The shuttering shall be of approved dressed timber of well seasoned wooden boards, to give a smooth and even surface and the joints shall not permit leakage of cement grout.
The timber shall be free from loose knots, projected nails, splits, adhering grout or other defects that may mar cement surface of the concrete. Opening for fan clamps and other fitting connected with services shall be provided in the shuttering as directed by Engineer-in-charge.
(i) Surface treatment for shuttering
The surface of timber shuttering that would come in contact with concrete shall be thoroughly cleaned and well wetted and coated with soap solution, raw linseed oil, or form oil of approved manufacturer, or any other approved materials such as polythene sheets, to prevent adhesion of concrete to formwork.
(ii) Camber
Suitable camber shall be provided in horizontal members of structures specially in long members to counteract the effects of deflection.
The camber for beams and slabs shall be 4mm per meter i.e, 1 in 250 and for cantilevers, at free and shall be 1/50th of the projected length or as directed by the Engineer-in-charge.
(iii) Removal of Formwork
The formwork shall be removed avoiding shock or vibration that may cause any damage to concrete. In a slab and beam constructions, side of the beam shall be stripped first; then the undersides of the slab and lastly underside of the beam.
The method of brick flat soling is very easy, in this process, the brick is laid into the foundation trenches. For Brick Soling, skilled workers are not required.
Method of Brick Flat Soling in Foundation Trenches
⇰ Picked Jhama or second class bricks in dry condition shall be laid on the foundation bed as headers with frog upward.
⇰ All bricks shall be laid closely with brick joints and the small gaps between them shall be field up with local fine sand and dry loose earth.
⇰ Brick-bats which are the permitted to be used only to provide break joints shall be placed at the edges of trenches.
⇰ The finished surface shall be levelled both longitudinally and transversely.
The following measures are taken to prevent deterioration of concrete:
1. From the consideration of permeability, the water-cement ratio is usually limited to 0.45 to 0.55.
2. The cement content should be such that it ensures sufficient alkalinity to prevent corrosion of reinforcement. For concreting under marine environment, minimum cement content of 350 kg/m or more is to be used.
3. The water-cement ratio and the cement content must provide enough paste to overfill the voids in compacted concrete.
4. Use of Portland slag cement or Portland pozzolana cement is advantageous for concreting in sea water.
5. Use of Portland cement having C3A content less than 5% is suitable for concreting under sulphate environment.
6. The super-sulphated cement provides acceptable durability against the acidic environment.
7. Addition of hydraulic additives is also helpful to prevent the deterioration of concrete.
8. It is possible to attain a marked improvement in the quality of concrete by encouraging natural or artificial carbonation of the surface layer.
9. Deterioration of concrete can also be prevented by treating the concrete with solutions of suitable salts or even acids in minor concentration.
10. The durability of concrete can also be increased by impregnating the pores with a suitable polymer.
Deterioration of concrete is caused due to the influence of both external and internal agencies. The external or environmental agencies causing the deterioration of concrete includes:
Weathering.
Attack by natural or industrial liquids and gases.
Acids in the form of water solution.
Fertilizers, insecticides and certain organic compounds.
Attack by biological agents.
The internal agencies responsible for the deterioration of concrete includes:
Harmful alkali-aggregate reactions.
Volume changes due to non-compatible thermal and mechanical properties of aggregates and cement paste.
Presence of sulphates and chlorides in the ingredients of concrete.
8 Factors Influencing the Choice of Mix Proportions
According to IS 456-2000 and IS 1343-1980, the designs of the concrete mix should be based on the following factors:
1. Grade Designation
The grade designation gives characteristic compressive strength requirements of concrete. It is the major factor influencing the mix design.
The concrete mix has to be designed for a target mean compressive strength which is somewhat higher than the characteristic compressive strength.
2. Types and Grades of Cement
The rate of development of compressive strength of concrete depends upon the type & grade of cement used. The choice of the type of cement depends upon the requirements of performance at hand.
