When should Forms and Shores be Removed Beneath Concrete?
What is Formwork?
Formwork is used to give concrete desired structural shapes like beams, slabs, columns and shells. It provides support to concrete when it is in plastic state up to a certain period until concrete gain sufficient strength to support its self-weight and other superimposed loads.
The formwork manufacturing team main include-
- General contractor
- Sub-contractor specialized in formwork
- Formwork engineers
- Formwork manufacturers
- Formwork suppliers
the contractor or manufacturer whoever supply formwork are responsible for design, to ensure safety during construction. But the engineers/architects have to make specifications about removal of formwork based on required concrete strength and maturity (age criteria). It is the responsibility of formwork contractor or engineers to furnish sequence of formwork or shore erection followed by removal clearly on the formwork drawings.
According to ACI committee 347, following recommendations are prescribed:
- Design office should furnish a minimum strength for specific concrete member that should be ensured before removal of shores and forms.
- This strength can be obtained by testing specimens cured in the field or on in-situ concrete.
- According to ACI 228.1R, other concrete procedures or tests can be used such as
- Penetration resistance,
- Maturity method and
- Pull-out tests.
Provided that the specimens/ methods used to test should represent actual concrete mix which should be approved by the architect/engineer and time to time justified by job cured specimens.
- The architects/engineers should specify responsibility of teams: who will take and prepare specimens from field placed concrete and who will execute tests. The following necessary information along with test result should be recorded:
- cylinder compressive strength
- cylinder size
- cylinder weight
- weather condition of the day when sample are made etc.
These information will be kept by contractor or other person as specified by design office which should also be mentioned on contract document. Such document should include minimum compressive strength after certain period of placement of concrete.
- After stripping of formwork, it should be monitored for excessive deflection or any type of distortion. Due to the activity of stripping or removal of shore, there should be no sign of damage in concrete.
- If forms have to be removed before specific period of curing, the curing process should continue providing sufficient thermal protection
- Special care should be taken for horizontal members so that supporting shores and forms must not removed from members like beams, slabs and other such members, before concrete achieving the stripping strength as specified by the architects/engineers.
- In case of post-tensioned concrete, shores should not be removed before adequate tension force is not exerted to support self-weight, formwork and other expected construction loads.