Specification for Placing of Concrete

Concrete shall be deposited as nearly as possible to its final position and shall be placed in layers not more than 30 cm thick. Concrete shall be placed at such a rate that it is at all times plastic and will flow readily around reinforcement and embedded parts. Once placing is started, it shall be a continuous operation until the lift is completed. The area of placement shall not be more than that which can be completed in one pour in compliance with this specification.

The time between placing of successive layers forming a lift shall be short enough to permit a vibrator to penetrate the lower layer while the upper layer is being compacted. The minimum time between completion of a lift and the commencement of placing of a succeeding lift shall be 72 hours.

Pneumatic placing and pumping of concrete may only be used with the approval of client. In such a case, discharge lines shall be horizontal or inclined upwards. The discharge end shall not be more than 3m from the point of placement and shall supply a continuous stream of concrete without air pockets. Aluminum piping shall not be used.

During hot weather, special precautions, as directed or approved by the owner, shall be taken to ensure that the temperature of the concrete when it is placed does not exceed 30 degrees Centigrade. These may include, but shall not be limited to, the use of chilled mixing water, cooling of aggregate or working at night.

Concrete shall not be placed in exposed areas during rain and, should it rain while placing is in progress, the placed concrete shall be covered with vinyl or other impervious sheets. Placing may be resumed only when rain stops well before initial set of the placed concrete occurs. Otherwise, placing operations shall cease and the boundaries of the pour shall be prepared as specified for construction joints.

Concrete shall not be placed under water except when specifically directed and authorized in writing by the owner. In such a case, placing shall be done from a bottom?discharging watertight bucket or by termite pipe penetrating the previously placed concrete so that fresh concrete is always deposited below the surface and the operation is continuous until completion of the pour.

Compaction

Concrete shall be compacted by mechanical vibrators electrically or pneumatically driven. Immersion vibrators shall operate, when fully immersed, at speeds of not less than 7,000 impulses per minute for vibrating heads of less than 10 cm diameter and 6,000 impulses per minute for heads 10 cm or greater in diameter. They shall be slowly immersed and withdrawn vertically at spacing of 30 cm to 50 cm, with vibration periods of 10 to 15 seconds for each penetration, and shall penetrate into the layer below. Vibrators shall not come into contact with reinforcement or embedded parts.

Where immersion vibrators cannot be used, the concrete shall be compacted by form vibrators, rigidly attached to the outside of the form, operating at not less than 8,000 impulses per minute.

Curing

Concrete surfaces shall be kept continuously moist for at least 14 days after placing and until the concrete temperature produced by heat of hydration of the cement has peaked and fallen at least 10 centigrade degrees. Initial curing of exposed surfaces shall be performed by atomized water spray positioned so that all surfaces to be cured are constantly moist, or by a continuous layer of burlap kept constantly wet. Hand sprinkling shall not be used except in conjunction with the burlap alternative.

Forms capable of moisture loss, such as plywood forms, shall be regularly sprinkled with water to avoid loss of moisture from formed surfaces of the concrete.