Contract documents should spell out who the accepting authority will be, typically the owner, architect, general contractor or site inspector. One person must have final authority on all issues of appearance. Acceptable ranges of color and shading should be determined when samples, mockups or initial production units are created. Components should be assessed for appearance during both wet and dry weather.
Minor defects and blemishes will occur on occasion, and precasters can adequately repair minor problems quickly. There are key defects or problems that are considered unacceptable in the fabrication of high-quality, PCI-certified architectural precast concrete. These should be addressed as soon as they appear to ensure the problem is corrected early in the production sequence. The unacceptable problems are:
- Ragged or irregular edges.
- Excessive air voids (“bug holes” or “blow holes”) in the exposed surface.
- Adjacent flat and return surfaces with noticeable differences in exposure from the approved samples.
- Casting lines evident from different concrete placement lifts and consolidation.
- Visible form joints or irregular surfaces.
- Rust stains on panel surfaces.
- Panels not matching the approved sample or non-uniformity of color within a panel or in adjacent panels due to areas of variable aggregate concentration and variations in depth of exposure.
- Blocking stains or acid stains evident on panel surface.
- Non-uniformity of color or texture.
- Areas of backup concrete bleeding through the face concrete.
- Foreign material embedded in the face.
- Repairs visible at 10- and 20-foot viewing distances.
- Reinforcement shadow lines.
- Visible cracks after wetting.
Precast concrete generally undergoes far less cracking than cast-in-place concrete. This resistance results in part from the high compressive and tensile strength of the prestressing activity.
A certain amount of cracking may occur without having any detrimental effect on the structural capacity of the member, and it is impractical to impose specifications that prohibit cracking. But cracks are unsightly and create potential locations for concrete deterioration, so any cracking should be avoided and inspected.
The best methods to minimize cracks are to provide proper reinforcement, prestressing and proper handling. Whether cracks are acceptable will depend on an evaluation of the cause of the cracking and the stress conditions that the crack will be under with the precast concrete unit in place.
The cement film on the concrete may develop surface crazing, which consists of fine and random hairline cracks. Crazing has no structural or durability significance, but it may become visually accented if dirt settles in these minute cracks. A relatively lean, properly consolidated concrete mix will show little crazing.
Tension cracks can be caused by temporary loads during production, transportation or erection of the products. The amount and location of reinforcing steel has a negligible effect on performance until a crack develops. While some of these cracks may be repaired and effectively sealed by pressure-injecting a low-viscosity epoxy, their acceptability should be governed by the importance and function of the panel.
Every effort should be made to promptly identify the cause of any cracking, particularly when several units display similar cracking. Such cracking often results from one problem, be it design, production or handling, and it can be corrected.
Erected panels not complying with range samples may require additional work. If the architectural precast concrete panels cannot be corrected to match the repair samples or repairs demonstrated on the mockup, they may be subject to rejection. A certain amount of repair is to be expected as a routine procedure. Repair and patching of precast concrete is an art requiring expert craftsmanship and careful selection and mixing of materials.
Repairs should be done only when conditions exist that ensure the repaired area will conform to the balance of the work’s appearance, structural adequacy and durability. Slight color variations can be expected between the repaired area and the original surface due to the different age and curing conditions of the repair. Time will tend to blend the repair into the rest of the component to make it less noticeable.
Should minor damage occur to clay-faced or veneer stone products during shipping, handling or erection, field remedial work can successfully be accomplished. Such repairs normally are done by the precaster with repair procedures developed in consultation with the product’s fabricator.
For more information, contact a PCI Certified Manufacturer.