Aggregates for concrete
Aggregate occupies 75% of the concrete mix. aggregate is cheap compared to cement.because it is less expensive than cement. it is desired to increase the amount of aggregate in the concrete mix. aggregate divided into coarse aggregate and fine aggregate. fine aggregate is the aggregate that passes sieve #4. any aggregate has a size larger than fine aggregate called coarse aggregate. size of aggregate shall not be more than the following:
- 3/4 of the reinforcing bar clear spacing.
- 1/5 of the narrowest structural unit
- 1/3 of the depth of the slab
The sources of coarse aggregates
- gravel: gravel can be in the following forms:
- uncrushed gravel: resulted from gravel disintegration
- crushed gravel: this produced by crushing gravels
- partially crushed gravel: produced by combing crushed and uncrushed gravel.
- Crushed rock: this produced by crushing of rocks
The sources of fine aggregates
- uncrushed or partially crushed sand
- uncrushed sand: resulted from natural disintegration of rock.
- partially crushed: produced by mixing of uncrushed and crushed sand.
- Crushed gravel sand: This produced by crushing gravel
- Crushed rock sand: this produced by crushing rock
British standard (BS882) provide grading for coarse and fine aggregate. for coarse aggregate, each size of aggregate shall be stocked alone and then blended together to produce the required grading. Table no:1 showing the gradation for coarse aggregate. gradation for coarse aggregate shall be within the limits given in table 1. Table 2 showing the gradation for fine aggregates.
The sources of aggregate shall be tested prior to using aggregates in concrete. BS812 part 102:1989 provide a provision for sampling and testing of aggregates. Table no:3 summarize the required test for aggregates. aggregates shall not contain large amounts of any light materials, organic impurities, and clay lumps because of their adverse effects on concrete strength and durability. aggregate strength and durability will significantly affect concrete. therefore, selected aggregate shall be tested sufficiently to verify their strength and durability.