Flowable Backfill 2

[jike]

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.

 

[civilperson]

This is too strong, can not be hand excavated. Requires concrete breaker or excavator to dig. Try 10-50 psi for a better product and needs less cement by weight for less cost.

 

[hokie66]

Control the rate of placement unless your walls are designed for the full hydroconcrete load of the flowable fill.

 

[RHTPE]

Hokie66 is correct - pay attention to the fluid pressure against foundation wall.

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.

 

[msquared48]

Again, too, it depends on the rate of pour and the time of setting, not curing, as to what the design lateral pressure for the wall should be. The 145 should be able to be lowered with a controlled placement plan.

Mike McCann
MMC Engineering

 

[RHTPE]

Regarding msquared48's comment - the density of the fluid will not be affected by the placement technique. The pressure exerted against the wall will be influenced only by the placement technique, the fly ash content, and whatever measures are taken to achieve the 11" slump. It would be best to limit the height of placement and place the flowable fill in several lifts a day or so apart.

RHT_PE m.ACI 347

 

[msquared48]

RHTPE:

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

 

[RHTPE]

Msquared48

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.

 

[msquared48]

Density, correct.

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

 

[RHTPE]

Final comment.

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).

 

[kslee1000]

RHTPE:

"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.

 

[RHTPE]

kslee1000:

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.

 

[kslee1000]

RHTPE: Thanks.