Mix Design
The purpose of a mix design is to group the aggregates in different proportions to achieve the desired strength. The components of a mix are proportioned so that the resulting concrete has adequate strength, proper workability for placing, and low cost. Low cost is achieved by using the minimum amount of cement required to obtain adequate properties. Admixtures are often used for special purposes. NOTE: This manual addresses mix designs for concrete paving mixes. See FM 5-428 for information on mix designs for structural concrete.
Criteria
10-1. The flexural (beam) and compressive strengths of a hardened mix are used for the concrete's design criteria. Flexural strength measures the bridging capacity and is used to design nonreinforced concrete pavement. Compressive strength measures the resistance to a direct load. Strength tests are usually made after 28 days for road pavement and after 90 days for airfield pavement.
Water-to-Cement Ratio
10-2. Select the proper water-to-cement ratio to ensure that a mix meets the requirements for flexural strength and durability. A durable mix has a long life, requires low upkeep, and is highly resistant to exposure and freezing. Figure 10-1 shows the relationship between age and flexural strength for Types I and III portland cement. Table 10-1 lists the recommended water-to-cement ratios for durability in various exposures. Select the lowest water-to-cement ratio that satisfies the requirements for flexural strength and durability.
10-3. Use the water-to-cement ratio shown in Figure 10-1 for flexural strength and adjust the ratio for durability. For example, to find the water-to-cement ratio for Type I portland cement with a flexural strength of 600 psi at 28 days, read from the bottom of the curve. The amount is 5 1/4 gallons of water per sack of cement. Table 10-1 shows that the durability requirement is 5 1/2 gallons of water per sack of concrete. Therefore, using 5 1/4 gallons as the lowest ratio will satisfy the requirements for flexural strength and durability. Once the water-to-cement ratio has been selected, do not change it except for air-entrainment adjustments.
Aggregate
10-4. FA is used to increase workability and to fill the spaces remaining in the CA. Very fine sand is uneconomical because it requires more cement paste, and very coarse sand produces unworkable mixes. In general, FA that has a smooth gradation curve produces the most satisfactory results. For economy, 10 percent or less of FA should pass a number 100 sieve; however, 3 to 4 percent passing a number 100 sieve provides optimum workability.
10-5. CA should be graded to the maximum size, which should not exceed one-third of the slab's thickness. Assuming FA and CA have smooth gradation, the larger the CA, the less paste is needed to produce satisfactory concrete. For most paving operations in a TO, the CA size is <2 inches.
Workability
10-6. The workability of a mix is largely governed by the amount of aggregate added to the mix. The sand's gradation and the relative percentage of sand to gravel also affect workability. Because more aggregate is required than cement, a stiff mix is more economical than a fluid one. If too much aggregate is used, the mix may contain voids and be dry, crumbly, and difficult to place in forms. Mechanical vibration can increase the workability of a stiff mix. If a mix is too fluid and contains insufficient aggregate, heavy aggregate particles settle to the bottom and fines rise to the top.
10-7. Conduct a slump test to measure the workability of a mix as described in FM 5-472 . However, remember that a slump test of air-entrained concrete will not yield a reliable measurement. A 1- to 2-inch slump for air-entrained concrete indicates about the same degree of workability as a 3-inch slump for non-air-entrained concrete.
Methods
10-8. There are two methods of mix design—book and trial batch. They are used to proportion the quantities of cement, water, and aggregates used in the concrete.
Book Method
10-9. The book method is a theoretical method of design that uses laboratory data. Because of the variation in materials, the book method is used as a design basis and adjustments are made in the field using the trial-batch method.
Selecting Mix Proportions
10-10. Use Table 10-2 to determine the quantities of ingredients needed for a trial mix of medium consistency and to compute the cubic-foot yield per sack of cement. The information in this table is based on a 3-inch slump with aggregate in a saturated, surface-dry condition.
Example: Based on the following specifications and using Table 10-2 , determine the quantities of each ingredient required per sack of concrete, the yield (cubic feet) per a one-sack batch, and the total materials required for a cubic yard of concrete:
fineness modulus of sand = 2.3
water-to-cement ratio = 6 gallons per sack
Adjusting for Slump Variation
10-11. Pavement mixes often require a slump other than 3 inches, so adjust the figures accordingly. For every 1-inch decrease in slump, decrease the sand by 3 percent and the water by 1 gallon per cubic yard of concrete. In the above example (paragraph 10-10), the mix adjustments for a 2-inch slump are:
cement = 5.5 sacks (no change)
Adjusting for Moisture
10-12. In the field, aggregates usually contain moisture in excess of the saturated, surface-dry condition. Excess moisture added to the mix will alter the water-to-cement ratio and reduce flexural strength and durability by increasing the capillary voids in the finished concrete. Normal surface moisture content is 2 to 6 percent for FA and 2 percent for CA. Excess moisture in FA or CA can change the water-to-cement ratio from 6 gallons to 8 1/2 gallons per sack of cement unless the problem is corrected. This increase in water would reduce the 28-day flexural strength of concrete by about 20 percent. Surface moisture content, however, is based on a saturated, surface-dry weight instead of a dry weight as in soils. Use the following formulas to determine the amount of moisture present in the aggregates:
M = excess surface moisture, in pounds
SMC = surface moisture content, in percent
Assd = weight of saturated, surface-dry aggregate (design weight), in pounds
Aw = weight of required wet aggregate, in pounds
Wa = adjusted volume of water, in gallons
Wd = design volume of water, in gallons
Example: The surface moisture content is 4 percent for FA and 1 percent for CA. Using the above formulas and Table 10-2 , determine the material requirements and calculate the yield, in cubic feet, per sack of cement.
