Turn your ideas into concrete! The specifications on this page will help turn your vision into a reality.
On the following pages are Architectural Precast Concrete Trim and Cast Stone specifications that are the result of many years’ experience. These specifications are for use in projects where it is imperative to have a high quality colored precast concrete or cast stone product. Many of the current specifications used to specify precast concrete trim are the same specifications that are used for large precast wall panel systems. Often they call for more extensive engineering and testing than is necessary in a trim project while at the same time being less demanding on the quality of the finish. The enclosed architectural precast concrete specifications are designed with trim-type pieces such as sills, headers, coping, watertable, and decorative panels in mind. These specifications should NOT be used for specifying precast wall panel systems and other large structural pieces unless they are modified to require engineering calculations done by a professional engineer and more extensive testing. The “Cast Stone” label is almost always used exclusively for trim-type units.
We believe that the enclosed specifications are strict enough to discourage inexperienced and unqualified manufacturers from bidding on a project, yet flexible enough to allow any capable manufacturer to be qualified as an acceptable fabricator by submitting standard sales materials and test reports. If adhered to, these specifications will help ensure that a satisfactory result is achieved. Also visit our Precast versus Other Materials page to see how precast products by Steps Plus compare with other materials used for architectural trim. See our Precast Concrete versus Vibrant Dry Tamped Cast Stone page to learn why you should never specify a Vibrant Dry-Tamped cast stone product in our northern environment.
This specification calls for a colored white-based mix consisting of white cement, white stone, white sand and color pigments. The use of all white ingredients ensures that almost any desired color can be achieved. Specifying just “color pigments” or “colored cement” while failing to call for a white based mix could result in precasters developing a price based on color pigments thrown into a mix of their standard dark concrete aggregates and gray cement. This drastically limits the range of colors that can be achieved. If white ingredients are not specified, yet a light colored product is desired, the precaster would be justified in requesting an extra to change his mix design to include the more expensive white ingredients.
When selecting a precast or cast stone color, the Architect should keep in mind that the selection of a light colored mix is preferable since color uniformity is better between light colored pieces than it is between dark colored pieces. This is partly because of the fact that white cement is generally more consistent in color than gray cement and partly because of the way the human eye sees these color variations.
In accordance with APA and PCI design suggestions, our specifications call for the precast and cast stone to be 5000 psi at 28 days when tested by cylinders per ASTM C39-86. Be aware that some cast stone manufacturers present test reports that are based on breaking cubes per ASTM C 1194 rather than cylinders per ASTM C39-86. This testing method is used primarily for dry-tamped cast stone which usually doesn’t contain any course aggregates. According to the PCI Architectural Precast Concrete Design Manual – 2nd Edition, compressive strength tests using cubes yield a result about 30% higher than tests done with cylinders. This means that a compressive strength of 6500 psi determined by 2″ cubes is equivalent to approximately 5000 psi for 6 inch cylinders.
These specifications call for all the cement paste to be removed by etching. This produces a smooth, fine-grained texture somewhat similar to natural stone. The sand in the mix is exposed yet only very small portions of the course aggregate are visible. Nearly all Cast Stone specifications call for this type of finish. This has a number of advantages over a “form” finish where the cement paste is left on the piece. Precast with a form finish often shows discoloration from the release agents used on the forms. A form finish also can take on a blotchy appearance as areas of paste are removed when the precast is scrubbed clean after it is installed. Over time, as form finish precast is exposed to the weather, the paste will start to crack, craze and wear off unevenly. Furthermore, patches are much more noticeable on a form finished piece than on a piece where the cement paste has been removed.
Deep exposed aggregate finishes that are common on panel work are not suggested for smaller trim pieces. The retarders used for this type of finish do not produce a consistent finish on verticals and returns which are common on trim-type pieces. Since retarders are affected by the heat of hydration of the concrete and since the heat of hydration varies with the thickness of the piece, each different trim piece type would end up with a different depth of exposure. Furthermore, a rough exposed aggregate surface is not suitable for window sills where a smooth window frame would have to be sealed against a rough precast sill. An exposed aggregate finish is usually quite a bit more expensive than an etched finish and the price increases dramatically if custom colored aggregates have to be purchased.
