Crushed aggregates

Crushed aggregates are produced in quarries by various rocks with proper mechanical and chemical properties. Rocks are classified into three general groups depending on how they were formed igneous rocks, sedimentary rocks and metamorphic rocks. 

Rocks formed by cooling molten elements (materials) are defined as igneous rocks. Depending on cooling rate, they are characterised as coarse textured, such as granite rocks (slow cooling rate), or fine textured, such as basalt (quicker cooling rate). 

Igneous rocks are distinguished into four main categories gabbros, basalts, granites and porphyries. 

Sedimentary rocks are classified depending on how they were formed. They are divided into four groups (a) clastic sedimentary rocks , (b) biochemical (or biogenic) sedimentary rocks, (c) chemical sedimentary rocks and (d) other sedimentary rocks formed by impact, volcanism and other processes. 

Clastic sedimentary rocks are mainly composed of quartz, feldspar, rock fragments (lithic), clay minerals and mica. Clastic sedimentary rocks are further subdivided into conglomerates (mainly composed of rounded gravels), breccias (mainly composed of rounded 

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It allows the reuse of treated soils in a variety of ways, including backfill at the site of origin, reclaiming aggregate or coal pits, general clean fill, crushed aggregate sales, asphalt mix sales, concrete mix sales, and cover for sanitary landfills. 

Hummel showed that the Fuller parabola (n = 0.5) does not yet furnish the optimal packing densities 14). In the case of spherical aggregates n = 0.4 ought to be chosen as the greatest density in the case of crushed aggregates n = 0.3 if the density maximum is to be reached for the aggregate (Figure 4). 

The subgrade used in the analysis was assumed to be a clay material (A-6 classification) with a moisture content of about 23% and an elastic modulus of 15,000 psi. The base course used in the analysis was assumed to be a dense graded crushed aggregate with an elastic modulus of 60,000 psi. These two materials were assumed to be elastic and are typical of those used in construction of many pavements throughout the U.S. 

The melting process must be controlled in a way that the structure of the crushed aggregate particles will not be leafy, foamy, or irregular. This does not mean, however, that the raw product before the crushing process must be homogenous and pore-free. 

The majority of igneous rocks, sandstones from sedimentary rocks and hornfels from metamorphic rocks are usually hard rocks. They typically produce durable crushed aggregates that can be used for all pavement layers. Special care should be given to the adhesion of bitumen to these rocks. 

Crushed aggregates derived from sedimentary rocks, such as limestone and dolomite, have very good to excellent properties and can be used in all pavement layers except for surface courses, because they have low resistance to polishing. 

The coarse crushed aggregates should have resistance to fragmentation by Los Angeles lower than 30%, when tested in accordance with ASTM C 131 (2006). 

Crushed aggregates, binder and additives (cellulose or mineral fibres) should be as specified by AASHTO M 325 (2012). 

The aggregates in the Type II mixture should be natural sand and gravels with or without crushed aggregate materials as per Type I. 

The base/sub-base materials are unbound crushed aggregates. Type I, or, if the thickness is 400 or 300 mm, selective granular material, Type II (see Tables 10.4 and 10.6 for... 

The elastic modulus, in contrast to compressive strength, does not noticeably increase with curing time, as it has been found by Croney and Croney (1991) testing specimens produced with crushed aggregates or gravels and cured for up to 520 and 104 weeks, respectively. Moreover, Croney and Croney (1991) state that the computed stresses in the concrete slab are not very sensitive to small changes in the elastic modulus. 

Unlike UK rigid pavement design methodology, the CRCP with asphalt overlay is not enforced to be used in all new constructions. However, the use of lean concrete, cement-stabilised crushed aggregate or dense asphalt concrete as a sub-base material in moderate to heavily trafficked roads is necessary for lightly trafficked roads, unbound granular sub-base may also be used. 

The crack-relief layer is typically a 90 mm thick layer of coarse and open-graded hot asphalt, containing 25% to 35% voids and made up of 100% crushed aggregate (Asphalt Institute MS-17 3rd Edition). The recommended aggregate grading limits are as shown in Table 15.18. 

Dolomite (i) crushed aggregate in concrete, railroad ballast, sewage filter beds (11) fluxing agent in smelting and refining of steel ... 

Friable mfr Gentle pressure is required to crush aggregates... 

ASTM D693-03a, Standard Specification for Crushed Aggregate for Macadam Pavements, 2003. 

The porous concretes are preferably composed with blast-furnace slag cements, gap graded crushed aggregate with grain size above 5 mm and without fines or minimizing sand volume fraction with superplasticizers and fly ash. Pavements with increased porosity are built in a few layers with different tasks. A minimum required permeability between 7-10 m/s and 3-10 m/s was proposed by Beeldens and Brishant (2004). 

Elvidge, C. B., Raymond, G. P. (1999). Laboratory survivabrhty of nonwoven geotextiles on open-graded crushed aggregate. Geosynthetics International, 6(2), 93—117. 

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