Concrete Repair FAQ

All about concrete repair.

What is it? How it works? Which products?

Common Questions

  • Although there are a couple of reasons for the surface to flake off, the main culprit is low density. A low density surface can be achieved through low quality cement, over finishing, poor finishing, too much water in the mix or adding water to the surface to extend finishing times. A common cause, but not the main one is also freezing and salt use during the curing stage of concrete.

    Once a weak surface is constructed, freeze thaw effects and moisture movement in the surface can create pressure to spall the surface of the concrete.

    Sometimes spalling can occur from expanding reinforcing bar. This flaking is seen more often in larger pieces and to a greater depth. A shallow buried bar or a leaking crack will create spalling at an accelerated rate.

  • The white powder often left on concrete is left through mineral deposits cause by water transmission often through cracks or pores in the concrete. Minerals in the cement of concrete and salts from road deicing are the most common content of this white powder.

    The formation of this white powder will help slow and sometimes prevent water transmission through concrete. The majority of the white powder is often calcium carbonate.

  • Seeing these form on your ceiling or walls indicates a long term unresolved water issue on the concrete. These are non-toxic but indicate problems that should be evaluated and corrected.

  • If you see orange staining or orange colour within the "white powder" or calcium carbonate, this indicates corrsion is occuring on the reinforcing bar.

    There are two common causes for corrsion.

    The common one is cracked concrete allowing water and air to directly contact the steel and create rust.

    The other common cause is carbonation of the concrete. Carbonation occurs when carbon dioxide enters the concrete, reacts with the calcium hydroxide in the concrete to produce calcium carbonate. Carbonation can not be easily avoided and will start as soon as the concrete is poured. The exposed surface will immediately be effected and over time it will seep into the concrete. As the CO2 seeps into the concrete, it changes the alkaline levels in the concrete which can remove the steel's resistance to rusting.

  • Concrete is a stiff and brittle material. In most cases, it does not flex well and easily cracks. Question is, is the cracking bad?

    Most of the time, cracked concrete isn't a bad thing, and more of an eye sore than anything. Concrete must move with temperature and ground movements; however, it does a poor job at handling this and allowances to control it such as cold joints and crack control joints are installed. Unfortunately, concrete doesn't always follow crack control joints.

    What about overhead cracks? They may be of greater concern, however, all suspended systems have to flex and cracking of beams can be designed to happen as the steel may not be fully working until the beam has a micro crack to put the steel in tension.

    Cracking can be subjective and we highly recommend you call us for an evaluation of cause, effect, and possible remediation if needed.

  • Concrete thickness will vary by the use, loading, reinforcing amount, and ground conditions.

    A general rule is, 4" for patios and sidewalks. 4" reinforced minimum for light vehicle loaded slabs. 6" non-reinforced as minimum for light vehicle loads. 6" reinforced for commercial slabs, forklift traveled slabs, light - med. truck slabs.

    Slabs over 6" get fairly subjective to the required thickness based on loading, ground conditions, and amount of reinforcing.

More Important Items

  • This can be dependent on the concrete mix used. Considering a typical concrete we recommend 48 hours for foot traffic, 72 hours from small hard wheeled objects such as toys, a minimum 5 days for car traffic, 7 days for light truck traffic and 14 days for heavy truck traffic.

    The times above can be almost cut in half with a fast set concrete mix.

  • You can under direction from the contractor. If the weather is cool (3 - 12 celcius) and moist generally no additional curing effort is needed. If a cure and seal is applied, it may not be needed. If the weather is dry, the concrete is sun exposed and there is a higher temperature (above 25) then it is probably a good idea.

    Water should be applied after the surface has an initial set. This time frame may be 4 to 12 hours after the concrete is finished. A general test would be if it is strong enough to walk on and it doesn't rub off easy with your finger.

    If water is unavailable, curing sprays and/or plastic poly sheeting will help. Older methods include saturated burlap or cotton sheets, or hay.

    Most benefits will occur after 3 days of wet curing, 5 days is a good average amount, and general maximum results will occur by 7 days of wet curing.

  • Yes. We always recommend sealing unless you are planning on coating it at a later date.

    For a stamped or a finish which you wish to pull the colour out, we recommend a film forming sealer.

    For a broom, non-slip, or trowel finish we recommend a penetrating sealer.

  • Salt does not damage concrete but the effects of salt can.

    Salt can increase freeze thaw cycles in freezing temperatures, create more pore pressures to cause concrete to pop or flake off.

    Flaking concrete from salt use is often due to low quality concrete or salting concrete less than 6 months old. Do not salt fresh concrete.

    Nitrate products or ammonium sulfate products can harm concrete and avoid their use.

    Ice Melt aka Chloride products can also be used and can have less detrimental effects to concrete, but often the harmful effects on steel and vegetation are accellerated. Often the difference between the harm of NaCl for CaCl to concrete are negligiable and the effects of use can be more important.

    Use of salts also attract moisture and hold it in the concrete. This can lead to extended freeze thaw cycles. Ice Melts often hold more moisture than rock salts.

