Concrete is one of the most reliable building materials in the world. It’s used to support some of the largest and heaviest structures built in the last 200 years. It’s cheap to produce and easy to manipulate so that it conforms to the needs of the builder and project. Overall, it’s an outstanding material.
However, concrete has a bit of a problem with the way it handles pressure. While it is amazing at supporting a lot of weight, it falls short when it comes to the distribution of tension. This inability to handle pressure causes a lot of problems for homeowners because the lack of elasticity in concrete is what accelerates settling and the destruction of concrete around a home.
Understanding why concrete does not stretch can help you learn more about a possible source of your concrete problems and avoid more future settling. Let’s take a look at why concrete cannot stretch.
Why Concrete Breaks Apart So Easily
The reason concrete works so well as a construction material is because it has great compression properties. Compression refers to the reduction of volume in an object due to pressure. It basically describes what happens when an object is squished together through force. Materials with very poor compression strength lose a lot of volume and become smaller the more force is placed in them.
Concrete is not like that at all. Because it has such great compression strength, it can withstand a great deal of force without losing too much of its volume. Some of the tallest buildings in the world are built on concrete slabs without there being any signs of extreme volume reduction. Compare it to rubber, which is easily compressible. If a 10-inch rubber slab was used as a foundation, with enough weight, it would compress and lose its volume by a few inches.
However, this compression strength comes at a cost. Solid objects tend to have very poor tensile strength. Tensile strength refers to a material’s resistance to breaking under pressure. On a concrete slab that is only held up on its edges, if you place a great deal of pressure on the center of the slab, it will compress where the pressure is being exerted (where it bows inward), but it will crack where the pressure is released on the other side (where it bows outward).
The reason solid objects like concrete have such poor tensile strength is due to the flexibility of their particles. Because the particles concrete is composed of are solid and unmovable, when pressure is applied and it ripples through the material in a burst of energy, the particles have no way to move aside. The pressure rips through the concrete and creates a crack as the pressure is released across the object.
For comparison, take a material-like rubber. Its particles can move about much easier. When pressure is applied to rubber, the particles make way for the tension that travels through the material. Because rubber stretches, the pressure disperses along the surface of the material until its tensile limit is finally reached and the rubber breaks.
Foundation Settling Can Be Caused by Concrete Not Stretching
Foundation settling occurs due to displacement that causes a loss in the amount of soil that makes up a foundation. With expansive soils, this happens when the soil takes in moisture, expands, then shrinks once it’s dry. Grand Junction is located to the west of the state, in an area known as the Colorado Plateau. The soil in this area is classified as Aridisols, which are characterized by their coarse, dry, sandy profile. Sandy soils do not expand because they drain the water instead of retaining it.
What causes settling with sandy soils like Aridisols is displacement through erosion. This is when water slowly washes away the soil. There’s also erosion through movement, like earthquakes. Colorado is in a relatively safe zone and doesn’t experience as many earthquakes as other parts of the country.
That said, it is close to seismic hazard zones located to the west. The strong earthquakes in those areas do affect Colorado, even if only slightly. The constant earth tremors shake up the loose soil, causing gaps to form underneath a foundation.
Most slab foundations are made to be four inches thick. When the slab is that thin, there’s usually no rebar added to make the concrete more flexible. Rebar is added to create what’s known as “reinforced concrete”. The steel rods, which have greater tensile strength than the slab, can absorb and evenly spread out the pressure throughout the slab so it has a tougher time breaking apart.
Unfortunately, most slab foundations are not reinforced, so when they settle, they break easily. Without a solid foundation, the slab begins settling and the weight of the house begins shifting to one side. Unable to handle the uneven pressure, the slab cracks and breaks. If it were reinforced with rebar, it would take longer to crack, and the settling wouldn’t cause such severe problems from the get-go.
Slab Settling Can Result from Concrete’s Inability to Stretch
Many driveways, patios, garages, and pool decks are made of slabs that are not reinforced. Because these structures only rely on the soil beneath them, once they give way, it’s very easy for them to break.
The slabs in these areas must support a lot of weight. Driveways and garages support the weight of cars and patios often have large, heavy furniture. Even just the weight of people walking across the slabs puts pressure on it, slowly weakening the concrete until it breaks apart. If concrete were able to stretch more and its tensile limit was greater, it would ease into the gap created by soil displacement every time it had to support weight but bounce right back into place once the weight was alleviated.
