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06 Nov 2019 by Permacolour
Although concrete is one of the most commonly used building materials there are quite a few misconceptions about what it is, what it can do, and how it can be used. Here we bust some of the most common concrete myths
This is a common misconception. Concrete finishes are not as cold as many think. Cement/concrete is actually fantastic at absorbing and retaining heat, to gradually release it throughout the day. Concrete is actually warmer than having tiles due to its natural ability to absorb heat.
Concrete floors are also suitable for use with underfloor heating. If this is something you would consider, care needs to be taken in the initial stages of using the system. There will be a waiting period to allow the concrete to sufficiently cure before you can turn on the heating system, as this is critical to maintaining your finish. The temperature should be gradually increased over the course of a few weeks.
It’s no secret that the cement industry is one of the world’s primary producers of carbon dioxide and this tends to mean that people believe that concrete is bad for the environment. In reality, concrete is the most durable building material on the market and a properly constructed concrete building can last for hundreds of years. In fact, the majority of greenhouse gases produced by a concrete building over its lifetime are produced by the electricity run in the house, rather than the structure itself.
Although pre-mix concrete is readily available in hardware stores and an array of DIY tutorials available online, the reality is that installing concrete correctly takes skill and experience. Obviously small jobs can be done with a DIY approach DIY provided you know what you are doing. Larger projects such as driveways, pathways, slabs or any sort of concrete construction and specialized finishing should be done by professionals.
Usually, when people think about concrete, they immediately imagine the dull, boring, grey type of concrete which is commonly used in construction or civil engineering. There are however so many options available in terms of coloring, textures, patterns, and finishing. Creating something truly unique and interesting has never been so easy.
It is true that concrete mixed with too much water will be weaker and less structurally sound as well as increasing the chances of cracking, flaking, and blistering among others. Concrete does, however, require water to hydrate and strengthen during the curing process. Concrete curing is not a drying process but one in which water is vaporized. As long as there is sufficient moisture and favorable temperatures, the hydration of concrete will continue for some time. When fresh concrete dries out (usually below about 80% relative humidity), hydration stops. If the temperature of fresh concrete approaches freezing (below 5°C), hydration slows dramatically. In high temperatures, rapid moisture loss can increase the risk of cracking, hence the importance of regulating the moisture availability for the concrete to cure sufficiently. The longer you cure concrete, the stronger and more durable it will become.
Decorative overlays are thin coatings and are not designed to disguise a crack or to prevent substrate blemishes from shadowing through. The phrase “a topping is only as good as what you put it on” is accurate when considering whether to overlay or not. Sometimes an overlay is considered due to some kind of surface defect, however suitable repair products, crack fillers and primers should be considered first prior to any decorative overlay coatings being applied.
Adding water to increase workability and slump will actually reduce the strength and substance of the concrete. Additional water dilutes the paste and increases the water-to-cement ratio (w/cm). Excessive water can also reduce concrete’s resistance to freeze-thaw cycles, increase drying shrinkage, and lead to other problems.
There are effective ways to increase concrete slump and workability. Aggregate gradation and the maximum size of the aggregate both greatly influence cement and water requirements, which affect mix workability. Water reducers and superplasticizers can also be used, to increase the slump while maintaining the water-to-cement ratio.
Truth is even the densest concrete is somewhat porous.
Water and other substances in liquid or vapor form can still pass through concrete. Permeability can be reduced by using mix designs with a low water to cement ratio, well-graded aggregate and chemical admixtures like plasticizers. Surface treatments, like sealers, can also aid in reducing permeability and water/liquid absorption.
Unfortunately, compressive strength alone does not determine the durability of the concrete.
Whilst compressive strength is an important characteristic of concrete, other factors are important for concrete durability in harsh environments. Usually, the principal causes for deterioration in concrete are corrosion of reinforcing steel, exposure to freeze-thaw cycles, alkali-silica reaction, and sulfate attack. Reducing permeability is the key to durability.
In areas of tension, structural reinforcement does not prevent cracking, but rather it holds the crack faces together. When concrete cracks, the tensile stress is transferred from the concrete to the steel, allowing reinforced concrete to withstand higher tensile loads than concrete alone.
Unfortunately, there is no absolute rule of thumb to determine the readiness of concrete for finishing. Improper finishing can cause surface defects like blistering, dusting, crazing, and delaminating. It takes skill and experience to know when to start the finishing process. Relying on “old rule of thumb” type methods for testing readiness may not always apply, given there are several external factors that need to be taken into account, such as different mix designs, weather conditions, and finishing tools. Relying on the absence of a sheen of water on the surface to determine when bleeding has stopped is not a reliable method, since bleeding may still be occurring even though it isn’t obviously visible. Bleeding must be completed for the entire slab thickness before finishing operations begin. Choosing the appropriate time to begin the finishing process takes skilled and experienced judgment and knowledge of the materials being used. The timing can change based on current weather conditions, mix designs, placement rates, and a variety of other issues. Experienced finishers take all those factors into account.
Concrete changes in volume while setting, hardening, and drying.
Curling of slab edges is caused by differences in the moisture content and temperature of the top and bottom of the slab. The edges of slabs at the joints tend to curl upward when the top surface of the slab is drier or cooler than the bottom surface. A “reverse curl” occurs when the top surface is wetter or warmer than the bottom. The possibility of curling occurring can be reduced by using techniques that minimize shrinkage differentials and the temperature and moisture-related volume changes that cause them.
Even after placement of the concrete, movement and/or settlement in the ground can all cause the concrete to sag over time, resulting in uneven, cracked concrete.
If only it were so! Structural reinforcement does not absolutely prevent concrete from cracking due to volume change.
Concrete that is prevented from moving during volume changes (during setting and drying) may crack since concrete is weak in areas of tension. Structural reinforcement does not prevent cracking, but rather it holds the crack faces together. When concrete cracks, the tensile stress is transferred from the concrete to the steel, allowing reinforced concrete to withstand higher tensile loads than concrete alone.