CMAT LE REVIEWER (CONCRETE) Flashcards
Artificial stone as a result of mixing cement, fine aggregates, coarse aggregates and water.
PLAIN CONCRETE
Concrete with reinforcement properly embedded in such a manner that the two materials act together in resisting forces.
REINFORCED CONCRETE
A binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together.
Concrete
The Father of Portland Cement
Isaac Charles Johnson
Laid the foundation for the chemical composition of Portland Cement
Louis Vicat
is those which harden by hydration in the presence of water. Limestone, clay, and gypsum are the main raw material to produce a _______. This raw material is burned at a very high temperature to manufacture __________.
HYDRAULIC CEMENT
The ______________ doesn’t require water to get harden. It gets with the help of carbon dioxide (CO2) from the air. This type of cement needs dry conditions to harden. Lime, gypsum plasters, and oxychloride are the required raw material to produce ____
NON-HYDRAULIC CEMENT
- In usual construction work, _____ is widely used.
- It is used for general construction purposes
- It is also used in most of the masonry works
Ordinary Portland Cement
The major raw materials used in the manufacture of cement are Calcium, Silicon, Iron and Aluminum. These minerals are used in different form as per the availability of the minerals.
Mixing of Raw Material
-The burning process is carried out in the rotary kiln while the raw materials are rotated at 1-2rpm at its longitudinal axis. The rotary kiln is made up of steel tubes having the diameter of 2.5-3.0 meter and the length differs from 90-120meter. The inner side of the kiln is lined with refractory bricks. The kiln is supported on the columns of masonry or concrete and rested on roller bearing in slightly inclined position at the gradient of 1 in 25 to 1 in 30. The raw mix of dry process of corrected slurry of wet process is injected into the kiln from the upper end. The kiln is heated with the help of powdered coal or oil or hot gases from the lower end of the kiln so that the long hot flames is produced.
-As the kiln position is inclined and it rotates slowly, the material charged from upper end moves towards lower end at the speed of 15m/hr. In the upper part, water or moisture in the material is evaporated at 400oC temp, so this process is known as Drying Zone. The central part i.e. calcination zone, the temperature is around 10000C, where decomposition of lime stone takes place. The remaining material is in the form of small lumps known as nodules after the CO2 is released
Burning of Raw Material
The cooled clinkers are received from the cooling pans and sent into mills. The clinkers are grinded finely into powder in ball mill or tube mill. Powdered gypsum is added around 2-3% as retarding agent during final grinding. The final obtained product is cement that does not settle quickly when comes in contact with water. After the initial setting time of the cement, the cement becomes stiff and the gypsum retards the dissolution of tri-calcium aluminates by forming tricalcium sulfoaluminate which is insoluble and prevents too early further reactions of setting and hardening.
Grinding of Clinkers
The grinded cement is stored in silos, from which it is marketed either in container load or 50kg bags.
Storage and Packaging
The size of the particles of the cement is its fineness. The required fineness of good cement is achieved through grinding the clinker in the last step of cement production process. As hydration rate of cement is directly related to the cement particle size, fineness of cement is very important.
Fineness of Cement
_________ refers to the ability of cement to not shrink upon hardening. Good quality cement retains its volume after setting without delayed expansion, which is caused by excessive free lime and magnesia.
* Test : Le Chatelier test and Autoclave Test
Soundness of Cement
- The ability of cement paste to flow is consistency
- It is measured by Vicat test.
- A cement is said to have a normal consistency when the plunger penetrates 10±1 mm
Consistency of Cement
Three types of strength of cement are measured – compressive, tensile and flexural. Various factors affect the strength, such as water-cement ratio, cement-fine aggregate ratio, curing conditions, size and shape of a specimen, the manner of molding and mixing, loading conditions and age.
Strength of Cement
- Cement sets and hardens when water is added. This setting time can vary depending on multiple factors, such as fineness of cement, cement-water ratio, chemical content, and admixtures. Cement used in construction should have an initial setting time that is not too low and a final setting time not too high. Hence, two setting times are measured:
- Initial set: When the paste begins to stiffen noticeably (typically occurs within 30-45 minutes)
- Final set: When the cement hardens, being able to sustain some load (occurs below 10 hours)
Again, setting time can also be an indicator of hydration rate.
