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Materials Estimation After proportioning the mix, you must estimate the total amount of material needed for the job. This is simply done by computing the total volume of concrete
Figure 17-4.Retaining wall. to be poured, adding a waste factor, and multiplying this volume times the amount of each component in the 1-cubic-yard mix design. The manner of doing this is described in the following example. Example Problem: Using the mix design deter-mined previously in this chapter, determine the total amount of materials needed to construct the 75-foot-long retaining wall shown in figure 17-4. The 1-cubic-yard mix design is recapped below. Cement: 6.49 sacks (Type IA) Water 36.6 gallons Coarse aggregate: 1,735.0 pounds Fine aggregate: 1,153.0 pounds Air content: 5.0 percent To determine the total quantity of each of the above ingredients needed for the retaining wall, you must first calculate the total volume of concrete required. As you should know by now, an easy way to do this is to break the retaining wall into simple geometric shapes and then determine and accumulate the volumes of those shapes. Since you should know how to do this, we will simply say that the total volume of the retaining wall is 63.7 cubic yards. To this figure you add a 10-percent waste factor so that the adjusted amount of concrete needed for the project is 70.07 cubic yards. (Had the initial volume needed been greater than 200 cubic yards, you would have used a 5-percent waste factor.) Now that you know the total amount of concrete needed, you can determine the total quantity of each of the concrete ingredients by simply multiplying the amount of each ingredient needed for 1 cubic yard by the total amount of concrete required for the retaining wall. As an example, you need 1,153 x 70.07 = 80,790.7 pounds, or 40.4 tons, of fine aggregate for the retaining wall. The other ingredients are computed in the same way. That being done, you find that the following quantities of ingredients are need for the project: Cement: 455.0 sacks (Type IA) Water: 2,567.0 gallons Coarse aggregate: 60.8 tons Fine aggregate: 40.4 tons BITUMINOUS MIX DESIGN Hot-mix bituminous concrete for pavements is a mixture of blended aggregate filled with bituminous cement binder. The materials are heated while being mixed to promote fluidity of the bitumen for thorough coverage of the aggregate particles. The design of a bituminous concrete mix consists of the determination of an economical blend and gradation of aggregates together with the necessary content of bituminous cement to produce a mixture that will be durable, have the stability to withstand traffic loads, and be workable for placement and compaction with the construction equipment available. The procedures described in this section are performed during the design of a hot-mix bituminous concrete. They include testing, plotting the results on graphs, and checking the readings against values from the design tables. Testing of the ingredients and the mix is started before and continued throughout the paving operations. Specific test procedures are not covered in this discussion; instead, you should refer to chapter 13 of this TRAMAN and to Materials Testing, NAVFAC MO-330.GENERAL PROCEDURES AND GUIDELINES The objective of hot-mix design is to determine the most economical blend of components that will produce a final product that meets specified requirements. The following is a list of general procedures: 1. Prepare a sieve analysis of each of the aggregates available. 2. Determine the aggregate blend that will achieve the specified gradation (Paving and Surfacing Operations, TM 5-337). Plot the selected blend proportions on a graph with the allowable limits to see that it conforms.3. Determine the specific gravity of the components. 4. Using selected percentages of bitumen (TM 5-337), make trial mixes, and compute the design test properties of the mix. 5. Plot the test properties on individual graphs using the selected bitumen percentages. Draw smooth curves through the plotted points. 6. Select the optimum bitumen content (AC) for each test property from the curves of the Marshall test results. For a discussion of the Marshall stability test, you can refer to chapter 13 of this TRAMAN and to NAVFAC MO-330. 7. Average the bitumen content values (from Step 6) and, from the graphs, read the test property value corresponding to this average. 8. Check these read values with the satisfactoriness of mix criteria. The selection of the mix ratios of materials is tentative. The bitumen should be the same as the one to be used in construction. The aggregates and fillers must meet definite requirements. In general, several blends should be considered for laboratory mix-design tests. Gradation specifications are based on limits established by the U.S. Army Corps of Engineers as satisfactory. Within these limits, the following variables are considerations that will affect the final mix design: 1. Use of mix (surface course, binder course, or road mix) 2. Binder (asphalt, cement, or tar) 3. Loading (low tire pressure100 psi and under, or high tire pressureover 100 psi) 4. Maximum size of aggregate (in stockpile or based on thickness of the pavement course) Once the gradation specifications have been selected, you should check the available materials to determine how to proportion the blend to meet these specifications. You should study sieve analyses of the available aggregates and compute a series of trial blends. You may have to make adjustment of the blend after testing the design and prepared mix. The considerations for establishing and adjusting the blend are explained in TM 5-337. The determination of optimum bitumen content is based on a definite design and testing procedure known as the Marshall method. This method is explained in chapter 13 of this TRAMAN and in NAVFAC MO-330. The final step is the preparation of a job-mix formula to be furnished to the construction unit. It is recognized that at times it will be necessary to shorten the design procedure as much as possible to expedite military construction. For additional information, refer to TM 5-337. |
 
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