  • Polymer concrete is the use of polymers to replace or enhance the properties of the portland cement. These polymers can be acrylics, epoxies, elastomers, and many others which modifiy properties to increase wear resistance, thermal resistance, chemical resistance, water permeability, bond capacity, and corrosion resistance.

  • The best concrete will be the concrete which suits your use best. The highest specification or one with additives may or may not be the best choice.

    MPa is a base point to select concrete. The higher the MPa the better refined cement or more cement may be added. This increased density allows for greater resistance to carbonation, freeze thaw and weathering. The downside to a higher MPa is sometimes low air void leading to thermal issues.

    For interior work, the MPa can be reduced to save money without causing much harm. Walls or low impact areas can handle 15-20MPa. For exterior applications subject to vehicles we recommend 32Mpa.

    A quality exterior mix we suggest would be 32-35MPa with 5%-7% air content.

    For patchwork, we normally recommend matching the MPa of the existing material. Sometimes using polymer concretes this is rather difficult to do as the specialty mixes are designed to set faster for quick loading. This will often mean a higher MPa. Concrete mixes are very specific to the use and specialty patching products must have an experienced person applying them.

Finishes

  • Broom finished concrete is exactly that, broom finished. It is the most common finish for exterior use and is seen in many locations, primarily sidewalks.

    Pros: Traction non-slip surface, timeless look, wears well, good for multiuse locations, hides curing blemishes, blends dirt marks. Relatively easy to clean.

    Cons: Holds dirt and stains easier. Bland appearance. Can sometimes wear uneven. Cracks easily show.

  • Exposed aggregate is a popular option for homeowners. It provides a decorative finish of low maintenance that hides stains.

    Pros: Easy to maintain, hides dirt and stains, cracks less visible, can be more attractive than the bland broom finish.

    Cons: Harder to clean than a broom finish to remove contaminants. Needs more maintenance with a sealer than other finishes to maintain a glossy rock look and to keep the concrete bonded to the aggregate. Long term wear and improper maintenance can lead to surface rock loss.

  • A trowel finish is a smooth finish for less traction and sometimes a more modern look. Preferable for forklift traffic to limit rubber tire abrasion and dusting. Better for hard tire use and washing.

    Pros: easy to maintain, easy to clean, smooth wearing surface, less dusting, long surface life. Good for a polished finish.

    Cons: can be slippery, easy to stain and show marks and blemishes in curing or different pours at application. Easily shows cracks and dirt.

  • Stamped is a preferred decorative finish providing a rock or pattern finish without using such products. Stamped offers ease of colour additives for highlighting patterns or "shadowing" to show relief.

    Pros: decorative, eye pleasing, value added product, moderate hiding of cracks and dirt.

    Cons: High maintenance, can be slippery, uneven wear, patching almost impossible to match.

  • A modified version between troweled and broom. Less relief than broom and more than trowel. Can be sometimes busy looking and suggested use would be for accents and borders.

Reinforcing

  • Rebar or steel reinforcing bar is the common go to for reinforcing. This is a long time trusted product that will provide some of the best performance characteristics with concrete.

    The drawback to rebar is corrosion. Corrosion of the steel will harm concrete and cause concrete failures. Past efforts of epoxy coated rebar has helped, but trends are now away from epoxy coated bar to stainless steel bar or glass fibre bar.

  • Welded wire is a common product to help reduce shrinkage cracking and general small stress cracking. It is often used in driveways or patio slabs to help hold the slab together.

    The drawback to welded wire is it is still made of steel and has corrosion issues. The smaller diameter of bar makes this product more suceptable to shear failure especially at corroded locations which often form at cracks. 10 gauge is a typical size of welded wire as an economical solution. 6 gauge would be a higher end product which would provide more life.

  • Glass fibre bar is better in many cases for reinforcing as it doesn't rust and has greater strength than steel. It lacks in two points which is direct shear and can't be bent on site. For this reason, sometimes steel is still required at certain locations.

  • Fibre can be includes into the concrete mix itself to help strengthen the product. In addition, it can also help reduce shrinkage cracking and increase abrasion, wear, and chipping resistance. It is not recommended as a complete replacement of reinforcing bar as the strength properties of a bar far exceed fibres.

  • A newer one to the market would be carbon fibre mesh reinforcement. This is a specialized product of greater cost and performance. Only special projects generally use this product due to the cost/benefit results.

  • In almost all cases, yes. Rust has negative effects to concrete if the rust occurs after the concrete is cured. Rust on steel prior to placement is often stopped as the concrete eliminates the air which allows rust to happen. Surface rust on a steel which is still structurally sound can increase performance of the steel by increasing the bond of the concrete to the steel.

DISCLAIMER

Not All Situations Are The Same

Information presented from this page are based from our experiences and knowledge. It is presented only as general information to assist in decision making and general procedures. This information may or may not provide accurate and/or appropriate solutions for any specific questions as each situation may vary and require different strategies. Use of the information provided does not hold Roaron Construction responsible for any liabilities, cost, wrong doing, or related expenses incurred from the use of information provided on this site.

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