Today’s concrete technology provides several ways to help limit the size and avoidance of cracks in concrete. Despite available preventative measures, such as a sound subgrade, designs for low-shrinkage concrete that incorporate shrinkage-reducing or shrinkage-compensating admixtures, using concrete with lower water-to-cementitious ratios, adding macro or microfibers, proper curing, using evaporation retarders and performing joint cutting operations as soon as possible, concrete can still maintain its tendency to crack.
Narrow cracks, typically in the 0.01-0.06 inch (10-60 mils) width range, in concrete slabs is an especially troubling concern when it occurs on bridge and parking decks, elevated floor slabs and similar high-use horizontal slab surfaces. Cracking can allow for the penetration of water, sulfates, chlorides and other harmful agents, accelerating surface spalling and the corrosion of steel reinforcement. This cycle of deterioration can shorten the service life of a concrete deck and require expensive repairs or full replacement of the deck in the future.
Common Methods for Treating Cracks
Protective Coatings
One way to treat narrow cracks in concrete is to use film-forming acrylic waterproofing coatings. Many of these coatings are quite flexible and have crack bridging properties. There are also additional benefits to using film-forming technologies to protect concrete in high traffic areas. Traffic deck coatings based on epoxy or urethane polymers are commonly installed on bridge decks, parking decks, factory floors, and loading docks. Many of these systems incorporate an aggregate broadcast to provide skid resistance and have the added benefit of waterproofing the concrete. However, protective coatings are applied to the concrete surface, which means that exposure to weather, abrasion or other damage will eventually lead to the deterioration of these materials. For this reason, many acrylic coatings, for example are primarily used on vertical surfaces and horizontal decks for decorative purposes or exposure to lighter pedestrian traffic.
Water Repellents
Another method of treating narrow concrete cracks is to use penetrating water repellent sealers containing silane, siloxane or a silane/siloxane blend. These sealers penetrate the crack and coat its sides to provide the substrate with a water-repellent barrier, but do not fill the cracks entirely or fully seal the concrete surface. Penetrating silane or siloxane sealers soak into the surface of the concrete where they chemically react to form a hydrophobic barrier within the pores that causes water and other liquids to bead off the surface (Fig. 1). By reducing the absorption of water, the surface becomes more resistant to cracking, spalling, pitting, freeze-thaw damage, mold and mildew growth, and efflorescence formation. These penetrating sealers will not change the appearance or color of the concrete and do not leave behind a visible surface film. When properly applied, silane and siloxane sealers can last for up to 7-10 years before the need to reseal.
Healer/Sealers
Another treatment for narrow cracks is to use a thin, chemically curing polymeric resin to fill the cracks. Penetrating by gravity alone, the resin fills the crack and seals out water, salts and other damaging elements. This method of crack repair is intended to seal cracks that are “static” or “non-moving”, such as shrinkage and settlement cracks that have stabilized. This method can also be used to protect the entire concrete deck by applying a flood coat of the polymeric resin on the entire surface. This essentially seals the deck while sealing or “healing” the cracks. The term healer/sealer is often used to describe the polymeric resin applied in this process. Concrete healer/sealers have typically been based on chemically cured methyl methacrylate, high molecular weight methacrylate, urethane or epoxy chemistry.
Healer/Sealer Material Properties
Although concrete healer/sealers have been around for decades, their ability to fully fill, seal and heal cracks often yielded mixed results. When healer/sealers were first developed, it was easy to look at the viscosity of the resin as the reason for these mixed outcomes. If the viscosity was too low, it could flow into the crack and out the bottom of the slab. If the viscosity was too high, it didn’t penetrate far enough to fill the crack. When a new generation of healer/sealers were developed, other factors were considered, including the method used to achieve the desired viscosity, modulus of elasticity and surface tension.
Viscosity Effects
Through laboratory testing and field trials, the best results were found when the crack healer/sealer material was found to be low viscosity. In formulating chemically curing polymeric sealers and coatings, the simplest way to lower the viscosity of a product is to increase solvent content. However, increased solvent typically means a higher volatile organic compound (VOC) content, increased hazards in mixing and handling, and an impact on physical properties.
Formulators can adjust the molecular weight and molecular weight distribution of the healer/sealer and this can have a profound effect on the viscosity of the finished product. Generally, the higher the molecular weight, the higher the viscosity. This is often not desirable for a crack healer/sealer.
Higher molecular weight also results in higher strength and durability of the final product. To solve this conundrum, the molecular weight distribution of the healer/sealer is adjusted, resulting in lower viscosity, without affecting the strength properties of the material. Adjusting the molecular weight distribution of a healer/sealer polymer can be compared to the gradation of aggregate in concrete and visualized by imagining a funnel with a large opening. If we pour just one size of aggregate into the funnel, it will flow through readily, but if we put in a full gradation of aggregate, it will clog the funnel and not flow. Similarly, a wide distribution of different sized polymer chains results in a healer/sealer with higher viscosity and therefore less ability to flow and penetrate cracks. Newer generation healer/sealers are formulated with a narrower distribution of polymer sizes, which produces a more flowable material.
