Preparing and Placing of Concrete PLAIN CONCRETE | Aci concrete
Preparing and Placing of Concrete PLAIN CONCRETE
Plain concrete is commonly known as (Aci concrete) concrete, is an intimate mixture of binding material, fine aggregate, coarse aggregate, and water. This was can be very easily molded to the desired shape and size before it loses there plasticity and its hardness. The Plain concrete is very strong in compression strength but very weak in a tension test. The tensile strength property is introduced in concrete its inducting different materials and this attempt has given rise to RCC, RBC, PSC, FRC, cellular concrete, and Ferro cement.
- This chapter is explained coordinating, blending, curing, properties, tests (lab test), and uses of plain concrete are dealt with in detail. The other improved versions of concrete are explained and their special properties and uses are pointed out (Aci concrete).
Major ingredients of concrete are:
- Binding material (like cement, lime, polymer)
- Fine aggregate (sand)
- Coarse aggregates (crushed stone, jelly)
A small number of admixtures like air-entraining agents, waterproofing agents, workability agents, etc. in this type also be added to impart the special properties into the plain concrete blending. Depending upon the proportion of ingredients, the strength of concrete varies. It is possible to determine the proportion of the ingredients for a particular strength by a mixed design procedure. In the absence of a mix design, the ingredients are proportioned as 1:1:2, 1:1 1 /2 :3, 1:2:4, 1:3:6, and 1:4:8, which is the ratio of the weight of cement to sand to coarse aggregate (Aci concrete).
In proportioning of concrete, it is kept in mind that voids in coarse aggregates are filled with sand and the voids in the sand are filled with cement paste. The proportion of ingredients usually adopted for various works are shown in the Table are given below.
The proportion of cement, sand, and coarse aggregates in concrete (Aci concrete)
|S. No.||Proportion||Nature of Work|
|1||1:1:2||machine foundation, footings for steel columns, and concrete underwater.|
|2||1:1 1/ 2 :3||Water tanks, shells, and folded plates, for other water retaining structures|
|3||1:2:4||Commonly used for reinforced concrete works like beams, slabs, tunnel lining, bridges|
|4||1:3:6||Piers, abutments, concrete walls, the sill of windows, floors|
|5||1:4:8||Mass concretes like a dam, a foundation course for walls, for making concrete blocks.|
Functions of Various Ingredients
Ingredients that are used are given below.
- Coarse aggregate
Cement is the binding material. After the addition of water, it hydrates and binds aggregates and the surrounding surfaces like stone and bricks. It was Generally a richer mix (with more cement) that gives more strength. The Setting time (initial setting time) starts after 30 minutes and it ended after 6 hours. Hence concrete should be laid in its mold before 30 minutes of mixing of water and should not be subjected to any external forces till the final setting takes place (Aci concrete).
Coarse aggregate consists of crushed stones. It should be well graded, and the stones should be of igneous origin. They should be clean, sharp, angular, and hard. They give bulk to the concrete and prevent shrinkage of the cement. Fine aggregate consists of river sand. It prevents the shrinkage of cement. When they’re surrounded by cement it gains mobility enters the voids (voids) in coarse aggregates and binding of ingredients takes place. It was added density to concrete that still fills the voids. The denser the concrete higher is its strength (Aci concrete).
Water used for making concrete should be clean. It activates the hydration of cement and forms a plastic mass. As it sets completely concrete becomes a hard mass. Water gives workability to concrete which means water makes it possible to mix the concrete with ease and place it in the final position. The more the water better is the workability. However excess water reduces the strength of concrete. Figure 3.1 shows the variation of the strength of concrete with the water-cement ratio. To achieve required workability and at the same time good strength a water-cement ratio of 0.4 to 0.45 is used, in case of machine mixing, and a water-cement ratio of 0.5 to 0.6 is used for hand mixing (Aci concrete).
Preparing and Placing of Concrete (Aci concrete).
The following steps are involved in the concreting:
- Transporting and placing
All measurement of materials for making concrete is known as batching. The following two methods of batching are practiced:
(a) Volume batching
(b) Weight batching.
- In this type of method cement, sand (fine aggregate), and concrete are batched by volume in a cubic foot. A wooden gauge (measuring box)box is made with wooden plates (now it was used iron boxes), its volume was being equal to that of one 50kg bag of cement. One bag of cement has a volume of 35 liters. The required amount of sand (fine aggregate) and coarse aggregate 20mm is added by measuring on to the gauge box (measuring box).
- The measuring quantity of water required for making this concrete is found after deciding the water-cement ratio. For example, if the water-cement ratio is 0.5, for one bag of cement (50 kg), the water required is 0.5 × 50 = 25 kg, which is equal to 25 liters. A suitable measure is used to select the required quantity of water. The volume of batching is not the ideal method of batching it’s very impotent. Wet sand has a higher volume for the same weight as dry sand. It is called bulking of sand. Hence it upsets the calculated volume required (Aci concrete).
Weigh Batching (weight calculation): This is one of the recommended methods of batching. A weighing (weight measuring) platform is used in the field to pick up a correct proportion of sand (fine aggregate) and coarse aggregates. Large weigh batching plants have automatic weighing equipment.
