There are two common methods of manufacturing cast stone : one is the dry tamp method and the other is the wet cast process. Both of these methods create a simulated natural cut stone look.
The most common finish for a dry tamp cast stone is limestone or sandstone while the wet cast method is capable of providing a wider array of possible finishes. The machine cast method of creating dry tamp cast stone is not discussed in this article.
What’s the difference between wet and dry cast stone?
The concrete mix for vibrant dry tamp cast stone has zero slump while the wet cast mix includes a higher water content and has 3” to 4” of slump. These two methods of making simulated natural stone are outlined below.
More about the dry tamp method
Manufacturing wet cast cast stone is just like baking a cake. Get your cake tin ready (mold) and pour in the cake mix batter (concrete mix meeting the requirements of ASTM C1364).
Here’s a list of important considerations when making wet cast cast stone.
- Molds need to be perfect.
- Mold release is important. Chemical release agents result in a better surface finish.
- The mix design of your concrete is important. It takes into account surface appearance, workability, surface defects (like bug holes), ease of casting and cost.
- No leaks can be in the mold. Otherwise, you will have color issues and edge quality deterioration.
- If there is an undercut in your piece, you may have to make complicated multi-piece molds
How do you make wet cast stone?
Professional manufacturers of cast stone products like Premier Precast have very strict ratios of ingredients as they strive to strike a delicate balance between creating a concrete mixture that is easy to work with for ornate molds and still meets the demanding requirements of ASTM C1364.
Many combinations of chemicals can be used to improve the performance of the mixture without having to dilute the mix with water. Good placement techniques including vibration ensure a quality finish.
Wet cast cast stone is produced by mixing the four common ingredients (sand, pea-rock, cement and water) together with carefully selected admixtures in a scientifically defined ratio and placing the mixture into professionally constructed precision molds.
The cast stone elements then sit for 24 to 48 hours to properly cure and strengthen before being removed from the molds for final cleaning and preparation.
What are the pros and cons of wet cast stone?
- Less effort to place the concrete in molds. You just pour it in vibrate and trowel.
- Less restrictions on choosing mold materials. Molds cost less and are quicker to make.
- Conducive to surface treatments such as retarders and texturing additives in the mold before casting.
- Different finishes can be achieved through post cure treatments such as acid etching, sand-blasting and other techniques.
- Less likely to be damaged during demolding.
- Can be cast to a smooth finish.
- Can incorporate larger aggregates into the concrete mix.
- Can only use a mold one time per day because the wet cast mix must set up and cure.
- Early strength not as high as dry tamp cast stone.
- Requires more detailing after demolding to be as clean looking as dry tamp method.
- Requires acid etching or other surface treatments to achieve the limestone look inherent with dry tamp cast stone.
More about the dry tamp method
As a kid, did you ever make a sand castle at the beach? You may have had a special sand castle bucket, and your dad may have shown you how to get the damp sand and compact it into the bucket and then flip the bucket over to make the castle.
The dry tamp method is just like making a sand castle at the beach.
The main difference is that you add cement to the damp sand and you use special compacting tools (usually air powered tampers) to compress the sand/cement mixture into the mold.
And just like making sand castles at the beach, once the concrete mix has been tamped into the mold, it can be flipped over, the cast stone part removed and the mold can be used again immediately… to make another sand castle (or cast stone part).
How do you make dry tamp cast stone?
A special mix of sand, cement, admixtures and a low amount of water make a damp sand like mix. This damp mix of sand and cement is placed loosely into a rigid mold and then hand or pneumatically tamped to pack the mix tightly into the mold in several layers.
Once the mold is full, the excess is scraped off and leveled smooth. Almost immediately, the cast stone part can be removed from the mold and the mold can be cleaned and used again.
After removal from the mold, most manufacturers introduce humidity to aid curing and early strength gain by putting parts into a water misting or steam curing room.
What are the Pros and Cons of Dry Tamp Cast Stone?
- Fast demold time.
- Can cast into the same mold several times a day.
- No treatment is required afterwards to achieve the limestone look.
- Result of cast stone is very clean and natural looking.
- High early strength.
- There is a time just after demolding when a repair is relatively easy.
- Steam curing or water misting can be costly to set up.
- No surface treatments such as mold applied textures or colors, can be achieved other than traditional limestone texture.
- Requires extra step of detailing during de-mold.
- Cannot create smooth finish cast stone
Please view the video of vibrant dry tamping cast stone from our friends at Advanced Architectural Stone.