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Sugars in formwork

Concrete surfaces can sometimes suffer from retardation of the hydration of the cement caused by sugars emanating from the timber form-face. This can leave a weak surface that can pull away as the form is struck or a dusty finish. Such defects are not considered of structural significance but may be an aesthetic problem.

Although sugars may be present from the manufacture of the form-face, their effect can be aggravated by the timber surface being exposed to strong sunlight prior to use, the effect of the sunshine being to convert relatively non-reactive sugars to reactive ones. To prevent this problem occurring all packs of plywood should be kept out of strong sunlight to prevent these sugars forming. New ply can be treated with a lime or cement (alkali) wash to neutralise the sugar prior to the application of release agent. This treatment is sometimes necessary for the first few uses. 

The problem has been known to occur with almost all types of plywood although Douglas Fir and some Malaysian hardwood plywoods seem to be more prone to the problem than other types.

Other references:Cement and Concrete Association Departmental Note – DN/2022, ‘Retardation of concrete surfaces cast against a plywood and timber forms’

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