The concrete most widely used today is a composite material made from a mixture of Portland cement, aggregate, and water. Once mixed, concrete is pliable and must be worked and then held in shape whilst it cures. Depending on need, concrete can be cast in place or purchased in precast hardened elements.
Pros and cons
Using concrete has numerous advantages; however, there is a downside, and a lot can go wrong when a concrete mix or job isn’t prepared and completed correctly.
- Durable – concrete structures can last two to three times longer than those built with other common materials.
- Requires little maintenance – it does not rust, rot or feed mildew and mould.
- Inexpensive – concrete’s key component is limestone, the earth’s most abundant raw material.
- Good compression strength – concrete is a particle-reinforced composite material, which means it has significantly higher compressive strength than tensile. The compressive strength of concrete varies from 2,500 psi for residential applications and 4,000 psi for commercial structures. In specialised applications, concrete compressive strength can reach over 10,000 psi.
- Can be made more environmentally friendly – when made with manufacturing by-products like slag cement, silica fume and fly ash, concrete’s footprint is reduced. It’s also recyclable. Once a concrete structure’s usefulness is at its end, the concrete can be crushed and recycled into aggregate for reuse.
- Workable – concrete is pliable in its initial state, which means you can form it in different shapes. As a foundation, concrete’s flexibility allows it to be poured directly onto moulded compressed subgrade and levelled, leaving no gaps between ground, concrete and structure.
- Non-combustible – can withstand high temperatures, making it fire-safe.
- Element resistant – its ability to resist the effects of wind and water makes it useful in structures regularly exposed to those elements and for buildings like storm shelters.
- Energy-efficient – tapping into concrete’s thermal mass can reduce the need for artificial heating and cooling.
- Minimal waste – it can be produced in the quantities needed per project.
- Healthy option – concrete doesn’t feed rot and mildew or give off volatile organic compounds. It can also help prevent dust, pollen and other airborne pollutants from entering a building.
- Environmentally harmful manufacturing process. While concrete embodies multiple, attractive sustainability qualities in use, the manufacturing process is still highly problematic. This can be offset by adding by-products, but there is still much work to be done if we are to continue relying as heavily on concrete as we do now.
- Requires stringent quality control during mixing, placing and curing. Mistakes made in any of those processes can have disastrous results.
- It’s vulnerable to cracking – cracking risk varies according to how/if a concrete element is cured and whether it is reinforced. Factors that cause cracking include thermal, chemical and mechanical processes, which result in shrinking, expanding or flexural stress.
- Low tensile strength when compared to other building materials – to avoid cracks, concrete often needs to be reinforced.
- Presence of soluble salt – when soluble salt is present in concrete, it can lead to efflorescence. Efflorescence is a transient, naturally occurring phenomenon in Portland cement. When introduced to moisture (like rain), the cement and water mix produces a chemical reaction that creates calcium hydroxide. As the concrete dries, the calcium hydroxide reacts to atmospheric carbon dioxide, producing calcium carbonate, which manifests as a white patch on the concrete’s surface. While it can be aesthetically undesirable, efflorescence does not affect the concrete’s strength or durability.
- Low strength-to-weight ratio – the weight of concrete is high compared to its strength.
- Long curing time – concrete needs three to seven days to cure and increase strength once put into place. Concrete that is water cured for seven days can achieve strengths as much as 50% stronger than concrete that has not been allowed to cure correctly. Water-curing for three days provides approximately 80% of the benefit of curing for the full seven days.
Admixtures are additions to a concrete mix that help achieve a specific goal. There are two main types: chemical and mineral.
Using chemical admixtures can:
- Reduce construction costs.
- Modify hardened concrete properties.
- Improve quality during mixing, transporting, placing and curing.
- Aid in overcoming operational emergencies.
Mineral admixtures can be used to:
- Improve economics.
- Increase strength.
- Reduce permeability.
- Influence other properties.
Some of the main admixture properties include:
Accelerators reduce setting time by varying amounts based on the type of accelerator used.
In contrast, retarding admixtures delay setting time. They are helpful in hot weather climates and conditions, when a job requires special finishing, or on difficult jobs where you need workability to last as long as possible.
Water-reducing admixtures reduce the amount of water needed by as much as 30%; however, the higher the range, the more expensive the product. Using water-reducing admixture results in stronger concrete.
Air entrainment admixtures are a must when concrete is exposed to freezing, thawing and de-icing salts. Microscopic air bubbles are entrained in the concrete so that when it freezes, the frozen water expands into the bubbles, preventing damage to the concrete. Using air entrainment admixtures helps improve workability and improve durability.
