In today’s construction, most builders normally use concrete for commercial and industrial building owners are starting to appreciate the sustainability of timber. Both are in fact used for sustainable construction but there are good reasons why bigger structures need concrete
Importance of Concrete in Construction
The importance of sustainable development is currently dominating headlines, and as a concept is frequently defined as the practice of meeting present needs without compromising the ability of future generations to meet their own needs. Produced from readily available raw materials, concrete’s strength, durability, and versatility ensure it provides solutions for the built environment that help achieve sustainable development.
Concrete is durable, low maintenance, and resistant to wind, water, and fire. Because of its ability to retain heat, it increases the energy efficiency of buildings and cuts heating/cooling bills.
Concrete is so integral to our communities because it is the only building material that cost-effectively delivers:
- the lowest carbon footprint for a structure or pavement over its life cycle
- unparalleled strength, durability, longevity, and resilience
- maximized energy efficiency via thermal mass
- durability in any environment
- a building material that doesn’t burn, rust or rot
- safety and security
- versatility – it can be molded into any shape, color or pattern imaginable
- no off-gas
- excellent vibration and sound insulating
- low maintenance costs
- 100 % recyclable, plus the materials needed to make concrete are abundant in just about every locale on the planet
Most concrete structures have a design life of 50 or 100 years. But unfortunately, we are finding that many of today’s structures are not living up to expectations. The premature deterioration of concrete infrastructure is one of the great hidden costs of our time.
It is rightly said that everything bad that happens to concrete happens as a result of water. Water carries chemical contaminants into the concrete, it can erode the surface, corrode the steel reinforcement, and contribute to expansive and disruptive reactions and more.
You will want to do everything you can to keep water out of your concrete and to enhance the durability of your concrete. So here are six suggestions on how you can improve the strength of your concrete project:
Mix Design
Durability starts with a concrete mix design that is properly matched to the service conditions that the concrete will be exposed to. No single mix design can be durable in every situation. For example, when concrete is exposed to sulfate contaminated soil, the mix design should consider using sulfate resistant cement. However, this mix design is not appropriate when a concrete structure is exposed to the cycles of freezing and thawing in a wet environment.
In this case, the mix design should include an air-entraining admixture since entrained air will help to protect the concrete under these conditions. The Portland Cement Association publishes Design and Control of Concrete Mixtures, which contains an excellent guide to the concrete mix design.
Construction Joints
Construction joints in a concrete structure are unavoidable. They are also a common weak point in the structure where water will often find its way in and through. Water penetrating through joints can deteriorate your structure in a variety of ways, but most notably by causing corrosion of the reinforcing steel. Be sure to design your structure with adequate, predetermined control joint locations in order to avoid random cracking. Cracks in concrete are just unplanned joints that the concrete makes for itself. Next, be sure that each joint employs a reliable waterproofing system of its own.
A joint waterproofing system that includes crystalline technology can provide the best long-term protection at joint locations. Note that crystalline technology can also self-seal random cracking and thus provide added reliability and protection.
Low Permeability
Even though mix designs must change to match the conditions, there is one factor that is common to all good concrete and that is low permeability. It is essential that water is kept out of your concrete structure. Lower permeability through good mix proportioning and by reducing the amount of free water in your mix using water-reducing admixtures or plasticizers.
Concrete that is exposed to water should also employ a waterproofing system. A crystalline waterproofing admixture such as Kryton’s Krystol Internal Membrane (KIM) has proven to be most effective at protecting concrete from water intrusion.
Proper Curing
Probably the most often overlooked step in achieving durable concrete is proper curing. Curing means maintaining conditions that will allow the concrete to harden and gain strength optimally. Proper temperature and humidity are the most important. The temperature should be above 10°C for strength development to proceed at a reasonable pace. Moisture must be maintained, especially in the first few days in order to promote cement hydration and keep the concrete from drying, shrinking and cracking. Properly curing your concrete will effectively pay for itself many times over by reducing permeability and increasing the concrete’s durability and life expectancy. ACI 308R-01: Guide to Curing Concrete provides appropriate guidance.
Concrete Placement
Concrete must be placed properly. The workability and consistency of the concrete mix must be sufficient to
place and consolidate the concrete into a solid mass that is free from any voids or rock pockets. Voids most commonly occur below or behind reinforcing steel bars – especially in areas where there are many bars together.
Dedication to proper vibration practices is the key to success here. Be sure to properly vibrate all concrete with special attention around congested reinforcement. Rock pockets are areas where there is not enough cement paste and fine aggregate to close the space between large aggregate. Make sure there is no standing water prior to concrete placement and do not move a concrete large distance from where it is initially poured. The use of self-consolidating concrete can alleviate many of these concerns.
Reinforcing Concrete
Reinforce a concrete pour with rebar or steel mesh. Rebar is the most common addition for strengthening the pour. In vertical or horizontal projects, the more rebar present in the concrete, the stronger the finished project will be. The standard application for installing rebar is on 2-foot centers, meaning that every 2 feet, you will place a rebar bar. Double the number of bars for greater strength.
The next thing to do is to raise the level of rebar in a flat pour so the steel is in the center of the wet concrete, instead of lying at the bottom. The slab becomes stronger when the rebar lies in the middle of the wet concrete as it sets. Use rebar chairs as you position the rebar to keep it a couple of inches off the sand base.
Note that it is important to consider the weather when pouring concrete. The hydration rate of wet concrete, which is the rate at which the concrete sets, depends upon many factors. On a hot sunny day with little or no humidity, concrete may set at a very fast rate, leading to shrinkage and a weaker structure. Test the concrete from the truck and ask the driver to add more water until you like the consistency.
Now, you have to mist new concrete with a water hose every hour on the day you pour it, and at least three times a day for the next two days. The slower the concrete cures, the stronger it is. By wetting the exterior of the concrete during the curing process over a period of three days, the concrete develops a strong internal bond.
When ordering concrete, ask the concrete company not to add calcium to your concrete on a cool day. Calcium is a common additive, added to the wet concrete when the truck is loaded to enable a quicker drying time in cold weather. This can speed up the process of the pour but it also decreases the strength of the concrete as it encourages air filtration in the wet concrete.
Finally, vibrate the wet concrete. Vibrating does two things to strengthen the concrete. First, it encourages the wet concrete to filter into voids in hard-to-reach places, such as the space below a basement window. Second, it removes tiny bubbles from the wet concrete, making the final product more solid. Use a pencil vibrator when pouring walls.
Conclusion
At its most basic, concrete is a mixture of aggregates and paste. The aggregates are sand and gravel or crushed stone; the paste is water and Portland cement. Portland cement is the generic term for the type of cement used in virtually all concrete.
Through a process called hydration, the cement and water harden and bind the aggregates into a rock-like mass. With appropriate mix design, concrete can be tailored for any construction requirement.
Our suggestions above are just some of the ways you can strengthen concrete further.
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