Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. In the past limebased cement binders were often used, such as lime putty, but sometimes with other hydraulic cements, such as a calcium aluminate cement or with Portland cement to form Portland cement (for its visual resemblance to Portland stone).Many other non-cementitious types of concrete exist with different methods of binding aggregate together, including asphalt concrete with a bitumen binder, which is frequently used for road surfaces, and polymer concretes that use polymers as a binder.
Water cement ratio plays an important role which influences various properties such as workability, strength and durability. Adequate water cement ratio is required for production of workable.
When water is mixed with materials, cement reacts with water and hydration reaction starts. This reaction helps ingredients to form a hard matrix that binds the materials together into a durable stone-like material.
Concrete can be casted in any shape. Since it is a plastic material in fresh state, various shapes and sizes of forms or formworks are used to provide different shapes such as rectangular, circular etc.
Various structural members such as beams, slabs, footings, columns, lintels etc. are constructed with concrete.
ACI 318 Building code requirements for structural concrete and ACI 301 Specifications for Structural are used in United States as standard code of practice for this construction.
There are different types of admixtures which are used to provide certain properties. Admixtures or additives such as pozzolans or superplasticizers are included in the mixture to improve the physical properties of the wet mix or the finished material.
Various types of this are manufactured these days for construction of buildings and structures. These have special properties and features which improve quality of construction as per requirement.
Components of Concrete
cement, sand, aggregates and water. Mixture of Portland cement and water is called as paste. So, concrete can be called as a mixture of paste, sand and aggregates. Sometimes rocks are used instead of aggregates.
The cement paste coats the surface of the fine and coarse aggregates when mixed thoroughly and binds them. Soon after mixing the components, hydration reaction starts which provides strength and a rock solid is obtained.
It is the process of mixing together the various ingredients—water, aggregate, cement, and any additives—to produce concrete. production is time-sensitive. Once the ingredients are mixed, workers must put the concrete in place before it hardens. In modern usage, most concrete production takes place in a large type of industrial facility called a concrete plant, or often a batch plant.
In general usage, concrete plants come in two main types, ready mix plants and central mix plants. A ready-mix plant mixes all the ingredients except water, while a central mix plant mixes all the ingredients including water. A central-mix plant offers more accurate control of the concrete quality through better measurements of the amount of water added, but must be placed closer to the work site where the concrete will be used, since hydration begins at the plant.
A concrete plant consists of large storage hoppers for various reactive ingredients like cement, storage for bulk ingredients like aggregate and water, mechanisms for the addition of various additives and amendments, machinery to accurately weigh, move, and mix some or all of those ingredients, and facilities to dispense the mixed concrete, often to a concrete mixer truck.
Modern concrete is usually prepared as a viscous fluid, so that it may be poured into forms, which are containers erected in the field to give the concrete its desired shape. Concrete formwork can be prepared in several ways, such as slip forming and steel plate construction. Alternatively, concrete can be mixed into dryer, non-fluid forms and used in factory settings to manufacture precast concrete products.
A wide variety of equipment is used for processing concrete, from hand tools to heavy industrial machinery. Whichever equipment builders use, however, the objective is to produce the desired building material; ingredients must be properly mixed, placed, shaped, and retained within time constraints. Any interruption in pouring the concrete can cause the initially placed material to begin to set before the next batch is added on top. This creates a horizontal plane of weakness called a cold joint between the two batches. Once the mix is where it should be, the curing process must be controlled to ensure that the concrete attains the desired attributes. During concrete preparation, various technical details may affect the quality and nature of the product.
Grades of concrete are defined by the strength and composition of the concrete, and the minimum strength the concrete should have following 28 days of initial construction. The grade of concrete is understood in measurements of MPa, where M stands for mix and the MPa denotes the overall strength.
Concrete mixes are defined in ascending numbers of 5, starting at 10, and show the compressive strength of the concrete after 28 days. For instance, C10 has the strength of 10 newtons, C15 has the strength of 15 newtons, C20 has 20 newtons strength and so on.
