PROPER METHODS FOR CONCRETE                                 PLACEMENT

Before any concrete is placed the entire placing programme consisting of equipment, layout, proposed procedures and methods is planned and no concrete is placed until formwork is inspected and found suitable for placement. Equipment for conveying concrete should be of such size and design as to ensure a practically continuous flow of concrete  during depositing without segregation of materials considering the size of the job and placement location.

Concrete is placed in its final position before the cement reaches its initial set and concrete is compacted in its final position within 30 minutes of leaving the mixer and once compacted it should not be disturbed.

In all cases the concrete is deposited as nearly as practicable directly in its final position and should not be re-handled or caused to flow in a manner which may cause segregation, loss of materials, displacement of reinforcement, shuttering or embedded inserts or impair its strength. For locations where direct placement is not possible and in narrow forms suitable drop and Elephant Trunks to confine the movement of concrete is provided. Special care is taken where concrete is dropped from a height especially if reinforcement is in the way particularly in columns and thin walls.

Concrete should be placed in the shuttering by shovels or other methods and should not be dropped from a height more than one metre or handle in a manner which will cause segregation.

Concrete placed in restricted forms by borrows, buggies, cars, sort chutes or hand shoveling should be subjected to the requirement for vertical delivery of limited height to avoid segregation  and should be deposited as nearly as practicable in it’s final position.

Concreting once started should be continuous until the pour is completed. Concrete should be placed in successive horizontal layers of uniform thickness ranging from 150 mm to 900 mm. These should be placed as rapidly as practicable to prevent the formation of cold joints or planes of weakness between each succeeding layers within the pour.

The thickness of each layer should be such that it can be deposited before the previous layer has stiffened. The bucket loads or other units of deposit should be spotted progressively along the face of the layer with such overlap as will facilitate spreading the layer to uniform depth and texture with a minimum of shoveling. Any tendency to segregation should be corrected by shoveling stones into mortar rather than mortar onto stones. Such a condition should be corrected by redesign of mix or other suitable means.

The top surface of each pour and bedding planes should be approximately horizontal unless otherwise specified in drawings.