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Mix designs for concrete block Proportioning using the fineness modulus method By Neal Jablonski roportioning the mix components for a concrete masonry unit (CMU) is an important step in producing high-quality units. A well-proportioned mix can improve a unit’s physical properties (compressive strength, unit weight, absorption) so they meet or exceed ASTM C 90 Standard Specification for Load-Bearing Concrete Masonry Units. Additionally, the right mix proportions can improve CMU durability and appearance. This article gives general guidelines for designing a concrete mix with locally available aggregates, and focuses on determining aggregate fineness modulus, proportioning aggregates, and choosing a cement-aggregate ratio. P The fineness modulus method The most commonly used method for designing mixes for concrete masonry is called the FM method. FM stands for fineness modulus, an index number roughly proportional to the average size of the particles in a given aggregate. The coarser the aggregate, the higher the FM. While all block mixes require at least one aggregate, a producer, for a variety of reasons, might decide to use as many as four aggregates in a mix. Such reasons include lack of well-graded aggregates nearby (and increased cost to obtain such aggregates) and the desire to produce a unit with better physical and aesthetic properties. However, because two-aggregate mixes are the most common, that combination is covered in this article. Manually calculating three- and four-aggregate blends is laborious, therefore spreadsheet software should be used. Following are the six steps involved in designing a mix using the FM method: Step 1: Determine the FM of each aggregate. To determine the FM of a single aggregate, follow the procedure outlined in ASTM C 136, “Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates.” The steps are as follows: 1a. Sieve approximately 500 grams of oven-dry aggregate through 3⁄8 inch, No. 4, No. 8, No. 16, No. 30, No. 50, and No. 100 sieve sizes. 1b. Starting with the largest sieve, add the percent retained on each successive sieve to arrive at the cumulative percent retained up to and including that sieve. 1c. Add the cumulative percent retained on each sieve and divide the sum by 100. Don’t include the pan in the sum. This will give you the fineness modulus. [See the example below.] Step 2: Proportion aggregates for proper FM. Aggregates are blended to obtain the desired FM for a specific Sieve analysis and calculating fineness modulus Using the sieve analysis result below, the fineness modulus can be calculated. The fineness modulus is the sum of the total percentage retained on each of a specified series of sieves divided by 100. A sample of fine aggregate weighing 508.5 grams is passed over the sieves shown below and the weights retained on each sieve are as shown. Sieve Weight Individual retained, grams % retained Cumulative % retained 3 8 ⁄ inch No. 4 No. 8 No. 16 No. 30 No. 50 No. 100 Pan 0 9.2 67.6 101.2 104.2 122.5 95.3 8.5 0 2 13 20 20 24 19 2 0 2 15 35 55 79 98 (100)* Total 508.5 100 Sum = 284 284 Fineness Modulus = --------- = 2.84 100 *Pan not included in fineness modulus class of unit. The industry-recommended FMs for various types of units and aggregates are: normalweight CMU, 3.70; medium-weight CMU, 3.67; and lightweight CMU, 3.84. (Grading charts are shown in Figure 1.) Aggregate blends shouldn’t contain excess fines or coarse particles. Blends with excess fines require more cement to coat the added surface area, while mixes with excess coarse aggregate will contain large interconnecting voids and be harsh. Calculate the proportion of fine and coarse aggregate for the desired unit using the following equation: X= A-B (_____ A - C) where X = percent of fine aggregate Y = percent of coarse aggregate A = FM of coarse aggregate (determined per step 1) B = FM of desired blended aggregate for block class C = FM of fine aggregate (determined per step 1) Step 3: Determine aggregate batch proportions. Calculate the batch weight of each aggregate by multiplying the total design batch weight (usually dictated by mixer size) by the percent of fine and coarse aggregate (per Step 2) in the mix. 100 Step 4: Determine moisture content in aggregate. Most aggregate contains moisture Y = 100 - X when it’s batched. Because design weights are for dry aggregate, batch weights must be increased to maintain the same dry cement-to-aggregate ratio. If aggregate batch weights aren’t adjusted to account for the moisture, you’ll get a lower yield (fewer units per pound of cement) and thus a less economical mix. To adjust aggregate batch weights, the moisture content of each aggregate must first be determined. 4a. Weigh a representative sample of each aggregate (500 to 2,000 grams, depending on particle size) and record the initial weights. 4b. Oven dry each sample to remove the moisture, then re-weigh and record this final weight. 