- - AGRICULTURE CORE CURRICULUM - - (CLF2000) Advanced Core Cluster: AGRICULTURE MECHANICS (CLF2500) Unit Title: CONCRETE/MASONRY ______________________________________________________________________________ (CLF2505) Topic: PLACING, FINISHING, AND Time Year(s) CURING CONCRETE 4 Hours 1 / 2 / 3 / 4 ______________________________________________________________________________ Topic Objectives: Upon completion of this lesson, the student will be able to: Learning Outcome #: (K-4) - Mix, pour, reinforce, finish, and cure concrete. (K-5) - Demonstrate the use of the basic tools needed to pour a concrete slab. Special Materials and Equipment: Brick trowel, edger, groover, wood or magnesium float, screed board, finishing trowel, wheelbarrow, bull float, tamper, push broom, slump cone w/rod, water hose, shovels, also possibly concrete ingredients, portable mixer, power trowel References: Burke, Stanley R., & Wakeman, T. J. (1990). MODERN AGRICULTURAL MECHANICS (2nd ed.). Danville, IL: Interstate Publishers. Cooper, Elmer L. (1987). AGRICULTURAL MECHANICS: FUNDAMENTALS AND APPLICATIONS. Albany, NY: Delmar Publishers. Editors of Reader's Digest. (1973). COMPLETE DO-IT- YOURSELF MANUAL (7th ed.). Pleasantville, NY: Reader's Digest Association. Phipps, Lloyd J., & Reynolds, Carl L. (1990). MECHANICS IN AGRICULTURE (4th ed.). Danville, IL: Interstate Publishers. Wagner, Willis H. (1979). MODERN CARPENTRY: BUILDING CONSTRUCTION DETAILS IN EASY-TO-UNDERSTAND FORM (4th ed.). So. Holland, IL: Goodheart-Willcox. Resources: Ahrens, Donald L. (1988). CONCRETE AND CONCRETE MASONRY. Available from: Hobar Publications, 1234 Tiller Lane, St. Paul, MN 55112 Phone: (612) 633-3170 (Catalog available) Evaluation: Teacher observation of student work, testing of project, and quiz by instructor. TOPIC PRESENTATION: PLACING, FINISHING, AND CURING CONCRETEA. Placing (pouring) 1. Pre-pour Guidelines a. Place concrete as close to its final position as possible. Get truck or wheelbarrow as close as possible. b. Place concrete in the forms as soon as possible, because concrete starts to harden about 15 minutes after it is mixed; in no case wait more than 45 minutes. Ready-mix companies charge a standby time if the truck is not emptied after 45 minutes. c. Before pouring, wet down the base material to prevent the water in the concrete from being drawn into the dry base material. Water loss will weaken the concrete. d. Check the forms for the last time with a carpenter's level. e. Have all tools at hand and a helper or two. 2. Tools and Equipment a. A square point shovel is used to move material and to fill in corners and low spots. b. A wheelbarrow is used to move material where the truck and chute cannot reach. 3. Pouring and Spreading a. Start placing concrete in a corner and work away from it. b. Use a shovel to spread concrete. Do not use a garden rake or hoe because they separate the coarse aggregate. c. Dump each succeeding load against the previous one; do not dump in separate piles and rake them together. d. The level of concrete should be higher than the forms. This will allow for settling, tamping, and screeding. B. Tamping 1. Process a. Tamping is a means of pushing the coarse aggregate down from the surface and bringing the fine aggregate to the top. b. Be careful not to tamp too much! c. At this time, run a shovel along the inside of the forms; this will help to push back the coarse aggregates from the edges. d. Use a hammer to tap along the forms to help vibrate the fine aggregates to the outside surface and fill in the voids. 2. Tools and Equipment a. Hand Tamp (Turkey's Foot) - This is a hand-held device made with a rectangular piece of expanded metal which is operated while standing in the wet concrete. The expanded metal is sized to push down the coarse aggregates and allow the fine aggregate to come up. b. Roller Tamp (Jitterbug) - This device has two rollers made of expanded metal; as it is rolled across the surface the gravel is pushed down. The roller type is operated from the outside edge of the slab and comes with extension handles to reach across larger distances. C. Screeding (striking off) 1. Process a. Screeding removes the excess concrete and fills in low spots. b. Bring the surface of the concrete even with the top edge of the form. c. Place a board across the edges of the form, and move it back and forth in a sawing motion. It is slowly advanced along the slab. d. Make sure there is a surplus of concrete against the front face of the screed board to fill in low spots. 2. Tools and Equipment a. Screed Board (Straight Edge) - a straight 2 X 4 or 2 X 6 plank NOTE: It is recommended that this be purchased the morning of the pour in order to insure a straight (unwarped) piece of lumber. b. Power Screed - a device that strikes off and compacts the concrete which is used for large pours D. Floating 1. Process a. The floating process brings the fine aggregates and cement paste (cream) to the surface which allows for further finishing. b. It also embeds the coarse aggregates just below the surface. c. Additionally, it aids in leveling ridges and filling voids left by the screed board. d. A hand float is used by holding it flat on the surface and swinging it in a sweeping arc; very light pressure is applied. e. Bull floats are pushed and pulled across the surface. f. Avoid overworking the concrete while it is soft since this may bring excess water to the surface. 