A by-product of coal-burning plants, fly ash can replace anywhere from 15–30% of the cement in the mix. Using fly ash helps:
- Improve workability
- Make it easier to finish
- Reduce heat generation
- Reduce cost
Adding speciality admixtures helps:
- Inhibit corrosion
- Control shrinking
- Add colour
- Inhibit reaction to alkali-silica
Concrete can be cast around steel bars (rebar, short for reinforcing bars) to improve tensile strength. The concrete provides its inherent compressive strength, while the steel provides tensile strength by helping the concrete resist bending and stretching. While, theoretically, numerous materials are suitable for reinforcing concrete, steel is favoured because it expands and contracts in heat and cold in almost fluid motion with the concrete itself, thus avoiding cracking.
Even though steel-reinforced concrete makes for a hardier building material than concrete alone, there are still issues, including the risk of becoming brittle and being prone to cracking. When placed in tension, reinforced concrete can also let in water, which causes the rebar to rust and the concrete to fail. Two methods help prevent this:
- Prestressing or pretensioning. Instead of placing rebar into wet concrete as is, bars are first pulled taut. As the concrete sets, the tight bars pull inward, further compressing the concrete to make it stronger.
- Poststressing or posttensioning. This method requires stressing the rebar after the concrete has started to harden.
Either method keeps the concrete in compression, helping to avoid cracks and limiting their ability to spread if they form. Another advantage to prestressing and poststressing is that you can use smaller, thinner pieces to carry the same load as ordinary reinforced concrete.
Fibre-reinforced concrete is a mixture of concrete and discrete, uniformly dispersed, randomly oriented fibres. The fibres can be flat or circular, usually with an aspect ratio (length to diameter) ranging from 30 to 150. Fibre-reinforced concrete is less expensive than rebar-reinforced and is mainly used for ground-level floors and pavements. Steel is the strongest fibre readily available, coming in various lengths and shapes. Materials used to create the fibres include:
- Steel – aids in tensile strength and improves resistance to disintegration, fracture and fatigue.
- Glass – is inexpensive and corrosion-proof; however, it is not as ductile as steel.
- Asbestos – is inexpensive and provides additional resistance to mechanical, chemical and thermal stressors. On the downside, asbestos fibre-reinforced concrete tends to have a low-impact strength
- Carbon – although more susceptible to damage than steel fibres, carbon fibres are often used because of their high elasticity and flexural strength.
- Organic – typically consists of cellulose fibres processed from pine trees.
- Polypropylene – a cheap and abundant polymer used because of its resistance to feeding chemical reactions
- Using recycled carpet fibres has also met with some success.
Whatever the material, the fibre’s purpose is to increase structural integrity. Different fibre types produce differing properties, and final product characteristics are based on several factors, including concrete type, type of fibre, geometry, distribution, orientation and density.
The benefits of using fibre-reinforced concrete include:
- Increases tensile strength
- Increases durability
- Aids in resistance to creeping (deforming under a sustained load)
- Reduces air and water voids
- Helps arrest cracks
- Improves static and dynamic properties
Hybrid concrete construction
Hybrid concrete construction uses cast in-situ concrete and precast concrete to take advantage of their different qualities – combining the speed, quality and accuracy of precast elements with the flexibility and economy of concrete cast in situ. The result is safer, faster construction, a higher quality product, better value for money, and more consistent performance.
The term ‘concrete cancer’ is an informal term used by some in the industry for concrete that has failed through a series of interrelated problems:
- Silica in the aggregate reacts with alkalis in the cement causing new crystals to form, slowly growing inside the cement. Because there is no room for these new crystals, the concrete begins to crack from the inside and spall from the surface, allowing water to get in. (In structures like roads exposed to de-icing salts during the winter, the water can be alkaline, exacerbating the problem.)
- The water reaches the steel reinforcement bars, potentially causing them to expand, rust and decay, creating fatal weaknesses in the concrete structure.
- When experiencing freezing temperatures, further expansion can occur, leading to additional cracking and water penetration.
- A vicious circle of degeneration and decay is created.
Defects and cracks
Macro v micro defects
Concrete defects are broadly classed as macro or micro. Concrete that develops macro defects has lower strength and is prone to rapid deterioration due to exposure to water or chemicals. Micro defects are invisible to the naked eye and are usually the result of a low-grade mix or too-high water-to-cement ratio. While micro defects are harmful to building health and can cause deterioration, they do not hold the immediate threat that macro defects do.
Types of cracking and their causes
As mentioned earlier, cracks can occur for several reasons. The risk severity depends on various factors, including how/if the concrete was cured and whether it’s reinforced.
- Plastic shrinking – short cracks spread unevenly across the surface and run to approximately mid-depth of the structural element. Plastic shrinking is usually the result of improper curing, with the surface being allowed to dry much faster than the rest of the slab.
- Hairline – thin, deep cracks usually formed from how the concrete settled during curing. Because of their depth, hairline cracks can result in more severe cracking once the concrete has hardened.
- Diagonal – diagonal corner cracking runs perpendicularly from one joint to another at a slab corner that results from the curled or warped corners due to temperature differences or moisture evaporation during the curing process. Once that happens, a gap forms beneath the corner, which increases the potential for cracking or breaking.
