Presentation on theme: "Concrete zDavid P. Shelton, Extension Agricultural Engineer zJames M. Harper, P.E., Field Engineer, Portland Cement Association zFor more materials contact."— Presentation transcript:
Concrete zDavid P. Shelton, Extension Agricultural Engineer zJames M. Harper, P.E., Field Engineer, Portland Cement Association zFor more materials contact the Portland Cement Association yPlease write or fax the Portland Cement Association on the letterhead from your educational institution, and ask for your educational package on portland cement. They will send you a full-color poster featuring original art created for PCA, as well as an instructional video. Portland Cement Association 5420 Old Orchard Road Skokie, Illinois 60077-1083 FAX: 847 966 8389 zAdapted to Powerpoint by Bill PannellBill Pannell zClick here to go to teaching ideasClick here to go to teaching ideas yReference: http://www.ianr.unl.edu/pubs/farmbuildings/g623.htm http://www.ianr.unl.edu/pubs/farmbuildings/g623.htm
Concrete zis a mixture of portland cement, water, aggregates, and in some cases, admixtures. zThe cement and water form a paste that hardens and bonds the aggregates together. zConcrete is often looked upon as “man made rock”.
zConcrete is a versatile construction material, adaptable to a wide variety of agricultural and residential uses.
zConcrete has strength, durability, versatility, and economy. zIt can be placed or molded into virtually any shape and reproduce any surface texture.
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.
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
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.
zWith proper materials and techniques, concrete 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.
zConcrete has substantial strength in compression, but is weak in tension. zMost structural uses, such as beams, slats, and manure tank lids, involve reinforced concrete, which depends on concrete's strength in compression and steel's strength in tension.
zSince concrete is a structural material, strength is a desirable property. 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.
Components of Concrete zPortland Cement zAggregate - sand, gravel, crushed rock zWater zAdmixtures - when necessary
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.
zThe production process for portland cement first involves grinding limestone or chalk and alumina and silica from shale or clay.
zThe raw materials are proportioned, mixed, and then burned in large rotary kilns at approximately 2500°F until partially fused into marble-sized masses known as clinker. zAfter the clinker cools, gypsum is added, and both materials are ground into a fine powder which is portland cement.
Three types of portland cement are used for agricultural applications:
zType I cement is the general purpose and most common type. Unless an alternative is specified, Type I is usually used. Type I
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
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
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.
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.
Admixtures zAdmixtures are ingredients other than portland cement, water, and aggregates. zAdmixtures are added to the concrete mixture immediately before or during mixing.
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:
Retarding admixtures: zare used to slow the rate of concrete hardening. zThey are useful for concrete that is placed during hot weather.
Accelerating admixtures zsuch as calcium chloride, are used to increase the rate of hardening--usually during cold weather.
zTo determine the most economical and practical combination of readily available materials. zTo produce a concrete that will meet requirements under specific conditions of use. Goals:
zThe majority of concrete used for agricultural applications is supplied by ready-mix producers. zWith an understanding of these goals, the customer can communicate better with the ready-mix supplier, and obtain concrete that is suited to the project at hand.
zA properly proportioned concrete mix will provide: yWorkability of freshly mixed concrete. yDurability, strength, and uniform appearance of hardened concrete. yEconomy
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
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
zThese properties--and thus the desired concrete quality--can only be fully achieved through proper placement and finishing, followed by prompt and effective curing.
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
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:
zThe lower limit of cement required is specified as a minimum cement content in bags per cubic yard. yA bag of cement weighs 94 lbs. Typical concrete mixtures include between 5 and 6.5 bags per cubic yard of concrete. yA minimum cement content assures desirable concrete properties, such as workability, durability, and finishability.
zA minimum amount of cement is required in order to adequately coat all aggregate particles and provide proper bonding.
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:
zShould be kept as low as possible z5-6 gallons per sack of cement is acceptable Water to Cement Ratio
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
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.
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.
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.
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.
zThe best curing method depends on: ycost, yapplication equipment required, ymaterials available, ySize and shape of the concrete surface.
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.
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.
zA perfect mix can result in poor quality concrete if correct placement, finishing, and curing techniques under the proper conditions of moisture and temperature are not used.
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,
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
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