Why Precision Hydration Is More Important than Wet vs. Dry Cast
We’ve all heard the words “dry cast” and “wet cast” thrown around in cast stone sales pitches and specs, but often these terms are used imprecisely, and even divisively. In fact, a given cast stone or precast producer will often tout the strengths of wet cast or dry cast merely based on which they themselves produce. It’s not a bad sales strategy, but it shouldn’t win your business. To argue that one casting method is inherently superior to the other showcases a lack of understanding with regard to cement hydration.
Believe it or not, even in an era of chemicals and additives, proper hydration, i.e. water content, is the most critical aspect of producing high-quality cast stone. When done right both methods should actually have a very similar water-to-cement ratio, and a manufacturers’ ability to measure this ratio precisely is what sets them apart. We’ll explain why that is.
The casting method is the wrong thing for the end user to be concerned with. Dry cast and wet cast can both be used to produce high quality stone given proper hydration. In all cases, the means and methods for specific parts should be left up to a trusted and experienced manufacturer.
When most people think of concrete, they think of wet cast concrete that’s used to pour a driveway. However, saucy, loose mixes with a high water content aren’t what a skilled mix-master will want associated with his product.
Though concrete has been around since ancient Rome, we credit scientist Duff Abrahams with determining the ideal water to cement ratio in 1918. He found that mixing at 0.3 parts water-to-cement, he could produce a very stiff concrete with a 0” slump.
In the industry, this zero slump concrete is known as “dry cast”, which causes confusion because zero-slump concrete is in fact perfectly hydrated concrete. In reality dry cast isn’t sandy or crumbly during production, but has the texture and feel of wet clay or beach sand. So, if perfectly hydrated cast stone has the consistency of dry cast, then is it possible to make quality wet cast? The answer “Yes”, thanks to a chemical called superplasticizer which alters the viscosity of dry cast without adding more water. To see plasticizer in action, take a look at the image below. In the production setting this is the type of wet cast that doesn’t suffer from the shortcomings of its overhydrated counterparts.
Hydration Horrors: It’s Not Easy
Mixing concrete too wet will have dramatic effects. Imagine the soupy cement you commonly see delivered by a concrete truck. If this concrete has a high water-to-cement ratio, it will have more slump. This runny mix design isn’t its only flaw; it will exhibit aggregate separation, shrinkage, and map cracking over time. It will have much lower compressive strength (PSI) and have a higher absorption rate more about issues with low psi and high absorption rate, which all lead to a shorter lifespan. Inconsistent moisture can also lead to color consistency issues as well.
The problem with high water content wet-cast stone is that it hides these faults very well for a short time after it is produced. Grossly overhydrated wet cast will start to show defects within a year or two. First it will shrink and begin cracking, then within five years it will begin map cracking. A decade later and you’re demolishing and installing a new facade entirely. You can see these issues in the picture below, which shows the base of a column at a prominent retailer in the midwestern US. It’s definitely not how you want your new building to look after less than a decade!
On the other hand, when mixed too dry, cast stone can be brittle. Dry cast is much less likely to exhibit aggregate separation, map cracking, shrinkage, or absorptive properties. Still, dry cast has gotten a bad rap in the industry as it’s been done wrong, causing many headaches. The reason is simple: few manufacturers have invested in the expensive tooling to be able to accurately regulate hydration, and dry cast made with too low of a water ratio will be brittle. You can spot improperly hydrated dry cast right away because it will crumble in your hand.
Hydration Mastery = Quality
At Marcstone we’ve learned from thousands of jobs that you cannot rely on weighed or metered batch water to determine the water-to-cement ratio of each batch. A quality end product depends on being able to precisely ensure the proper base hydration throughout, so the added water for batching must adjust up or down in real time to achieve optimal hydration, and for that you need high tech equipment.
Hitting Abrahams perfect ratio requires high tech equipment because aggregates and sands hold different amounts of moisture day-to-day, batch-to-batch, and even minute-to-minute. Wet aggregates can easily push you into overhydrated territory, dry aggregates can easily push you into under hydrated territory.
That’s why Marcstone uses a state-of-the art microwave moisture probe technology, coupled with digital water dosing meters that can analyze the batches moisture 24 times per second and adjusts the added batch water up or down by the drip. So, here at Marcstone, it doesn’t matter if the aggregates are wet as a rainstorm or as dry as desert sand, each batch comes out with absolutely perfect hydration; every time.
At Marcstone we don’t want to be perceived as proponents of Dry Cast or Wet Cast method, and instead we focus on what matters: proper hydration. The parts are analyzed and the best method for each part is established. Unfortunately this level of sophistication is not at all common in the cast stone industry. So, next time someone tells you wet cast is better than dry cast just envision Ricky Bobby with a garden hose hollerin’: “That there looks about right!” There’s a good chance you just envisioned their batch master.