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A Beginner's Guide to ACI 318-14 Chapter 2 - Materials © 2018 T. Bartlett Quimby |
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Section 2.2 Concrete Materials Last Revised: 04/07/2018 It is assumed that you have a fundamental understanding of concrete and concrete mix design from prerequisite courses, so this is largely a summary of those principles plus a view of the ACI 318-14 provisions that govern the selection of concrete materials. Fundamental Concepts Concrete consists primarily of course and fine aggregates, Portland cement, and water. The proportions of this materials, along with small amounts of various additives, can be adjusted to obtain a concrete mix that meets a variety of specifications. Additionally, the process of mixing, placing, and curing of the concrete mix can have a large impact on the various properties of the final product. It is not the intent of this text to cover concrete materials and mix design in detail as this is generally covered in properties of materials or specific concrete materials courses. For the application of the design equations, the most critical material property is the 28-day compressive strength, f'c, of a 6" diameter x 12" long standard concrete specimen cured under laboratory conditions according to ASTM specifications. There are situations where field cured specimens might also be required. 28-day Compressive Strength, f'c As mentioned above, the value of f'c is the 28-day compressive strength of a 6" diameter x 12" long standard concrete specimen cured under laboratory conditions according to ASTM specifications. The specimen is taken in the field as the concrete is being placed on site. The test is done to ensure that the concrete delivered by the concrete producer is capable of developing the design strength. Field curing conditions will have an impact on strength of the concrete in place, however, the final strength of concrete in place is rarely verified. While this sounds critical, it turns out that the strength of RC components (particularly bending members) is more controlled by the strength and volume of reinforcing steel than the strength of the concrete. As result, if the concrete is properly field cured, any variation in concrete strength has little impact on member strength and is within the acceptable limits of variation. In prior edition of the Code, statistical mix proportioning was covered in detail. In the current edition of the Code reference is made to ACI 214R-11, Guide to Evaluaton of Strength Test Results of Concrete. Typically, the target f'c is higher than the specified f'c to account for statistical variation in test results. This means that your actual f'c is likely to be a bit larger than what you specify. As you do your design before you have the construction test results, the concrete supplier's goal is to ensure that you have at least the strength that you specified. Note that most ACI 318 equation use the square root of f'c. It is important to note that in all cases, the result of the square root of f'c is in unit of pounds per square inch, psi. This means that you must use f'c in psi units to get the right answer. Note that the square root of f'c is much different if you use f'c in kips per square inch, ksi, units. When in doubt about the units to use for any variable in ACI 318, refer to ACI 318-14 Chapter 2 - Notation and Terminology where the units are always listed. Weight of Concrete The weight of concrete is controlled by the type of aggregates chosen. The more common concretes are listed as either normalweight or lightweight. Occasionally heavyweight concrete is used. Note that the variable, wc, used in ACI 318 is just the density of the concrete (i.e. without the steel) and is used in computing the Modulus of Elasticity Ec. Normalweight concrete is taken as 145 pcf. Note that the normal weight of reinforced concrete (the weight used in load calculations) is taken to be 150 pcf and accounts for the weight of the steel in the composite member. As the steel does not have in impact on the properties of the concrete, we use the weight of just the concrete in computation related to just the concrete properties. Lightweight concretes vary from about 90 pcf to 115 pcf. When specifying lightweight concrete is best to consult with the concrete suppliers before starting design in order to determine what weights they can deliver based on the obtainable aggregates. Heavyweight, or high-density, concretes can range from 190 pcf to 260 pcf and higher. Typically, the aggregates consist of heavy materials, including such things as iron punchings. Modulus of Elasticity, Ec The Modulus of Elasticity, Ec, is a function of the weight of the concrete (wc) and the 28-day strength (f'c). ACI 318-14 19.2.2.1 specifies how Ec is to be determined. Equation 19.2.2.1.a is the general equation with equation 19.2.2.1.b being the special case for wc equal to 145 pcf (actually for 144 pcf, but rounded off to two significant figures as that is about all the accuracy justified in this case.) Concrete Components As stated above, the primary components of concrete are: course and fine aggregates, Portland cement, and water. Aggregates: Aggregates take up most of the volume in the concrete mix. Typically two sizes of aggregates are used. The relative proportions of these two aggregate sizes are adjusted to create a maximum packing density, thus minimizing the need for more expensive cement. Aggregates can also be chosen in different weights so as to create either lightweight or heavyweight concrete mixes. The integrity of the aggregates is also important in the design of the mix. ACI 318-14 26.4.2.1(a)(4) limits maximum aggregate size based on:
Typically, the minimum specified clear distance between forms will control the aggregate size. ACI 318-14 25.2 also puts an effect limit on the maximum size of aggregates. The language of the section is in terms of a limit on rebar spacing, however, it is fairly typical to limit aggregate size to meet these requirements. For example, section 25.2.1 requires the clear spacing between bars in a horizontal layer by the following formula: Minimum Clear Spacing > min[1", db, or (4/3)dagg] This can be rewritten to be: dagg < (3/4)(Minimum Clear Spacing) = (3/4) max[1", db] As the minimum bar size will control the formula, most mix designs limit maximum aggregate size to 3/4" Other sub-sections of ACI 318-14 25.2 can result in other maximum aggregate sizes, however you need to ensure that all the cases are met and, since it is unlikely that separate mix designs will be used for each structural element, the controlling size for the project will the one that has the smallest maximum aggregate size. Similarly, ACI 318-14 25.7.2.1 and ACI 318-14 25.7.3.1 also limit maximum aggregate size based on bar spacing Cement: There are four basic types of Portland cement. Your choice of cement will depend on economics and your project needs. The basic types include:
Water: Typically, any available potable water will work, however alkali waters must be avoided. Water content is a important variable effecting strength and durability of the final product. Generally it is advisable to keep the water:cement ratio as low as possible and still allow the fresh concrete to workable. Useful References There a quite a few good references to guide the design and placement of concrete mixes. Some particularly helpful references include:
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