M30 grade concrete is often used for post-tensioned members, whereas M40 grade concrete is typically used for pretensioned members. The losses in prestress members owing to various causes range from 250 N/mm2 to 400 N/mm2. Thus, a post-tensioning cable with 50,000 psi (354 MPa) breaking strain will require a minimum diameter of 1/8 inch (3.2 mm). For a pretensioned member, the required cable diameter would be 1/4 inch (6.4 mm).
Concrete has high tensile strength in its early stages of development and increases in length before it begins to stretch out. This is called "creep" and occurs because of two factors: thermal expansion and chemical hardening. As the concrete heats up during daylight hours, it expands slightly; this can be as much as 8 percent of its original size. Concrete also hardens over time when water evaporates or chemical agents are added; this process also causes the material to expand.
If unconfined creep is not taken into account, then concrete posts will likely fail after only 20 years instead of the expected 100. To prevent this, steel bars are placed within the concrete at regular intervals to provide support and hold it in place while the material creeps away from the first barrow beam.
Concrete kinds and their applications
|Concrete Grade||Mix Ratio (cement : sand : aggregates)||Compressive Strength|
|M10||1 : 3 : 6||1450 psi|
|M15||1 : 2 : 4||2175 psi|
|M20||1 : 1.5 : 3||2900 psi|
|Standard Grade of Concrete|
The letter "M" stands for mix, and the number following it represents the minimum compressive strength that the structure will achieve after 28 days of casting. So, M40 signifies that the building built with this grade of concrete will have a minimum strength of 40 N/mm sq. Or Mpa after 28 days. The higher the number, the stronger the material.
Concrete's strength increases as it cures (hardens) - just like steel. If you walk on a freshly poured slab, it will be very soft because there is no water to harden the cement. As it cures, it becomes more solidified and less likely to collapse.
When a contractor calls for M40 grade concrete, they are asking for a low-cost option for creating a strong foundation. This concrete should not be used in areas where excessive force will be applied to the structure, such as foot traffic centers of buildings or main entrances. The open area around these features can erode over time due to freezing and thawing of the ground which can cause premature failure of the concrete.
M40 concrete should only be used in applications where durability is not a major concern, such as driveways and small sidewalks. These structures do not experience heavy traffic or any type of loading so they do not need ultra-strong concrete.
The M symbol followed by numbers refers to the minimum required compressive strength of the cured concrete.
High early strength concrete (2500-3500 psi compressive strength in 24 hours) is often achieved with Type III high-early strength cement (see Table 1), a high cement content (600-1000 lb/cu yd), and lower water-cement ratios (0.3 to 0.45 by weight). High-early strength cements have higher calcium carbonate contents than standard cements (approximately 15% vs 12%). They are also more alkaline (pH 10.5-11.5) than standard cements (pH 13.5-14.5).
Type III cements have higher heat release rates and burn temperatures than Type I or II cements. This means that they can be used in hotter climates and for larger projects. However, because of their high alkalinity, they should not be mixed in an oxygen-deprived environment such as a vacuum bagging plant or dry dock.
Type IV cements have lower calcium carbonate contents (130-220 kg/m 3 ) than Type III cements but still provide sufficient time to place the form work before the cement sets too much. They are most commonly used in colder climates where Type III cements would freeze during cold weather events. Type V cements have even lower calcium carbonate contents (70-110 kg/m 3 ) and are used when maximum strength is desired immediately after placement.
Residential concrete compressive strength needs range from 2500 psi (17 MPa) to 4000 psi (28 MPa) and greater in commercial projects. Certain applications need higher strengths of up to and surpassing 10,000 psi (70 MPa). Concrete that meets these requirements is called high-strength concrete.
Concrete's compressive strength depends on several factors such as the type of aggregate used, the amount of cement, the age of the concrete, among others. Concrete's tensile strength determines how much weight it can support before it breaks; however, compression stresses often exceed tensile forces so concrete generally remains strong after it has been loaded by weights or people.
High-strength concrete is more resistant to damage caused by objects falling onto it or environmental conditions such as heat and humidity. The extra strength also allows building owners to use thinner walls, which reduce construction costs. Thicker walls would be required if high-strength concrete had not been developed.
The minimum requirement for high-strength concrete is 3500 psi (24 MPa), but most builders use 5000 psi (34 MPa) or even 6500 psi (44 MPa). This article uses the number 50000, which means 50 km-lbs/in2 (56 MPa).
Loads above the average person's weight will cause low-strength concrete to crack or split.
The high-tensile steel used for prestressed concrete members is often made up of wires, bars, or strands...
I Because of its high tension, shear bond, and bearing resistance, high strength concrete is necessary for prestressing concrete. High-strength concrete is less prone to shrinkage cracks and has a lower modulus of elasticity and ultimate creep strain, resulting in less prestress loss in steel. High-strength concretes also have higher compressive strengths after curing.
II To produce strands capable of withstanding the tensile stress caused by the pre-stressing operation itself, which can be as high as 20 times the yield strength of conventional low-strength concretes.
III To produce anchors that can firmly hold the strand in the hole, which must be able to withstand the combined action of the horizontal component of the load and the tension imposed on the strand.
IV So that the concrete surface can support some of the weight of an overlying structure.
V To prevent the strand from moving when the concrete is under compression, thereby preventing loss of tension.
VI To produce strands that are flexible enough for installation in confined spaces.
VII To produce strands that can be colored to match existing structures or to provide additional protection against corrosion for structures exposed to corrosive environments.
VIII To produce strands that do not affect the overall performance of the concrete during the prestressing operation or the loading of the final structure.
In residential and commercial constructions, the compressive strength of concrete typically ranges from 2500 psi (17 MPa) to 4000 psi (28 MPa) and higher. Several applications also make use of pressures greater than 10,000 psi (70 MPa). The tensile strength of concrete is about 20% of its compressive strength.
Cement is a powder that when mixed with water forms an adhesive that can be shaped into any number of useful products including mortar, which is used to bind together the elements in a brick wall or concrete floor; grout, which is used to fill in between tile pieces in a bathroom or kitchen floor; and paint, which is made by adding pigment to cement. Cement is also used in plaster, which is a dry mixture of clay, sand, and cement that's used to cover walls and ceilings. When water is added to plaster, it dries into a hard surface suitable for painting or otherwise decorating.
The term "cement" comes from the Latin word calx, which means lime. Cements are made by mixing a powder with water to form a paste and then allowing it to harden into a solid mass. The powder component consists mainly of silica and alumina combined with small amounts of other substances such as iron oxides, calcium sulfate, and magnesium oxide.