PREPARATION FOR PLACING
In preparation for the placing of concrete, all construction debris and extraneous matter shall be removed from the interior of forms. Standing water on areas to receive concrete shall be removed before concrete is placed. All exposed reinforcement shall be free from loose rust, scale and wind blown salts and spray.
Placing of concrete shall not be commenced until the Engineer has inspected and passed the formwork or other areas to receive concrete and any reinforcement, cast in fixings etc., against which the concrete is to be placed. Any such given approval shall not relieve the Contractor of any of his responsibilities under the Contract.
Where concrete is to be cast against an existing concrete face, that face shall have been prepared to expose the aggregate and all loose particles removed. This surface shall be wetted prior to receiving concrete. This preparation shall be subject to inspection by the Engineer.
PLACING OF CONCRETE Concrete shall be placed in the shortest time if possible after mixing is completed and before it has taken an initial set. It shall be placed as close as possible to its final position to avoid segregation of materials and displacement of reinforcement.
The concrete placing must ensure the following requirements:
• Do not deviate position of reinforced steel, formwork and concrete thickness for protecting reinforced steel. • Do not use vibrating rammer to move concrete horizontally in formwork • Concrete must be placed continuously until completing any structure specified in the design
In order to avoid layer segregation, the free dropping height of the concrete mixture when placed is not over 1.5 m
When placing concrete with the free dropping height greater than 1.5m, need to use inclining gutter or trunk spout. If the dropping height is over 10m, need to use the trunk spout with the vibrating equipment.
When using the trunk spout, the spout needs to be inclined deviation compared to the upright direction not over 0.25m per 1m high; in all cases, need to ensure for the lowest spout is upright.
When using inclining gutter, it must be tight and flat. The width of gutter in not less than 3-3.5 times of the biggest aggregate seed’s diameter. The slope level of gutter need to ensure for the concrete mix not stuck, fast slid arising the segregation. At the end ofthe gutter, put an upright hopper to drive the concrete mix dropping vertically in the placing position and regularly clean the cement mortar inside the inclining gutter.