Aw = 1,100 + 44 = 1,144 pounds of FA
Aw = 2,200 + 22 = 2,222 pounds of CA
Calculate the quantity of water.
Wa = 33 - 0.12(44 + 22) = 33 - 0.12(66) = 33 - 8 = 25 gallons of water
After adjustments for moisture, the mix ingredients for 1 cubic yard of concrete are:
Adjusting for Entrained Air
10-13. One way to adjust for air entrainment is by strength correction. This method results in a slump reduction that maintains a constant workability. For each percent of air, decrease the water by 3 gallons and the sand by 10 pounds per sack of cement. The sand is decreased because the air bubbles cause oversanding.
Example: Using the mix specifications in paragraph 10-10, adjust the ingredients for an air content of 4 percent.
Solution: After adjustments for entrained air, the mix ingredients for 1 cubic yard of concrete are:
cement = 5.5 sacks (no change)
sand = 1,100 - (5.5 x 10 x 4) = 880 pounds
gravel = 2,200 pounds (no change)
water = 33 - (5.5 x 0.25 x 4) = 27.5 gallons
The yield of the mix is changed by the entrained air. To determine the adjusted yield, divide the design yield by 100, minus the percent of entrained air, as follows:
Adjusted entrained-air yield = design yield (100 - percent air)
Example: Determine the yield of the mix design in paragraph 10-10 if the entrained air is 4 percent.
Trial-Batch Method
10-14. The trial-batch design method is a simple field method that is based on experience. It is more reliable than the book method because the mix can be adjusted until it is satisfactory. Record data as described in FM 5-472 for the final mix design, and calculate yield from the absolute volume of materials.
Mixing a Trial Batch
10-15. Select the water-to-cement ratio based on experience or by using Table 10-1 . Select the workability based on the guidance in paragraphs 10-6 and 10-7. If the slump criteria is not established, make the mix as stiff as possible while maintaining a homogenous, voidless mass.
10-16. Trial batches can be as large as the mixer allows, but small quantities are more convenient. Use about one-tenth of the sack batch. For example, if the water-to-cement ratio is 5 gallons per sack, then use 0.5 gallon of water and one-tenth sack of cement (9.4 pounds). Mix the cement and the water to form a paste. Then mix the sand and the gravel with the paste until the desired consistency is obtained. Ensure that the FA and the CA are in a saturated, surface-dry condition.
10-17. Obtain the weights of the sand and the gravel by weighing each container filled with aggregate before running the trial batch and by weighing the container with the remainder of the aggregate after the run. The difference is the weight of the aggregate used in the trial batch. Test the consistency of the trial batch using the slump test (see FM 5-472 ). After determining the required amounts of sand, gravel, and water needed for one-tenth sack of cement, multiply the weight of each ingredient by 10 to obtain the amount needed for a one-sack batch of concrete.
Calculating the Yield
10-18. Convert the weights of the ingredients to absolute volumes. To calculate the absolute volumes of the ingredients in a mix, determine the specific gravity of the materials. Portland cement normally has a specific gravity of 3.15. See FM 5-472 for the standard tests used to determine the specific gravities of sand and gravel. The sum of the absolute volumes is the concrete yield from a one-sack batch of concrete.
Establishing the Cement Factor
10-19. Establish the cement factor to determine the quantity of each ingredient necessary to batch a mixer or to estimate the total amount of each ingredient required. To do this, divide the volumetric capacity of the mixer or the job by the yield and multiply the quantities for a one-sack batch by the cement factor.
Example: Determine the amount of each ingredient required to batch a 16S mixer, which has a capacity of 16 cubic feet with no overload. The assumed yield is 4 cubic feet.
Solution: Use the following formula to determine the amount of ingredients needed:
Applying the Trial-Batch Method
10-20. The following example illustrates the trial-batch method of mix design and yield calculation. Use the Chapman flask test to determine the specific gravity of sand, the pycnometer test to determine the specific gravity of gravel, and the suspension method to determine the specific gravity of CA (see FM 5-472 ).
Example: Using Type I cement, design a concrete mix for a 10-inch concrete pavement with a flexural strength of 550 psi at 28 days. The specific gravities are assumed to be 3.15 for cement, 2.65 for sand, and 2.66 for gravel. The pavement will be located in an area that has a severe climate.
Solution: For Type I portland cement and a specified flexural strength of 550 psi, use 6 1/4 gallons of water per sack (see Figure 10-1 ). For a 10-inch pavement slab placed in a severe climate (durability factor), use 5 1/2 gallons of water per sack (see Table 10-1 ). The lowest acceptable ratio that will satisfy the requirements for flexural strength and durability is 5 1/2 gallons of water per sack.
The required slump is 1 1/2 to 2 inches. Mix the cement and the water together. Add the sand and the gravel to the paste until a well-proportioned plastic mix is obtained. For the initial trial batch, use one part sand and two parts gravel. More than one trial batch may be necessary to get the required slump. A slight variation in the slump is not detrimental as long as the mix is plastic enough to be finished without excess mortar. To correct excess slump, add more aggregate.
Compute a one-sack batch of mix by multiplying trial-mix calculations by 10 as follows:
water = 5.5 gallons (0.55 x10)
gravel = 362 pounds (36.2 x 10)
Determine the yield by computing the absolute volume of each component using the following formulas:
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