Most projects, even residential buildouts, use some type of concrete for one or more elements of the project. Whether it's a slab for the house, slabs for the floors or walls, footings for the foundation, steps, patios and driveways, beams for porches and decks—concrete is used in nearly any construction process. But how does one make concrete? No need to worry—knowing the basics will not only help you understand what to expect when working with concrete but it will also help you ask the right questions when working with your contractor, engineer or client.
Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time.
Fresh concrete is plastic that can be molded, cast into forms, and shaped.
Water, ice, and other liquids reduce the strength of cured concrete by dissolving the hydrated cement.
The strength of concrete is dependent on numerous factors, including the amount and type of cement used.
Cement paste hydration. The greater the water content in the concrete mix, the more capable it becomes of making its own water, which serves as a sort of "fertilizer" for its micro-organisms, increasing the strength of the concrete over time.
Carbonation. The carbonation process starts when water reacts with the binder (the material that holds together the aggregate and cement in concrete) to create carbon dioxide gas. Carbon dioxide gas is what makes concrete expand when it sets and contracts when it dries out. This expansion and contraction causes the reinforcing steel to bend, which strengthens the concrete.
Crystallization. When you pour concrete, there's a period where it's just sitting there — not hardening or even solidifying yet. This stage is called "primary drying".
Drying. As the concrete is getting harder, it gets drier — less likely to absorb water from rain or groundwater when wet. Drying can be accelerated by adding materials such as portland cement that have a high water absorption rate to reduce free water in the mix so that less will go into the drying and crystallization phases later on.
Drying and curing is an essential part of any concrete construction job. There are several ways to dry concrete and in the long run, these different methods can have a huge impact on the performance of your concrete. Concrete needs to cure; this is the chemical reaction between the water, cement, and other materials used to create the concrete. These reactions could continue for up to 28 days, which means that you need to be patient before using it. The curing process can take more than 45 days in humid conditions.
Concrete has many different properties that can be exploited for the final benefit of the end-user, but in order to understand what is required, it is essential that the client or engineer has a full understanding of these properties and how they affect each other. In most cases, a combination of these properties will need to be taken advantage of when using concrete as an end product, so it is important that you have a good understanding of what you want from your concrete. Below we take a look at all the different types of property that you need to consider and give advice as to which type may be best for your project.
Now that you understand the basic components of concrete, it's time to delve into some of the finer details. Let's begin with a simple question: What is the chemical composition of concrete?
The answer to this could be surprisingly simple. The chemical composition of concrete is identical to the chemical composition of cement. Concrete and cement are essentially the same product; in fact, the word cement is used to refer only to a slurry made from Portland cement and water, which was applied as mortar between courses of masonry or stones. Nowadays we usually use "cement" as a more general term for any cement-based binder - including concrete and mortar - but technically speaking, it's pretty much synonymous with Portland cement itself.
The term “hydration” is used to describe the chemical reactions between water and cement, resulting in the formation of crystalline compounds. Hydration is an exothermic process that consumes energy from the surroundings. It is important for concrete professionals to understand hydration because it affects all properties of hardened concrete, such as strength and permeability, contributing to changes in volume that may cause cracking or other damage.
Hydration mechanisms occur at a molecular level by means of complex chemical reactions between cement minerals (cement paste) and water molecules. These complex reactions are usually divided into two categories: 1) nucleation and 2) crystal growth (ASTM C114). However, this paper will not go into details about these mechanisms because they are very difficult concepts to grasp without specialized training in chemistry or mineralogy. A more useful way of explaining hydration processes involves only a basic understanding of atomic bonding behavior; specifically, ionic bonding behavior between atoms of different elements within compounds made up of these atoms (or ions).
Concrete has a number of adverse characteristics that can limit its use or lead to problems in service. Many of these are associated with the formation of microcracks, and together they are often referred to as deleterious processes.
There are three main types:
Sulfate attack is the expansion of cement paste by sulfate ions, resulting in cracking and loss of strength.
Alkali-silica reaction occurs when alkali cations from the pore solution penetrate siliceous aggregates, causing them to swell and crack.
Carbonation occurs when atmospheric CO2 diffuses into concrete, converting calcium hydroxide into calcium carbonate (limestone) and releasing water. This reduces the pH within pores and cracks.
