture ythe largest practical maximum size of aggregate ythe optimum ratio of fine-to-coarse aggregates Minimizing water and cement requirements:

51. zA bag of cement weighs 94 lbs. Typical concrete mixtures include between 5 and 6.5 bags of cement per cubic yard of concrete. zA minimum cement content assures desirable concrete properties, such as workability, durability, and finishability.

52. zAggregate size depends on the end use: yThe maximum aggregate size should be no larger than one-third the thickness of the concrete. yAggregate size should also be less than three-fourths the clear space between reinforcing bars where rebar is used. Determining Aggregate Size:

53. zShould be kept as low as possible z5-6 gallons per sack of cement is acceptable Water to Cement Ratio

54. zConcrete that has been specified, batched, mixed, placed, and finished "letter-perfect" can still be a failure if improperly or inadequately cured. zCuring is usually the last step in a concrete project and, unfortunately, is often neglected even by professionals. Curing

55. zCuring has a major influence on the properties of hardened concrete such as durability, strength, water-tightness, wear resistance, volume stability, and resistance to freezing and thawing.

56. zProper concrete curing for agricultural and residential applications involves keeping newly placed concrete moist and avoiding temperature extremes (above 90°F or below 50°F) for at least three days. zA seven-day (or longer) curing time is recommended.

57. Two general methods of curing can be used: zKeep water on the concrete during the curing period. zThese include yponding or immersion, yspraying or fogging, and ysaturated wet coverings. ySuch methods provide some cooling through evaporation, which is beneficial in hot weather.

58. zPrevent the loss of the mixing water from concrete by sealing the surface. zCan be done by: ycovering the concrete with impervious paper or plastic sheets, yapplying membrane-forming curing compounds.

59. zThe best curing method depends on: ycost, yapplication equipment required, ymaterials available, ySize and shape of the concrete surface.

60. zBegin the curing as soon as the concrete has hardened sufficiently to avoid erosion or other damage to the freshly finished surface. zThis is usually within one to two hours after placement and finishing.

61. Summary zConcrete is a highly versatile construction material, well suited for many agricultural applications. z It is a mixture of portland cement, water, aggregates, and in some cases, admixtures. zStrength, durability, and many other factors depend on the relative amounts and properties of the individual components.

62. zWhen specifying and ordering concrete, the customer should be prepared to discuss such things as: y1. Amount of concrete required, y2. use of the concrete, y3. type of cement,

63. y4. minimum amount of cement per cubic yard y5. maximum water-cement ratio y6. any special admixtures, y7. amount of air entrainment, y8. desired compressive strength, y9. amount of slump, and y10. any special considerations or restrictions

65. Teaching Ideas: zTo introduce this lesson get cement, sand, crushed rock and water in jars for the students to look at. zShow powerpoint presentation and video if you acquire it. zFind questions in a textbook and have students look up answers from book. Test or quiz if desired. zArrange tour of a redi-mix plant and a site which is being poured. zAllow student to mix, place, form and cure concrete in the shop making patio blocks. Forms for round blocks can be made by cutting 3” sections from a 5 gallon plastic bucket with a bandsaw. zAcceptable concrete can be produced by purchasing bags of ready to mix concrete or by mixing from individual components. zReturn to first pageReturn to first page