4c. Determine the moisture content of each aggregate. Subtract the final (dry) weight of the aggregate Designing a block mix Following the steps outlined in the article, let’s design a two-aggregate, normal-weight block mix with an industry-recommended FM of 3.70. The mixer can handle a total dry aggregate batch weight of 4,500 pounds. Step 4: Moisture content. Moisture content of the sand is 5% and the gravel 2%. We determine the adjusted batch weights per Step 4d as follows: (for sand) 2,430 lbs. 1.05 = 2,552 lbs. (for gravel) 2,070 lbs. 1.02 = 2,111 lbs. Step 1: Aggregate FMs. As indicated in the table below, our coarse aggregate (gravel) has an FM of 4.82 and our fine aggregate (sand) has an FM of 2.75. Step 5: Cement content. We desire a moderate cement-to-aggregate ratio of 1:10 (per ratio listings on page 368). For the required aggregate design batch weight of 4,500 pounds the required cement is: 1 = 450 lbs. ___ cement weight = 4,500 10 Step 6: Water content. Our experience in producing similar units indicates that a good total water content to start with is about 5.5% of total batch weight (4,500 pounds of aggregate + 450 pounds of cement) or in this case 272 pounds of water. We know, per Step 4c, that the sand contains 5% moisture (122 pounds) and the gravel contains 2% moisture (41 pounds). This 163 pounds of water already contained in the aggregate is subtracted from the required amount of water (272 pounds), leaving 109 pounds of water to be batched. Since water weighs 8.34 pounds per gallon, about 13 gallons of water will be used in the first trial batch. Total initial batch weights are as follows: Cement 450 lbs. Sand (5% moisture) 2,552 lbs. Gravel (2% moisture) 2,111 lbs. Added water 109 lbs. (about 13 gallons) Percentage retained on each sieve 3 8 ⁄ 4 8 16 30 50 100 Pan FM Fine —- 1 10 15 27 32 14 1 2.75 Coarse —- 24 48 21 2 1 1 4.82 Material 3 Step 2: Aggregate proportioning. Knowing the FMs we can calculate the aggregate proportions for the mix: 4.82 - 3.70 1.12 X = __________ (100) = _____ (100) = 54% 4.82 - 2.75 2.07 X = 54% fine aggregate Y = 46% coarse aggregate Step 3: Batch proportions. Our dry aggregate design batch weight is 4,500 pounds, so we calculate: 4,500 54% = 2,430 lbs. of fine aggregate 4,500 46% = -------2,070 lbs. of coarse aggregate 4,500 total design batch weight of aggregate from the initial (wet) weight, and divide it by the final weight. This is the moisture correction factor. 4d. Adjust the batch weight of each aggregate to maintain the proper mix proportions. Multiply the design batch weight for each aggregate (3a) by the moisture correction factor (4c) and add this weight to the original design batch weight. Or simply multiply the design batch weight for each aggregate by one plus the moisture correction factor (found in 4c). Step 5: Determine cement content. Cement is the final component needed to produce a high-quality CMU. Cement binds aggregate particles and partially fills spaces between them. 5a. Choose the cement-to-aggregate ratio that will achieve the necessary CMU properties with aggregates being used in the mix. Below are ranges of cement-to-aggregate ratios that can be used for various Figure 1. Aggregate gradation for concrete masonry units Normal-weight Sieve size 3 8 ⁄" 4 8 16 30 50 100 Pan Min. Max. 0 20 10 10 10 10 0.5 0.2 5 30 23 20 20 20 15 10 Ideal 0 25 15 15 15 15 10 5 FM = 3.7 Medium-weight Sieve size Min. Max. Ideal 3 8 0 12 18 16 11 5 5 7 0.3 22 27.5 25 19 13 11 13 0 17 23 20 15 9 7 9 FM = 3.67 ⁄" 4 8 16 30 50 100 Pan Lightweight Sieve size 3 8 ⁄" 4 8 16 30 50 100 Pan Min. Max. Ideal 0.5 17 21 13 7.5 5 5.3 0.7 5 28 30 21 15.8 13 10.5 13.1 0.5 21 25.5 17 11.5 9 6.5 9 FM = 3.84 Source: Besser Co., “Concrete Masonry Technology Blockmakers Workshop Series” types of aggregates. All of the ratios are based on dry weight. Type of aggregate Range of ratios (cement:aggregate) Sand and gravel Limestone Pumice Cinders Slag (expanded) Slag (air cooled) Clay (expanded) 1:8 to 1:12 1:7 to 1:12 1:4 to 1:6 1:6 to 1:8 1:5 to 1:7 1:8 to 1:12 1:6 to 1:9 5b. Determine the required total aggregate design batch weight by adding the design batch weight of the fine and coarse aggregate as found in Step 3. 5c. Determine how much cement is required in the mix. Multiply the weight determined in 5b by the desired cement-to-aggregate ratio. Aggregate design batch weight cement-to-aggregate ratio = cement (in lbs.) For example: 4,500 lbs. 1 —— = 450 lbs. 10 Step 6: Determine water content. The total amount of mixing water needed to make a high-quality unit will vary depending upon the type of aggregate used, cement content, and desired appearance. The producer is left to determine this amount through trial batches. Just a start The mix design techniques described above can be used to design mixes for normal-weight, medium-weight, and lightweight units. Extra care should be taken when designing lightweight and medium-weight mixes. If a calculated blend contains too much or too little lightweight or medium-weight aggregate, the desired density might not be achieved. A well-designed mix helps producers meet the increasing demand for high-quality low-cost CMUs. The fineness modulus method provides a good starting point when designing a mix for any type of unit. However, whether producers use the fineness modulus method or any other, they need to know that choosing the best mix design usually requires a trial-and-error approach in which mixes are tested and adjusted until a desired result is attained. ✥ Neal Jablonski is a technical service specialist for Grace Construction Products in Milwaukee. References: 1. Grant, William, Manufacturer of Concrete Masonry Units, second edition, Concrete Publishing Corp., 1959 2. Besser Co., Concrete Masonry Technology Blockmakers Workshop Series 3. Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates, ASTM C 136-95, 1995 PUBLICATION #J960363 Copyright © 1996, The Aberdeen Group All rights reserved