Can construction loads on a concrete slab exceed its design service load?
Yes: the load a concrete slab carries during construction frequently exceeds its design service load, and that load is applied to concrete that has not reached its specified strength. A slab in service carries its own weight plus the occupancy it was designed for. The same slab during construction may carry its own weight plus the weight of a fresh pour above transmitted through shores, plus the shoring, plus materials and equipment.
For a concrete slab in a building under construction, the construction condition, not the finished condition, governs — and the construction condition is analyzed less often than it should be.
How is construction load shared between shored levels of a concrete building?
When a concrete slab is poured on shores bearing on slabs below, the construction load distributes among those shored levels according to their relative stiffness. Analyzing this properly requires treating the shored levels as an interconnected system rather than checking each slab in isolation, and the distribution changes every time a level is stripped or reshored.
Simplified approaches to analyzing load sharing between shored levels exist and are widely used. Where the actual shoring sequence departs from the assumptions those simplified approaches rest on, the actual distribution of load between levels can differ substantially.
Is a fixed number of days a safe criterion for stripping shores from a concrete slab?
No: a fixed number of days is not a safe criterion for stripping shores, because concrete gains strength as a function of both time and temperature, so its strength is a function of maturity, not of days. The same concrete mix reaches a given strength far more slowly in cold weather, and a stripping schedule built around calendar days can strip slabs in winter that would have been ready on the same day in summer.
Cold-weather pours, unheated enclosures and overnight temperature drops are recurring factors in early-stripping failures of concrete slabs.
What do field-cured concrete cylinders tell you about whether a slab is ready to strip?
Field-cured cylinders stored alongside a concrete slab come closer to the slab’s in-place strength than standard-cured cylinders do, though field-cured cylinders still differ from the slab in thermal mass. Standard-cured cylinders are used for acceptance and represent the mix’s potential, not the slab.
For stripping decisions, the relevant question is the strength of the concrete in the structure, which depends on the conditions that concrete actually experienced. The maturity method, calibrated for the specific mix and using temperature sensors embedded in the slab, tracks in-place strength more directly, as do pull-out and other in-place tests.
Is reshoring the same as leaving shores in place?
No: reshoring means removing shores and reinstalling them, which allows the concrete slab to deflect and take up its own weight before the reshores are set, so the reshores then carry only subsequently applied load.
Backshoring, where supports are removed and replaced one at a time without letting the slab deflect, behaves differently from reshoring. Confusing reshoring and backshoring, or installing reshores with a preload they were not meant to carry, changes the load distribution between shored levels in ways the shoring analysis did not assume.
What is the usual failure mode when shore loads overload a concrete slab?
Punching shear is the usual failure mode: flat plate slabs under concentrated shore loads are vulnerable to punching shear at the shore locations and at columns, and punching failures are sudden and offer no warning.
Because the punching shear mechanism is local and brittle, a single overloaded location can initiate a progressive collapse through several floors as debris from each level loads the one below. This is why early-stripping failures in concrete frames tend to be severe rather than localized.
Who decides when to strip shores from a concrete slab, and on what basis?
Stripping and reshoring are normally the contractor’s means and methods, with a shoring design prepared by a qualified engineer and a strength criterion specified.
An investigation of an early-stripping failure asks what stripping criterion applied, what testing supported the stripping decision, who made it, and whether the stripping and reshoring sequence followed the shoring design. A stripping decision made on schedule pressure without strength verification is a specific, documentable finding rather than a general observation about site practice.
How do you reconstruct the shoring and stripping sequence after a concrete frame fails?
Reconstructing the shoring and stripping sequence after a concrete frame fails requires the actual pour dates for each level, the shoring and reshoring configuration at the time of failure, when each level was stripped, the strength test results available at each decision point, and the temperature history.
Daily reports, pour logs, testing laboratory reports and site photographs together usually establish the shoring and stripping sequence, and site photographs are often the most reliable record of what was actually standing where.
What evidence should be preserved after a shored concrete slab collapses?
After a shored concrete slab collapses, the evidence to preserve starts with the shoring components and their arrangement as found, level by level, with spacing recorded, and concrete from the failed slabs for in-place strength testing, taken from locations of known original position.
The testing records should also be preserved, including field-cured and standard-cured cylinder results and any maturity data, along with the shoring design and calculations, the specified stripping criterion, daily reports, pour logs, temperature records and site photographs covering the days before the collapse.
This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.