If underground ducts are proposed, make enquiries as to the nature of the ground. Ordinary concrete is not waterproof and is porous. In heating plants with underground ducts there has been trouble with attainment of temperature due to evaporation of moisture in ducts.
Waterproof cement rendering will obviate trouble against normal drainage:
1 part cement, 3 parts washed sand; mix with soapy water (50 g soft soap per litre). The free lime in the concrete combines with the alkali in the soap forming a calcium compound which fills the pores in the concrete.
Posted in Fans Ventilation A Practical Guide