2. Tools and Equipment a. Concrete Float - made of wood, aluminum, or magnesium. It consists of a flat rectangular base of wood or metal with a handle attached; the edges of the base are chamfered. Hand floats are used mostly for small jobs and outside edges. b. Bull Float - consists of a rectangular wooden board (or lightweight aluminum plate) for a base approximately 3-4 feet long. A swivel pole handle is attached to the base. It comes with extension handles to reach across larger slabs and is operated while standing around the edge of the slab. NOTE: The following finishing procedures should be completed after the water sheen has disappeared from the surface and the concrete has stiffened slightly. E. Edging 1. Process a. Rounds the corners of concrete in the form. b. Prevents edges from chipping off. 2. Tools and Equipment a. Concrete Edger - The blade is made of flat sheet steel with a 3/8" edge turned down along the long side to form a rounded corner. The ends are turned up slightly. Some are equipped with handle attachments so they can be used on large pours. F. Jointing 1. Process a. By cutting grooves or placing joints in the slab, the location of cracks can be controlled. 1) A planned break permits the concrete to expand and contract without cracking. 2) If cracks do occur with grooves, it will usually happen below the surface under the groove. b. Control joints are needed every 4 to 5 feet apart in sidewalks. c. Joints are needed 10 to 15 feet apart for floors, drives, or patios. d. Cut a joint down the center of 10 foot and longer slabs. e. The depth of the grooves should be at least 1/5 of the thickness of the concrete. 2. Tools and Equipment a. Joint Material - The joint can be created using a 3/8" softwood board, 1/2" asphalt strip, or by pouring sections at different times and using a compound to prevent the sections from sticking together. b. Groover - The bottom blade is flat with a raised ridge down the center while is 1/2 inch wide and comes in different depths. The base is fabricated of cast or sheet steel. Some type of straight edge must be used. c. Power Saw with Masonry Blade - used to create grooves in slabs, usually on large jobs. It cannot be done until 18-24 hours after concrete is poured. G. Finishes 1. What type of surface is needed? a. Slab uses b. Interior or exterior c. Non-slip or smooth d. Sealed for sanitary reasons e. Water washing f. Decorative 2. Popular Surfaces a. Floating - The surface should be floated a second time after the concrete has been set so the water and fine material will not be worked to the top. 1) It is done with a wood or metal float. Wood floats produce a rougher finish than metal floats. 2) Floating gives a medium non-slip surface, good for concrete which is constantly exposed to outside weather and meant for general use. 3) This floating procedure is usually conpleted before doing another surface finish such as troweling. b. Brooming is used if a rougher finish is required than floating. 1) It is done after the second floating operation. 2) A damp, stiff-bristled (coarse-straw) broom is pulled lightly across the surface of the concrete. 3) This gives a rough surface, provides excellent footing and traction; it is especially good for outside areas which become slick from rainy weather or where there is constant use of water such as in a milking barn. 4) For a finer texture, steel trowel the surface first and then use a soft-bristled broom. c. Troweling produces a smooth, hard surface since this action brings the cement paste to the surface. 1) It is done immediately after floating when the surface is semi-hard. 2) A troweled surface allows for easy cleanup and is ideal for sanitary conditions; however, it is extremely slippery when wet. 3) It is completed with a steel hand trowel which is a rectangular piece of sheet metal with a handle atached. a) For the first troweling, hold the trowel flat against the surface, making sweeping arcs back and forth with even pressure. b) For the second troweling tilt the leading edge of the trowel upward. This step should be done when a hand pressed against the surface leaves only a slight impression. 4) This process can also be completed with a power troweler which produces a similar surface to the hand trowel and is excellent for large jobs. To operate the power troweler, the concrete must be hard enough to stand on. 5) Additional trowelings will make the surface smoother and harder. However, excessive troweling will leave the surface weak and easily damaged by frost and chemicals. H. Curing 1. Guidelines a. To cure properly, the concrete must be protected from drying air, excessive heat, and freezing temperatures for several days after pouring. 