Gradation: The importance of the granulometric curves in concrete and roads
Gradation refers to the proportions of different-sized particles in a given aggregate. Unforeseen changes in proportions often pose perplexing problems. Under current road-building practice, for example, an aggregate mix sampled from belt or stockpile may fully meet specifications at the plant ,yet probe unacceptable “in the job” because of particle size changes produced by degradation or segregation of delivery handling.
To calculate the correct combinations of the mixtures of the aggregates (their proportions), the granulometric curves are used, which are graphical representations of the proportions of aggregates according to the size of the particles that form it.
Scarcity or quality aggregates, in many areas, makes the use of alternate marginal material a degradation hazard.
Makers of concrete need clean, hard aggregate….comprising specified proportions of particles in sizes that promote close fit or smaller pieces among larger ones. Such degradation brings three important advantages:
- Less space between particles
- More aggregate per cubic unit , hence a corresponding rise in durability .
- Mix uniformity, which improves the set and hardening of the concrete.
Gradation directly affects both quality and costs of concrete, because to bind aggregates into one solid unit demands that each particle be completely coated by the cementing paste and, in addition, that paste fill all space between the particles. Excessive space hinders making a mix uniform, and requires additional paste to fill useless voids. Its result: cost rise while quality of the concrete falls.
Discussion of processes and size analysis follows.
Fig. 1: Left: One-size particle. Middle: incorrect gradation. Right: Gradation with a correct granulometric curve.
Fig 2: This photo shows the different sized of aggregates. This separation and proportions are mandatory for well-graded sand.
Behavior of an aggregate during any sort of handling or process operation usually predicts the performance it will give as a construction material. Brittle materials tend to degrade faster than tough ones. This characteristic is a critical factor in determining treatment needed to meet size specification at the stipulated sampling point.
Reports blame excessive fines as a cause of many road failures in the past. Such records bespeak fundamental need to control the proportions of different-sized particles in aggregate entering a given job….whether dam, road, roof, bridge or burial vault. Normally, the thicker the selection, the coarser the aggregate used.