Coarse Aggregates
Coarse aggregates Composition
The stone crushing or gravel building tends to improve quality and bond characteristics and generally results in a higher flexural strength of concrete. When mixture proportioning studies or local experience indicates that a low flexural strength will be obtained with uncrushed gravel or stone, the possibility of obtaining higher strength by crushing the material will be investigated.
Coarse Aggregates Sizes And Grading
The nominal maximum size of the coarse aggregate used in pavement concrete should not exceed 1/4 of the pavement thickness. In no case will the coarse aggregate exceed a 2-inch nominal maximum size. When the nominal maximum size of coarse aggregate is greater than 1 inch, the aggregates will be furnished in two size groups as shown in table 2-1, with grading within the separated size groups conforming to the requirements of table 2-2. Where local practice provides size-group separations other than shown in table 2-1, local size grading may be specified if approximately the same size ranges are obtained and the grading of coarse aggregate when combined and batched for concrete is as required by mixture proportioning. State specifications for grading may be used in place of the Corps of Engineers grading. Usally, coarse aggregates sizes is 2.0 to 4.75mm, and mediu aggregates size is 0.425 to 2mm.
Coarse Aggregates Deleterious Substances
Deleterious substances in coarse aggregate are unintended materials contaminating the desired aggregate. These substances will reduce concrete strengths, increase the chance for popouts and weatherouts, decrease workability of the plastic concrete, and lead to abbreviated life spans of the concrete pavements. Time will not allow extensive testing of aggregate stockpiles, but if deleterious substances are suspected, washing (Sand washing machine)of the aggregate should be specified. This should take care of most of the objectionable material. If unwanted materials are still present and to such a degree as to cause considerable problems, a new source for aggregate should be sought. Some deleterious substances which are possible in concrete aggregates are listed in table 2-3. The amounts of these substances in each size group of coarse aggregate should not exceed the limits shown, determined in accordance with ASTM C 117, ASTM C 123, ASTM C 295 applicable only to material coarser than 3/8 inch, and ASTM C 142.
Slag Aggregate
Before blast-furnace slag is used in concrete (Iron Slag Concrete), stockpiles of the material must be saturated with water for at least 24 hours.
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