At present, the cement is widely used across the world in the construction of various engineering structures. It has proved to be one of the leading engineering material of modern times and has no rivals in production and applications. Cements may be used alone (i.e., “neat,” as grouting materials), but the normal use is in mortar and concrete in which the cement is mixed with inert material known as aggregate to form a strong binding material. Following are various possible applications or uses of cement:
The most significant use of cement is production of concrete and mortar.
Cement mortar can be used for masonry work, plaster, pointing, etc.
Cement concrete can be used for laying floors, roofs, constructing lintels, beams, weather sheds, stairs, pillars, etc.
It can be used for construction of important engineering structures such as bridges, culvert, dams, tunnels, storage reservoirs, light houses, docks, etc.
It can also be used for construction of water tanks, tennis courts, septic tanks, lamp posts, roads, telephone cabins, etc.
It can be used for making joints for drains, pipes, etc.
It can be used for manufacturing precast pipes, garden seats, artistically designed urns, flower pots, dust bins, fencing posts, etc.
It can be used for preparation of foundations, watertight floors, footpaths, etc.
It can be used for creating fire-proof structures in the form of concrete. Also, it can be used for making acid-resistance and waterproof structures.
Colored cement can be used for decorating or coloring the structures.
It can be used for shotcreting the tunnel or geological walls to strength the structure.
Despite so many uses of cements, it has few demerits. However, its usability far overcomes its demerits. Some of the negatives of cement are as follows:
Structure once build out of cement are difficult to be displaced or reused. They can’t be easily recycled like plastics or steels.
Cement structure are very heavy. So, while building skyscrapers, it can’t be totally build on cement. Instead steel structures are placed.
The entire manufacturing process in a modern plant is now controlled through a microprocessor based programmable logic control system to maintain uniform quality of cement and a high rate of production. The entire operation of the plant is controlled centrally in a single control room and the plant employs minimum of manpower.
The modern plants have also taken adequate care to prevent the environmental pollution and dust nuisance to its surrounding areas. The cement mills have electro-static precipitators (ESP) installed to check the dust emissions. The bag filters and glass bag houses are located at various locations to prevent dust emission and to ensure healthy and hazard-free atmosphere.
Following three distinct operations are involved in the manufacturing of normal setting or ordinary or Portland cement:
The raw materials such as limestone or chalk and shale or clay may be mixed either in dry condition or in wet condition. The process is accordingly known as the dry process or the wet process of mixing.
Dry process (modern technology)
In this process, the raw materials are first reduced in size of about 25mm in crushers. A current of dry air is then passed over these dried materials. These materials are then pulverized into fine powder in ball mills and tube mills. All these operations are done separately for each raw material and they are stored in hoppers. They are then mixed in correct proportions and made ready for the feed of rotary kiln. This finely ground powder of raw materials is known as the raw mix and it is stored in storage tank.
The dry process has been modernized and is widely used at present because of following reasons:
Competition: At present, several dry process cement plants are vying with each other. The cement consumers in general and the practicing civil engineers in particular are greatly benefited by such competition.
Power: The blending of dry powders has now perfected and the wet process, which required much higher power consumption can be replaced with confidence.
Quality of cement: It is found that the quality of the production no longer depended on the skilled operators and workmen because temperature control and proportioning can be done automatically through a centralized control room.
Technology: There has been several advances in instrumentation, computerization and quality control.
Following is the procedure of manufacturing cement by dry process using modern technology:
Boulders of limestone upto 1.2m size are transported in huge dumpers upto 300kN capacity and dumped into the hopper of the crusher.
The hammer mill crushers of single stage are now used for crushing. The crushed limestone now of 75mm size is moved from crusher by a series of conveyors for stacking. The stacker helps in spreading the crushed materials in horizontal layers and the reclaimer restricts the variation of calcium carbonate in crushed limestone to less than 1% thereby minimizing quality variation in the materials.
The argillaceous or clay materials found in the quarry are also dumped into the crusher and stacked along with the limestone.
