Manufacturing Procedure

Manufacturing Procedure

Plant Layout

The physical layout of the plant includes production, handling, storage and shipment of concrete products which can be done in a safe and efficient manner with minimal product damage.

Housekeeping

Plant shall have an active housekeeping program. The purpose of this program is to provide a clean and safe environment so that quality precast concrete products can be manufactured efficiently.

Handling Equipment

Equipment such as hoists, overhead crane, forklift trucks shall be used to lift and handle products which weigh less than the rated capacity of the equipment.

Pre-Pour Operations

1. Cleaning of Forms
Forms shall be cleaned after each use. Concrete, tape, and other materials adhering to the forms shall be removed.

2. Application of Form Release Agent
orm release agent shall be applied after the forms are cleaned and if necessary, the seams sealed. Reinforcement and other items to be embedded in concrete shall be free of form release agent. Special care is attained to avoid over-application of form release agent.

3. Positioning of Reinforcement
Reinforcing steel shall be positioned as specified by the design, and concrete cover must conform to the product requirements.

4. Positioning of Miscellaneous Embedded Items
Embedded items shall be positioned at locations specified in the design. Inserts, plates, lifting devices/hook and other items to be embedded in precast concrete products shall be held rigidly in place during casting operations.

Casting of Concrete

Conveying Concrete
Concrete may be conveyed from the mixer to the casting locations by any means that does not contaminate the concrete or cause excessive segregation. Concrete discharged directly from the mixer into the forms is permitted. Conveying equipment shall be capable of providing a supply of concrete to place of final deposit without separation of ingredients and without interruptions sufficient to permit loss of plasticity between successive increments.

Depositing Concrete into Forms
Conventional concrete shall be deposited into forms as near as possible to the final location, keeping free fall of concrete to a minimum. Concrete that has practically hardened or been contaminated by foreign materials shall not be deposited.

Consolidating Concrete
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

Secondary Pours
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.

Stripping Products From Forms

1. Minimum Strength Requirement
Products shall not be removed from the forms until the concrete reaches the designed compressive stripping strength or at least one-day.

2. Product Damage during Stripping
Products damaged during stripping shall be evaluated by qualified plant personnel to determine if repairs are necessary, and if so, repair is required before shipping.

3. Formed Surfaces
Formed surfaces shall be considered satisfactory if they are relatively free of air voids and honeycombed areas, unless the surfaces are required by the design to be finished.

4. Post-Pour Inspection
A post-pour inspection shall be made of each product. After products are stripped from the forms, they shall be inspected for conformance with the design. Items to be repaired shall be classified as “major” or “minor” defects, or as “honeycombed areas.”

5. Inspection of Repairs
Products that require repairs of “honeycombed areas” or “major” repairs shall be inspected while repairs are made. QC Inspector shall perform checks of repairs.

Marking, Storage and Shipment of Products

  • Product Marking

Products shall be marked as required by project specifications.

  • Storage Areas

Areas used for storage of products shall be firm enough and level enough to avoid causing damage to stored products. It is good practice to place only products that are ready for shipment in storage, thus minimizing product handling.

  • Shipment of Products

Trucks and other conveyances used to transport precast concrete products from the plant to the location designated by costumer shall be equipped and maintained to deliver those products without damaging them to the extent that they must be repaired or rejected. Trucks and other delivery equipment should be inspected periodically to ensure products will be delivered without damage.

  • Final Inspection

Prior to shipment, products shall be inspected to assure design conformance and proper identification. The QC Inspector shall inspect the storage area and the stored product on a daily basis.