Pouring concrete slabs over biological material 1

[jrmort]

When I was in my teens (long,long ago...) I worked on a concrete crew that poured pool decking (Kool Deck, Exposed Aggregate and Salt Finishes). At that time, my employer engrained in me that one must never pour concrete over areas with any biological material in them (grass, weeds, etc). Since that time, I have held to this but I do not know where in "official" literature I might find something to back this up. I have a colleague in the swimming pool industry who insists that if one scrapes the majority of grass off the top of the sub-base, one can put a little soil on top of the grass and pour over it.

I emphatically disagree with this and would appreciate any feedback.

 

[Rjeffery]

organic material rots and loses volume. The decay can be aerobic or anaerobic (with or with out Oxygen). concrete is, ingeneral very strong in compression but weak in tension. i.e. You can't pull or bend it and expect to hold together.

The lime and orher basic components assist in the decomposition of the organic matter.

Removing the topsoil will reduce possable volume change from the grass, thatch and humis (duff).

ACI International and PCA has several (inexpensive) booklets on slab on grade construction that will give a more difinitive explanation of why organic material should be removed. The basic reasion is differential settlment.

Hope that helps.