Sealers and a warning about deicers
With a few exceptions, Architectural Precast Concrete and Cast Stone building trim does not need to be sealed. Along with the cost of the sealer and its application, sealers can also change the appearance of the trim and any surrounding masonry that it accidentally comes in contact with during the application process. Sealers also wear off over time so a variation in appearance may develop as the sealer wears off at different rates depending on each piece’s exposure to the weather. Air entrained, high strength concrete with a low rate of absorption as called for in our specifications will yield a product that is resistant to normal freeze-thaw cycles without being sealed.
However, an exception to this is that exterior stair treads and other grade or low-level trim that may be exposed to salt or other deicing compounds should be sealed with a penetrating sealer. The use of any deicing compound, even if it is not chemically reactive with concrete, can cause spalling and pop-outs by subjecting the concrete to many more freezing and thawing cycles than would occur naturally. New concrete that is less than a year old is more susceptible to damage than older concrete and only a few applications of a deicer can cause problems. Unsealed precast concrete will often show damage from the use of deicers more quickly than adjacent cast-in-place concrete for two reasons:
1) most cast-in-place concrete is coated with a liquid curing membrane (such as Cure ‘N Seal) after casting which also acts as a sealer, and
2) etched or sandblasted precast concrete has only a thin layer of sand/cement paste over the coarse aggregates due to it being cast upside down and then flipped over when it is installed. The finishing process for precast products, typically etching or sandblasting, decreases the thickness of the paste even more. It is often just this thin layer of paste that pops off of the surface of the coarse aggregates when spalling occurs. Cast-in-place concrete is more resistant to this type of spalling because the coarse aggregates are pushed under a thick layer of paste as the concrete is finished by troweling.
Since sealing is not done by the precast manufacturer and since there are so many sealers on the market today with different properties, our specifications refer to a sealer in general terms. The Architect should select a specific sealer that is best suited for each particular project. Sealers must be applied by the installer after the precast or cast stone is installed for the following reasons: most sealers call for the concrete to be at least 28 days old before being sealed, sealers can prevent the mortar or sealants in the precast joints from bonding, sealers prevent patches from adhering, and sealers applied after installation will protect not only the precast but also the adjacent mortar joints.
A Note About Freeze-Thaw Testing
Some specifications currently in use call for freeze-thaw testing to be done per ASTM C666. A number of the spec writers including this requirement in their specifications do so without knowing much about it. This rather complicated test involves freezing and thawing samples of concrete 300 times and performing periodic monitoring tests to establish the cumulative percent mass loss. Because of all the cycles and the special handling of the spalled fragments which involves prolonged oven drying, the test can take over a month to do and is very expensive ($1,500 to $2,500). Most test labs do not even offer this ASTM C666 test. (No test labs in our Central New York State area perform it.) On many small projects, the freeze-thaw testing is worth more than the product itself. Due to the cost, extended duration of the test, and lack of availability, Steps Plus strongly recommends that this freeze-thaw testing requirement be left out of all precast concrete and cast stone specifications. As an alternative, compressive strength and absorption test reports should be required. High strength concrete with a low absorption will be very durable and will be resistant to damage from freeze-thaw cycles. ASTM C 1364-97 (Standard Specification for Architectural Cast Stone) states that proven field performance of similar products is also an acceptable way to confirm that a manufacturer’s product will have adequate resistance to freezing and thawing.
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Architectural Precast Trim Specifications – Paragraph Version for Unit Masonry Section
This section in paragraph form is formatted to be inserted into your own Unit Masonry specification section. While only 12 sentences long, it covers all the basics.
Architectural Precast Trim Specifications for Masonry Unit
This section is nearly the same as our SHORT VERSION 03450 specification (below) but its outline format is designed to be inserted into your own Unit Masonry specification section. Approximately 2 pages long.
03450 Specifications for Architectural Precast Concrete
This covers all the critical areas necessary for specifying top quality Architectural Precast Concrete. Approximately 2 1/2 pages long.
03450 Specifications for Architectural Precast Concrete- Standard Version
This goes into more detail than our short version specs. Approximately 5 to 6 pages long.
Architectural Cast Stone Specs
The outline format of this section is designed to be inserted into your own Unit Masonry specification section. Approximately 2 pages long.
04720 Specs for Cast Stone
This is our standard length Cast Stone specification. Approximately 3 pages long.