Languages

Concrete Basics

Works best with 120L CONCRETE MIXER

In its simplest form, concrete is a mixture of paste and aggregates. The paste, composed of cement and water, coats the surface of the fine and coarse aggregates. Through a chemical reaction called hydration, the paste hardens and gains strength to form the rock-like mass known as concrete.

Within this process lies the key to a remarkable trait of concrete: it's plastic and malleable when newly mixed, strong and durable when hardened. These qualities explain why one material, concrete, can build skyscrapers, bridges, sidewalks and superhighways, houses and dams.

Proportioning
The key to achieving a strong, durable concrete rests in the careful proportioning and mixing of the ingredients. A concrete mixture that does not have enough paste to fill all the voids between the aggregates will be difficult to place and will produce rough, honeycombed surfaces and porous concrete. A mixture with an excess of cement paste will be easy to place and will produce a smooth surface; however, the resulting concrete is likely to shrink more and be uneconomical.

A properly designed concrete mixture will possess the desired workability for the fresh concrete and the required durability and strength for the hardened concrete. Typically, a mix is about 10 to 15 percent cement, 60 to 75 percent aggregate and 15 to 20 percent water. Entrained air in many concrete mixes may also take up another 5 to 8 percent.

Portland cement's chemistry comes to life in the presence of water. Cement and water form a paste that coats each particle of stone and sand. Through a chemical reaction called hydration, the cement paste hardens and gains strength. The character of the concrete is determined by quality of the paste. The strength of the paste, in turn, depends on the ratio of water to cement. The water-cement ratio is the weight of the mixing water divided by the weight of the cement. High-quality concrete is produced by lowering the water-cement ratio as much as possible without sacrificing the workability of fresh concrete. Generally, using less water produces a higher quality concrete provided the concrete is properly placed, consolidated, and cured.

Hydration Begins
Soon after the aggregates, water, and the cement are combined, the mixture starts to harden. All Portland cements are hydraulic cements that set and harden through a chemical reaction with water. During this reaction, called hydration, a node forms on the surface of each cement particle. The

node grows and expands until it links up with nodes from other cement particles or adheres to adjacent aggregates. 
The building up process results in progressive stiffening, hardening, and strength development. Once the concrete is thoroughly mixed and workable it should be placed in forms before the mixture becomes too stiff.

During placement, the concrete is consolidated to compact it within the forms and to eliminate potential flaws, such as honeycombs and air pockets. For slabs, concrete is left to stand until the surface moisture film disappears. After the film disappears from the surface, a wood or metal hand float is used to smooth off the concrete. Floating produces a relatively even, but slightly rough, texture that has good slip resistance and is frequently used as a final finish for exterior slabs. If a smooth, hard, dense surface is required, floating is followed by steel troweling.

Modern concrete consists primarily of four components: Portland cement, sand, gravel and water. A common misconception with concrete is that it dries and gets hard. Actually, the hydraulic cement reacts with the water in a chemical process called hydration. As an example, concrete can be placed underwater and will still change from a liquid state to a solid state and achieve full strength.

Many additional ingredients can be added to the basic concrete mix in order to change the properties of the resulting concrete. The following list shows some common admixtures (additives) and additional ingredients and their basic purposes:

Admixtures

  1. Accelerators speed up the hydration, or hardening, of the wet concrete. Often used in colder temperatures so the concrete crew has less waiting time between placing and finishing the concrete.
  2. Retarders slow the hydration, or hardening, of the wet concrete. Often used in hotter temperatures so the concrete doesn’t set too quickly, allowing the concrete finishing crew to get the proper finishing work completed.
  3. Air entraining agents add and help distribute tiny air bubbles throughout the concrete. These tiny air bubbles help the concrete resist the freeze-thaw cycles with much less cracking and damage.
  4. Plasticizers and Superplasticizers improve the workability of the concrete during the wet (or plastic) stage allowing the concrete to flow more easily. They are particularly helpful when placing concrete around congested rebar arrangements. Alternatively, Plasticizers and Superplasticizers can be used to lower the water content in the concrete while keeping a decent level of workability.
  5. Pigments change the color of the concrete for aesthetic reasons.


Additions

  1. Fly Ash can replace about half of the required amount of Portland cement. Fly Ash is a by product of coal fired electric generating plants, so is often readily available and economical. Concrete made with Fly Ash and Portland cement can have higher strength and improved chemical resistance and durability. The use of Fly Ash concrete is considered environmentally sound, since most fly ash otherwise ends up in landfills and the energy to produce the replaced Portland cement can also be saved.
  2. Ground granulated blast furnace slag (GGBS or GGBFS) can also replace part of the required Portland cement. GGBS is a by product of the steel production process. GGBS has had the most use in Europe and Asia.
  3. Silica Fume can also replace part of the required Portland cement. Silica Fume is a by product of the manufacture of silica alloys. The particle size of Silica Fume is 100 times smaller than that of Portland cement. Silica Fume improves concrete strength, abrasion resistance and corrosion resistance to chemicals, particularly to salts.

What is Reinforced Concrete?

In the mid-1800s, builders began adding steel in the concrete to carry the tension forces. This reinforced concrete became a phenomenally popular building method. There are several reasons why the combination of reinforcing steel and concrete works so well:

  1. The coefficient of thermal expansion is similar for concrete and steel, so when reinforced concretes freezes or gets hot, the two materials contract and expand similarly. If they didn’t, the combination would tear itself apart over time.
  2. The bond between reinforcing steel bars (rebar) and concrete is strong and efficient. The rebar has surface deformations (ridges) to further improve that bond. Due to the strong bond, the concrete effectively transfers stresses to the steel and vice versa.
  3. When the cement paste contacts the steel rebar, it forms a non-reactive surface film that inhibits corrosion. This process helps rebar from corroding inside the reinforced concrete.
  4. The location of the rebar in the structure depends on the use. Simple beams and slabs often only have rebar only on the tension (bottom) side. When a continuous beam spans over top of columns, the tension is at the top of the beam, so rebar is needed at the top of the beam over column supports.

Column footings are interesting to consider. Many people don’t know where the tension side exists on the footing. As a simple way to remember, hold out your left hand with the palm facing up. Now take the index finger of your right hand and poke down into the middle of the outstretched palm. Cup your left hand a bit, as if reacting to the downward force of your index finger.

It’s easy to see that the skin at the bottom of your left hand becomes taut (goes into tension) and the skin on top of your hand gets wrinkly (goes into compression). Therefore, the bottom of a simple concrete footing is in tension right under the column. So the rebar needs to be near the bottom of the footing.

It is important that the reinforcing steel have enough concrete cover so that the concrete bonds to the rebars and allows the concrete and steel to act together as a monolithic structural unit. The concrete cover also protects the reinforcing steel from excessive moisture or chemical corrosion. The American Concrete Institute Building Code recommends the following

Curing begins after the exposed surfaces of the concrete have hardened sufficiently to resist marring. Curing ensures the continued hydration of the cement and the strength gain of the concrete. Concrete surfaces are cured by sprinkling with water fog, or by using moisture-retaining fabrics such as burlap or cotton mats. Other curing methods prevent evaporation of the water by sealing the surface with plastic or special sprays (curing compounds).

Special techniques are used for curing concrete during extremely cold or hot weather to protect the concrete. The longer the concrete is kept moist, the stronger and more durable it will become. The rate of hardening depends upon the composition and fineness of the cement, the mix proportions, and the moisture and temperature conditions. Most of the hydration and strength gain take place within the first month of concrete's life cycle, but hydration continues at a slower rate for many years. Concrete continues to get stronger as it gets older.