The most effective way to prevent these processes is to reduce water permeability by increasing cement content or using less permeable materials in combination with ground granulated blast furnace slag (GGBS) or fly ash.
Alkali-silica reaction (ASR) is a complex chemical reaction that occurs when moisture and alkali are present in concrete. The alkali reacts with certain siliceous rocks, such as quartz, opal, chalcedony, or chert. ASR is a major problem that affects the durability of concrete structures.
The first phase of the reaction is the formation of a gel which swells as it absorbs water and expands to create internal pressure. Eventually, this causes cracking and spalling. The extent of damage depends on the relative amount of reactive silica in the aggregate, how much alkali hydroxide is available for reaction, the amount of water entering the system, and the chemical composition of pore solution found within concrete.
The American Concrete Institute (ACI) has identified five different types of ASR based on mineralogical characteristics: Type A - amorphous silica; Type B - cristobalite; Type C - tridymite; Type D - Opal C (cubic); and Type E – Opal CT (trigonal).
When exposed to carbon dioxide, the calcium hydroxide Ca(OH)2 in the paste reacts with the gas to produce calcium carbonate CaCO3, which is insoluble and occupies more space than the original compound. This carbonation process reduces alkalinity and causes a reduction in pH value. Carbonation can cause corrosion of steel reinforcement within concrete because, as shown by the equation below, it consumes hydroxyl ions (OH−) which are needed to maintain a protective layer of non-soluble iron oxide on the steel surface. Corroded reinforcement has a reduced cross-section which can result in structural failure. A good analogy is a bicycle chain; when lubricant deteriorates due to heat or age, segments of the chain rub against each other and quickly wear out.
Sulfate attack in concrete can be "external" or "internal". External: due to penetration of sulfates from outside the concrete into the concrete. Internal: due to a reaction between cement and sulfates already present in aggregates, groundwater, etc. The products formed by the reactions are expansive, which may result in cracking and loss of cohesion of the cement paste that binds aggregate particles together.
So you've decided to construct a new structure, and this is your first concrete project. Congratulations on your decision to become a building contractor! You're about to embark on an impressive journey into the world of concrete.
If you haven't heard of concrete before, let me give you a quick rundown. Similar to the way that you might use an oil-based lubricant to move metal parts, concrete is a mixture of cement and water. Concrete provides the structure for buildings by filling voids between rocks or other materials with mortar. The basic parts of concrete are cement (the stuff that makes cement paste), water (the liquid component), aggregates (pieces of rock and mineral matter added for reinforcement or aesthetic reasons), and mortars (hardener). If you're only interested in learning how much work it takes to mix concrete, skip ahead a section or two; all the other sections will still be relevant. For our purposes, we'll discuss just three types of concrete: normal, heavyweight, and lightweight.
This type of concrete is made from a mix of lime, cement, sand, water, and other materials. The mixture is poured into a mold and then cut into blocks. The blocks are steam-cured.
It has numerous advantages:
It is lightweight and fire-resistant.
Because it can be cut with a trowel or saw in the field, it can be used to form complex shapes.
It has good soundproofing qualities because of its low density.
Classification of concrete according to the mixture is given below,
According to the cement-aggregate ratio
According to the quality of aggregates
According to the method of mixing
According to the nature of compaction
According to the duration of setting
With appropriate mixes of components, there are many special properties that can be either engineered into the concrete or reinforced with fiberglass. Some of these are:
very high and very low strengths
very low density
very high electrical and thermal resistance
very high tensile strength
very high fire resistance
very high damping (a property desirable in earthquake-prone regions)
Very high durability (in seawater environments)
Very low heat of hydration (to reduce cracking in massive structures, such as dams and nuclear reactors)
Very low permeability to gases or liquids
First things first: understanding concrete! Concrete is a strong, artificial material used in construction. It can hold up extremely heavy structures. The actual chemical makeup of the concrete is made up of cement and fine aggregate (usually sand or gravel). Concrete parts are comprised of a mixture of cement, water, and aggregate.
Other ingredients can be added to change the properties - these are admixtures. The most common type of admixture is an air-entraining admixture which prevents damage from freezing and thawing cycles in cold weather climates by introducing microscopic air bubbles that disperse when frozen, preventing large-scale expansion that could crack it permanently.
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