The crushed materials are checked for calcium carbonate, lime, alumina, ferrous oxide and silica contents. Any component found short is added separately.
The additive material and crushed limestone are conveyed to the storage hoppers. The raw materials are fed to the raw mill by means of a conveyor and proportioned by use of weigh feeders which are adjusted as per the chemical analysis done on the raw materials taken from the hoppers time to time.
The materials are ground to the desired fineness in the raw mill. The fine powder which emerges as a result of the grinding in the raw mill is blown upwards, collected in cyclones and fed to the giant sized continuous blending and storage silo by use of aeropole.
The material is dropped merely by gravity from the blending to the storage silo thereby conserving power.
The material is then once again pumped using an aeropole into the preheater with temperature increased from 60°C to 850°C by blowing hot gas at temperature of 1000°C.
The maerial from the bottom of the preheater is fed to the rotary kiln.
Burning
In modern technology of dry process, the coal brought from the coal fields is pulverized in vertical coal mill and it is stored in silo. It is pumped with required quantity of air through the burners. The preheated raw materials roll down the kiln and get heated in such an extent that the carbon dioxide is driven off with combustion gases. The material is then heated to temperature of nearly 1400°C to 1500°C when it gets fused together. The fused product is known as the clinkers or raw cement.
The size of clinkers varies from 3mm to 20mm and they are very hot when they come out of burning zone of kiln. The clinker temperature at the outlet of kiln is nearly 1000°C. A rotary kiln of small size is provided to cool down the hot clinkers. It is laid in opposite direction and the cooled clinkers having temperature at about 95°C are collected in containers of suitable sizes.
Grinding
The clinkers as obtained from the rotary kiln are finely ground in ball mills and tube mills. During grinding, a small quantity, about 3 to 4 percent, of gypsum is added.
The gypsum controls the initial setting time of cement. If gypsum is not added, the cement would set as soon as water is added. The gypsum acts as a retarder and it delays the setting action of cement. It thus permits cement to be mixed with the aggregates and to be placed in position.
The grinding if clinkers in modern plants is carried out in the cement mill which contains chromium steel balls of various sizes. These balls roll within the mill and grind the mixture which is collected in a hopper and taken in the bucket elevator for storage in silos.
The cement from silos is fed to the packer machines. Most of the modern plants have electric packing plant having provision to account for the weights of empty bags of different types and to ensure a 50kg net weight of cement bag within ± 200g limit. Each bag of cement contains 50kg or 500N or about 0.035m3of cement. These bags are automatically discharged from the packer to the conveyor belts to different loading area. They are carefully stored in a dry place.
Initial setting time is that time period between the time water is added to cement and time at which 1 mm square section needle fails to penetrate the cement paste, placed in the Vicat’s mould 5 mm to 7 mm from the bottom of the mould.
Final setting time is that time period between the time water is added to cement and the time at which 1 mm needle makes an impression on the paste in the mould but 5 mm attachment does not make any impression.
INITIAL AND FINAL SETTING TIME We need to calculate the initial and final setting time as per IS: 4031 (Part 5) – 1988. To do so we need Vicat apparatus conforming to IS: 5513 – 1976, Balance, whose permissible variation at a load of 1000g should be +1.0g, Gauging trowel conforming to IS: 10086 – 1982.
Procedure to determine initial and final setting time of cement i) Prepare a cement paste by gauging the cement with 0.85 times the water required to give a paste of standard consistency. ii) Start a stop-watch, the moment water is added to the cement. iii) Fill the Vicat mould completely with the cement paste gauged as above, the mould resting on a non-porous plate and smooth off the surface of the paste making it level with the top of the mould. The cement block thus prepared in the mould is the test block.
A) INITIAL SETTING TIME Place the test block under the rod bearing the needle. Lower the needle gently in order to make contact with the surface of the cement paste and release quickly, allowing it to penetrate the test block. Repeat the procedure till the needle fails to pierce the test block to a point 5.0 ± 0.5mm measured from the bottom of the mould.The time period elapsing between the time, water is added to the cement and the time, the needle fails to pierce the test block by 5.0 ± 0.5mm measured from the bottom of the mould, is the initial setting time.
