Aggregate is gap-graded when intermediate sizes are essentially absent from the gradation curve. Usual gap-graded mixes contain aggregate retained on a 3/4- or 1 1/4-inch sieve, and particles passing the No. 4 sieve. Gap-graded mixes are used to obtain uniform textures for exposed-aggregate concrete and can also increase strength and reduce creep and shrinkage. Though the intermediate sizes usually can be omitted without making the mix unduly harsh or prone to segregation, choose the fine-aggregate percentage with care for gap-graded concrete. Use about 25% by volume with rounded aggregate and 35% with crushed material. Air entrainment usually is required to improve the workability of low-slump, gap-graded mixes.

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Gap graded means you have two or more aggregates of the same size. Such aggregates aren't good to use in concrete or as foundation.

Missing in immediate size particals, lead to segregation in concrete mixture.

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