Q. What is Cast Stone?

Cast stone is economical. The more complex the components, the less expensive each piece becomes. Color and texture matching for historical restorations is possible. There are shorter lead times. Consistent Repeatability is better depending on the limestone and tooling used by the manufacturer. Cast stone offers greater design flexibility and can outlast limestone. Its color is retained longer. Whenever detail and repletion are required, cast stone is more cost effective. Tri-Stone has a line of components typically used by masons that is competitively priced to limestone (See our Product Line).

Precast concrete can refer to wall panels, floor and roof slabs, catch basins, landscaping and retaining walls as well as building products such as: lintels, sills and coping. Cast Stone is a refined architectural concrete building unit manufactured specifically to simulate natural cut stone.

Specifications

Precast – Usually job specific because of the wide variety of products that fall under its category. General accepted specifications are:

• Bug holes up to 5/8” are acceptable
• Finishes range from: Swipe (CMU), Retarded finish (washed aggregate), Sand blasted, Acid etched, Honed or machined

Cast Stone –

• No bug holes more than 1/32”
• Finish variation to be equal to good natural cut limestone
• Dimensions not to deviate more than 1/8”

The process involved in making precast stone and cast stone are similar, but to meet the specification for cast stone, higher quality molds are used, the design mix needs to be different and the finish process is more extensive.

Although it is possible for a precast manufacture to produce stone that meets the Cast Stone specifications it is unlikely, because once a manufacture can attain those specifications they will use the term Cast Stone in describing their products.

There are many factors to consider when purchasing a stone product. You want to find the best. An important question to consider is: “Do I use wet cast stone or dry cast stone?” The wet cast method is better to use because of the reliable manufacturing process, the endless detail capabilities, and the impressive strength.

Manufacturing Process
In order to make dry cast stone, a concrete mixture is compacted into a mold; then immediately taken out of the mold to be put into a curing room with humidity levels of 95% for 24 hours or more. This process leaves a lot of variables. According to the article “Architectural Precast Concrete vs. Cast Stone” in the November 2007 issue of Masonry, the immediate removal of the dry cast stone from its mold “accelerates the hydration of the concrete, which severely compromises quality”. Because of the tamping method used to make dry cast stone, it is very difficult to reinforce with structural inserts and cages, which results in it being limited to smaller pieces. Also, during the tamping process, different employees may do it differently, causing subtle variations.
Wet cast, however, is made much more reliably. The concrete mixture is poured into a mold and then cures in the mold for 24 hours or more. It is taken out when it has reached 65% of its final strength, which makes for strong edges and details. Unlike the dry cast curing method, the wet cast method does not have the risk of improper curing, ensuring quality stone every time. It is quite simple to reinforce wet cast stone, unlike dry cast. Anchors, rods, inserts, and plates are easily cast into wet cast stone.

Appearance
Dry cast stone has very few decorative options. Its size, shape, and surface are very limited. It has to be a small piece, it has to be a very simple shape, and its surface is restricted to that of rough, imitation limestone. Quite the opposite, wet cast has relatively endless possibilities. Tri-Stone has made parts smaller than a brick and pieces weighing thousands of pounds. Because the wet cast method allows for many complex shapes, lion’s heads and odd shapes are not even questioned as to achievability. The potential for surfaces using the wet cast method are, again, very extensive.

Strength
Coarse aggregate, like small rocks, is an important key to the durability of the stone. Dry cast contains, if any at all, very little coarse aggregate. The wet cast mixture contains coarse aggregate throughout the piece, withstanding rough handling and environmental factors. Its original appearance is sustained for longer periods than dry cast stone.
To meet ASTM C 1194, dry cast has to have the strength of 6500 psi at 28 days, while wet cast is specified to be 5000 psi at 28 days according to ASTM C39-86. This may make it appear that dry cast is stronger than wet cast; however, the tests are not controlled. Dry cast is tested by breaking cubes whereas wet cast is tested by breaking cylinders. The book, Strength and Related Properties of Concrete: a Quantitative Approach, by Sándor Popovics, reported that “compressive strength values obtained by cylinders, prisms, cubes, and modified cubes, and compressive strengths determined by other methods are neither interchangeable nor necessarily comparable”. Simply put, you cannot compare the results of the two different methods of testing. Besides, stone at Tri-Stone has the psi between 9,000 and 10,000.

 Tri-Stone, Inc. has perfected the wet cast method of cast stone manufacturing. The wet cast method is superior to the dry cast method because the manufacturing process is more reliable, there is more of a range for the appearance options, and it is stronger.