Lean concrete backfill

[slickdeals]

We have a project where a backfill of 1.5 m is required. This is over a large area (12 m x 25 m), which was over-excavated previously. Pit locations and core locations have changed since project went on hold. Contractor wants to use M15 PCC (2000 psi lean concrete) to backfill.

Are there any restrictions in terms of maximum thickness of layers? I was thinking of specifying 3 layers of 500 mm thickness. What should be the time duration between placement of various layers? For example, should the second layer be placed 6 hours after the first layer has been placed or some such?

 

[EngineeringEric]

Is this true back-fill? I mean is it non-structural and could just be compacted stone?... If so why limit the depth of lifts, a lean concrete mix if poorly cured could just be sub-par comprehensive strength.. But is that an issue with this application?

Just wondering.

 

[cvg]

the only reason to place in lifts is
a) avoid floating structures such as pipes
b) reduce thermal stress and cracking

do you really care about cracking? and do you really need 2,000 psi?

a leaner mix with say 100 - 200 psi may be more than strong enough, cheaper and would greatly reduce heat of hydration, also reducing cracking

 

[slickdeals]

Seems like contractor wants to use it so that he wouldn't have to put soil down in small layers and compact it. And then get the compaction test done.

The lean concrete will carry the weight of the structure above. 15 MPa has enough compressive strength. Raft design bearing pressure is only around 70 tons/m2.

Reason to limit the depths is so that each layer can be properly vibrated and consolidated.

 

[dik]

We often use CLSM with strengths of 0.7, 1.0 and 2.0 MPa 1 MPa approx 145 psi.

and a trial mix:



CLSM TRIAL MIX PROPORTIONS

MIN CEMENT 50 PCY
MIN FLY ASH (TYPE F) 150 PCY
FINE AGGREGATE 2700 PCY TO ASTM C33
MAX WATER 290 PCY

FLYASH SHALL NOT HAVE A HIGH ALKALI, SULPHATE OR REACTIVE MATERIAL CONTENT

 

[dik]

Should have added, because it forms a triangular mass... the load can often be away from the foundation wall... and can be used to minimise pressures on walls that are compromised.

Dik

 

[cvg]

typically they do not vibrate CLSM since it is designed to be a flowable fill.

 

[JStephen]

Supposedly, one advantage of using flowable fill instead of lean concrete is that you can dig it out later if you need to. Way back when I was an estimator and priced it, it wasn't much different from concrete, though. As far as I know, you'd just dump it in the hole and be done.

 

[dik]

JS... it's still expensive in these areas...

 

[JedClampett]

We do this all the time. Sometimes we make the contractor place slurry, lean concrete or controlled low strength material (its got lots of different names) in places where we're worried about buildings bearing partially on backfill. We never limit the lifts.
The contractor's grumble when it's not their idea, but I think they eventually see the advantages of not having to compact backfill in a tight space.

 

[RHTPE]

Don't forget to evaluate the fluid pressure against the structure being back-filled.

5' depth of CLSM would exert a total force of 1.88 kips/ft against the structure. If this is a simple foundation wall, this CLSM placement pressure may cause problems. Which may justify placement in lifts with adequate time between lifts for the CLSM to reach its initial set.


Ralph
Structures Consulting
Northeast USA