While Architectural Cast Stone can be made by two different methods, the vibrant dry tamp method and the wet-cast method, Steps Plus uses only the wet-cast method. We feel that our wet-cast Architectural Precast Concrete and Cast Stone products have numerous advantages over Vibrant Dry Tamped (VDT) Cast Stone. While the VDT method may seem attractive due it’s ‘sugar-cube’ finish and the fact that the same mold can be used numerous times in one day making it cheap to produce, it’s more prone to a number of potential problems than Wet-Cast Cast Stone. An article titled “What is Cast Stone” by Matthew Farmer (an engineer that routinely investigates construction failures) in the July/August 2008 issue of the Concrete Repair Bulletin, states that;
“There are a number of common problems that can occur with cast stone. Whereas some of these occur in cast stone produced using both wet casting and the VDT method, the majority of problems observed in modern construction are associated with the VDT method of manufacture.” [1]
Government Notes Problems with VDT Cast Stone
In the U.S. General Services Administration’s Historic Preservation Technical Procedures article titled ‘CAST STONE: CHARACTERISTICS, USES AND PROBLEMS”, it is stated that facing delamination and hairline cracks are common problems with dry-tamp cast stone [2]. The U.S. Department of the Interior’s Nation Park Service issued “Preservation Brief 42: The Maintenance, Repair and Replacement of Historic Cast Stone” in which they state that:
“Separation of the facing and core layers of dry tamp units is not uncommon, and often reflects fabrication defects such as poor compaction, lengthy fabrication time, or improper curing.” [3]
How “Vibrant Dry Tamped” Cast Stone is Made
The “Vibrant Dry Tamped” (VDT) method of making cast stone involves the vibratory ramming of earth moist, zero-slump concrete against a rigid mold in layers until it is densely compacted and the mold full. The cast stone unit is then immediately removed from the mold. The process is similar to making a sand castle at the beach by inverting a bucket of damp sand and then pulling off the bucket. The unit is then hand tooled to patch any edges that are damaged during the stripping process. The bad thing is that this zero-slump mix often has very small coarse aggregates or no coarse aggregates at all. According to an article in the November 1991, Concrete International Magazine titled “Freeze-Thaw Durability of Cast Stone”, the authors state that this type of cast stone is
“properly termed precast mortar due to its high cement content and its use of small size aggregate”. [4]
Moisture loss is a big concern in the “Vibrant Dry Tamped” method of making cast stone. In order for the unit to hold its shape when pulled from the mold immediately after tamping, the mix must be only slightly moist. Once the piece is removed from the mold, all the surfaces now exposed may become even dryer as moisture starts to evaporate from the newly exposed faces. However, the cement in the cast stone unit requires water to hydrate (harden) and this is why zero slump VDT cast stone units require a special curing procedure. To ensure that the cement in a VDT product gets enough water to hydrate, the Architectural Precast Association (APA) specifies the following curing procedure for VDT units: Vibrant Dry Tamped units should be kept in a warm curing chamber approximately at 100ºF (37.8ºC) at 95 percent relative humidity for approximately 12 hours, or cured in a 95 percent moist environment at a minimum 70ºF (21.1ºC) for 16 hours after casting. Additional yard curing at 95 percent relative humidity shall be 350 degree-days (i.e. 7 days @ 50°F (10°C) or 5 days @70°F (21°C)) prior to shipping.
If the above curing procedure is not done properly, the quality and durability of the unit may be significantly affected. You may end up with a surface that deteriorates quickly or units that are hard on the outside but soft and powdery on the inside. The photos below show a number of Vibrant Dry Tamped Cast Stone projects that have started to exhibit defects after a relatively short period of time, in some cases while still on the shipping pallets.
CRAZING in Vibrant Dry Tamped Cast Stone
As can be seen in the following photos, crazing cracks are a very common problem with vibrant dry tamped cast stone. According to the article titled ‘CAST STONE: CHARACTERISTICS, USES AND PROBLEMS” by the U.S. General Services Administration, it is stated that these hairline cracks are
“often caused by volume differences between the facing and backup material, or improper proportioning of the facing mix. When aggregates of uniform size are used, the cast stone tends to be more porous and less durable.”[2]
DELAMINATION and DEEP CRACKS in Vibrant Dry Tamped Cast Stone
Tamping the VDT mix into the molds can result in a strata effect between the different layers of material. This can result in return ends that are not as durable as the piece faces. This strata effect could also result in the layers delaminating and/or cracking as shown in the photos below. The strata effect can be made worse if the manufacturer tries to cut corners by using only a thin layer of colored mix for the visible surfaces and a different cheaper mix for the backup. Since the facing and backup mixes may have different expansion and contraction properties, the risk of delamination and cracking increases.