Impact of Modulus of Elasticity and Surface Tension
Modulus of elasticity is a measure of stiffness, with higher-modulus materials exhibiting less deformation under load compared to low-modulus materials. A low-modulus crack healer/sealer can provide better resistance to mechanical or thermal movement of the concrete deck. Since many treated cracks are found on bridge and parking decks, often subjected to this type of movement, a low modulus of elasticity product is optimal in many instances, as it will withstand a certain amount of movement and help prevent re-cracking.
In addition, low surface tension is an important factor in allowing the healer/sealer to penetrate the crack. Healer/sealer products are formulated using specially designed agents to reduce surface tension, which allows the resin to penetrate the concrete cracks more readily. A liquid with high surface tension contains molecules that are more attracted to each other than they are to the surface upon which they are applied. Also, the molecules at the surface of a high surface tension liquid have no molecules attracting them from above, so these surface molecules can only be attracted down and in. This attraction to itself coupled with strong surface tension causes the liquid to bead up instead of spreading out on the surface. Concrete healer/sealer formulators incorporate surface tension reducing additives called surfactants to ensure the healer/sealer spreads out onto the concrete surface instead of beading up and resisting flow and penetration.
Healer/Sealer Application
The healer/sealer application begins with proper surface preparation. The concrete surface must be structurally sound and free of grease, oil, curing compounds, soil, dust and other contaminants. New concrete and masonry must be at least 28 days old. Surface laitance must be removed. Concrete surfaces must be roughened and made absorptive, preferably by mechanical means, and then thoroughly cleaned of all dust and debris.
If the surface was prepared by chemical means (acid etching), a water/baking soda or water/ammonia mixture, followed by a clean water rinse, must be used for cleaning to neutralize the substrate. The Concrete Surface Profile (CSP) should be CSP 2-5 in accordance with ICRI 310.2R[1], published by the International Concrete Repair Institute (ICRI). Following surface preparation, the strength of the surface can be tested if quantitative results are required by project specifications. A tensile pull-off tester may be used in accordance with ASTM C1583[2], with a required tensile pull-off strength commonly specified to be at least 250 psi (1.7 MP).
The application of the materials includes pretreating large cracks if necessary, flood coating with the low-viscosity, low-modulus epoxy, distributing the epoxy onto the substrate, removing excess epoxy, broadcasting fine sand onto the wet epoxy, removing the excess sand when resin has cured, and opening the deck to traffic.
After properly mixing the material, cracks may be pre-treated by gravity feeding the healer/sealer by hand directly on top of 
the crack (Fig. 2) or by ponding the material over cracks (Fig. 3), permitting it to sink in and seal the crack onto the properly prepared surface in a wave form and spread uniformly with a squeegee or a short nap roller to fill voids, cracks, and porous areas (Fig. 4).
Allow epoxy to penetrate the surface, re-applying to cracks and porous areas if necessary. Before the epoxy becomes tacky, excess epoxy that has not penetrated the surface can be removed with a squeegee. 
Broadcast clean, oven-dried silica sand (recommended 16/30 or 20/40 mesh) into the wet epoxy (Fig. 5) to provide a skid-resistant surface, or where subsequent toppings or coatings will be applied. Before opening to traffic, and when the healer/sealer has cured, remove any loose aggregate (Fig. 6).
Summary
Despite current concrete technology to limit the size of or avoid cracks in concrete, cracking in concrete slabs is especially troubling when it occurs in bridges and parking decks, elevated floor slabs and similar high-use horizontal slab surfaces. Current methods for treating and healing narrow cracks include film-forming coatings; penetrating water repellent sealers; or filling the crack with a thin, chemically curing polymeric resin, engineered to ensure ideal viscosity, modulus of elasticity, and surface tension, for effectively treating narrow cracks in concrete.
References
- ICRI 310.2R, Selecting and Specifying Concrete Surface Preparation for Sealers, Coatings, Polymer Overlays, and Concrete Repair, International Concrete Repair Institute, St. Paul, MN, 2013.
- ASTM C1583/C1583M-20, Standard Test Method for Tensile Strength of Concrete Surfaces and the Bond Strength or Tensile Strength of Concrete Repair and Overlay Materials by Direct Tension (Pull-off Method), ASTM International, West Conshohocken, PA, 2020.
About the Author
Jennifer Crisman is the Director of Marketing Services at Euclid Chemical, a leading manufacturer of specialty concrete and masonry construction solutions. A 20-plus-year industry veteran, Crisman manages the marketing communications activities for Euclid’s expansive line of admixtures, fiber reinforcement, concrete repair products, flooring materials and decorative concrete systems.