Concrete Stretching and the Freeze-Thaw Effect
Concrete slabs are more likely to break when settling in places where the temperature falls below zero. This is due to the freeze-thaw effect. The freeze-thaw effect occurs when water freezes and expands, displacing the materials around it. When it comes to concrete settling, the soil is usually displaced by ice. Once the ice melts, the gaps it leaves behind cause settling.
When it comes to concrete’s tensile limits, the freeze-thaw effect makes things a lot worse. When concrete is saturated with water and the water freezes, the ice displaces the concrete’s particles on a micro-scale. The tears it creates are small, but it still makes an impact on the concrete’s health. Because it tears open the concrete, the pressure it must withstand now has more avenues to flow from as it rips through the slab from the point of compression.
Grand Junction does not see a lot of freeze-thaw because the temperature only falls below freezing about three times a year. Still, being affected by frost damage a few times a year still weakens the slab significantly. If you allow your concrete to be exposed to snow, its tensile strength will decrease, and breaking will be more likely after settling.
What Can Be Done About Cracked Slabs?
Cracked concrete slabs are a safety hazard and you’ll want to get them repaired right away. Walking on an uneven and broken path puts you and your family at risk of tripping and getting injured. You might be wondering what can be done if you have cracked concrete. Can it be repaired, or does it need to be replaced?
When it comes to slabs on driveways, patios, garages, and pool decks, settled concrete slabs can be repaired even if they are cracked. Concrete lifting solutions like PolyRenewal™ can set the broken slab back into place. It involves drilling holes into the slab and injecting polyurethane foam. The foam fills the gap until it raises the slab back into place. The polyurethane foam creates a firm foundation that discourages cracks due to pressure.
The foam is firm but flexible enough that when the tension from the weight reaches the foam, instead of bursting out of the concrete and cracking it, the foam absorbs this tension. Indirectly, the polyurethane foam increases the slab’s tensile limits. Polyurethane foam can be used if the slab is broken, but only if a large section has been torn apart. The slab would have to be replaced if there were too many small pieces that have been broken off.
Do I Have to Replace My Slab Foundation?
When experiencing foundation issues, many homeowners believe that the only solution would be to break the existing foundation and replace it with a new one. This involves raising and supporting the house, destroying the existing foundation, building a new one, and setting the house back into place. Many slab foundations are not reinforced with rebar, so when thinking of replacing their foundation, homeowners believe the solution is to build a reinforced foundation slab. This is can be an unnecessary, time-consuming, and expensive solution.
This replacement would also be short-lived in some cases. Reinforced concrete may have greater tensile strength than non-reinforced concrete, but that doesn’t mean that it can never break. Settling will occur again and once it gets severe enough, the new foundation will crack, and you’ll continue to have the same problems.
For slab foundations, these can be repaired with polyurethane foam injections as well. However, this can only be done depending on the extent of the settling. Often, it’s much better to use slab piers. Slab piers are galvanized steel rods that are driven into the ground and used to lift and support settled concrete slabs.
They help slabs disperse the pressure from large loads by absorbing the tension. The piers are driven into the ground until they reach dense, load-bearing soil. Anytime the concrete experiences pressure, the piers absorb it and release it onto the load-bearing soil.
This solution is cheaper and more effective than replacing your entire foundation. The entire job can be done in a day, depending on the scope of the project. It ensures your foundation does not suffer any more damage from pressure again.
Concrete Can Not Stretch
In short: the answer depends on whether or not the concrete has already been poured and cured.
- During the Production Process
If a new house that you’ll move into is being built, there is one thing you can do to strengthen the concrete slabs on the property. Adding steel rods, or rebar, is the best way to increase the tensile limits of concrete. Rebar has excellent tensile distribution and can bend a lot before snapping in half. By adding rebar through the concrete, whenever the slab has to withstand pressure, the energy gets released and distributed through the slab, but a lot of the tensile load is handled by the rebar itself. With rebar, concrete slabs are less likely to break from tensile pressure.
There is no material that can be mixed into the concrete in order to reinforce it. Concrete needs to be firm enough to withstand a large amount of weight. If you add anything that makes it more elastic, you run the risk of weakening its compression properties. Making structurally sound concrete is all about mixing the right amount of the right materials. Experimenting with different materials might actually be more detrimental to the health of the slab.
