The Public Road Administration (PRA) system is based on Particle-size composition and Plasticity characteristics.

Explanation:
The PRA system is a method used to determine the suitability of soil for road construction and maintenance. It involves analyzing the particle-size composition and plasticity characteristics of the soil.

Particle-size composition:
- The particle-size composition refers to the distribution of different-sized particles in the soil.
- It is determined by conducting sieve analysis, in which the soil sample is passed through a series of sieves with different mesh sizes to separate the particles.
- The percentage of particles retained on each sieve is then calculated, and a particle-size distribution curve is plotted.

Plasticity characteristics:
- Plasticity characteristics refer to the ability of soil to undergo deformation without cracking or breaking.
- This property is primarily influenced by the clay content in the soil.
- Clay particles have a high plasticity due to their ability to absorb water and form a cohesive mass.
- The plasticity characteristics of soil are determined using tests such as the Atterberg limits test, which measures the moisture content at which the soil transitions between different states (liquid, plastic, and solid).

Importance of particle-size composition and plasticity characteristics in road construction:
- Particle-size composition and plasticity characteristics play a crucial role in determining the engineering properties and behavior of soil.
- These properties influence the soil's strength, permeability, compaction, and stability, which are critical factors for road construction and maintenance.
- For example, soils with a high clay content have high plasticity and tend to shrink and swell with changes in moisture content, leading to pavement distress and stability issues.
- On the other hand, soils with a well-graded particle-size distribution (containing a range of particle sizes) provide good compaction characteristics and improved load-bearing capacity.

Conclusion:
In conclusion, the Public Road Administration (PRA) system is based on the analysis of particle-size composition and plasticity characteristics of soil. These properties are essential for assessing the suitability of soil for road construction and maintenance, as they influence the soil's strength, stability, and other engineering properties.