On-Site Magazine

Treating cracks in concrete

By JENNIFER MIZER   

Concrete Construction

There are a variety of current methods for treating and healing narrow cracks.

Pretreating cracks. (Photo courtesy of Euclid Chemical)

Today’s concrete technology provides several ways to limit the size of cracks, and to prevent them. 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 microfibres, proper curing, using evaporation retarders, and performing joint cutting operations as soon as possible, concrete can still maintain its tendency to crack.

Narrow cracks in concrete slabs, typically in the 0.01 to 0.06-inch width range, are especially troubling when they occur 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 life of a concrete deck, requiring expensive repairs or full replacement of in the future. There are several common methods for treating cracks, however.

 

Advertisement
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.

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.

Since protective coatings are applied to the concrete surface, exposure to weather, abrasion or other damage will eventually lead to the deterioration of these materials. As such, 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

Penetrating concrete sealers repel water. (Photo courtesy of Euclid Chemical)

Penetrating water repellent sealers containing silane, siloxane or a silane/siloxane blend 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, reducing the absorption of water.

These sealers will not change the appearance or colour of the concrete, and do not leave behind a visible surface film. When properly applied, silane and siloxane sealers can last for up to 10 years before resealing.

 

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.

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.

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, which 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 though.

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.

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.

 

MODULUS OF ELASTICITY

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 found on bridge and parking decks are 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.

 

HEALER/SEALER APPLICATION

Spreading material over deck slab. (Photo courtesy of Euclid Chemical)

Application of a healer/sealer 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 thoroughly cleaned of 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 Guideline 310.2R from the International Concrete Repair Institute.

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, with a required tensile pull-off strength commonly specified to be at least 250 psi.

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 the 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 or by ponding the material over cracks, 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.

If necessary, allow the epoxy to penetrate the surface, re-applying to cracks and porous areas as needed. Excess epoxy can be removed with a squeegee before it becomes tacky. Broadcast clean, oven-dried silica sand (recommended 16/30 or 20/40 mesh) into the wet epoxy 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.

Despite current concrete technology’s aim to avoid cracks in concrete, and to limit the size of any such cracks, they still happen. 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.

There are methods for treating and healing narrow cracks, however including film-forming coatings, penetrating water repellent sealers, and chemically cured polymeric resin.

 

Jennifer Mizer is the director of marketing services at Euclid Chemical, a manufacturer of specialty concrete and masonry construction solutions that includes admixtures, fibre reinforcement, concrete repair products, flooring materials and decorative concrete systems.

Advertisement

Stories continue below