Where very high compressive strength is required, Portland cement of grades 43 and 53 will be found suitable.
3. Maximum Nominal Size of Aggregates
The workability and compressive strength of concrete greatly depend upon the maximum size of aggregates. The workability increases with an increase in the maximum size of the aggregate.
On the other hand use of the large maximum size of aggregate, requires a smaller quantity of cement for a particular water-cement ratio. However, the smaller size aggregates provide a large surface area for bonding with the mortar matrix which increases the compressive strength.
4. Grading of Combined Aggregate
The grading of combined aggregate i.e., the relative proportion of the fine & coarse aggregates in a concrete mix is one of the important factors affecting the strength and workability of concrete. For dense concrete, it is essential that the coarse and fine aggregates be well grade.
5. Water-Cement Ratio
At a given age and under normal temperature, the compressive strength of concrete depends primarily on the water-cement ratio. The lower the water-cement ratio, the greater the compressive strength & vice-versa.
6. Workability
Insufficient workability of concrete may be liable for incomplete compaction of concrete which ultimately affects the strength, durability & surface finish of the concrete.
7. Durability
The durability of concrete is its ability to resist deterioration due to weathering action, chemical attack, abrasion etc. The requirements of durability are achieved by restricting the minimum cement content & the maximum water-cement ratio.
From the consideration of permeability, the water-cement ratio is usually restricted to 0.45 to 0.55. For a given water-cement ratio, the cement content should correspond to the required workability considering the placing conditions and the concentration of reinforcement.
In addition, the cement content is chosen to ensure sufficient alkalinity to provide a passive environment against the corrosion of steel.
8. Quality Control
The strength of concrete may vary from batch to batch over a period of time. The source of variation in the strength of concrete may be considered due to the variation in the quality of materials, mix proportion, mixing equipment, supervision & workmanship. The factor controlling this variation is quality control. The degree of control is ultimately evaluated by the variation in test results.
Necessity of Adding Gypsum (CaSO4) in Cement Manufacture
Portland cement is obtained from grinding clinkers and usually sets and hardens immediately after the addition of water. In order to slow down the setting time of cement, gypsum @ 3% to 4% is added during the process of clinker grinding.
If the quantity of gypsum(CaSO4) is more, the cement becomes very slow setting. It also hardens slowly which results in delay the removal of formwork. Gypsum combines with tricalcium aluminate and prevents flash setting.
But, if more gypsum is added with cement, the excess amount after combining with tricalcium aluminate remains free in cement. It expands and makes the cement unsound.
The tendency of water to rise to the surface of freshly placed concrete is called bleeding. Bleeding causes the formation of a porous, weak and non-durable concrete layer at the top. It also increases the permeability of concrete.
Bleeding can be reduced by adopting the following measures:
Using pozzolanic materials.
Proper proportioning of ingredients and uniform & complete mixing.
This test determines the relative quantity of mixing water that will bleed from a sample of freshly mixed concrete.
Apparatus
The apparatus used to conduct the bleeding test of concrete are:
1. A cylindrical container having inside diameter of 250mm, inside height of 280mm and capacity of 0.01m³(approximately).
2. A tamping rod made of steel of 16mm in diameter and 600mm long.
3. A pipette.
4. A graduated jar of 100cm³ capacity.
Procedure
Following are the procedure for bleeding test of concrete:
A sample of freshly mixed concrete is obtained. The concrete is filled in the cylindrical container in equal five layers and each layer is tamped by tamping rod. The top surface is made smooth by trowelling.
The test specimen is weighted and the weight of the concrete is noted. Knowing the total water content in 1m³ concrete. The quantity of water in the concrete in the cylindrical container is calculated.
The cylindrical container is kept in a level surface free from vibration at a temperature of 27℃ ± 2℃. Water accumulated at the top is collected by pipette at regular interval till bleeding ceases. All the bleeding water is stored in a measuring jar. Then
Bleeding Water Percentage = (Total quantity of bleeding water/Total quantity of water in the sample of concrete)×100%
