Specifications and Tolerances in Precast Concrete Construction

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Specifications for Precast Concrete:

Structural units of precast concrete is manufactured in the grade of concrete and to the sizes and details shown on the Drawings. Where the installation of precast concrete units in any structure is such that the faces of the units are to be left exposed either internally or externally, the exposed surfaces of the units as finished shall be uniform in colour and in texture. All cement, aggregates and other materials used in the manufacture of the units should be obtained from the approved sources throughout the period of manufacture. The precast concrete manufacturer should have full details of proposed method of carrying out all operations connected with the manufacture and assembly of precast concrete structural members, including:

• A description of the types of casting bed, mould and formwork for the various types of members;
• The procedure for concrete casting and the method of curing the concrete;
• The procedure for transporting, handling, hoisting and placing of each type of precast structural member;
• Particulars of the temporary supports deemed necessary to ensure adequate stability during erection and fully proof against the effects of construction loads, wind load and other transient loads.

Fig: Precast concrete building and elements

Dimensional tolerances in precast concrete members:

Dimensions and shapes of precast concrete structural members shall comply with tolerances given on the drawings or, when not so given, with those stated hereunder:

Description	Tolerance
Length
a) Not exceeding 3m	± 6mm
b) Between 3m and 4.5m	± 9mm
c) Between 4.5m and 6m	± 12mm
d) Additional for every 6m	± 6mm
Cross-section (each direction)
a) Not exceeding 500mm	± 6mm
b) Between 500mm and 750mm	± 9mm
c) Additional for every subsequent 250mm	± 3mm
Straightness or bow (deviation from intended line
a) Not exceeding 3m	6mm
b) Between 3m and 6m	9mm
c) Between 6m and 12m	12mm
d) Additional for every subsequent 6m	6mm

Squareness of Precast Concrete Members:

When considering the squareness of a corner the longer of the two adjacent sides being checked shall be taken as the baseline, and a line perpendicular to the baseline shall be taken as the checkline. The shorter side shall not vary in its distance from the checkline so that the difference between the greatest and the shortest side is not more than the distance specified as under:

Length of shorter side	Tolerance
Not exceeding 1.2m	6mm
Between 1.2m and 1.8m	9mm
Exceeding 1.8m	12mm

For the purpose of this requirement, any error due to lack of straightness shall be ignored; squareness shall be measured with respect to the straight lines which are most nearly parallel with the features being checked. When the nominal angle is other than 900 the included angle between the baseline and the checkline shall be varied accordingly.

Twist of precast concrete members:

No corner of a nominally plane surface shall deviate from the plane containing the other three corners by more than:

Surface dimension	Twist
Between 600mm wide and upto 6m long	6mm
Between 600mm wide and for any length	12mm

Flatness of precast concrete members:

The deviation from a 1.5 m straight edge placed in any position on a nominally plane surface should not exceed 6 mm. Read More on Precast Concrete