The purpose of dry mix design is to quantify the different sizes of mineral aggregates to be used in order to obtain a mixture of maximum density.
There are three important steps involved in dry mix design, viz.
- selection of sets,
- aggregate upgrades, and
- ratio of sets,
1. selection of sets
The desirable properties of bituminous paving mix depend to a large extent on the nature of the aggregate used. Aggregates are classified as coarse, fine and filler.
The function of coarse aggregates in contributing to the stability of bituminous paving mixtures is largely due to the interlocking and abrasion resistance of adjacent particles. Similarly, fines or sand contribute to the stability failure function to fill in the gaps between coarse aggregates. Mineral filler is largely touted as a void filling agent.
Crushed aggregates and sharp sands produce a higher consistency of the mixture than do gravel and round sand.
2. gross upgrade
The properties of bituminous mixtures, including density and stability, are heavily dependent on the aggregates and their grain size distribution. Gradation has a profound effect on mixing performance. It may be reasonable to believe that the best gradation is the one that produces the maximum density. This would involve a particle arrangement where smaller particles are packed between larger particles, thus minimizing the void space between particles. it makes more particle-to-particle contact, which will increase stability in bituminous pavements and reduce water infiltration. However, some minimum spaces are required:
- Provide enough volume for the binder to occupy,
- promote rapid drainage, and
- Provide resistance to frost action for base and sub base courses.
A dense mixture can be obtained when this particle size distribution follows fuller’s law which is expressed as:
p = 100*(d/d)n
P The percentage by weight of the total mixture passing through any given sieve size is
D is the largest particle size in that mixture, and
n The parameter is based on the size of the aggregate (0.5 for perfectly round particles).
on the basis of this law Fuller-Thompson Gradation The charts were developed by adjusting the parameter n For the fineness or coarseness of the aggregate.
Practical considerations such as construction, layer thickness, workability, etc., are also considered. for example Figure 1 Provides a distinctive upgrade to bituminous concrete for a thickness of 40 mm.
3. ratio of sets
After selecting the sets and their classification, the set has to be proportional and the following are the common methods of proportioning the sets:
trial and error process,
Change the proportion of ingredients until the required overall upgrade is achieved.
There are two graphical methods in common use for the ratio of sets, triangular chart method And Roach’s method, The former is used when only three ingredients are to be mixed.
In this method a system of equations is developed based on the numbering of each set, the required gradation and solved by numerical methods. With the advent of computers, this method is becoming popular. The resulting solution gives the ratio of each type of material needed for the given overall upgrade.
Dr. Tom V Matthews (IIT Bombay)