1. Class Day Nineteen Class Day Nineteen

2. Introduction to Reinforced Concrete  Concrete is a mixture of graded aggregates held together by a matrix.  Concrete used in construction is a mixture of graded aggregates:  Large aggregates: Hard gravel ranging in size approximately from ¼” to ¾”. Clean and free of dust & small particles.  Small aggregates: Clean washed sand free of dust and small particles.  Aggregates form a mixture of larger stones, with voids filled with smaller stones, with voids filled with smaller stones, with voids filled with sand particles, all coated and held in place by a properly mixed paste made from portland cement and water.

3. Concrete STRUCTURAL CONCRETE  Ingredients  Portland Cement  Course Aggregate  Fine Aggregate  Water Admixtures are not standard and are used for special purpose, such as ease of placement or protection from freezing.

4. Even though concrete is “rocklike”, the final result can appear pliable, and assume most any shape.

5.  THE STRENGTH OF CONCRETE IS DEPENDENT UPON FOUR CONDITIONS: 1 The hardness of the aggregates. Obviously, the hardness of the aggregate is important to strength in compression, since concrete could fail by forces that crush the aggregates. 2 The degree of proportion of distribution of the aggregate sizes. The variation of aggregate size is important in that proper distribution is instrumental in eliminating voids of hard material, making the mix as homogeneous as possible.

6. 3 The ratio of the amount of water to that of cement. The hardening of concrete is not a drying process, but a chemical reaction called HYDRATION. It is the reaction between water and portland cement, and a concrete mix requires only a small amount of water for hydration to occur. Too much water dilutes the hydration process. Too little water limits the completeness of the process. If the water in concrete freezes before hydration is complete, the process stops and the resulting concrete is weak and poor in wearability.

7. 4 The care given to the concrete after it has been placed. After concrete is placed, it must be given the conditions to “cure” which is the completeness of the hydration process. Concrete placed where the hot sun evaporates the water near the surface causes uneven hydration and results in poor durability of the outer surface. Concrete footings placed against dry earth lose water due to absorption into the dirt, causing uneven curing.

8. PORTLAND CEMENT The manufacture of cement for concrete was first developed in Portland, England in 1824. That process has developed in all parts of the world using the natural ingredients necessary for the finished product. The designation, ‘portland’ is one of definition of ingredients rather than a proper name. Portland cement is a mixture, primarily of ingredients that contain lime, iron, silica, and alumina – all natural elements found in various formations of limestone, marble, marl, seashells, clay, or shale.

9.  Portland cement is manufactured in seven types, each containing admixtures for specific requirements such as high early strength, air entraining, low heat of hydration, and resistance to sulfates. The general use of cement for concrete in construction is Type 1 – normal cement.  AS A DEVELOPING ARCHITECT, or AS SOMEONE WHO IS SUPPOSED TO POSESS INTELLIGENCE: please do not ever refer to concrete items as “cement walkways or cement footings or cement driveways or cement roads.” Cement is an ingredient in the manufacture of concrete – just as flour is an ingredient in bread – or just as sugar is an ingredient in ice cream.

10. MIXING AND PLACING CONCRETE A MIXING: Concrete should be mixed, using a reference of proportioning of aggregates by weight. The condition of aggregates is important, in that the amount of moisture present in gravel or sand contributes to the amount of water for the mix, and excessive water is detrimental to the strength of the mix. Concrete for construction is generally mixed at a batching plant, set up for the scientific methods of proportioning the mix. The ingredients are placed in a large rotating drum on a truck, so the mix is complete by the time it gets to the job site.

11. CONCRETE TRANSIT TRUCK

12. You as a project architect will specify a strength required for concrete. The contractor in turn will order the concrete from a manufacturing plant and hold them responsible for the strength of the mix. You, the architect, will then get a copy of the details of the concrete mix design. Often the concrete batching manufacturer will send a representative ( other than the truck driver) to the job site to see that the concrete is placed properly, and that no additional amount of water is added to the mix after it leaves the plant.

13. B PLACEMENT OF CONCRETE The depositing of concrete at the site is extremely important to its strength. If concrete is placed in excavations that can absorb water, then part of the water needed for hydration might be lost. Concrete placed in forms and around reinforcing steel must be worked and tamped into place to eliminate voids in the final mix, and to assure a complete bond between the concrete and reinforcing steel.

14. If concrete is allowed to free-fall for a long distance from the end of the chute, such as to the bottom of a deep footing, the aggregates will separate and the mass will not have a properly distributed mix of ingredients. In large placements, the deposited concrete must be worked into place with vibrators to fill voids of cement paste around exposed surfaces of the aggregates.

15. Several methods of depositing concrete are employed:  Where the truck can have ready access to the placement site, the material is deposited from a curved, adjustable, rotateable, metal chute.  If the placement site is not accessible by the truck, concrete can be pumped through a flexible tube to reach distant areas, or to locations on upper stories of buildings.  Large quantities of concrete that must be placed in remote areas can be transported in steel buckets equipped with trap doors, moved to the deposit site by overhead cranes.