Different mixes (M) come in various mix proportions of the various ingredients of cement, sand and coarse aggregates. For instance, M20 comes in the respective ratio of 1:1:5:3. You can see other examples below in the table.
|Concrete Grade||Mix Ratio (cement : sand : aggregates)||Compressive Strength|
|Grades of Concrete|
|M5||1 : 5 : 10||5 MPa||725 psi|
|M7.5||1 : 4 : 8||7.5 MPa||1087 psi|
|M10||1 : 3 : 6||10 MPa||1450 psi|
|M15||1 : 2 : 4||15 MPa||2175 psi|
|M20||1 : 1.5 : 3||20 MPa||2900 psi|
|Standard Grade of Concrete|
|M25||1 : 1 : 2||25 MPa||3625 psi|
|M30||Design Mix||30 MPa||4350 psi|
|M35||Design Mix||35 MPa||5075 psi|
|M40||Design Mix||40 MPa||5800 psi|
|M45||Design Mix||45 MPa||6525 psi|
|High Strength Concrete Grades|
|M50||Design Mix||50 MPa||7250 psi|
|M55||Design Mix||55 MPa||7975 psi|
|M60||Design Mix||60 MPa||8700 psi|
|M65||Design Mix||65 MPa||9425 psi|
|M70||Design Mix||70 MPa||10150 psi|
Choosing the right concrete grade for the construction job:
So what can these grades be used for and which is best for the job at hand? Below is a list of a few of the initial concrete grades and what they are best used for.
Used for: Patio slabs, pathways and non-structural work
Type: Domestic & commercial use
Used for: Pavement kerbs and floor blinding
Type: Domestic & Commercial
Used for: Domestic floors and foundations (where the weight of structure will be lighter). Also good for workshop bases, garages, driveways and internal floor slabs.
Used for: Construction in all areas. Multi-purpose concrete mix, usually used for foundations.
Type: Domestic & Commercial
Used for: Pathways and roadways (this is the lowest grade concrete mix that can be used for this purpose). More durable than the grades that have come before, and thus is much more weather-resistant and can take heavy road traffic.
Used for: Commercial structures. This heavy concrete mix is usually used for creating external walls and slabs, as well as for structural piling.
Used for: Commercial construction sites, creating foundations and beams for structural support and roads. The most durable in this list, C40 can withstand chemical corrosion also, so is frequently used on farms where slurry could corrode structures, or in septic tanks.
How to Make Concrete?
Concrete is manufactured or mixed in proportions w.r.t. cement quantity. There are two types of concrete mixes, i.e. nominal mix and design mix. Nominal mix is used for normal construction works such as small residential buildings. Most popular nominal mix are in the proportion of 1:2:4.
Design mixed concrete are those for which mix proportions are finalized based on various lab tests on cylinder or cube for its compressive strength. This process is also called as mix design. These tests are conducted to find suitable mix based on locally available material to obtain strength required as per structural design. A design mixed offers economy on use of ingredients.
Once suitable mix proportions are known, then its ingredients are mixed in the ratio as selected. Two methods are used for mixing, i.e. Hand Mixing or Machine Mixing.
Based on quantity and quality required, the suitable method of mixing is selected. In the hand mixing, each ingredients are placed on a flat surface and water is added and mixed with hand tools. In machine mixing, different types of machines are used. In this case, the ingredients are added in required quantity to mix and produce fresh concrete.
Once the it is mixed adequately it is transported to casting location and poured in formworks. Various types of formworks are available which as selected based on usage.
Poured concrete is allowed to set in formworks for specified time based on type of structural member to gain sufficient strength.
After removal of formwork, curing is done by various methods to make up the moisture loss due to evaporation. Hydration reaction requires moisture which is responsible for setting and strength gain. So, curing is generally continued for minimum 7 days after removal of formwork.
Types of Concrete Construction
Concrete is generally used in two types of construction, i.e. plain concrete construction and reinforced concrete construction. In PCC, it is poured and casted without use of any reinforcement. This is used when the structural member is subjected only to the compressive forces and not bending.
When a structural member is subjected to bending, reinforcements are required to withstand tension forces structural member as it is very weak in tension compared to compression. Generally, strength of concrete in tension is only 10% of its strength in compression.
It is used as a construction material for almost all types of structures such as residential concrete buildings, industrial structures, dams, roads, tunnels, multi storey buildings, skyscrapers, bridges, sidewalks and superhighways etc.
Example of famous and large structures made with concrete are Hoover Dam, Panama Canal and Roman Pantheon. It is the largest human made building materials used for construction.