This document discusses the proper interpretation of clause 11.3 of IS-456-2000 regarding when to strip formwork from concrete slabs. It notes that both following the time guidelines in the clause's table and requiring a certain strength percentage are incorrect approaches. The key criteria is that formwork should not be removed until the concrete has achieved at least twice the stress it will experience upon removal. A simple calculation method is provided to determine the required concrete strength based on factors like slab thickness, span length, and imposed loads. Following this clause correctly allows for reduced project timelines and costs compared to the alternative incorrect interpretations.

1. ACCURATE TECHNICAL INTERPRETATION OF CLAUSE-11.3 OF
IS-456-2000 FOR STRIPPING OF FORMWORK.
This is a common problem to all the sites that regarding de shuttering of slab ,client or
consultant either refer Table under clause 11.3.1 of IS-456 ,in which guidelines of de
shuttering has been given in terms of age of concrete or consultant / client
consider achievement 70%-100% strength is the criteria for de shuttering of slab.
Both the above criteria are semi - correct and without any technical sense, which badly affect
project schedule, time cycle and underutilization of resources or implementation of very
uneconomic mix to achieve the wrong strength criteria.
The Fact
Quote from IS-456-2000, CL-11.3
“Forms shall not be releaseduntil the concrete has achieved a strength of at least twice
the stress to which the concrete may be subjected at the time of removal of formwork..”
Unquote.
Cl-11.3.1 also says that the above criteria is the only guideline for stripping and the table of
stripping time is established , based on consideration that it will deem the above criteria.
Hence instead of following the table it would be better to follow the exact clause
of stripping, which is very advantageous for the constructor. Most interestingly it depends
upon span length, thk of concrete and imposed load of scaffolding etc., irrespective of grade
of concrete.
Simple Calculation to know the required strength.
A……Load CALCULATION
Load during stripping (considering 1 mtr strip)/mtr =
a) Thk. of slab x 1X Density of conc.= 0.200x 1x 25kn/cum= 5.0 KN
for a slab of 200 mm thk
b) Imposed load( load due to scaffold and forms above = 2.5 KN
W= 7.5 KN/m
B……. Max Bending Moment =( Wl X l)/8= (7.5 x8x8)/8 = 59
KN/m ,
Considering 8 mtr span of slab.
C.. ….. Section Modulus = Z= bd2 /6= ( 1mtr X 0.2x 0.2) /6= 0.0066
m3 for 200 mm d ( thk) and 1m srip.
D….. Stress = B.M. max/Section Modulus= 59/.0066=8.99 n/mm2= 9 n/mm2
E. … . Compressive strength requirement at de shuttering = 2 x9 = 18 N/mm2.

2. In most of our cases slab span is 3-4 mtr. and as such the strength requirement will not be
more than 4-5 N/mm2.
However the minimum strength requirement should not be considered less than 10 Mpa as
per
BS-8110. The strength requirement is expected to actual strength of structure not cubes
strength, hence apply 30 % safety factor on cubes strength while application.
Hence the min. Cube comp. strength requirement is 13 Mpa.
The above requirement of code is identical both in IS or BS code and widely used for miwan
or tunnel formed concrete. Hence it should be also applied in System or conventional formed
concrete
The write up should be thought upon, understood and let all concern understand the
requirement correctly. It will be highly beneficial for the contractor / developer and
purchaser in terms of reduced time cycle of slab, ultimately the project duration and
cost.
K.K.PATHAK
K.K.PATHAK
Head -QA/QC ,Godrej The Tree project
L&T Construction, B&F IC
Author: K.K. Pathak, a Construction Quality Specialist with 23 years of professional
experience exclusively in QAQC and QMS with the construction and Real Estate sectors.
Author is presently serving with L&T Construction, B&F IC. Prior to joining with L&T,
KKP served in Pratibha- Capacite Group as Head –QA/QC, with M/s Phoenix Mills Ltd,
HCC, Simplex, and Petron. Innovative selection of alternative materials or adaption of
alternative work procedures and specifications which adds values to the organization or
projects is the motto of the author. Product Quality and innovative construction techniques
implemented by Mr. Pathak have been well appreciated by Print Media on several occasions
.Mr. Pathak has been awarded with “50 MOST IMPACTFUL QUALITY PROFESSIONAL”
award by WORLD QUALITY CONGRESS in the year 2016.