By Eric Carleton, P.E.
Editor’s Note: Last issue, Precast Inc. went in-depth on formed surface imperfections. This article will discuss unformed surface imperfections and how to prevent them.
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 bootprint 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
Shrinkage cracks can form either before or after final hardening. If they occur before final hardening, they are caused by water evaporation exceeding bleed water replenishment. Blocking wind or direct sun or providing indirect misting can help prevent these cracks. You can even use evaporation retardants. Shrinkage cracks forming after the final hardening are often caused by concrete drying and small volumetric changes creating tensile stresses to a point of cracking. You can solve this problem by modifying cementitious materials, aggregate type and gradations and admixture use. Shrinkage-reducing admixtures may also be used.
Crazing is defined as very fine, shallow interconnected cracks (sometimes called map cracking). There are multiple causes of crazing, including high evaporation and drying rate during curing, excessive floating, finishing while concrete is bleeding or bleed water remains on the surface, adding water to the surface during finishing to wet the concrete, and adding cement to the surface to dry the concrete. The best way to prevent crazing is to properly float and finish at the proper times.
Small bumps appearing on the unformed surface of concrete during finishing is known as blistering. Blistering is caused by excess air or bleed water trapped under a concrete surface that was sealed by excessive floating or troweling, or improper use of a steel trowel. Blistering also occurs when the surface dries and out and crusts, which seals the surface. To prevent blistering, you should reduce or eliminate surface drying conditions and improve bull float and troweling processes to reduce potential surface sealing.
Delamination and scaling
Delamination and scaling appear as thin layers of cement paste on the product surface that flake away from the concrete mass. The cause of these problems can be attributed to sealed concrete surfaces not allowing the release of pore water pressure – delamination early with bleed water, scaling later in service with saturated concrete and expanding frozen water under the surface. Again, properly floating and finishing at the right time is the best way to prevent these issues.
Dusting is quite simply the presence of a powdery material on the surface of concrete. It can be caused by excessive floating creating too much surface paste as well as finishing while the concrete is still bleeding. It can also be caused by carbonation when heaters blow carbon dioxide onto the concrete surface disrupting the hydration process, when water is applied at finishing or if there is rainwater exposure during finishing. By implementing correct finishing and curing processes in your plant, you can easily prevent dusting.
Discoloration, light or dark patches and other color inconsistencies, can be caused by delayed or early finishing, changes in finishing practices, the presence of calcium chloride in the mix and even if plastic sheeting comes in contact with the concrete during curing. Keep mix designs the same for all the respective castings and ensure the finish wait time is based on concrete set time and not just bleed water to prevent discoloration.
Low spots in your concrete can be caused by an inadequate amount of concrete placed in the form or even poor form setup. This is a problem that can easily be prevented by checking your form setups for grade and level and verifying your concrete pour for complete form filling. You should always check and verify that there are no open areas under the screed during strike off. It could even be as simple as using a wider float tool or increasing plant area lighting to better view concrete surfaces for smooth level surfaces free of variability.
Concrete has shown through thousands of years of use to be a perfect building material. However, it takes knowledge, skill and care to mix, cast, cure and deliver the best precast concrete product free of surface imperfections. Regardless of the product application, a precast concrete company’s name and reputation is based on the appearance of each product on a truck or job site for public view. Though absolute perfection may not be attainable, excellence is within all our grasps.
Eric Carleton, P.E., is NPCA’s director of codes and standards. He is an ASTM Award of Merit recipient and currently serves as vice-chairman of ASTM C13, Concrete Pipe.