B) FINAL SETTING TIME Replace the above needle by the one with an annular attachment. The cement should be considered as finally set when, upon applying the needle gently to the surface of the test block, the needle makes an impression therein, while the attachment fails to do so. The period elapsing between the time, water is added to the cement and the time, the needle makes an impression on the surface of the test block, while the attachment fails to do so, is the final setting time.
Concrete admixtures are used to improve the behavior of concrete under a variety of conditions and are of two main types: Chemical and Mineral.
Chemical Admixtures
Fritz-Pak Corporation in Dallas, TX
Chemical admixtures reduce the cost of construction, modify properties of hardened concrete, ensure quality of concrete during mixing/transporting/placing/curing, and overcome certain emergencies during concrete operations.
Chemical admixtures are used to improve the quality of concrete during mixing, transporting, placement and curing. They fall into the following categories:
air entrainers
water reducers
set retarders
set accelerators
superplasticizers
specialty admixtures: which include corrosion inhibitors, shrinkage control, alkali-silica reactivity inhibitors, and coloring.
Mineral admixtures make mixtures more economical, reduce permeability, increase strength, and influence other concrete properties.
Mineral admixtures affect the nature of the hardened concrete through hydraulic or pozzolanic activity. Pozzolans are cementitious materials and include natural pozzolans (such as the volcanic ash used in Roman concrete), fly ash and silica fume.
They can be used with Portland cement, or blended cement either individually or in combinations.
ASTM Categories – Concrete Admixtures –
ASTM C494 specifies the requirements for seven chemical admixture types. They are:
Type A: Water-reducing admixtures
Type B: Retarding admixtures
Type C: Accelerating admixtures
Type D: Water-reducing and retarding admixtures
Type E: Water-reducing and accelerating admixtures
Type F: Water-reducing, high range admixtures
Type G: Water-reducing, high range, and retarding admixtures
Note: Changes occur in the admixture industry faster than the ASTM consensus process. Shrinkage Reducing Admixtures (SRA) and Mid-Range Water Reducers (MRWD) are two areas for which no ASTM C494-98 specifications currently exist.
CHEMICAL ADMIXTURES FOR CONCRETE
Definition: what are chemical admixtures?
The definition of RILEM (International Union of Testing and Research Laboratories for Materials and Structures) is:
• In most advanced countries, admixtures have become as essential ingredient of concrete as cement, aggregate and water themselves.
• Their effect on cement concrete, mortar or paste can be assessed by (the usual) simple tests on cement, mortar and concrete.
• It is rather difficult for a civil engineer to understand them through their chemical nature.
• They are unlikely to be pure compounds but associated with some other minor chemical compounds or they may be mixed formulations.
• Admixtures are usually very complex compounds chemically excepting a few.
• They modify one or more properties of concrete, mortar or paste either in fresh or hardened state.
• Admixtures interact with hydrating cement by physical, chemical or physico-chemical actions.
• These are added to the normal components of a mix not normally exceeding 5% by mass of cement or cementitious materials
• Admixtures for concrete, mortar or paste are inorganic or organic materials
Why admixtures? The need.
• We may need high flowability either due to reinforcement congestion or narrowness of sections or the inability to use vibrators• We may have to transport concrete over large distances or to great heights• We may have to observe strictly a low w/c ratio as per design requirement• But, in many other situations we cannot get away with such concrete or such an attitude.• If we have a problem later on, there is little we can do to rectify it.• Its durability, water tightness and consistent strength will be suspect.• If the cost of materials and labour are paid entirely by the owner (or some one else) at whatever rates we have quoted, then, Admixtures will make the concrete more expensive. But then our concrete will be an indifferent one,• If we are pouring a low grade concrete, say M20, If we are not unduly concerned about its water content nor its water-cement ratio,
CIVIL ENGINEERING 