Flowable Backfill
Flowable Backfill
(OP)
What are the considerations for approving a flowable backfill between foundation walls in lieu of compacted fill? 200 psi at 28 days, 11" slump, 100# cement, 400# fly ash, fine aggregate, water, air entrainment and flow agent.






RE: Flowable Backfill
RE: Flowable Backfill
RE: Flowable Backfill
Assuming 145 pcf flowable fill against a 12' tall wall, you'll be seeing a resultant force in excess of 10 kips per lineal foot of wall applied 4' up from the bottom.
RE: Flowable Backfill
Mike McCann
MMC Engineering
RE: Flowable Backfill
RHT_PE m.ACI 347
RE: Flowable Backfill
Thatr's exactly what I was getting at with the rate of pour comment - the rate of vertical progression of the wall pour with respect to the time of curing. My comment stands.
Mike McCann
MMC Engineering
RE: Flowable Backfill
Sorry to be critical Mike, but your statement was "The 145 should be able to be lowered with a controlled placement plan."
In actuality, the density remains a constant, be it 145 lbs per cubic foor (pcf) or some other value. The placement rate will have an affect on the pressure (and total force) applied against the wall.
RE: Flowable Backfill
But I thought part of the original the question concerned an implication regarding the actual pressure seen by the wall from the CDF, which varies with a lot of things, rate, water content, rate of fill, additives and temperature of the mix and air to mention a few. Consider the following attached table from the ACI publication "Formwork For Concrete".
This gets into what Hokie said, and that is what I am addressing.
Mike McCann
MMC Engineering
RE: Flowable Backfill
Your earlier statement impied the density (the 145 pcf) would vary - it does not. It is a constant. The pressure exerted on the wall will vary. That pressure is a function of:
- rate of placement
- rate of hydration, which is affected by:
- type(s) of cementitious material(s) used
- types of admixtures used
FYI, I've attached the pages from the latest ACI (347-04) Guide to Formwork for Concrete which pertain to concrete pressure. Committee 347 has incorporated factors that attempt to reflect the effects that unit weight (density) and use of admixtures/other cementitious materials has on placement pressure. Concrete or CDF or CLSM - they all behave in the same manner. As the material begins to set up, it supports the weight of additional material added above without exerting additional lateral pressure against a form (in this case, the wall).
RE: Flowable Backfill
"In actuality, the density remains a constant, be it 145 lbs per cubic foor (pcf) or some other value."
Only for personal curiosity, is the density of wet flowable concrete identical (stay constant) to its end product (hardened concrete)? I believe there is a small difference in between the two states. However, the diferences are very small, thus is negligible for all practical purposes.
RE: Flowable Backfill
There is a very slight loss of water due to evaporation (more so for slabs perhaps) and a small amount of bleed water, but essentially all of the constituent materials that go into concrete remain in the concrete. Any loss would be insignificant with regard to density.
RE: Flowable Backfill