Wet concrete load during a pour behind retaining wall

[Redacted]

Hi there, I am trying to approximate a wet concrete load behind a wall during a pour.

Would I just consider the wet concrete load as 25 kN/m3 and calculate as a normal earth pressure or would I need to consider 24 kN/m3 + 10 kN/m3 (where the 10 kN/m3 is the density of water and 24 kN/m3 the density of concrete).

Thanks

 

[Lomarandil]

For relatively short (time or height) pours, concrete will act like a fluid (just a heavy one). So you would calculate it like water pressure (or "earth pressure coefficient = 1"), at a density of 25kN/m3.

If your pour is very tall or very slow, you may be able to reach a limiting maximum pressure rather than increasing the fluid pressure linearly with wall depth over the full height. The idea being that at some point, the concrete begins to set and stops acting like a fluid. This is described in ACI 347 (among others), but I've found that it doesn't usually apply outside very large scale projects.

----
just call me Lo.

 

[oldestguy]

In English units wet concrete weight is 150 lb/c.f. At rest soil maybe 50. That's at least a factor of three.

 

[Redacted]

Thanks that is helpful. For a temporary case such as for a short concrete pour behind a wall, would you normally use the full ULS safety factors for overturning and stability calculations? Or because it is a short duration (only a few hours) set the safety factor to 1?

Is there any guidance/literature on this that you know of?

 

[dik]

From my project notes:

FALSEWORK AND FORMWORK

ALL WORK TO CONFORM TO CSA S269.1 FALSEWORK AND FORMWORK, AND CSA S269.2 ACCESS SCAFFOLDING FOR CONSTRUCTION PURPOSES, AS REQ'D

FORMWORK SHALL NOT BE LOOSENED UNTIL 24 HOURS HAS ELAPSED AND UNTIL CONC HAS ATTAINED SUFFICIENT STRENGTH TO PREVENT DAMAGE

COL AND WALL FORMWORK SHALL NOT BE REMOVED BEFORE CONC HAS ATTAINED SUFFICIENT STRENGTH TO PREVENT DAMAGE, WITH THE FOLLOWING BEING THE MIN:
-ARCH CONC - 10 MPA
-STRUCTURAL CONC - 8 MPA

UPON REMOVAL OF FORMWORK, PROVIDE A CURING MEMBRANE AS REQ'D

SLAB AND BEAM FORMWORK SHALL NOT BE REMOVED UNTIL CONC HAS ACHIEVED A STRENGTH OF 20 MPA MIN. OR AS DIRECTED BY THE [OWNER | CONSULTANT | ENGINEER]

STRENGTH OF CONC FOR STRIPPING TO BE DETERMINED BY FIELD CURED SAMPLES

RESHORING SEQUENCE TO BE APPROVED BY THE [OWNER | CONSULTANT | ENGINEER] BEFORE STRIPPING

WORK AND MATERIALS SHALL BE IN CONFORMANCE WITH THE FOLLOWING STANDARDS/SPECIFICATIONS: CSA STDS O86.1, O121, O151, O177, S269.1, AND S269.2

PLYWOOD FOR FORMWORK SHALL BE [EQUIVALENT TO COFI COFIFORM-PLUS | COFIFORM-PLUS | AINSWORTH POURFORM 107] AND SHALL BE GRADE STAMPED ACCORDINGLY

FORM RELEASE AGENT SHALL BE MULTI-REACTIVE, NON-STAINING, VOC COMPLIANT AND SHALL BE DUOGARD AS MANUFACTURED BY WR MEADOWS

CONCRETE FORMWORK SHOP DRAWINGS SHALL BEAR THE SEAL OF AN [OWNER | CONSULTANT | ENGINEER] REGISTERED IN THE PROVINCE WHERE THE PROJECT IS LOCATED WHO WILL BE RESPONSIBLE FOR THE DESIGN, FABRICATION AND INSTALLATION OF SAME

LOCATION OF CONSTRUCTION OR CONTROL JOINTS SHALL BE REVIEWED BY THE [OWNER | CONSULTANT | ENGINEER] AND SHALL BE SHOWN ON SHOP DRAWINGS

FORMWORK DESIGN DEFLECTION HAS BEEN LIMITED TO L / 180 WHERE L IS THE SPAN OF THE SUPPORTING FORMWORK ELEMENT. WET SERVICE CONDITIONS HAVE BEEN ASSUMED FOR FORMPLY DESIGN. SHORT TERM LOADING CONDITIONS HAVE BEEN ASSUMED FOR ALL FORMWORK ELEMENTS

MATERIAL FOR STUDS SHALL BE EQUIVALENT TO SPECIES S-P-F, GRADED #2 OR BETTER AND SHALL BE GRADE STAMPED ACCORDINGLY

MATERIAL FOR WALES SHALL BE EQUIVALENT TO SPECIES S-P-F, GRADED #2 OR BETTER AND SHALL BE GRADE STAMPED ACCORDINGLY

FORMWORK DESIGN PRESSURES ARE IN ACCORDANCE WITH THE CURRENT EDITION OF ACI 347. FOR FORMWORK LESS THAN [15FT | 5.0M] HIGH, THE MAXIMUM PRESSURE IS Cw x Cc x [150 + 9000 * R / T] AND FOR FORMWORK GREATER THAN [15FT | 5.0M] HIGH THE MAXIMUM PRESSURE IS Cw x Cc x [150 + 43400 / T + 2800 * R / T], WHERE R = RATE OF DEPOSITION AND T = TEMPERATURE. THE MINIMUM DESIGN PRESSURE OF 600 PSF HAS BEEN WAIVED TO COMPLY WITH CANADIAN PRACTICE