Setting Time of Cement
When water is added to cement, the reaction that takes place is called hydration. Hydration generates heat, which can affect the quality of the cement and also be beneficial in maintaining curing temperature during cold weather. On the other hand, when heat generation is high, especially in large structures, it may cause undesired stress. The heat of hydration is affected most by C3S and C3A present in cement,
and also by water-cement ratio, fineness and
curing temperature. The heat of hydration of Portland cement is calculated by determining the difference between the dry and the partially hydrated cement (obtained by comparing these at 7th and 28th days).
Heat of Hydration
Heating a cement sample at 900 - 1000°C (that is, until a constant weight is obtained) causes weight loss. This loss of weight upon heating is calculated as loss of ignition. Improper and prolonged storage or adulteration during transport or transfer may lead to pre-hydration and carbonation, both of which might be indicated by increased loss of ignition.
Loss of Ignition
When cement is mixed with water, the water replaces areas where there would normally be air. Because of that, the bulk density of cement is not very important. Cement has a varying range of density depending on the cement composition percentage. The density of cement may be anywhere from 62 to 78 pounds per cubic foot.
Bulk Density
Specific gravity is generally used in mixture proportioning calculations. Portland cement has a specific gravity of 3.15, but other types of cement (for example, portland-blast-furnace-slag and portland- pozzolan cement) may have specific gravities of about 2.90.
Specific Gravity (Relative Density)
Proportioning concrete mixture is done in two different ways: by weight or by volume method. The most common and convenient way is by the volume method using the empty bag of cement or by a measuring box for sand and gravel. Measuring the aggregates and water by weight is sometimes used in a concrete batching plant for ready-mix concrete or as specified in the plan.
CONCRETE PROPORTION
Selection of the proper quantity of cement, coarse aggregate, sand and water to obtain the desired quality is known as proportioning of concrete. Concrete is formed by successive filling of voids in aggregate by sand, the voids in the sand by cement and voids in cement by water and undergoing a chemical reaction.
PROPORTIONING OF CONCRETE
After fixing, the desired proportion of the number of required ingredients, viz., cement, coarse- aggregate, fine aggregate, cement and sand, has to be measured out in batches for mixing. This process of measuring out ingredients is called batching. Batching may be done by weight or by volume. Volume batching is inferior to weight batching as using the former is liable to change the volume of sand in bulking or aggregate constant void feasibility.
BATCHING OF MATERIALS
In this batching method, all the ingredients of concrete are directly weighed in kilograms. As the weight of cement bag is 50kg, 20 bags are needed for 1 ton of cement.
WEIGHT BATCHING
In this batching method, two units of measurement are employed in L and solids in cubic meters. That is all ingredients are measured in cubic meters.
VOLUME BATCHING
- Mixing of concrete may be done by hand or by a machine. Mixing should be done thoroughly so that the ingredients are uniformly distributed, and this can be judged by the uniform colour and the consistency of concrete.
- On a clean, hard and water-tight platform cement and sand are mixed dry using shovels until the mixture shows a uniform colour. Then aggregates are added and the ingredients are thoroughly mixed. Based on the water-cement ratio, the quantity of water required is calculated and added to the dry mix. The mass is then turned to obtain a workable mass and placed in the required area within 30 minutes. Hand mixing can be used for small quantities of concrete, or due to the non-availability of a machine or where the noise of the machine should be avoided. In general, extra cement of 10% is used to compensate for the possible inadequacy.
Mixing of concrete
- The mixed concrete should be conveyed to the concreting yard as early as possible but within the initial setting time of the cement. The choice of conveyance depends on several factors, viz., nature of work, distance from the mixing place to the construction site, height to be lifted, type of cement, etc.
TRANSPORTING AND PLACING
Compaction of concrete has to be done as early as possible after placing the concrete in place. The purpose of compaction is to expel air and bring the particles closer so as to reduce the void and make the concrete denser. This increased density will give higher strength and make the concrete impermeable. Over-compaction leads to segregation while under-compaction makes the concrete lean. To check for correct compaction, the compaction should be stopped as soon as the cement paste starts appearing on the upper surface of the concrete.