This type was produced uniform and good concrete, it is necessary to mix cement, sand, and coarse aggregate, first in dry condition and then in wet condition after adding water. The following methods are practiced: (a) Hand Mixing (b) Machine Mixing (Aci concrete).
Hand Mixing (manual mix): In this method, the Required amount of coarse aggregate (20mm) for a batch is weighed and is spread on an impervious platform. Then the sand (fine aggregate) required for the batch is spread over the coarse aggregate mix. They are mixed in dry conditions (because if water mixed the weight is increased) by overturning the mix with shovels. Then the volume of cement required for the batch is spread over the dry mix and mixed with shovels (for easy mix). After the uniform texture is observed water is added gradually and mixing is continued. A full amount of water is added, and mixing is completed when uniform color and consistency is observed. This method is the process of mixing is completed in 6 to 8 minutes of adding water. This method of mixing is not very good but for small works, it is commonly adopted (Aci concrete).
Machine Mixing (mixer machine): In large construction work and important works machine mixing is preferred. Figure 3.2 shows a typical concrete mixer. Required quantities of sand and coarse aggregates are placed in the drum of the mixer. 4 to 5 rotations are made for dry mixing and then the required quantity of cement is added and dry mixing is made with another 4 to 5 rotations. Water is gradually added, and the drum is rotated for 2 to 3 minutes during which period it makes about 50 rotations. At this stage uniform and the homogeneous mix is obtained.
- Transporting and Placing of Concrete
After mixing the concrete should be transported to the final position for placing. In many small works, it is transported in to iron pans from labor hand to hand of a set of workers. Wheelbarrow and hand carts also may be employed. In large-scale concreting chutes and belt conveyors or pipes with pumps are employed. In the transporting to the required place, care should be taken to see that segregation of coarse aggregate from the matrix of cement does not take place. Concrete is placed on form works. The form works (shuttering work) should be cleaned and properly oiled on the inner side. If the plain concrete is to be placed for foundation work, the soil bed should be compacted well (for avoiding bulking) and is made free from loose soil. The plain Concrete was dropped on its final position as closely as possible on placing. If it is dropped from a height, the coarse aggregates fall early and then the mortar matrix. This segregation results in weaker concrete.
Compaction of Concrete:
- In the process of placing concrete, the air is entrapped. The entrapped air reduces the strength of concrete up to 30%. Hence it is necessary to remove this entrapped air. This is achieved by compacting the plain concrete after placing it in its final position shrinkage. Compaction (using a vibrator) can be carried out either by hand or with the help of a vibrator’s machine.
- (a) Hand Compaction (using bamboo or rod): In this method, concrete is compacted by ramming (planks), tamping, spading, or by slicing with tools. In intricate portions, a pointed steel rod of 16 mm diameter and about a meter long is used for poking the concrete.
- (b) Compaction by Vibrators: Concrete can be compacted by using high-frequency vibrators. Vibration reduces the friction between the particles and set the motion of particles. As a result, entrapped air is removed, and the concrete is compacted. The use of vibrators reduces compaction time.
- When vibrators are used for compaction, the water-cement ratio can be less, which also helps in improving the strength of concrete. Vibration should be stopped as soon as cement paste is seen on the surface of the concrete. Over vibration is not good for the concrete.
- The following types of vibrators are commonly used in concrete: (a) Needle or immersion vibrators (b) Surface vibrators (c) Form or shutter vibrators (d) Vibrating tables. Needle vibrators are used in concrete beams and columns.
- Surface vibrators and form vibrators are useful in concrete slabs. Vibrating tables are useful in preparing precast concrete elements (Aci concrete).
Curing of Concrete
The Curing process may be defined as the process of maintaining satisfactory moisture and temperature conditions for freshly placed plain concrete for some specified time (minimum 7 days) for proper hardening of concrete. In this type Curing in the early ages of the concrete members is more important. Curing for 14 days is very important. Better to continue it for 7 to 14 days more. If anyone method of curing is not done properly, the strength of the concrete reduces. After Cracks develop due to shrinkage. The durability of concrete structure reduces (Aci concrete).
The following curing methods are employed:
(a) Spraying of water
(b) Covering the concrete members’ surface with wet gunny bags (curing bags), straw, etc.
(d) Steam curing and
(e) Application of curing compounds.
Spraying of water (curing): In the construction works Walls, columns, plastered surfaces are cured by sprinkling water using pipes.
Wet covering the surface (vertical members): In construction works Columns and other vertical surfaces may be cured by covering the surfaces (for a long time wet) with wet gunny bags or straw.
Ponding (filling): They have used horizontal surfaces like slab and floors are cured by stagnating the water (using gunny bags) to a height of 25 to 50 mm by providing temporary small hands with mortar (curing band).
Steam curing (boiled): In the pre-cast concrete manufacture of prefabricated concrete units steam (boiled) is passed over the units kept in closed chambers. It accelerates the curing process, resulting in the reduction of the curing period (7 to 14 days).
Application of curing compounds (chemical curing): Compounds (admixture) like calcium chloride may be applied on the curing surface top. The admixtures show affinity to the moisture and retain it on the surface. It keeps the concrete surface wet for a long time.