All county highways constructed under the provisions of this chapter shall be of such type, and upon such grade as to permit of the improvement and building up of such highways from time to time as funds are available to a type equivalent to the secondary highways of state highway system, and of such type as to permit of ultimate hardsurfacing in the manner, and in accordance with the specifications of the state highway or chief engineer of state transportation department for hardsurfacing of primary roads of the state highway system.