Questions from the Field

Questions from the Field is a selection of questions NPCA Technical Services engineers received from calls, emails and comments on blog posts or magazine articles on precast.org.

If you have a technical question, contact us by calling (800) 366-7731 or visit precast.org/technical-services.

Chris writes:

Is 3D printing a technology that is used in the precast concrete industry?

NPCA Technical Services engineers answered:

Some plants use 3D printing to print forms or formliners. In terms of using 3D printing in manufacturing precast concrete, we are not aware of any NPCA member plants using the technology. However, the American Concrete Institute has formed a committee on 3D printing. ACI Committee 564, “3-D Printing with Cementitious Materials,” met for the first time at ACI Fall Convention 2018 and plans to bring together all stakeholders to discuss the many challenges and opportunities with this new technology. ACI Fall Convention 2020 plans to have a session titled, “Current Global Advances in 3D Printing of Concrete.” There are also some conferences focused solely on this technology. The Transportation Research Board is organizing and sponsoring the First International Conference on 3D Printing and Transportation on Nov. 20-21, 2019, in Washington, D.C.

So far, the majority of projects and examples using 3D printed concrete have been cast-in-place applications and not precast concrete. The NPCA Engineering and Technology Committee is monitoring this technology to determine its potential applications in the precast industry.

Image by Christian Reil from Pixabay

David writes:

Is there a cross-section and size recommendation for the butyl sealant used with butt joints? I have seen several cross-section shapes including square, rectangular, flat, and round, but none were specific to butt joints.

NPCA Technical Services engineers answered:

There is no specific size calculation for the correct geometry of butyl rope sealants. Many butyl rope manufacturers’ installation recommendations state the sealant should be compressed at least 50% from the initial height. Therefore, it is important to know the assumed maximum distance between the interface gap between the two joining concrete surfaces. If the space will be 1/2 inch, then the sealant rope should be at least 1 inch in height. It is also important to note that typically rope sealant nomenclature corresponds to the equivalent cross-sectional area of a circular section. For example, a 1-inch rope sealant may not be 1 inch high when measured, but equivalent to the area of a 1-inch circular section which is 0.785 in². Therefore, a butyl sealant that is measured to be 1 inch tall may actually be called some value greater than 1 inch. Be aware of the sealant’s actual height/thickness and clearly specify within the specified provisions the furnished measured height of the butyl sealant.

Within rubber gasket design there is typically both a minimum and maximum allowable deformation. The maximum limit is established to ensure the gasket strain limits won’t be exceeded, which would lead to splitting, rubber deterioration, etc. However, this is not the case with a butyl rope sealant. When highly compressed, preformed sealants will typically have the ability to move laterally, a slightly larger sealant size should not be a problem.

NPCA file photo