SECTION III. AGGREGATES
2-35. Aggregates make up 60 to 80 percent of the concrete's volume. Their characteristics considerably influence the mix proportions and economy of the concrete. For example, very rough-textured or flat and elongated particles require more water to produce workable concrete than do rounded or cubed particles. Angular particles require more cement paste to coat them, making the concrete more expensive. Aggregates should be clean, hard, strong, durable, and free from chemicals or coatings of clay or other fine materials that affect the bond of the cement paste. The most common contaminating materials are dirt, silt, clay, mica, salts, humus (decayed plant matter), or other organic matter that appears as a coating or as loose, fine material. You can remove many contaminants simply by washing the aggregate. However, test CAs containing easily crumbled or laminated particles. The most commonly used aggregates are sand, gravel, crushed stone, and blast-furnace slag. They produce normal-weight concrete (concrete that weighs 135 to 160 pounds per cubic foot). Normal-weight aggregates should meet the specifications for concrete aggregates that restrict contaminating substances and provide standards for gradation, abrasion resistance, and soundness. Aggregate characteristics, their significance, and standard tests for evaluating are given in Table 2-1.
Table 2-1. Aggregate characteristics and standard test
2-36. Abrasion resistance is essential when the aggregate is subject to abrasion, such as in a heavy-duty concrete floor.
FREEZE AND THAW RESISTANCE
2-37. The resistance to freezing and thawing relates to aggregate porosity, absorption, and pore structure. If particles absorb too much water, there will not be enough pore space for the water to expand during freezing. The undesirable result is that this water will expand anyway, cracking the concrete. You can predict aggregate performance during freezing and thawing in two ways: pasted performance and freeze-thaw tests of concrete specimens. If aggregates from the same source have been satisfactory under those conditions in the past, use the aggregate. You can determine the performance of unknown aggregates by subjecting concrete specimens to both freeze-thaw and strength tests.
2-38. In aggregates, this characteristic means that they do not react unfavorably with cement, and that external sources do not affect them chemically. Good field service records are generally the best predictor of nonreactive aggregates. If no service record exists and you suspect an aggregate is chemically unstable, laboratory tests are necessary.
PARTICLE SHAPE AND SURFACE TEXTURE
2-39. The particle shape and surface texture affect the properties of plastic concrete more than those of hardened concrete. Very sharp, rough aggregate particles or flat, elongated particles require more fine materials (hence more cement) to produce workable concrete than do rounded or cubed particles. Avoid stones that break up into long sliver pieces or limit them to a maximum of 15 percent in either FAs or CAs.
AGGREGATE SIZEDISTRIBUTION AND GRADING
2-40. The aggregate's size distribution and grading affect the concrete's workability, economy, porosity, and shrinkage. Prior experience has shown that exclusive use of very fine sands produces uneconomical mixes, whereas exclusive use of very coarse sands produces harsh, unworkable mixes. The proportioning of the different particle sizes is called grading an aggregate. Grading is controlled by the aggregate producer. The aggregates particle-size distribution is determined by separation with a series of standard sieves. The standard sieves are numbers 4, 8, 16, 30, 50, and 100 for FAs and 6, 3, 1 1/2, 3/4, and 3/8 inch and number 4 for CAs (other sieves can be used for CAs). The number of a FA sieve corresponds to the number of meshes (square openings) to the linear inch that the sieve contains; the higher the number, the finer the FA sieve. Any material retained in the number 4 sieve is considered CA, and any material that passes the number 200 sieve is too fine for concrete. The finest CA sieve is the same number 4 used as the coarsest FA sieve. With this exception, a CA sieve is designated by the size of one of its mesh openings. The size of the mesh openings in consecutive sieves is related by a constant ratio. Size-distribution graphs show the percentage of material passing each sieve (see Figure 2-1). Figure 2-1 also gives the grade limits for FA and for one designated size of CA. Normal CA consist of gravel or crushed stone, whereas normal FA is sand.
Figure 2-1. Limits specified in ASTM C33 for FA and one size of CA
2-41. The fineness modulus indicates the fineness of a FA but is not the same as its grade. Many FA gradings can have the same fineness modulus. To obtain the fineness modulus of a FA (see Table 2-2), quarter a sample of at least 500 grams of sand and sieve it through the numbers 4, 8, 16, 30, 50, and 100 sieves. Record the individual weights of the materials retained on each sieve and the cumulative retained weights. Add the cumulative percents and divide by 100. The result is the fineness modulus of the sample. A sand with a fineness modulus falling between 2.3 and 3.1 is suitable for concrete (see Table 2-3). FA having either a very high or a very low fineness modulus is not as good a concrete aggregate as medium sand. Coarse sand is not as workable, and fine sands are uneconomical. Take care to obtain representative samples. The fineness modulus of the aggregate taken from one source should not vary more than 0.20 from all test samples taken at that source.
