New forms are a capital expense and often put on the back burner. When dated formwork is not replaced, the need to properly maintain, inspect and store old formwork becomes crucial.
By Bryan Cousino, P.E. | Photos courtesy of Spillman Co. (www.SpillmanForm.com)
Under the gun and in a flurry of non-stop movement, a precast concrete production crew preps forms to meet an impending deadline. Old caulking remains on one of the panel forms before the pour. What is the result? Production meets the client’s deadline but at the cost of a poor finish and product appearance. This precast company’s reputation slips a notch, because most people, not just owners and contractors, judge the quality of a precast concrete product in large part by its outward appearance.
This article lists the required daily and monthly inspections and the correct way to prep, store and level forms to ensure a good-looking concrete finish that will protect your company’s reputation for quality products.
Formwork inspection items and frequency
1. Weld inspection – inspect for separation, cracking and bending:
a. Lifting loops
c. Vibrator mounts
2. Casting surfaces – inspect for cleanliness and defects (blemishes in the skin, pitting and rust)
3. Chamfer – inspect for alignment and general damage
4. Springs, hinges and moving parts – confirm proper operation and range of motion
5. Shims – confirm all shims are in place
6. Trueness of level – inspect using a level or surveying equipment on formwork each time it is removed from the slab
1. Weld inspection – inspect for separation, cracking and bending:
b. Form components (hinge plates and clamped connections)
c. Connection of reinforcing structure to form skin
2. Trueness of level – inspect with a level or surveying equipment on forms that are permanently anchored to the floor
3. Anchors to floor slabs – inspect
4. Signs of rust (under structure) – inspect twice monthly on forms subjected to accelerated curing
Formwork maintenance prior to storage
1. Remove any built-up concrete on the form, paying particular attention to the casting surfaces.
2. Coat form with a weatherproof rust inhibitor (form oil is not a suitable long-term rust inhibitor). Most rust inhibitors must be reapplied every six months (three months in marine environments).
a. Coat internal parts (pallets, headers and bulkheads) and moving components (hinges, clamps and rollers) with suitable grease or rust inhibitor.
3. Inspect painted surfaces and re-coat as necessary.
4. Store on a flat, level surface with appropriate hardwood dunnage; typically, supports on 5-ft increments are adequate.
5. Keep forms from high-traffic areas and exposure to the elements.
6. Do not let standing water collect on the form.
7. Cap ports on hydraulic cylinders and retract cylinders to avoid rust and pitting.
8. Inspection – Form storage is a good opportunity to do a thorough form inspection. Signs of rust on the contact surfaces should be removed by sanding or grinding. Never sandblast the casting surfaces. Signs of rust on non-contact surfaces can be removed by sanding, grinding or sandblasting (always use a NIOSH-approved respirator when sandblasting). Be sure to protect all surfaces, as rust forms rapidly on unprotected steel surfaces.
Proper leveling of formwork
A form that is not adequately leveled can cause nonconformances in casting tolerances, improper form operation, difficulty in stripping and damage to the casting or form components. Forms should be installed on a flat surface capable of supporting the form. It is never recommended to install a form on a gravel surface.
Forms less than a 6-ft maximum casting dimension can generally be leveled with a 4-ft level. Forms greater than 6 ft may need to be shot in with a laser level. Forms are produced on a level surface, and as little as 1/16 in. in variation can cause performance problems in certain forming applications. These problems are compounded the taller and longer the form. A 1/16-in. variation in levelness at the bottom of a form that produces an 8-ft-tall product can cause a 1/4 in. or more variation from specifications at the top of the form.
It is important to check the levelness of a form to the casting surfaces. Structural elements used for reinforcement have mill tolerances in excess of typical precast and prestressed castings and cannot be used as reference points for leveling a form. Additionally, these steel mill tolerances may cause the bottom of the form to vary and should be shimmed as necessary to level the casting surfaces.
In conclusion, older formwork can remain in service to produce a quality precast concrete finish as long as these inspection and maintenance guidelines are rigorously followed.
Bryan Cousino, P.E., is engineering manager at Spillman Company in Columbus, Ohio. He is responsible for formwork design and is the primary contact for technical questions on installation, performance and form maintenance. Contact him at [email protected]
Thanks Bryan for the article, very interesting! Formworks maintenance is always the best solution in order to prevent future damages or quality lowering. in the structure.
Dear Mr.Bryan Cousino,
Good Day. I have questions that is What will be the earliest time to remove the precast form work from the Port Land Precast Cement wall.
The hours of casting.Secondly do I have use Plasticize and hardener to reduce the curing Time and to recast a new product.
Sara Geer says
Thank you for the comment Batumalai. Claude Goguen, one of our technical engineers, has provided the following response:
“The minimum stripping time is a function of concrete strength. Usually, the method used to determine stripping time involves comparing the actual strength gained to the required strength for stripping the structure. The required stripping strength is sometimes specified in the contract documents. In precast, the stripping strength in usually listed in the plant specific quality control manual.
The NPCA Quality Control Manual, section 4.6.1 reads as follows: “Products shall not be removed from the forms until the concrete reaches the designed compressive stripping strength. If no such requirement exists, the plant shall define product-specific minimum stripping strengths that must be obtained prior to stripping. These requirements shall be defined in the plant specific quality control manual discussed in Section 1.1.2. In addition, one-day, or stripping compressive strength tests shall be performed for each mix design at least quarterly in order to confirm that adequate stripping strengths are being attained. These requirements do not apply to dry-cast and/or machine made products.”
The earliest time to remove formwork is when the concrete has achieved enough strength to support its own weight and the weight of anticipated loads. From ACI 547 – Guide to Formwork for Concrete – “When forms are stripped, there should be no excessive deflection or distortion and no evidence of damage to the concrete due to either removal of support or to the stripping operation. If forms are removed before the specified curing is completed, measures should be taken to continue the curing and provide adequate thermal protection for the concrete.”
So unfortunately, I can’t give you an amount of hours. Various concrete mixes will have different rates of set. You just need to cure cylinders in the same environment as the structure and periodically break one until the desired strength has been reached. The time required to reach that strength can then be used as a basis for further stripping for that particular mix design.
As for your second question in regards to the use of plasticizers and hardeners to reduce curing time, plasticizers generally have little effect on setting times. These are used to increase workability and/or reduce water content of concrete. ASTM C494 type E admixtures can provide water reduction while accelerating set. If you’re concerned about setting times, generally you would use a set controlling admixture such as an accelerator. Other ways to increase concrete strength development are using high early strength cements such as a type III and curing at higher temperatures.”
It’s amazing that so many different components can affect the quality of precast concrete so much. What I’m a little confused about are the form components that would need to be inspected once a month. I’m not quite sure that the purpose of hinge plates are in the precast process. Are they like plastic components that actually help shape the concrete? I know that plastic is often used in concrete formation because of the fact that it doesn’t stick to the concrete.
Mason Nichols says
Thank you for your comment. Evan Gurley, one of our technical services engineers, offered the following response to your question on hinge plates:
“In this article, hinge plates refers to components of a form (typically steel or aluminum forms) that permit the formwork to open and close or swing, allowing for setup and stripping. The steel or aluminum formwork itself helps shape the final design of the precast product being manufactured. As long as proper preparation and maintenance is performed on the steel or aluminum formwork, concrete will not “stick” to the form during casting or stripping operations.”