- 1 The advantages of epoxies
- 2 Applying epoxy resin
- 3 Type of cracks
- 4 The final word
It would be difficult, if not otherwise impossible, to imagine a world without concrete. From homes to highways, either poured or prestressed, Australia alone uses more than 29 million cubic metres of it annually. To put that into perspective, that’s more than 300 times as much concrete as there is in the ramps, roadbed and pylons of the Harbour Bridge.
For all its inherent strength and durability though, concrete isn’t without weaknesses – not the least of which comes in the simple form of water. The same hydrating process that’s so critical to proper curing and hardening the weeks after a batch of concrete is poured, is also responsible for introducing some of the destructive reagents that ultimately result in cracking, corrosion, and eventually compromise. Even intentionally created control and expansion joints can provide an unintended pathway for water to leach through. Once it begins, it’s practically impossible to stop.
The advantages of epoxies
Because the damage caused by water incursion can potentially begin years before being detected, taking action at the first signs of cracking is an absolute imperative to preventing even further deterioration from occurring. Fortunately, there’s no shortage of concrete epoxy and epoxy mortar gap filling mixtures on the market for making urgent repairs and for taking preventative action.
Admittedly, the idea of using a plastic mixture to repair concrete seems almost counterproductive, but nothing could be further from the truth. Concrete can’t be repaired with more concrete. In fact, not only are concrete aggregates too large to fill most breaks, but even with admixtures, there’s simply no way of knowing whether it’s flowing into every fissure.
It’d be necessary to make the crack even larger just to know that it was being filled. Epoxies are regarded as the best materials for repairing breaks and filing seams primarily because of their flow characteristics. There’s no question about whether they’re going to fill every crevice, but the advantages don’t stop there.
They cure quickly
With an average working time of 30 minutes and curing time of 24 hours, both at 25°C, most epoxies cure quick enough to ensure their application doesn’t cause a disruption.
They have excellent adhesion
Fresh concrete doesn’t adhere well to concrete that’s already cured. But because of its porous nature, the plasticizer used in concrete epoxies permeates and adheres better than freshly poured concrete does on its own.
Compared to concrete’s standard compressive strength of 4,000 – 5,000 psi, the compressive strength of epoxy is around 10,000 psi, creating a bond whose tensile strength is greater than the concrete itself. And unlike concrete mixtures, epoxies don’t shrink.
Concrete mixes shrink as their water content evaporates, and repairing a crack or a break using the exact same medium that’s guaranteed to shrink and pull away from the edges (creating a brand new series of cracks) doesn’t make for a very effective repair. Epoxies remove that variable from the equation altogether.
Applying epoxy resin
Depending on their usage and application method, epoxy resins typically come in three packaging variations:
These low-pressure injection tubes don’t require any mixing, they fit standard sized caulk guns, possess five times the compression strength of concrete, and are the quickest way to fill structural and surface cracks.
Epoxy bag and tub mixtures come in ready-mixed quantities of resins and aggregates to which there’s a liquid hardener added, applied either by pouring or by trowel. Sold in four and 20 litre quantity kits, these mixes can be prepared in less than 10 minutes, and can be used for everything from crack and structure repairs, to piling splicing. Sand or other aggregates can also be added to these mixtures to create epoxy mortar as well as non-skid coatings.
Multi-viscous epoxy gels can be used on vertical and horizontal concrete bodies with injection port systems that essentially flood a crack or break with adhesive material. This is the most intensive application method. And as it’s also used to make blind-side repairs, it also requires the most time for preparation and positive identification of the root cause of degradation to ensure structural integrity hasn’t been compromised.
Type of cracks
With their high tensile strength and high adhesion, epoxies can be used with virtually any type of break that wouldn’t require a complete structural replacement. But that concrete breaks at all tends to surprise most people.
The reality is that even the strongest concrete mixes are extremely porous. 12% -18% of the volume of a cured concrete body isn’t actually solid material: it’s pores that have been left by evaporating water. The irony is, once more water finds a way to permeate its way through those pores (or the tensile or compressive stresses become too great) cracks are going to occur.
As concrete cures – and that 12% -18% of water begins to evaporates – air pockets left due to shrinkage can be significant. Especially with roundly poured bodies and around corners. Cracks are formed as the tension at the edge of those surfaces retreat and those air pockets connect.
Expansion joints are meant to allow concrete a certain amount of flexibility due to temperature changes, but when they no longer have the ability to move and begin to bind against each other, cracking’s inevitable.
Once water gains access to the steel rebar or reinforcement that’s fixed inside a concrete body, rust and degradation are inevitable to include the leaching of other water-borne reagents that begin to slowly dissolve the base components of the concrete.
Concrete’s strong, but not indestructible. An average compressive strength of 5,000 psi is pretty hardy, but the ground under a slab doesn’t need to erode very much for it to crack if something heavy either rolls over it or is dropped on it.
Rapid or inconsistent temperature changes during the curing process can only lead to uncontrolled setting and hardening rates due to uneven water evaporation. The result will be early crack formations and a hugely under-strength body.
In short, there’s always a need to understand the chain of events that precipitate concrete degradation. Understanding the type of degradation that has taken place will ensure the right type of epoxy is used to make the necessary repairs.
The final word
At the end of the day, it’s not always possible to determine precisely why a concrete body has begun cracking. Although poorly mixed and poorly prepared concrete batches can occur, very often it’s just a combination of factors – and plain old water – that lead to degradation.
When it begins to take place though, rest assured that there’s a concrete epoxy mix available that can help address the problem as well as isolate it. If you have a concrete problem, now’s the time to take concrete steps to fix it.