blind concrete

huangyhg

blind concrete ?
to have a layer of mostly cohesionless soil under foundations is good except maybe where such layer is of too fine sand and washable through extant or yearly watertable currents. when well done and compacted and under slabs on the ground they are even more benefitial. in my practice i have never met a case where we have used crushed stone or the described cohesionless -normally well graded- sublayers that has shown to be a source of problems.
respect the preparatory layer of concrete uses to be benefitial in having a clean surface where to work and lay and measure correct covers, being also normally a contributor against the potential corrossion of the main rebar at the bottom of footings, the one more in risk to have contact with apport water.
one could argue that the big total cover implied by this concrete sublayer may mean that cover and the sublayer act together and then be a combination that more easily produces cracks going to the main steel. given that this is the ordinary practice, and even if for some cases such could be what happens, for most cases the solution of having the concrete preparatory sublayer seems to be more benefitial than detrimental to the foundation's life.
footings and ground slabs are designed for ground with an assumed or known ground bearing pressure (gbp).  
if there is a soft spot at the bottom of the excavation, it must be removed.  levels can be made up with crushed stone (hardcore), sand or, sometimes, low strength concrete.
after excavation the ground will remain open for several days while the reinforcement and shuttering are prepared and the clients engineer approves the work before concreting..
this delay allows time for the bearing stratum to weather and soften, thus reducing the gbp.
to seal and protect the surface of the bearing stratum it is usual to pour 50 or 75 mm of 10 or 15 n/mm2 concrete.
this also (1)provides a firm base for erecting shuttering true to line and level and (2)keeps the rebar clear of a possible muddy bottom and (3)prevents the loss of water and fines from the lower layers of structural concrete being absorbed into the ground.
if you are a gambler and have a sound bottom to the excavation, you may like to gamble that there will be no rain before you pour the structural concrete - but if it rains, no-one will thank you for the work of removing shuttering and rebar and digging out the soft mud; plus the cost of the extra concrete to make up the lost levels.
in the uk it is never, never worth the risk to ignore the blinding concrete; and elswhere in the world, even in dry climates i have always felt the blinding concrete was worth the small extra cost because of the better quality of rebar and shuttering achieved.
in some cases, e.g. under slabs, it is possible to use polythene film instead of blinding concrete; particularly where mesh rebar can be rapidly place in position and the excavation will only be open for a very short time before the structural concrete is poured.
warning:
(1) polythene should never be used under foundations that depend on friction with the ground, e.g. base of retaining wall designed to resist sliding.
(2) blinding concrete should always be sprayed with water before the structural concrete is poured to prevent absorption of water from the lower layers of the structural concrete.