20/09/2021
CONCRETE TOP 9 MISTAKE
Here we’ve compiled the top 9 mistakes people make when installing concrete in hopes of helping you avoid some costly and frustrating headaches.
1. Thinking all concrete is the same.
There is much more to concrete than meets the eye. Depending on the purpose of the concrete, different mix designs may be required.
Concrete is composed of the same three basic ingredients:
Portland cement
Water
Aggregates (sand as a fine aggregate and gravel as a coarse aggregate)
Beyond the three basic components, there are also additional ingredients known as admixtures. Admixtures give concrete additional properties or strengths and make it suitable for a range of different applications.
Admixtures fall into one of seven major categories:
- Accelerating additives make the curing process, which can take anywhere from 3 to 14 days, happen much faster. This is particularly useful when trying to install new concrete in adverse weather conditions, as the curing process is crucial for concrete to reach its potential strength and can be affected by freezing temperatures
- Air entrainment is a type of additive that improves concrete’s ability to withstand the freeze-thaw cycle, thereby reducing cracking. Air entrainment offers a number of other benefits, including improved workability, improved durability, reduced bleeding and reduced segregation.
- Corrosion-inhibiting admixtures are used in reinforced concrete to slow the corrosion of the steel reinforcements. Corrosion-inhibiting admixtures are particularly useful in underwater concrete structures or those that are exposed to the elements, like highway bridges and parking garages.
- Retarding additives give the opposite effect of accelerating additives, because they slow the concrete’s setting rate. They are useful in hot weather conditions, which make concrete dry out and harden faster than desired.
- Shrinkage-reducing admixtures combat the shrinkage that occurs when concrete dries, thus reducing cracking and increasing durability.
- Superplasticizers give temporary workability to low-slump concrete, making it easier to pour and place. Superplasticizers only work for around 30 to 60 minutes, after which the concrete returns to its original state of low fluidity and becomes very difficult to work.
-Water-reducing admixtures affect the water/cement ratio in order to produce concrete that is easy to work with but that has the properties of concrete with less water and a denser cement paste.
2. Underestimating the importance of properly supported concrete forms.
Concrete forms provide a “container” for the fluid concrete and hold it in the proper shape until it can hold its own weight after fully hardening and drying. Forms have to have tight joints to prevent concrete from leaking through, must be rigid to prevent the concrete from bowing out, and must be strong enough to hold the concrete firmly in place.
Crucially, forms must also be level where appropriate and must have adequate support or bracing. Careful construction of concrete forms ensures that the finished concrete product will be correctly shaped, which affects its overall strength and durability.
Forms are most often made from softwoods like spruce, fir and pine, but they can also be made from fiber, clay or metal. When using wood forms, smoothing the side that faces the concrete is recommended, as is creating a watertight tongue-and-groove joint. Braces, spreaders, stakes, tie wires, tie rods and other forms of support keep the forms in their proper shape until the concrete has been poured and/or hardened.
To promote easy form removal, treat the forms with light bodied petroleum oil (or shellac if using plywood). The oil not only prevents the concrete from sticking to the forms, but also prevents wooden forms from absorbing water, which could lead to warping. During the curing process, the concrete should naturally contract slightly, pulling away from the forms and making it easy to pop them off. Using a wooden wedge to help pry the form away from the concrete in order to avoid breaking the concrete edge.
3. Failing to use reinforcement when needed.
Concrete may seem like a strong, durable, everlasting material, but it does have one major weakness: it is prone to cracking. Reinforcing concrete with steel is a common practice because the properties of steel complement those of concrete, and the two materials are easy to work with. Steel has high tensile strength and concrete is capable of bearing heavy loads without being compressed. Together, they create a superior building material: reinforced concrete.
Reinforced concrete allows skyscrapers to reach unbelievable heights, but it is just as smart to use it in projects a little closer to the ground. Bridges, staircases, tunnels and floors all benefit from steel reinforcement. For longevity and safety, most non-decorative uses of concrete require some level of steel reinforcement.