FORMWORK DESIGN PRESSURES ARE IN ACCORDANCE WITH THE RECOMMENDATIONS OF THE COFI WHICH ARE BASED ON THE CURRENT EDITION OF ACI 347. THE MAXIMUM PRESSURE, FOR A RATE OF PLACEMENT LESS THAN 2 METRES PER HOUR, IS 7.2 + 785 * R / (T + 17.8) , WHERE R = RATE OF DEPOSITION IN METRES PER HOUR AND T = TEMPERATURE IN DEGREES CELSIUS. THE MINIMUM DESIGN PRESSURE OF 600 PSF HAS BEEN WAIVED TO COMPLY WITH CANADIAN PRACTICE

THE CHEMICAL COEFFICIENT (Cc) HAS BEEN ASSUMED TO BE 1.2; CONCRETE SHALL BE TYPE GU, GUb, OR HSb WITH A MAXIMUM OF 25% FLY ASH

THE CHEMICAL COEFFICIENT (Cc) HAS BEEN WAIVED TO COMPLY WITH CANADIAN PRACTICE

THE UNIT WEIGHT COEFFICIENT (Cw) HAS BEEN ASSUMED TO BE 1.0 FOR NORMAL WEIGHT CONCRETE

THE UNIT WEIGHT COEFFICIENT (Cw) HAS BEEN WAIVED TO COMPLY WITH CANADIAN PRACTICE

CONCRETE RETARDERS SHALL NOT BE USED

MAXIMUM HEIGHT OF FORMWORK SHALL NOT EXCEED [15FT | 5.0M] UNLESS NOTED

LIMITATIONS ON THE HEIGHT OF FORMWORK HAS BEEN WAIVED TO COMPLY WITH CANADIAN PRACTICE

MAXIMUM RATE OF POUR SHALL NOT EXCEED [7FT | 2.1M] PER HOUR

MAXIMUM CONCRETE TEMPERATURE SHALL NOT EXCEED 20 DEGREES CELCIUS

CONCRETE DENSITY SHALL NOT EXCEED [150PCF | 2400KN]

MECHANICAL VIBRATION SHALL BE LIMITED TO THE TOP [4FT | 1.2M] OF THE CONCRETE POUR

DESIGN FORMWORK PRESSURES ARE AS SHOWN ON THE DOCUMENTS

[3/4" | 19MM] FORM PLYWOOD SHALL BE FASTENED TO STUDS AT [24" | 600MM] MAX SPACING USING 6d COMMON WIRE NAILS. FORM PLYWOOD LESS THAN [3/4" | 19MM] CAN BE FASTENED TO STUDS AT [24" | 600MM] MAXIMUM SPACING USING 5d COMMON WIRE NAILS. THE EDGES OF ALL PANELS SHALL BE SUPPORTED BY STUDS. PROVIDE SOLID BLOCKING AT EDGES PERPENDICULAR TO STUD SPACING. TAPE JOINTS AS REQUIRED

STUDS SHALL BE MINIMUM 4 SPAN CONTINUOUS ACROSS WALE MEMBERS. SPLICE SHALL OCCUR BETWEEN WALE MEMBER PAIRS

WALES SHALL BE SPLICED AT EDGE OF FORMPLY PANELS ONLY

FORM PANELS SHALL BE SECURED TO SUPPORTING STRUCTURE USING [3/8" | 10MM]%%C ANCHORS COMPLETE WITH OVERSIZED WASHERS AT [16" | 400MM] ON CENTRE

FORMWORK TIES SHALL HAVE A MINIMUM SAFE WORKING LOAD OF 3000 POUNDS WITH A MINIMUM FACTOR OF SAFETY OF 2.0 AND SHALL BREAK AT CONCRETE SURFACE WITH NO SHARP PROJECTIONS. IN AREAS WHERE CORROSION IS AN ISSUE FORMWORK TIES SHALL BE STAINLESS STEEL. IN AREAS WHERE WATER PENETRATION IS AN ISSUE FORMWORK TIES SHALL HAVE AN INTEGRAL WATERBAR

FORMWORK TIES SHALL BE CENTRED BETWEEN THE DOUBLE WALE AND SHALL BE LOCATED A MAXIMUM DISTANCE OF [1-1/2" | 40MM] FROM A STUD

Some of the above is applicable. Also ACI has a publication SP-4 on formwork.

Dik

Dik

 

[Redacted]

Thanks Dik, that's very helpful

 

[msquared48]

Yes, the ACI has a design manual for forms and pour rates. I would get one if you do this often.

Very helpful publication.

Mike McCann, PE, SE (WA, HI)

 

[IDS]

My very first work experience of things not going quite to plan was on a pour to line an old canal wall, probably about a 4 or 5 m pour height.

The contractor was very pleased with the rate of progress until the formwork failed at the base, and all the still wet concrete started to discharge into the canal.

I remember being quite impressed with the quick thinking of the contractor's project manager who immediately called the fire brigade who turned up promptly and flushed all the wet concrete into the remaining trickle of a stream at the base of the canal.

So the message is, be conservative with the calculated pressure and the assumed pour rate.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[dik]

Glad to... even though the reference to standards and form pressures are the only sig part... I included the whole section in case others are involved with formwork design.


Dik