REINFORCING CONCERNS in Vibrant Dry Tamped Cast Stone
Because the VDT mix is tamped down into the mold in layers, it is impossible to tamp in around a reinforcing cage such as required for some long trim units and structural lintels. Structural lintels with cages should always be made using the wet-cast process. In his article, “What is Cast Stone”, Matthew Farmer expresses concern that reinforcing of any type in a VDT piece is a potential problem by stating the following:
Placement of reinforcing steel is of particular concern in VDT-cast stone panels and should be avoided. Usually the face mixture is placed first, then the steel is set, and the backup mixture poured around it. Therefore, this process does not allow for the reinforcing steel to engage the face mixture. If the backup mixture is dry-tamped as well, it is nearly impossible to achieve adequate consolidation around the reinforcing steel that is sufficient to develop its strength. If it must be used, wet-cast methods of fabrication are preferable so that the reinforcement can be fully encapsulated in cementitious material.[1]
PREMATURE DETERIORATION in Vibrant Dry Tamped Cast Stone
A big danger with vibrant dry tamped cast stone is that it does not have enough moisture in the mix when it is cast for all the cement to hydrate. This is why the wet curing procedure described above is so important for VDT cast stone. The photos below show a couple examples of cast stone that was evidently not cured properly and started to disintegrate in less than one year.
CHIPS in Vibrant Dry Tamped Cast Stone
The lack of coarse aggregate in most Vibrant Dry Tamped Cast Stone reduces the product’s resistance to chipping. By it’s nature, the tamping process used to fill the molds makes it difficult to get good compaction at corners and bottom edges.
DEFECTS DUE TO TAMPING PROCESS in Vibrant Dry Tamped Cast Stone
In the tamping process, the sand-cement mix is put down in relatively thin layers and then physically tamped into the mold to achieve adequate compaction. The Concrete International Magazine article “Freeze-Thaw Durability of Cast Stone” explains why the tamping process contributes to durability problems:
“The dry-tamp process, using a low slump mix, makes it difficult to achieve good compaction at the molded finish and corners of the panel. Poor compaction at the molded finish allows the surface to absorb more water than the body material of the cast stone. Due to the greater water absorption and the poor compaction of the molded finish, its resistance to a harsh freezing and thawing environment is reduced and and the molded finish becomes susceptible to deterioration”.[4]
The fact that physically tamping the material is not always consistent can lead to visible crazing patterns as Matthew Farmer explains in his “What is Cast Stone” article when he says:
“The process of tamping also contributes to crazing by creating centers of high compaction (where the tamper impacted the material) surrounded by rings of lower compaction. Crazing appears to be more concentrated in the areas of lower compaction or density”.[1]
This effect is clearly shown in the photo below where the crazing pattern matches the spiral pattern that the tamper used when making the base panel. Tamping in many multiple layers also increases the likelyhood of having lift lines as can be seen in the column shaft photo below.
Some other limitations of Vibrant Dry Tamped Cast Stone are as follows:
- L-shape and U-shape pieces are extremely hard to tamp, if not impossible.
- Vibrant dry tamped panels generally can not be made as large as wet-cast panels.
- Structural inserts and weld plates that are easily cast into the backs of wet-cast units are difficult to put into the backs of tamped pieces.
- Different tamping patterns by different employees (for example, left to right tamping versus top to bottom or spiral patterns) may cause subtle variations in the finished piece.
Strength – Different tests yield different results.
Dry tamp cast stone and wet cast products are typically tested using different methods which can cause confusion because they yield different results. Dry tamped cast stone is typically specified to be 6500 psi at 28 days when tested in accordance with ASTM C 1194 (done by breaking cubes) while wet-cast cast stone is typically specified to be 5000 psi at 28 days when tested in accordance with ASTM C39-86 (done by breaking cylinders). 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 for dry tamped cast stone is equivalent to approximately 5000 psi for 6 inch cylinders for wet-cast products. Don’t make the mistake of thinking that dry-tamped cast stone has a higher compressive strength than wet-cast products because of this difference in testing procedures.
Steps Plus only uses the Wet-Cast Method
Steps Plus uses only the Wet-Cast procedure for manufacturing precast and cast stone. In this method, measurable slump concrete is vibrated into a mold until is is densely compacted. The product is typically removed from the mold the following day. The mix contains enough water for complete hydration of the cement and the mold walls prevent the moisture from evaporating during the concrete’s initial set. Units are then cured with sufficient protection from moisture evaporation to achieve the specified design properties. Since wet-cast products already have enough moisture in them for complete hydration of the cement, the complicated wet curing procedure absolutely vital for VDT cast stone is not necessary for wet cast products. Our wet-casting process avoids all of the other drawbacks of the Vibrant Dry Tamped Cast Stone listed above as well.
References
- Farmer, Matthew C., “What is Cast Stone?”, July/August 2008 Concrete Repair Bulletin, pages 18-25.
- U.S. General Services Administration Historic Preservation Technical Procedures, ‘04720-01 CAST STONE: CHARACTERISTICS, USES AND PROBLEMS’, downloaded on 06-23-10 from http://w3.gsa.gov
- Pieper, R., “The Maintenance, Repair, and Replacement of Historic Cast Stone,” Preservation Brief No. 42, Technical Preservation Services, National Park Service, U.S. Department of the Interior, Washington, DC, 2001.
- Kaskel, B., Wonneberger, B., and Bortz, S., “Freeze-Thaw Durability of Cast Stone”, November 1991, Concrete International Magazine, pages 32-37.
