Concrete Tips

1. Always implement proper safety procedures before beginning your project, and take care to handle and dispose of concrete mix and chemicals appropriately.
2. Order a mix of air-entrained concrete, blended to suit your precise project specifications and environmental conditions.
3. Start by leveling the intended surface as much as possible, and remove all debris from the area.
4. Implement forms that allow for sufficient drainage. This usually requires that slabs are set to slope a minimum of 2%.
5. Avoid high slump concrete, in which the water-to-cement ratio is disproportionate. (Consult an expert for guidelines)
6. Carefully monitor and measure slab thickness to ensure uniformity
7. Bullfloating, or darbying, should be performed as quickly and efficiently as possible after you strike off concrete to reduce excess moisture
8. Placing contract joints, with a groover at one-fourth slab thickness, is easiest when concrete is fresh (as opposed to sawing after hardening)
9. Finish concrete appropriately, and avoid hard troweling exterior concrete or concrete that contains higher concentrations of entrained air (over 3%)
10. Always properly cure your concrete and ensure you are waiting the optimal length of time before finishing

On Vancouver Island, winter temperatures occasionally drop below a point that compromises the chemical composition of freshly poured concrete. During the first 24 hours, even partial freezing can ruin the future strength and durability of your concrete. Monitoring the pouring process is critical to your project’s success if temperatures are expected to dip below 4 degrees Celsius. Some precautions that help reduce the impact of cold weather include:

• Ordering air entrained concrete, at a higher MPA, designed to withstand freeze-thaw conditions
• Never placing concrete on frozen ground
• Considering the use of heated enclosures, for both worker comfort and preserving concrete integrity
• Increasing ventilation and exposing concrete to supplementary heat sources
• Protecting freshly poured concrete from frost with windbreaks or other means
• Utilizing low-slump concretes
• Adjusting curing times, as appropriate, to accommodate for temperature fluctuations
• When possible, delaying pours during cold snaps

Concrete is precisely blended in a laboratory to suit particular conditions, and unexpectedly hot weather can be almost as detrimental as cold. Concrete is a product with an exothermic reaction and temperatures over 25 degrees Celsius can negatively affect a concrete pour. On Vancouver Island, scorching hot summers can lead to rapid evaporation of water within concrete mixes – which impacts concrete slump, curing time and risk of cracking.

Setting times must be adjusted to compensate for the rapidity of drying. This process can be delayed with cooling aggregates, water-reducing additives and retarders. Project sites can be protected, as much as possible, with sunshades and concrete pours can be timed strategically (for early or late in the day) to limit exposure to the most extreme summer temperatures.

As concrete consolidates, shrinkage and temperature fluctuations will often cause the mixture to crack. The solution is to place control joints within poured slabs, in order to engineer precise placement of the cracks.

Reinforcing slabs with steel is not recommended, but properly placing predetermined control joints can ensure your concrete cracks in a uniform manner. Accurate joint spacing, appropriate depth and proper timing are all critical to placing joints successfully.

• Joints can be cut into concrete either at the time of placement, performed at first pass and then again successively, or sawed in within 24 hours after the concrete has hardened (which presents a higher risk of uneven fracture)
• Carefully plan joint placement patterns to guard against re-entrant cracks and to effectively camouflage joints beneath walls or under areas that will receive additional layers of flooring
• Joints should be spaced no more than 2-3 times the thickness of a slab
• Dowels, or similar load transferring tools, are recommended when spacing joints wider than 15 feet across
• Joints should be cut at 25% depth of the slab

Bugholes (surface voids) are unsightly air cavities, resembling craters, that arise when entrained air migrates to the surface of concrete. These surface voids primarily detract from concrete aesthetics, although larger bugholes can potentially compromise surface integrity.

Bugholes are attributed to the air, water and chemical release agents within a concrete mixture. As a concrete mixture consolidates, pressure densification forces any trapped air and water upwards. Improper vibration, which disturbs the ratios of these elements, is a common cause of these cavities, as are unsuitable mix designs that impede concrete consolidation.

Bughole formation is often preventable with some careful preparation and the correct materials. Some suggestions to reduce to likelihood of surface voices include:

• Maintaining proper vibration techniques, while consolidating the concrete, by vibrating both the inside and outside of the mould
• Reducing the amount of air entrapment through mix design by incorporating eco-friendly additives, like fly ash.
• Carefully monitoring release agent applications, ensuring that layers are thinly applied
• Extending mix time to achieve optimal consistency

Properly curing concrete is essential to ensuring that your pour is durable and resilient enough to serve its intended purpose. Effective curing protects against freeze-thaw conditions, scaling, abrasion, moisture absorption and cracking – and this can be even further enhanced, if the concrete is sealed after being cured.

Once concrete is poured and finished, it must cure under a strictly monitored combination of temperature, moisture level and timeframe in order to maximize its lifespan. A wide variety of factors will determine each project’s individual concrete curing specifications, including: concrete mixture, concrete MPA, parameters of the project and its future exposure to weather and temperature.

The American Concrete Institute recommends a minimum curing period of the time needed for the concrete to attain 70 percent of its specified compressive strength.

Various sealing and curing agents are available. Contact us with your specific needs and we can recommend the appropriate sealing or curing agent to maximize the life your concrete.

Air entrained concrete contains a controlled infusion of billions of microscopic air bubbles into concrete, by introducing a chemical agent. Hardened concrete is porous and filled with tiny capillaries that cause surface scaling if exposed moisture and freezing. Entrained air permits concrete to withstand expansion. Our team of experts infuses the entrained air into concrete mixes to maximize the strength and durability of your exterior concrete. The mix of air entrainment varies per project, but between 4% and 7% is best suited for the freeze-thaw conditions of the Comox Valley.

We advise against using hard troweling to finish air entrained concrete. Hard troweling is an optional finishing technique that utilizes a steel trowel in order to smooth the surface to a desired finish. It can produce a polished appearance, not unlike a sealant. The danger is that hard troweling exudes pressure that decreases air content, by pushing the entrained air, upward or downward, within the concrete mix. This can lead to densification that drastically reduces resistance to freeze-thaw damage. Additionally, when exposed to moisture, the smooth surface of hard troweled concrete becomes a slipping hazard.

Cracks in concrete surfaces occur during the drying process. Imagine a sponge left to dry. As the surface of the sponge dries out, the top beings to shrink and curl because the underside of the sponge—not being exposed to the air—is still dry. A concrete surface is similar. In addition, the underside is prevented from shrinking over its full length, as it has adhered to the subsurface.

Unplanned cracks can be unsightly and are never in desired locations. With proper planning, joints can be added at regular intervals which can force cracks to appear in the optimal locations.

Another way to prevent cracking is through the use of fibre-reinforced concrete. Polypropelene fibres have over a decade's use in the concrete industry and they reduce the risk of cracking and add further durability to the product.

Curing is another step you can take to prevent cracking. The process of curing involves keeping the surface of the concrete project wet and safe from damaging temperatures. Uncured concrete is more likely have cracks than properly cured concrete. A minimum of three days for the curing process is the minimum, though better results can be achieved after the recommended seven days.

Contact us with your specific needs and we can recommend products that will maximize the life your concrete.