Concrete Explained

Concrete is comprised of fine and course aggregates bonded together by a cementitious paste. Fine and course aggregates like sand and crushed rock make up approximately 60% of its volume followed by 25% water and 15% cement. When mixed, the water initiates a chemical reaction with the cement permanently bonding the aggregates together. However, it is not as simple as it sounds, and not all concrete is the same. There are endless batch designs and engineering considerations that go into creating a compound that will perform to its desired need.

Cement is the binder within the concrete and essentially the key ingredient. Portland cement is the generic term for the type of cement comprised of mostly lime, silica, and alumina. These raw materials are extracted from mines/quarries, crushed, dried, and ground into a fine powder. Once the cement has been mixed with additives for developing different types of cement it is delivered to ready-mix concrete producers and stored in silos ready for use. When Cement is mixed with water it creates a chemical reaction known as Calcium Hydroxide. This reaction is ultimately responsible for the compressive strength and hardening properties of concrete.

Fly ash is a waste product derived from burning and crushing coal in power generating plants. During combustion, the impurities in the coal rise with the exhausted gases and solidify into Fly ash when cooled. Fly ash has a similar appearance to that of cement; however, the chemical composition is different, as it reacts with the byproduct Calcium Hydroxide released from the reaction between cement and water. The additional reactions created by fly ash improve some of the concretes properties such as harness and strength. It is incredible how one industry can utilize a waste product derived by another, in order to improve the concrete products we use every day.

Concrete additives are introduced into the concrete during the batching process to change the characteristics and/or control the behavior of concrete. Additives can come in a liquid or powdered state and all serve a unique purpose:

Water Reducers: These additives reduce the amount of water required in a mix; thereby reducing the water-cement ratio and increase workability. Water reducers are used on large projects where high early strength gains are required and/or reinforcing steel within the formwork requires workability. These additives can reduce the amount of water used in a mix by 5-10%.

Accelerators: Designed to speed up the chemical reaction and increase the rate of hydration. Calcium is a common accelerating agent used in concrete, especially in colder climates.

Retarders: Designed to delay the rate of hydration and prolong the formation of Calcium Hydroxide. Utilized extensively in warmer climates, as it helps workability and placement.

Silica Fume: Like Fly ash, Silica Fume is a waste product from the production of silicon and ferrosilicon alloys. These particles are extremely small and create a Pozzolanic reaction with the Calcium Hydroxide, to form additional chemical reactions within the smaller voids around the hydrated cement particles. As a result, the addition of Silica Fume improves compressive and flexural strength and makes for a more durable product because of its dense matrix.

https://www.norchem.com/silica-fume-concrete.html

Air-Entrainment: Designed to be added into concrete to help prevent cracking and scaling during freeze/thaw cycles. Air entraining additives are essentially soap based compounds that create millions of microscopic air bubbles within the concrete itself. These bubbles not only allow moisture to expand without harming the concrete but also reduce bleed water and segregated aggregates.

Shrinkage reducing: All concrete is prone to shrinkage as it undergoes hydration and water loss. Shrinkage reducers are added to the mix when cracking must be reduced for durability or aesthetic reasons. These admixtures help reduce the internal stresses placed upon the concrete during the chemical reaction process.

Corrosion-Inhibiting: Designed to be used on structures that have large amounts of reinforcing steel and chloride salts are present (i.e. bridges, parking and marine structures). These admixtures work for many years after the concrete has set and aid in reducing the risk of the reinforcing steel rusting within the concrete.

Super Plasticizers: Designed to reduce water content by 12 – 30 percent and create a concrete that is extremely workable which can be placed with little to no vibration. These additives are usually added to the mixer truck onsite as they only have a 30 – 60 minute working time.

