Concrete shall be consolidated in such a manner that segregation of the concrete is minimized. Vibrators used to consolidate concrete and shall have frequencies and amplitudes sufficient to produce well-consolidated concrete. Internal vibrators shall be lowered vertically into the concrete without being forced downward until the tip of the vibrator reaches the bottom of the form or until it penetrates into a previously consolidated lift. Vibrate the concrete until air bubbles within the vibrator’s field of action essentially stop coming to the surface. Do not use vibrators to move concrete laterally. Proper use of vibrators to consolidate concrete requires trained operators 3s3.
Unformed surfaces of wet-cast precast concrete products shall be finished as specified
For products that require secondary pours, procedures shall be established to assure that concrete cast during the secondary pour adequately bonds to the precast concrete product and becomes an integral part of the product.
The surfaces of the product against which the secondary pour is to be made should be free of laitance, dirt, dust, grease or any other material that will tend to weaken the bond between the original and new concretes. If the surface is very smooth, it should be roughened to help promote good bond.
Two critical elements in curing concrete are: maintaining correct moisture content and maintaining correct concrete temperature.
Proper curing is important in developing strength, durability, chemical resistance and water tightness – all important considerations for underground utility structures. The nature of precast operations poses unique challenges to proper curing. According to ACI 308R, “Guide to Curing Concrete,” strip the forms at the beginning of the next workday. That is an acceptable standard. The time necessary to develop enough strength to strip the forms is highly dependent on ambient temperature in the casting area. The Portland Cement Association (PCA) lists three methods of curing:
1. Maintaining water moisture by wetting (fogging, spraying, wet coverings, etc.)
2. Preventing the loss of water by sealing (plastic coverings or applying curing compounds)
3. Applying heat (often in conjunction with moisture, with heaters or live steam) Choose the methods that best suit the particular production operation.
All three are permissible, but preventing the loss of water (method 2) is the simplest choice for utility structures that is being used currently. Curing compound is applied because the bleed water is no longer present on the surface. During the curing process, it is ensured that the concrete temperature does not exceed 150º F. Admixtures such as retarders are often used to slow down hydration to prevent rapid set.