- After the Curing Process
Once concrete has been poured and cured, there’s nothing you can do to strengthen it. Concrete slabs strengthen on their own over time. For a small slab, it can take about a month to fully finish strengthening while large slabs can take up to a year. You may have heard that pouring water on the slab will strengthen it, but this is only true if the slab is in the curing process, not after.
If you have a slab that has cracked and settled due to pressure, there is a way to fix it and reinforce it so that it doesn’t break again. It can be lifted using polyurethane foam, which is the best material to lift settled concrete slabs. The foam creates an even foundation for the slab to rest on, so that even when pressure is applied, it doesn’t break.
There are two main methods of concrete lifting used today: mudjacking and polyurethane foam injections. Polyurethane foam has many advantages over mudjacking. It lasts longer and the material not only discourages further settling, but it also discourages tensile pressure.
Mudjacking involves pumping a cement slurry underneath a settled concrete slab until it lifts it up. The cement then hardens and supports the slab after a few hours. With mudjacking, the cement slurry has a rough texture that has the potential to create complications for the slab. The slurry is so rough and heavy that it’s not able to evenly distribute itself underneath the concrete and many gaps are left. Because there are gaps, it creates an uneven foundation that affects the way the slab reacts to tension. With mudjacking, there’s always a risk that the slab will be unable to withstand pressure and cracks again.
Cement is also very heavy, so it compresses the soil below. Soil compression is responsible for concrete settling, so you definitely don’t want to set the soil under a material that will only make things worse. Like the slab above it, the cement slurry is also permeable, so water can get through it, reach the soil, and cause more settling.
- Polyurethane Foam
Polyurethane foam injections are similar to mudjacking, except instead of a cement slurry, polyurethane foam is used to fill the gap under the slab. It’s light enough to not press down on the soil underneath it, effectively reducing soil compression. It’s also impermeable, meaning that no moisture will make its way through to the soil. Polyurethane foam is initially very soft as it’s distributed under the slab. It expands and is able to fill the gap under the concrete completely.
Because it leaves no gaps, when the slab experiences pressure, it is less likely to crack because the tension is distributed evenly. This is because the slab is being supported by an even foundation, the foam, leaving no room for the pressure to overwhelm a specific part of the slab.
It is impossible to prevent concrete deterioration. All concrete experiences some sort of damage over time, especially in areas where it is frequently exposed to water, like a foundation, driveway, or pool deck. As a matter of fact, even when it’s only a few days old, concrete still develops micro-tears because of the material it’s made from.
Given the fact that soil shrinkage and displacement is also inevitable, you shouldn’t be surprised if your concrete slab settles and breaks. Is there any way to avoid this?
When soil shrinks and compacts, the particles become displaced and loosen up. As the soil disperses, it’s no longer able to support the slab. When the slab is put under pressure, more than it can withstand, it breaks apart and settles against the displaced soil.
Soil displacement is inevitable, but there are things you can do to discourage it. Grand Junction’s soil is incredibly dry, but it’s coarse and sandy. This means that soil expansion and shrinkage is not common, but displacement due to erosion is likely. Having proper yard drainage and making sure your soil stays as dry as possible can help deter this natural process.
Making sure your concrete is structurally sound is the best way to avoid cracks and breakage. If there is soil erosion, you can prevent your slab from breaking by limiting exposure to weakening substances. Protect your concrete from water, snow, and salts to make sure it doesn’t slowly deteriorate.
Driveways, patios, and pool decks should be covered to limit climate exposure. You should also avoid placing heavy objects on the concrete, and if you must, make sure the weight is distributed as evenly as possible. This means that cars should be parked in the middle of the driveway and no heavy furniture should be placed on one side of the patio.
Call Foundation Repair of Western Colorado for Your Concrete Lifting Needs
If the concrete on your property is settling and you don’t know what to do, look no further. Foundation Repair of Western Colorado is here to bring you the most innovative concrete lifting and foundation repair methods. Since 2005, we’ve been lending our expertise to the homeowners of Grand Junction, CO, so they can live in a structurally sound home. Contact us today to schedule a free inspection, and we’ll send one of our concrete experts out to your home for a no-cost, no-obligation evaluation of your property.