This article discusses uniformed surface imperfections and how to prevent them.
An article excerpt from the September 2019 issue of Precast Inc., by Eric Carleton, P.E., NPCA's director of codes and standards.
We always aim for perfection when producing precast concrete products, but sometimes it is just out of reach. This story continues our pursuit of eliminating the causes of imperfections on the surfaces of precast concrete.
New product lines equal more unformed surfaces
Many precast products cast vertically have a limited amount of unformed surfaces. As the precast concrete industry expands to include more plant-cast flatwork – such as precast bridge deck panels, paving slabs and traditional markets such as precast steps and docks – increased knowledge of best practices to eliminate unformed surface imperfections becomes increasingly important.
For the precast producer, unformed surface issues can manifest themselves as shrinkage cracks, crazing, blistering, scaling and delamination, dusting, discoloration or low spots. Technical literature and recommended industry practices contain a common thread of recommendations to reduce the likelihood of these imperfections occurring.
Those recommendations include starting with a proper mix for both the application and environment, proper curing and careful timing when finishing. Though each item is critically important to produce the best precast product, the emphasis for this article will be placed on curing and timing.
For precasters familiar with only pouring and consolidating concrete within closed formwork, the proper finishing techniques needed for unformed surfaces to provide the strongest, most durable concrete with the desired surface texture is not necessarily intuitive.
Less is more
After casting the concrete, screeding the concrete to level, and possibly an initial float to eliminate low and high spots, it is time to wait. The concrete will begin its bleeding time – the mix will have the heavier constituents settle and consolidate on their own, squeezing excess mix water up and eventually out to the surface. It is most important to discontinue finishing operations until the concrete mix has begun its initial set and bleed water has stopped appearing. In good conditions, this can be identified when the initial sheen of the water has dissipated on the concrete. However, the atmospheric conditions around the concrete surface will dictate how quickly the surface bleed water will dissipate (or evaporate), if at all.
A high ambient temperature, direct sunlight, arid or low humidity and/or windy conditions can lead to surface water evaporating faster than new bleed water can be brought to the surface. That leads to drying of the surface concrete and creates a host of potential surface issues later for the hardened concrete. This premature drying of the concrete can also give the false impression that the bleeding has stopped.
Manufacturing products within a controlled environment is a big benefit for this critical time for the concrete mix. It is important to base the wait time on more than simply bleed water appearance. A true "rule of thumb" is if a gloved thumb pressing onto the concrete surface leaves a 1/4-inch indent or less, then the concrete is ready for final finishing. Similar indent rules have been used in the ready-mix industry for a foot or boot print with a 1/4-inch indent.
This wait time is perfect for setting up and pouring the next form, cleaning the concrete tools, having a safety conversation, etc. However, each precaster will need to determine the optimum wait time based on their mix and production processes and revise as needed when things change. It is a tricky optimization requiring careful thought, record keeping and training, as starting final finishing too early can create serious surface issues and starting too late will mean working with a stiffened concrete, which may be difficult to finish.
More is more
Curing is another important process within the early life cycle of concrete and cannot be overemphasized. It should begin immediately after the final finishing is completed. Proper curing of precast products with unformed concrete surfaces is like that of formed surfaces with respect to temperature conditions. However, large exposed surface areas need special considerations to ensure those areas remain in a moist condition by maintaining a high-humidity environment. At this point, following the initial set and finishing, any added water to the surface is beneficial. Therefore, direct gentle water spray, water-soaked coverings, or liquid membrane curing compounds in accordance with ASTM C309, "Standard Specification for Liquid Membrane-Forming Compounds for Curing Concrete," applied topically on all the exposed surfaces are all good methods to maintain moist conditions. Additional best practices and information on concrete curing can be found within ACI 308R-16 "Guide to External Curing of Concrete."
Common surface imperfections: Causes and solutions
For the full article, please visit the Precast.org site.
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