- Map cracking – another curing-related type of cracking, map cracking (also known as checking or crazing) creates a web of fine, shallow cracks across the surface. Similar to plastic shrinking, this results from the surface drying faster than the interior concrete. However, because map cracking is shallow, it’s more of an aesthetic problem than a structural one.
- Pop-outs – Pop-outs happen when some of the aggregate used is excessively absorbent, and its location near the surface of the piece exposes it to water. The result is expanding aggregate that “pops out” of the concrete.
- Scaling – depressions that form on the surface of the concrete, usually the result of water penetration or delamination. In the first instance, it is caused by inadequate finishing. The second is a case of too much water remaining in the concrete due to insufficient curing or vibrating. In both cases, the water forms pockets just below the surface that form blisters that either break or freeze and expand, thus breaking off pieces of the surface.
- Spalling – these depressions are more significant than those formed by scaling and are more likely to appear in reinforced concrete. Causes include increased pressure from beneath the concrete surface and rebar corrosion. As rust forms, it creates pressure that pushes away chunks of concrete, exposing the corroded metal. Spalling can cause severe problems because decay accelerates once rusted rebar is exposed.
- D-cracking – moisture entering a joint can cause cracks to form, and they are much deeper than surface cracks.
- Offset – the result of concrete settling onto an uneven surface, causes include pouring onto a subgrade that hasn’t been appropriately compacted or pressure caused by tree roots or previously placed concrete.
Concrete slabs are used as support for everything from buildings to pedestrians. Because of this, any project that calls for a concrete slab must be efficiently planned and undergo considerable consideration before the placement begins. Some of the most common mistakes made when creating a slab-on-grade include:
- Not compacting the subgrade properly – Neglecting this step and experiencing a ground-saturating rain risks plumbing and utility trenches collapsing, leaving the slab without support.
- Wrong water-cement ratio – if the water/cement ratio is over .50, the concrete can become overly permeable, leading to problems like excessive cracking and moisture-associated flooring issues: vinyl flooring not adhering correctly, yellowing or mould and mildew forming underneath. On tiled floors, the grout may become wet.
- Not using an air entrainment admixture – in cold weather climates, any water inside the concrete will expand when temperatures hit freezing and cause fractures to form.
- Improper curing – when concrete is improperly cured, cracks that wouldn’t otherwise occur can happen (because curing delays shrinkage until the concrete is strong enough to resist cracking). Concrete allowed to moist cure for seven days is also approximately 50% stronger than concrete exposed to dry air for the same amount of time
- Too far out of level – out-of-level concrete results in expensive corrective work. The discrepancy will be conspicuous if the corrections aren’t made at specific points – for instance, where the wall meets the ceiling.
- Sloppy placement of control joints – improperly spaced and placed control joins can lead to cracking, which can result in damaging vinyl flooring and breaking tile grout.
- Lack of mesh supports – using wire mesh without providing blocks to support the mesh can cause the mesh to end up in the ground rather than in the concrete. This leads to cracks, which open doors to problems with water permeability and can affect flooring. An alternative to wire mesh is fibres added to the concrete mix.
There are arguments both for and against concrete as a sustainable material. While the cement manufacturing industry has made strides in reducing CO2 emissions related to manufacturing, it is still a significant offender in that arena, primarily since it relies heavily on fossil fuels to produce. That said, lacking viable alternatives, concrete has several things going for it:
- While manufacturing is energy-intensive, construction consumes minimal materials, energy or other resources.
- Concrete is a durable, low-maintenance material that has a long lifespan.
- It doesn’t burn, which helps reduce waste materials and fire-associated toxic emissions harmful to people and the environment.
- Concrete’s structural integrity can be designed to resist water penetration, which is essential given our increased issues with heavier rainfalls and increased flooding caused by climate change.
- Taking advantage of concrete’s thermal mass (passive heating and cooling) allows designers and specifiers to create more environmentally friendly assets that do not rely as heavily on active heating and cooling systems.
More about NBS
In the article Using NBS to specify sustainable outcomes on projects, NBS explores how the Plan of Work and NBS Chorus can be used to tailor a project to meet sustainability expectations across the project timeline. Through Chorus, you can access your specifications across locations and organisations due to its flexible, cloud-based functionality. No software to install, no fuss, just access to your specs anywhere, at any time and on any device. All that is required is a modern web browser and an internet connection.
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NBS is continually pushing for more sustainability information in all manufacturer listings. Today the requested information includes aspects such as embodied carbon, country of manufacture and country of material origin, recycled content and end-of-life data. We are also seeing a rise in manufacturers providing EPDs, which can be accessed in the listings third-party certificates section.
If you are a specifier, you can view NBS Source here. Alternatively, if there are products you would like to see but occasionally cannot find within the platform, please drop us a note via email@example.com with the details, and our team will reach out to the manufacturer on your behalf.
The Construction Information Service
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NBS: Sustainable Futures Report 2022
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