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Engineering Geology

Engineering Geology - geomuseu

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E n g i n e e r i n g G e o l o g y<br />

pattern of cracking develops on the surface. These troubles can be avoided if a preliminary<br />

petrological examination is made of the aggregate. In other words, material that contains over<br />

0.25% opal, over 5% chalcedony, or over 3% glass or cryptocrystalline acidic to intermediate<br />

volcanic rock, by weight, will be sufficient to produce an alkali reaction in concrete, unless<br />

low-alkali cement is used. This contains less than 0.6% of Na 2 O and K 2 O. If aggregate<br />

contains reactive material surrounded by or mixed with inert matter, a deleterious reaction<br />

may be avoided. The deleterious effect of alkali aggregate reaction can also be avoided if a<br />

pozzolan is added to the mix, the reaction taking place between it and the alkalis.<br />

Reactivity may be related not just to composition but also to the percentage of strained quartz<br />

that a rock contains. For instance, Gogte (1973) maintained that rock aggregates containing<br />

40% or more of strongly undulatory or highly granulated quartz were highly reactive, whereas<br />

those with between 30 and 35% were moderately reactive. He also showed that basaltic<br />

rocks with 5% or more secondary chalcedony or opal, or about 15% palagonite, showed deleterious<br />

reactions with high-alkali cements. Sandstones containing 5% or more chert behaved<br />

in a similar manner.<br />

Certain argillaceous dolostones have been found to expand when used as aggregates in<br />

high-alkali cements, thereby causing failure in concrete. This phenomenon has been referred<br />

to as alkali–carbonate rock reaction, and its explanation has been attempted by Gillott and<br />

Swenson (1969). They proposed that the expansion of such argillaceous dolostones in<br />

high-alkali cements was due to the uptake of moisture by the clay minerals. This was made<br />

possible by dedolomitization that provided access for moisture. Moreover, they noted that<br />

expansion only occurred when the dolomite crystals were less than 75 microns.<br />

It usually is assumed that shrinkage in concrete should not exceed 0.045%, this taking place<br />

in the cement. However, basalt, gabbro, dolerite, mudstone and greywacke have been shown<br />

to be shrinkable, that is, they have large wetting and drying movements of their own, so much<br />

so that they affect the total shrinkage of concrete. Clay and shale absorb water and are likely<br />

to expand if they are incorporated in concrete, and they shrink on drying, causing injury to the<br />

cement. Consequently, the proportion of clay material in a fine aggregate should not exceed<br />

3%. Granite, limestone, quartzite and felsite are unaffected.<br />

Road Aggregate<br />

Aggregate constitutes the basic material for road construction and is quarried in the same<br />

way as aggregate for concrete. Because it forms the greater part of a road surface, aggregate<br />

has to bear the main stresses imposed by traffic, such as slow-crushing loads and rapid-impact<br />

loads, and has to resist wear. Therefore, the rock material used should be fresh and have<br />

high strength. In addition, the aggregate used in the wearing course should be able to resist<br />

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