1) Preventing the moisture loss during the early stages of hardening (hydration) is extremely important. 2) Hot, dry weather can cause the water to evaporate too rapidly, so the concrete will not set properly. 3) Extreme cold can cause the water to freeze and expand the concrete. b. Concrete must be kept moist and warm while it is setting (hardening). 1) Concrete should dry slowly. 2) This will greatly increase the strength and durability and will prevent the failure (cracking, crumbling, or breaking) of the concrete before the end of its expected lifetime. c. Curing time varies from 5 days (warm weather) to 7 days (cold weather), with 6 days being a satisfactory average. 1) Concrete kept moist for seven days is about 50% stronger than concrete which is allowed to dry during the same time. 2) If moisture is maintained for one month, the concrete is double the strength of that which is allowed to dry immediately. d. Form boards should remain in place as long as possible; if feasible, they should remain until the curing is completed. 2. Different Methods of Curing Concrete a. Seal the surface by covering it with plastic sheeting or waterproof paper; joints must be sealed and covering must lay flat and be anchored at the edges. b. Cover the concrete with burlap, canvas, sawdust, straw, sand, or another type of insulating material. This covering must be kept continuously wet during the curing time. c. Keep the concrete constantly moist with a lawn sprinkler or a soaker hose. d. Spray the concrete with a plastic-based curing compound that forms a continuous membrane over the surface. It is essential the coverage be complete. ____________________________________________________________ ACTIVITY: 1. Bring students to a construction site where concrete is being poured. This could be coordinated with Activity #1 from the last lesson. 2. Continuation of Activity #2 from the last lesson:demonstration of placing, finishing, and curing can be completed. If necessary, mixing can also be included (Supplementary Handout #1 was included to help with site mixing). 3. Complete a slump test on the concrete to be poured. Follow the guidelines presented in lesson CLF2502, Concrete Proportions. 4. Set up small concrete projects (half-pint milk cartons) which can be used to demonstrate the effects of reinforcement, mixing, temperature, and curing on concrete strength. A small testing unit using a hydraulic press or jack can be built so some quality destructive testing can be performed. Remember to keep safety in mind! ____________________________________________________________ Supplemental Handout #1 - Mixing Concrete A. Which method should you use to mix concrete? 1. Site Mixing (by hand or mixer) a. All ingredients need to be present. b. To measure ingredients, construct a cubic foot measuring box or obtain a metal bucket that has been marked to show one cubic foot. c. Another option is to measure ingredients using shovelfuls; on the average there are 10 shovelfuls per cubic foot. This can vary according to individual and size of shovel. This method is recommended only for individuals who have experience using shovelfuls. 2. Using a Portable Mixer a. With mixer stopped, add in all coarse aggregate and half the water. - The mixer's capacity (batch size) is approximately 60% of its total volume. - Never exceed the batch size of the mixer. b) Start the mixer, and then add sand, cement, and the rest of the water. c) Continue to mix for three minutes or until materials are throughly mixed and the concrete is a uniform color. d) Place batch into wheelbarrow and move to pour site. e) Wash out the drum after each batch; scour at the end of the day adding water and several shovelfuls of coarse aggregate while turning; rinse it out and then dry. 3. Mixing by Hand a. Measure out ingredients. b. Spread sand in a ring on the mixing area, add cement, and mix until there is a uniform color without streaks. Add coarse aggregate and turn the mix over three times. c. Form a depression in the middle of the heap, add some premeasured water, and work it into the center of the mixture. Add more water as needed and mix until the entire pile is damp. d. Shovel the dry part of the mix into the depression until the water has been absorbed. Be careful not to let the walls collapse on the ring. Then turn the whole heap over until it is evenly moist. e. Form a new depression if there is a need to add more water. If only a little water remains to be added, sprinkle it on the surface of the heap. Never change the ratio of cement and aggregate. f. To test the mix, draw the shovel backward over the heap in a series of jabs; try to leave clear cut ridges. If mix is too dry, they will be indistinct; if too wet, they will level out. g. Shovel batch into wheelbarrow and move to pour site. h. Clean up by scouring the mixing area and equipment. 7/23/91 JD/tf #%&C