Author: Melissa Rushin

Concrete Colorado Springs are everywhere around us, but we rarely think about it. This ubiquitous material is complex and requires testing and careful management.

Concrete is made from a mix of dry ingredients that grow together into a hard, durable material. A lot of complexity goes into designing a specific concrete formula with the right ratios and characteristics.

Water is an essential part of the concrete mix. It is used to form a paste that binds the aggregates together and hardens the concrete through a chemical process called hydration. The amount of water in the concrete determines many fresh and hardened properties, including workability, strength, permeability, drying shrinkage, and durability.

The water-to-cement ratio is the most important factor in the design of concrete mixes. The higher the w/c ratio, the more porous and permeable the concrete will be. A low w/c ratio creates concrete that is stiff and not easily workable, which is why chemical admixtures like plasticizers and superplasticizers are added to the mix. The w/c ratio can also be adjusted with the addition of air-entraining admixtures.

Water in the concrete is used for a variety of reasons: washing during the quarrying of coarse aggregates, quenching of the ground granulated blast furnace slag (GGBS), mixing and transporting of the concrete, cleaning the plant, dust suppression measures, and finally as the curing medium for the concrete. The quality of the water is crucial for the success of the hydration process, and it must be clean and free of salts, chlorides, and other chemicals that may interfere with the reaction and weaken the final cured concrete.

Concrete hydration is a two-stage process. During the first stage, the cement compounds react with water molecules in the form of a hydrate known as calcium silicate hydrate. The hydration of these compounds results in a significant increase in temperature. This first stage is referred to as the “dormancy period” and lasts up to three hours.

During the second stage of hydration, the unreacted cement compounds separate from the hydrated forms, which leaves behind empty spaces or voids in the concrete. These voids in the concrete are filled with water and calcium hydroxide. As a result, the permeability of the concrete increases while its strength decreases.

Too much water in the concrete reduces its strength, especially at elevated temperatures. When using ready-mixed concrete, it is best to order the desired slump with the mix design optimized for that purpose so that there is no need for additional water at the job site. In addition, adding too much water decreases the strength of the concrete and makes it more susceptible to cracking later in its life.

Cement

Cement is the main binder in concrete, a construction material that acts like glue to hold other materials together. It is a gray powder that hardens when mixed with water and aggregate to form concrete. The cement industry has been around for thousands of years, but modern technology allows manufacturers to make it more quickly and with a lower environmental impact. The basic recipe for cement consists of limestone, clay, silica sand, and iron ore that are heated to high temperatures and ground into very fine powder. The powder is called cement because it hardens when mixed with water. Cement was a significant innovation in the building industry, allowing structures to support large loads and withstand weather conditions.

The paste of concrete binds the aggregates with a chemical reaction known as hydration. This creates the remarkable trait that makes concrete plastic and malleable when it is newly mixed but extremely strong and durable once it dries. The hydration process is controlled by the ratio of Portland cement to water, which can be varied to change the material’s characteristics.

Professionals can also mix in a wide variety of additives to alter the properties of cement for specific applications. For example, class “G” oil well cement is made with additives that change the behavior of the cement for use in deep underground conditions. The cement industry is constantly innovating to make its products more environmentally friendly. It has developed ways to reduce the energy required to grind and dry the raw materials and to increase the efficiency of kilns.

Despite the great advantages of concrete, it is not without its drawbacks. One is that it must be kept damp for a long period of time after pouring to prevent it from drying out and losing its strength. It is also vulnerable to damage from frost and low temperatures.

Another disadvantage is that cement production is associated with significant environmental impacts at every stage of the manufacturing process, from airborne pollution to blasting in quarries and contaminating the countryside. Nevertheless, cement manufacturers are increasingly paying attention to sustainability issues and have developed equipment to reduce dust emissions and noise, as well as technologies that capture and separate exhaust gases. They are also reintegrating old cement quarries into the countryside by using them for aggregate or re-cultivating them as forests.

Aggregates

Aggregate is a granular rock-like material that is used with a hydraulic cementing medium to produce concrete or mortar. It consists of a wide variety of materials, such as gravel, crushed rock, sand, recycled concrete, and blast furnace slag. These materials are mined, processed, and then used with a cementing agent to produce concrete. The aggregate in concrete is responsible for the strength, workability, and durability of the finished product. Several factors, such as aggregate size, shape, bulk unit weight, moisture content, specific gravity, reactivity, and soundness, are important in the production of quality concrete.

There are two main types of aggregate: coarse and fine. Coarse aggregate is made up of gravel, crushed rock, or blast furnace slag. It is extracted from natural sources such as quarry rocks, riverbeds, or seabeds. It is then crushed, sized, and washed to ensure cleanliness and proper gradation. Coarse aggregate also has to be free of any deleterious substances that can deteriorate the final concrete product.

Fine aggregate consists of natural sand or crushed rock fines, with most particles passing the 9.5 mm (3/8 in.) sieve and a good percentage retained on the 75 um (No. 200) sieve. The fine aggregate in concrete acts to fill the voids between the coarse aggregate particles and act as a workability agent. Fine aggregate should have a rounded shape to maximize workability and economy. It is important to note that while a specific aggregate may meet the requirements of a particular mix design, it might not be suitable for all applications due to its varying properties.

The gradation of the aggregate is another important factor in the production of quality concrete. This is because the gradation determines how much water must be added to the concrete to achieve the desired slump. If the aggregate has a uniform size, it will require more water to produce the desired slump than if it were composed of a range of sizes.

The maximum aggregate size should be chosen to suit the needs of a particular mix design. The maximum aggregate size should not be too large, as this will create more void space in the concrete. The aggregate should have a range of particle sizes to allow for optimum gradation, which will reduce the amount of cement required and enhance the strength and durability of the concrete.

Rebar

Rebar, also known as reinforcing steel bar or mesh, is what makes concrete so strong. The rods are installed before the concrete is poured and are manufactured with ridges that the poured concrete can adhere to. It’s these ridges that make it possible for large slabs of concrete to stand up to the weight of vehicles and machinery without sagging or breaking.

While concrete has great compression strength, it doesn’t have a lot of tensile or torsional strength. Rebar can strengthen these areas and reduce cracking that can derail a project or cause the structure to fail. The rebar can also be strategically placed to ensure that any stress is evenly distributed throughout the concrete and that there aren’t spots that are more prone to failure than others.

Concrete surfaces that are expected to support heavy loads, like driveways or patios, benefit the most from rebar reinforcement. The extra strength can prevent the concrete from cracking under tension forces that would otherwise weaken it and cause sagging, breaking, or other damage. While rebar isn’t necessary for every concrete surface, it is an essential part of any large construction that requires added strength to support heavy loads.

Typically made from carbon steel, the rebar can be either black or epoxy-coated. The epoxy coating protects the rebar from corrosion and can make it up to 1,700 times more resistant than standard carbon steel rebar. Regardless of the type used, rebar must be carefully placed to ensure that the concrete can adequately withstand the force that will be exerted on it.

A concrete contractor knows how to properly place rebar in a concrete pour, and a well-written blueprint will include the exact specifications needed for each project. Getting this step right can help save time, money, and labor costs in the long run.

Rebar can be found in a wide variety of concrete structures, including roads, bridges, skyscrapers, and homes. Historians disagree on who first utilized reinforced concrete, but it’s been used in construction projects for over 150 years. The process can be complicated and require specialized equipment, but it can help build a stronger, longer-lasting, and more durable concrete structure.