A Critical Study of Inaccuracies Encountered in Precast Concrete Construction

Dr . Lalit Gumashta

Accuracy in precast concrete components and in technique of construction is vital to the quality of the finished structure. Precast concrete elements used in buildings and civil engineering works may exhibit inaccuracies of the following types.

• Variation in section and length, including local mould deflections.

• Camber and deflection on finished product.

• Twist and wind, faces out of plane.

• Irregularities in line of face.

• Wrongly constructed, misplaced and displaced reinforcement and projecting bars.  

Any of these irregularities will create delays in construction, prevent the proper assembly of elements and may result in unsafe practice at site or production of sub – standard structure.

Classification of Inaccuracies in Precast Concrete Components

Inaccuracies can be classified into two types, inherent types and included type.

Inherent Inaccuracy

Cement concrete is a mobile material and its dimensional changes affect the fit of the elements in or on to a structure. The following significant factors affect the accuracy of an elements at any one time.

• Creep of concrete under sustained load

• Elastic deformation under load

• Thermal movement

• Drying shrinkage and moisture movement

These factors in turn are influenced by properties of coarse and fine aggregates, mix proportions, water cement ratio and curing method employed in production. Precast concrete elements consisting of different materials for facing and structural backing may become distorted due to different movements.

Inaccuracy may be induced into concrete elements during the manufacturing process by inaccurate moulds. In the course of erection, elements exhibiting small amounts of twist and warping can introduce difficulties in the installation of gaskets and vapour barriers, sometimes leading to the failure of such vital parts of the construction.  

Inaccuracy may be induced by the curing method (or by lack of curing), and the methods employed in stacking and storage, variations in pre-stressing force, movements promoted by detail such as location of ducts and openings in the element.

Materials used in mould manufacture and methods of mould construction have to be selected to match the required accuracy. A major factor in the achievement of accuracy is the provision of moulds having a minimum number of parts. Each joint in a mould is prone to grout penetration and failure to clear grout from such joints results, at the time of reuse, in mould distortion, causing inaccuracy, further grout infiltration and leakage.

In the course of a contract, inaccuracies arise due to inherent characteristics of mould materials and due to the way in which to moulds are used. These include bowing caused by grout infiltration in joints, distortions caused by bad mould storage between uses and mould displacement due to uplift generated by concrete in course of vibration.

• All those concerned with precast concrete construction profession associated with design, production and erection are made aware of specified standards, allowable tolerances, site methods, work instruction before the beginning of any such project.

• Information provided must be up to date.

• For achieving desired accuracy of jigs and templates in the course of elements production is extremely essential in cases where elements are cast to close tolerances for speedy erection at site, location of connections and projecting steal being critical.

• The inclusion of simple angle brackets and stops on the stop ends or mould sides to immobilize fittings and preserve accuracy of location at the time of mould design.

• Duplicate jigs and templates can be used at site to locate and check the position of bolts, fixing and fastening in the in situ concrete.  

It may be concluded that by observing some important factors highlighted above, economic, speedier, safe and durable structures can be ensured. These will also contribute to better adaptability of this construction technique, so vital for speedy infrastructural development works.