Cover Image

Not for Sale

View/Hide Left Panel
Click for next page ( 59

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement

Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 58
58 per day (162). Besides the reduced production rate, the other drawback of this type of production process is the generation of excess crushed fines. Many crushing operations, including this German example, produce more crushed fine aggregate than they can sell. A related concept to the reduced reduction ratio is the con- cept of producing product close to the close-side setting of the crusher. A typical crushing operation in the United States might produce ASTM No. 57 stone in combination with ASTM No. 8 stone (159). In this case, the close-side setting of the crusher would be set to approximately 29 mm. Obser- vations indicate that the No. 57 stone would tend to have the best particle shape and that the smaller No. 8 stone would tend to be more flat and elongated. This phenomenon is illus- trated in Figure 24, which is produced with data from Jahn (163) for a granite No. 57 stone. The aggregate was tested using the multiple shape ratio method. Producing the No. 8 stone separately from the No. 57 stone could eliminate the effect. However, this process would increase the production Figure 22b. Multi-layer crushing. of fines and would require additional equipment or reduced production. Particle shape can also be improved with the use of impact 2.10.3 Applications of Crushers crushers. Shergold (164) states: German specifications require that all coarse aggregate It is thought that the good particle shape obtained with used in HMA have less than 20% 3 to 1 particles (162). Good impact breakers can be explained with the assumption that, particle shape that meets this criterion can be produced in impact breaking, the stresses have a more or less random distribution, whereas in compressive crushing the stresses through the used of compression-type crushers, even with are concentrated in relatively closer spaced planes near the granite-diorite aggregates. One German quarry uses a jaw surface. primary crusher and 5.5-ft cone secondary crusher, after which aggregate for HMA passes through a series of four to five Huber et al. (34) demonstrated the improvement in shape short head cone crushers to improve shape (162). Storage is when the same aggregate was crushed with a cone and a ver- provided to maintain a consistent feed rate. The resulting tical shaft impact crusher. No. 57 stone was produced from product is an 8-mm to 11-mm aggregate with typical percent the same Indiana limestone source in each crusher. Neither F&E greater than 7% to 9% 31 ratio. This is an example of crusher produced flat and elongated maximum to minimum using smaller reduction ratios to improve aggregate shape. particle ratios greater than 51. The cone crusher produced Using this method, the quarry produces 3,000 to 4,000 tons 19.4%, and the vertical shaft impact crusher produced 9.0% per day. However, when specialty stone was produced for an particles exceeding the 31 ratio. One potential drawback of open-graded friction course requiring less than 5% 31 par- using impact-type crushers to produce coarse aggregate is the ticles, production levels were reduced to less than 150 tons resulting effect on the fine aggregate. Iowa DOT is conducting research to compare the aggre- gate properties and properties of the resulting aggregate pro- duced with a cone or hammermill (a form of horizontal shaft impact crusher) crusher (165). The study evaluated three aggregate sources. Preliminary results indicate no significant difference in aggregate shape between particles crushed with the cone or hammermill crusher. However, the VMA of the HMA produced with aggregate crushed in the hammermill crusher was consistently 0.7% higher than the VMA of the HMA produced with the cone crusher. 2.10.4 Influence of Shape on Performance Figure 23. Vertical view of gyratory action of The production of more cubical coarse aggregate can gyratory/cone type crusher. produce more cubical fine aggregate (166, 167). Studies have

OCR for page 58
59 35 Percent by Mass of Particles Exceeding Specified Ratio Granite #57 Stone 30 25 20 3:1 Ratio 5:1 Ratio 15 10 5 0 19 12.5 9.5 4.75 Sieve Size (mm) Particles Retained On Figure 24. Example of shape variation with particle size. indicated that cubical fine aggregate particles may pack these products often compete. Cubical coarse aggregates are similarly to round fine aggregate particles, producing low believed to be desirable for the production of HMA. However, percent uncompacted voids as measured by AASHTO T304 the production of cubical coarse aggregates may result in Method A (127, 54). Vertical shaft impact crushers have been Aggregate base that packs more tightly, reducing drain- used to produce more cubical fine aggregate for use in Port- land cement concrete (157, 168, 169). A Virginia stone pro- age capacity; More cubical fine aggregate, resulting in lower uncom- ducer investigated the effects of vertical shaft impact crush- pacted voids; ers to produce more cubical fine aggregate. The resulting fine Reduced LA abrasion loss; aggregate packed more tightly than that previously produced More cubical aggregates may pack closer in HMA, by the quarry. This resulted in an undesirable increase in the resulting in lower VMA; unit weight of concrete block produced with the aggregate, More cubical fine aggregate, which packs closer, result- resulting in a loss of sales (157). In North Carolina, a quarry ing in higher density block; and separated sand-size particles prior to crushing with a vertical More cubical fine aggregate that reduces water demand shaft impact crusher to sell to HMA and concrete block plants for Portland cement concrete, resulting in higher strength. (168). This illustrates the sometimes divergent requirements of aggregates between different industries. Although aggregate shape could seemingly be customized Aggregate is produced for numerous applications including for a given application, in practice this is not possible or, at base, HMA, and Portland cement concrete. The needs of least, is cost prohibitive.