16. If concrete is to be placed in a deep footing such as a drilled pier shaft, or if concrete is to be placed underwater, the material may be conveyed through a tube called a “TREMIE.” This type of placement assures that the mix will not be allowed to free-fall and separate the ingredients, and in the case of underwater, will prevent the mix from being diluted.

17. Concrete Placement Direct From the Transit Mixer

18. Placement with a Concrete Pump

19. Placement of concrete using a Crane & Bucket

20. SAMPLING CONCRETE FOR TESTING TO VERIFY STRENGTH Two procedures are done at the jobs site – and are done each time for a specified quantity of concrete that is placed. You as the architect may direct samples to be taken for a specified number of cubic yards of concrete placed, and that each placement will be documented as to specific location on the job. These samples are taken by a representative of a testing laboratory.

21. The Slump Test: A cone shaped metal cylinder approximately 12” tall, 4” open top diameter, 8” open bottom diameter, is placed on a clean, level surface. Concrete is inserted into the top and tamped full with a steel rod. The top surface is struck smooth. The slump cone is then lifted upward, allowing the concrete to remain. The concrete mass, still being wet, will slump down due to its weight. The distance from the top of the cone to the top is the mass is measured. Too much slump indicates too much water – a clue to see if the mix has inferior strength. Slumps should be 6” or less.

22. Sample collected Slump Measured Cone Removed and Concrete Allowed to ‘Slump’ Slump Cone Filled THE SLUMP TEST

23. Concrete Test Cylinders: You the architect will specify that three test cylinders will be taken for each specific batch of concrete placed. A cardboard or plastic cylinder 6” in diameter, 12” tall is filled with concrete, tamped solid and struck smooth at the top. Three of these will be done for each batch. The cylinders are allowed to “set”, then placed in plastic bags, identified and sent to an independent testing laboratory, where they will remain for your specified length of time.

24. At age 7 days one cylinder will be tested in a machine to determine its strength. It should attain approximately ½ to 2/3 of the compressive strength you specified. At 21 days another cylinder will be tested for strength. The results should be in the vicinity of 80% to 90% of specified strength. At 28 days (the age of fully cured concrete) the last cylinder will be tested to ascertain its matured strength. If strengths are high after the 7 day test, you may choose to skip the 21 day test. But in any case, the 28 day test should be made.

25. Failures are rare, but the consequences are left to you, the architect’s specification. If a failure occurs after 28 days, You may have required by your specification that on-site samples of the material be taken by the laboratory of the failed batch, then tested for matured strength. These will be samples, circular in shape, cut by a machine made for the purpose. If the cored samples do not test to the required strength, the Contractor is required to remove all the quantity of the failed batch and replace it with new concrete. For these reasons, documentation of batches of concrete placed is important.  Obviously, that is why the Contractor requires the concrete manufacturer to be responsible for the strength of the mix.

26. Test Cylinders Filled with a Sampling of the Concrete

27. Test Cylinders in bags for curing Preparation for breaking a test cylinder

28. CONCRETE REINFORCING STEEL Steel bars are made specifically to resist tensile and shearing diagonal tension in concrete structural members. They are made in several strength grades, so identified, and have a deformed surface to aid in bonding with concrete. Steel bar size: Eleven Standard Diameters 3, 4, 5, 6, 7, 8, 9, 10, 11, 14, 18 3, 4, 5, 6, 7, 8, 9, 10, 11, 14, 18 Number refers to eighths of an inch Number refers to eighths of an inch Steel bar yield strength grades: 40,000 p.s.i. 50,000 p.s.i 60,000 p.s.i

29. Steel reinforcing is placed inside concrete forms according to details of placement requirements determined by structural calculations, and secured in place with wire ties. Sufficient room must be allowed between bars and between bars and forms to allow the installation and consolidation of the concrete mix. Since steel is subject to weakening by fire, a concrete cover of specific thickness on the outside of reinforcement is required for maintaining the fire integrity of concrete structures.

30. Reinforcing supports placed inside forms for the purpose of holding steel bars in position are called ‘chairs’. They are made in numerous configurations for specific conditions, constructed of wire electronically welded together. Reinforcing steel for underground concrete footings is sometimes required to be suspended from above in order to support the steel. A solid connection such as a reinforcing steel bar stuck in the soil as a support would leave a channel through the concrete for moisture to enter and deteriorate the reinforcing steel.

31. ORGANIZATIONS FOR REINFORCED CONCRETE STANDARDS include: CRSIThe Concrete Reinforcing Steel Institute governs the development and standards for the manufacture of steel reinforcing. ACIThe American Concrete Institute publishes the specifications for the design, reinforcing, placing, and control of structural concrete.

32. Improperly consolidated Concrete

33. Segregation at the bottom of the pour (also note the trash at the bottom of the wall)

34. Extensive Reinforcing Can Make Placement and Consolidation Difficult

35. Reinforcing Supports