Concrete strengths are measured in MPa (megapascals) which is the metric equivalent to PSI (pounds per square in). Upland Ready-Mix suggests the following strengths by location:

(This table is to be used as a guide only. Strength requirements may differ depending on your project, location, and engineering requirements).

https://vancouverreadymix.com/mpa-explained/

Pouring concrete in cold and/or freezing temperatures can result in a poor product if not properly controlled. Introducing one of more of the following items will help ensure that you are left with a strong and stable concrete product:

1. Use of hot water instead of cold water will help accelerate the hydration process and prevent early onset freezing.
2. Additional cement to the mix to make up for any strength loss due to cold temperatures.
3. Heating the aggregates prior to batching will help delay freezing.
4. Adding accelerators to help speed up the rate of hydration.
5. Using water reducers to help limit the amount of bleed water.
6. Insulating the concrete with heavy tarps or insulation will help keep the heat created from the chemical reactions within the concrete for effective curing.

Pouring concrete in warm weather can also have adverse effects on the final product. Heat drastically accelerates the rate of hydration and may result in an unworkable product that is prone to shrinking and cracking. The following consideration must be taken into account when pouring concrete in temperatures that exceed 25 degrees Celsius:

1. Have enough personnel onsite to help expedite the finishing process.
2. Try and shade the area that was recently poured and keep it out of direct sunlight.
3. Pour first thing in the morning or at night when temperatures are cooler.
4. Have a retarding admixture onsite and ready for use prior to pouring.
5. Use ice in the initial batch to slow the chemical reaction.
6. Dampen the sub-grade or formwork with water prior to pouring.

A slump test is use to determine the consistency of the mix and to identify how much water has been added. Concrete is added to a steel slump cone then rodded to ensure it has been compacted, and finally leveled off. The cone is then lifted off of the concrete and placed beside it as a reference point. The distance between the top of the cone and concrete is measured; therefore, indicating the slump of the concrete. The higher the slump the more distance between the cone and concrete identifying a higher water/cement ratio. Low slump will have less distance between the cone and concrete and showing a lower water/cement ratio. The slump test also helps determine the workability of the concrete; as well as, its true, collapsible, and shear characteristics.

Upland Ready-Mix has a mixer truck fleet consisting of 12 vehicles capable of delivering anywhere from 1 to 10 cubic meters of concrete per load. The trucks are outfitted with a corkscrew fin within the drum itself, allowing the concrete to be thoroughly mixed and remain within the drum. When the drum is spinning in the discharge orientation, the fins grab the concrete and send it out the chute.

Upland Ready-Mix is outfitted with a dry-mix batch plant which utilizes a weigh hopper to weigh out the dry materials individually prior to discharging them into the truck, where they meet the liquid components for mixing. A wet batch plant is outfitted with a mixing drum which blends the wet and dry material together prior to loading. Dry batch plants are extremely efficient as they allow the mixer truck to do the mixing while traveling to site.

Colored concrete is growing in popularity, as there are several ways to introduce color into the product. Popular methods include using stains, pigments, hardeners, and dyes. The most common method of coloring fresh concrete is to add a pigment into the mix during batching to achieve an even and uniform color throughout the entire load. Stains are popular on new and old concrete surfaces as they tend to be earthy tones that blend well with their surroundings. When considering the use of colored concrete on your next project, we suggest that you speak with a concrete finishing company that has extensive experience working with these compounds and able to provide you with the correct fit and finish.

Concrete sealers help protect the surface of the concrete and seal out anything that may cause damage such as stains, dirt, salts, oils, and/or freeze/thaw cycles. We suggest the use of sealing compounds on all exterior flat work, as it is an additional barrier of protection.

Concrete is an amazing material as it can be finished in many different ways. The most popular driveway finish creates an exposed surface that gives texture to the driveway and excellent traction for vehicle and/or foot traffic. On the other hand, garage slabs usually have an ultra-smooth float and trowel finish, which increase the surface hardening properties. Exterior walkways and sideways tend to have a broom finished for traction. Concrete can also have a stamped finish giving texture to the finished product. Common stamps include a brick, slate, stone or even a wood grain look. As with color, we suggest you speak with an experienced concrete finisher when considering different finishing options.