- Compaction by hand may be performed by rodding, tamping, ramming or hammering. Wooden steel hammers are used for ramming massive concrete works. Tampering is adopted for compacting slabs or other horizontal surfaces. Rodding is followed for compacting vertical sections.
- Compaction by machines is performed using mechanical vibrators.
COMPACTION OF CONCRETE
_______ is a method by which the concrete is protected against loss of moisture required for hydration and kept within the recommended temperature range. Curing will increase the strength and decrease the permeability of hardened concrete. Curing is also helps in mitigating thermal and plastic cracks, which can severely impact durability of structures.
CURING OF CONCRETE
is an on-the-spot test to determine the consistency as well as workability of fresh concrete. This test plays a vital role in ensuring immediate concrete quality in a construction project. It is used almost in every construction sites.
concrete slump test
just subsides shortly and more or less maintain the mould shape. This type of slump is most desirable.
true slump concrete
If one-half of the cone slides down in an inclined plane, it is called a shear slump. Shear slump indicates lack of cohesion in the concrete mix. Shear slump may occur in the case of a harsh mix.
Shear slump
In this case, fresh concrete collapses completely.
Collapse Slump
If concrete maintains the actual shape of the mould, it is called _____ which represents stiff, consistent and almost no workability.
Zero Slump
The ________ is the only reliable condition to get an idea about the workability of concrete. If other types occur, the test should be repeated.
true slump
- The_________ of the concrete cube test provides an idea about all the characteristics of concrete. By this single test one judge that whether Concreting has been done properly or not. Compressive strength of concrete depends on many factors such as water-cement ratio, cement strength, quality of concrete material, quality control during the production of concrete, etc.
COMPRESSIVE STRENGTH TEST
ASTM C39 - STANDARD TEST METHOD FOR COMPRESSIVE STRENGTH OF CYLINDRICAL CONCRETE SPECIMEN
ASTM C805 – REBOUND HAMMER OR SCHMIDT HAMMER
ASTM C803 – PENETRATION RESISTANCE TEST
ASTM C597 – ULTRASONIC PULSE VELOCITY
ASTM C900 – PULLOUT TEST
ASTM C42 – DRILLED CORE
ASTM C1074 – WIRELESS MATURITY SENSORS
COMPRESSIVE STRENGTH TEST
The rebound hammer test is done using a spring release mechanism that activates a hammer and drives a plunger into the surface of the concrete. The rebound distance from the hammer to the surface is then measured and given a value between 10 and 100. That measurement is correlated to the concrete’s strength, but the measurement can not be taken accurately unless the rebound hammer is calibrated using cored samples.
ASTM C805 – REBOUND HAMMER OR SCHMIDT HAMMER
A penetration resistance test uses a testing device that drives a small rod into the surface of the concrete. The force used to break the surface of the concrete, and the depth of the hole is then used to measure the compressive strength. This test is highly influenced by the type of concrete mix used and surface conditions. The measurement can not be taken accurately unless the penetration resistance device is calibrated using multiple core samples.
ASTM C803 – PENETRATION RESISTANCE TEST
- Drilled core tests are considered one of the most accurate tests because cores are taken from the concrete slab being tested they cured under the same conditions as the entire slab. Once the core is removed from the slab, it undergoes a compression test in a hydraulic press, which is done in a concrete testing lab.
ASTM C42 – DRILLED CORE
Precast concrete is a construction material created by pouring concrete into a preshaped mold. The concrete is then left to cure in an appropriate environment. Once cured, the mold is removed and reused.
The precast concrete product is then taken to the job site and used for the project. For instance, if designed to be walls, the precast concrete walls are typically lifted into position by a crane on site.
PRECAST
Ultrasonic pulse velocity tests the velocity of vibrations through a concrete slab. The velocity of the vibrations gives data on the concrete’s elasticity, density, and resistance to stress. The data can then be used to determine the overall strength of the concrete. This test is great because it is one of the few non-destructive concrete testing methods, and can quickly help test for flaws that aren’t visible to the human eye, such as internal cracks and honeycombing.
ASTM C597 – ULTRASONIC PULSE VELOCITY