Rebar, steel mesh, steel fiber, cellulose fiber, recycled carpet fiber and glass fiber are most commonly used and each has its own unique way of being integrated with the concrete for best results.
4. Placing steel mesh reinforcement incorrectly.
Steel mesh is just one concrete reinforcement option, but it is particularly popular because it can be bent to fit many shapes, like columns or walls.
But for best results, steel mesh needs to be contained in the center of the concrete, not near the bottom or near the top.
Instead, a little more than half of the concrete should be poured before the steel mesh is placed on top. Then the remainder of the concrete is poured on top. The weight of this concrete will push the steel mesh down slightly, therefore landing it more or less in the center. Steel mesh “sandwiches” can also be placed before any concrete is poured, but for a single sheet of mesh, the above method is sufficient.
5. Using an incorrect water / cement ratio.
The water / cement ratio compares the weight of water to the weight of cement in the overall concrete mixture. The water / cement ratio affects the concrete’s workability, setting time, strength and durability.
Uniform Building Codes dictate what the proper water / cement ratio should be in various circumstances. In general, a water / cement ratio of .50 is sufficient. Higher water ratios can lead to cracking due to excess shrinkage from water evaporation, whereas lower water ratios can result in concrete that is difficult to pour and maneuver. Admixtures can be used to improve the workability of concrete without affecting the water / cement ratio.
6. Trying to pour concrete in poor weather conditions.
Warm, rain-free days are ideal for pouring concrete. In fact, a stretch of at least three days without extremely hot weather, or rain in the forecast is preferred.
The temperature of the concrete within the first 24 to 48 hours determine whether the concrete will reach its strength potential.
Hot weather, on the other hand, causes water to evaporate off of the concrete too quickly. The curing process requires concrete to stay wet for a number of days. Sprinklers can be used to keep the area constantly moist, or a covering can be placed over the concrete to retain moisture.
Of course, rainy days pose a challenge because the extra water can skew the delicate cement / water ratio of the concrete when it is poured. It is wise to reschedule pouring outdoor concrete in such weather conditions, but a waterproof enclosure may suffice.
In the end, there are usually more days in a year that are troublesome for concrete than days in which the weather is perfectly suited for the purpose. Know that the weather affects concrete’s strength in the early days and which precautions to take.
7. Removing forms before the concrete is truly dry.
The forms or “molding” that holds the concrete in place until it dries should be left alone for at least two days to ensure that the concrete is completely dry. If the forms are removed too soon, the concrete can begin to sag, crack and collapse, especially if conditions like temperature affected its strength. As exciting as it is to see the finished product, be patient. In fact, as you’ll find out below, concrete requires attention even after the forms are taken off.
8.Overworking the concrete.
Concrete does need a little bit of extra attention after it has been poured, but if it is moved around too much or “overworked,” multiple problems can occur. In addition, trying to move through the finishing steps too soon can compound those problems. Avoid touching the concrete after bull floating until the bleed water disappears, as the following can occur:
Concrete that is overworked can become dry and cracked.
Concrete that is overworked before the bleed water has risen to the surface can trap that extra water within the mix, resulting in future problems with scaling.
Overworked concrete can also cause aggregates to separate from the cement, which results in too much fine material near the surface.
9. Being impatient during the curing process.
Pouring, setting and finishing concrete is hard work, and it’s tempting to consider the project “done and dusted” once the concrete is visibly dry and load bearing. However, it needs additional care, and this is called curing.
Curing concrete ensures it reaches its desired strength without cracking or turning brittle. Different types of concrete have different minimum curing times.
Concrete does not reach its greatest strength when there is a disparity between moisture content during the curing process. The water integrated into the original mix evaporates from the surface and may cause cracking if some of the water is not replenished to restore some of the balance between the interior and surface of the concrete.
Curing can be accomplished by keeping the concrete continuously wet, by using a covering to prevent evaporation, or by using a chemical enhancement.
All things considered, it’s important to know what type of concrete you are working with and to be patient enough to allow the concrete to cure for the minimum recommended timeframe.