Table 2-2. Typical fineness modulus calculation
Table 2-3. Fineness modulus ranges for FAs
2-42. The selection of the best FA grading depends on the application, the richness of the mix, and the maximum size of the CA used. In leaner mixes or when small-size CA is used, a FA grading near the maximum recommended percentage passing each sieve is desirable for workability. In richer mixes, coarser FA grading is desirable for economy. If the W/C ratio is kept constant and the ratio of FA to CA is chosen correctly, a wide range of FA grading can be used without much affect on strength. Grading is expressed as the percentage by weight passing through the various standard sieves. The amount of FA passing the number 50 and 100 sieves affects workability, the finished-surface texture, and water gain or bleeding. Bleeding is the rise of water to the concrete's surface. For thin walls, hard-finished concrete floors, and smooth concrete surfaces cast against forms, the FA should contain not less than 15 percent material passing the number 50 sieve and at least 3 or 4 percent (but not more than 10 percent) material passing the number 100 sieve. These minimum amounts of FA give the concrete better workability, making it more cohesive, and they produce less water gain or bleeding than lower percentages of FA. In no case should the percentage passing a number 200 sieve exceed 5 percent, and only 3 percent if the structure is exposed to abrasive wear. Aggregate grading falling within those limits are generally satisfactory for most concrete.
2-43. The grading of CA of a given maximum size can vary over a wide range without much effect on cement and water requirements if the proportion of FA produces concrete having good workability. Table 2-4 gives the grading requirements for CA. If CA grading varies too much, the mix proportions will need to vary to produce workable concrete. If the variance continues, it is more economical to request that the producer adjust his operation to meet the grading requirements. CA should be graded up to the largest practicable size for the job conditions. According to the American Concrete Institute (ACI) 318-83, the nominal maximum size of the CA should not be larger than 1/5 the narrowest dimension between the sides of forms, 1/3 the depth of slabs, and 3/4 the minimum clear spacing between individual reinforcing bars or wires, bundles of bars, or prestressing tendons or ducts. These limitations may be waived if, in the judgment of the engineer, workability and consolidation methods are such that concrete can be placed without honeycomb or voids. (These are undesirable areas; however, a smooth finish is desired even though CA is visible. Honeycomb or voids are usually first observed when the formwork is removed.) The larger the maximum size of the CA, the less paste (water and cement) that is required to produce a given quality. Field experience shows that the amount of water required per unit volume of concrete for a given consistency and given aggregates is nearly constant, regardless of the cement content or relative proportions of W/C. Furthermore the amount of water required decreases with increases in the maximum size of the aggregate. The water required per cubic yard for concrete with a slump of 3 to 4 inches is shown Figure 2-2. The figure demonstrates that for a given W/C ratio, the amount of cement required decreases as the maximum size of CA increases. In some instances, especially in higher-strength ranges, concrete containing smaller maximum-size aggregate has a higher compressive strength than concrete with larger maximum-size aggregate at the same W/C ratio
Table 2-4. Grading requirements for coarse aggregate
Figure 2-2. Water requirements for concrete of a given consistency as a function of CA size
2-44. Certain particle sizes are entirely or mostly absent in gap-graded aggregates. The lack of two or more successive sizes can create segregation problems, especially in non-qair-entrained concrete having slumps greater than 3 inches. However, for a stiff mix, gap-graded aggregates can produce higher strengths than normal aggregates in concrete mixes having comparable cement contents.
BULK UNIT WEIGHT
2-45. This is the weight of the aggregate that fills a 1-cubic foot container. This term is used because the volume contains both aggregate and voids of air spaces.
2-46. This is the ratio of aggregate weight to the weight of an equal volume of water. Normal-weight aggregates have specific gravities ranging from 2.4 to 2.9. The internal structure of an aggregate particle is made up of both solid matter and pores or voids that may or may not contain water. The specific gravities used in concrete calculations are generally for saturated, surface dry (SSD) aggregates; that is, when all pores are filled with water, but no excess moisture is present on the surface.
ABSORPTIONAND SURFACE MOISTURE
2-47. Both absorption and surface moisture must be known to control the net water content of the concrete and determine correct batch weights. Clearly, dry aggregate requires more concrete mixing water. The four moisture conditions of aggregates are as follows (see Figure 2-3)
Figure 2-3. Moisture conditions of aggregates
2-48. Bulking occurs when damp FA is handled. Bulking is the undesirable increase in volume caused by surface moisture holding the particles apart. Figure 2-4 shows the variation in the amount of bulking with moisture content and grading. Sand is normally delivered in batch quantities in a damp condition; but, due to bulking, actual sand content can vary widely in a batch volume, often not in proportion to the moisture content of the sand. Therefore, be very careful when proportioning by volume. Too much moisture on the aggregate surfaces also adds to the concrete mixing water. The amount can be considerable, especially the excess water in FA.
Figure 2-4. Variation in fine-aggregate bulking with moisture and aggregate grading
2-49. Aggregates can contain such impure substances as organic matter, silt, clay, coal, lignite, and certain lightweight and soft particles. Table 2-5 summarizes the effects of these substances on concrete
Table 2-5. Impurities in aggregates
HANDLING AND STORING
2-50. Figure 2-5 shows both the correct and incorrect methods of handling and storing aggregates. You must handle and store aggregates to minimize segregation and prevent contamination by impure substances. Aggregate normally stored in stockpiles builds up in layers of uniform thickness. Stockpiles should not be built up in high cones or allowed to run down slopes because this causes segregation. FA remains at the top of the stockpile while heavier aggregate rolls toward the bottom. Do not allow aggregate to fall freely from the end of a conveyor belt. To minimize segregation, remove aggregates from stockpiles in horizontal layers. When you are using batch equipment and storing some aggregate in bins, load the bins by allowing the aggregate to fall vertically over the outlet. Chuting the materials at an angle against the side of the bin causes particle segregation.
Figure 2-5. Correct and Incorrect aggregate handling and storage
|David L. Heiserman, Editor||
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Revised: June 06, 2015