Lean concrete backfill
Lean concrete backfill
(OP)
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?
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?






RE: Lean concrete backfill
Just wondering.
RE: Lean concrete backfill
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
RE: Lean concrete backfill
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.
RE: Lean concrete backfill
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
RE: Lean concrete backfill
Dik
RE: Lean concrete backfill
RE: Lean concrete backfill
RE: Lean concrete backfill
RE: Lean concrete backfill
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.
RE: Lean concrete backfill
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