تم تحميل الملف من http://geel.frbb.net
Lecture No. 06 Subject: Sources of Aggregates
Objectives of Lecture • To explain the sources of aggregates used for making concrete.
Aggregates • The total aggregates (fine aggregates + coarse aggregates) are used in concrete as filler and generally occupy 60 % to 75 % of the concrete volume (70 % to 85 % by weight). • Fine aggregates generally consist of natural sand or crushed stone with most particles smaller than 0.2 in. • Coarse aggregates consist of one or a combination of gravels or crushed aggregate with particles predominantly larger than 0.2 in. and generally between ⅜ and 1½ in.
Sources of Aggregates • Freshly mixed normal weight concrete (2200 to 2400 kg/m3) can be produced using: • Natural gravel and sand are usually dug or dredged from a pit, river, lake, or seabed. • Crushed aggregate is produced by crushing quarry rock, boulders, cobbles, or large size gravel. • Crushed air-cooled blast-furnace slag is also used as fine or coarse aggregate. • Recycled concrete, or crushed waste concrete, is a feasible source of aggregates and an economic reality where good quality aggregates are scarce.
Various light weight materials such as expanded shale, clay, slate, and slag are used as aggregates for producing lightweight concrete (1350 to 1850 kg/m3). • Other lightweight materials such as pumice, scoria, perlite, vermiculite, and diatomite are used to produce insulating lightweight concretes (250 to 1450 kg/m3). • Heavy weight aggregates such as barlite, magnetite and iron are used to produce heavy weight concrete and radiation-shielding concrete.
Lightweight Aggregates • Expanded clay (left) • Expanded shale (right)
Constituents in Naturally Occurring Aggregates • Naturally occurring concrete aggregates are a mixture of rocks and minerals (see Table 5-1) • Minerals • Silica (ex. Quartz) • Silicates (ex. Clay) • Carbonate (ex. Calcite, dolomite) • Igneous rocks • Granite • Basalt • Sedimentary rocks • Sandstone • Limestone • Shale • Metamorphic rocks • Marble • slate
Amount of cement paste required in concrete is greater than the volume of voids between the aggregates.
Aggregates can occasionally contain particles of iron oxide and iron sulfide that result in stains on exposed concrete surface.
Influence of Adding mineral admixture on alkali-silica reactivity (ASR)
Local Aggregates Sources Eastern Province: Fine aggregates: • Most of the fine aggregate in the eastern province is dune sands with silica contents ranging from 79% to 98%. Coarse Aggregates: • The coarse aggregates are limestone and they contain high content of calcite and some quartz. Central Province: Fine aggregates: • Good quality fine aggregates are available throughout the central province; they contain quartz, feldspar, and calcite. In general, the fine aggregates contain 82% to 99% silica. Coarse Aggregates: • The coarse aggregates are limestone, diorite, and amphibolites. These aggregates contain calcite, quartz, and dolomite.
Western Province: Fine aggregates: • The fine aggregates contain quartz, feldspar, calcite, and mica. In general, the fine aggregates in the western region contain less silica (60% to 76%) compared to sands from eastern and central regions. Coarse Aggregates: • The coarse aggregates are amphibolites, hornblende diorite, etc. They contain about 50% SiO2.
Unwashed local aggregate is the largest contributor of chlorides in concrete in the Gulf region. The local aggregate in the eastern region of Saudi Arabia is composed of crushed limestone which is usually porous, absorptive, relatively soft and excessively dusty. The dust and fines are heavily contaminated with sulfate and chloride salts.
Table 1. Mineralogical composition of the selected coarse aggregates determined by X-ray diffraction technique. The following Tables present some test results on selected local coarse aggregates:
Table 2. Materials finer than ASTM # 200 sieve in the selected coarse aggregates.
Table 3. Specific gravity and water absorption for the selected coarse aggregates.
Table 5. Chloride and sulfate concentrations in the selected coarse aggregates.
In conclusion • Tests conducted on the selected coarse aggregates indicated that the quality of coarse aggregates from quarries on the Riyadh road is relatively better than the coarse aggregates from quarries in Hofuf and Abu-Hadriyah. • The quantity of fine materials in all the coarse aggregates was less than the allowable value of 1%. However, the quantity of fine materials in the coarse aggregates from quarries in Abu-Hadriyah was more than that in the coarse aggregates from quarries in Hofuf and on the Riyadh road. • The loss on abrasion in all the coarse aggregates was less than the allowable value of 40%. The loss on abrasion in the coarse aggregates from quarries in Abu-Hadriyah was generally more than that in the coarse aggregates from quarries in Hofuf and on the Riyadh road.
In conclusion • The water absorption in the coarse aggregates from quarries in Abu-Hadriyah was more than that in the coarse aggregates from quarries in Hofuf and on the Riyadh road. • The chloride concentration in the coarse aggregates from the Al-Suhaimi quarry in Abu-Hadriyah was two times the allowable value of 0.03%, while in other coarse aggregates, the chloride concentration was less than the threshold value. • The sulfate concentration in all the coarse aggregates was less than the value specified by the Saudi Aramco specifications.