What type of block shall be concrete of a mix not leaner than one part cement, two and one-half parts sand, and five parts stone?

The correct answer is that the type of block specified is the Thrust Block. Thrust blocks are essential components in the installation of fire service mains, particularly at changes in direction and at fittings like tees and elbows.

The concrete mix requirement, which dictates a ratio of one part cement, two and one-half parts sand, and five parts stone, ensures that the thrust blocks have the necessary strength and durability to withstand the forces exerted by the pressurized water flow in the pipes. This mix specification is designed to provide a robust structural support that can absorb and transfer the thrust generated by the flow of water, which is particularly critical in maintaining the integrity of the system and preventing dislocation of joints.

In the context of private fire service mains, thrust blocks play a vital role in ensuring that the system operates safely and effectively, preventing potential failures that could arise from inadequate support.