Er. Kaushal Kishore

By
KAUSHAL KISHORE
Materials Engineer, Roorkee

It is supposed that concrete should give satisfactory service to its entire life. However problems arises, if care is not taken during construction blemish appears on the surface of a concrete slab, it will likely to be one of these: bilisters, cracking, crazing, curling, delamination, discoloration. DUSTING, efflorescence, low spots, popouts, scaling or spelling. This paper will give the details about dusting.

DUSTING
Formation of loose powder resulting from disintegration of surface of hardened concrete is called dusting or chalking and this is composed of water, cement and fine particles. The concrete surface powder under any kind of traffic, and also surface can be easily scratched with nail or even by sweeping.
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By
KAUSHAL KISHORE
Materials Engineer, Roorkee

CAPPING THE CYLINDERS
It is required that the cylinders ends must be plane within 0.050 mm. The most common way of achieving this planeness requirement is to cap the ends of the cylinder as per ASTM C617⁶ with suitable materials. Three different capping materials are permitted (a) A thin layer of stiff Portland cement paste may be used on freshly molded specimens. (b) on hardened cylinders, either high-strength gypsum plaster or sulfur mortar may be used (c) A third method is, an elastomeric pad is placed within a metal retaining ring, and the assembly is then placed over the specimen end. The pad conforms to the shape of the cylinder end, but is prevented from spreading laterally by the metal retaining ring. This provides a uniform load across the specimen ends.

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By
KAUSHAL KISHORE
Materials Engineer, Roorkee

Fundamental principle
A pulse of longitudinal vibrations is produced by an electro-acoustical transducer, which is held in contact with one surface of the concrete under test. When the pulse generated is transmitted into the concrete from the transducer using a liquid coupling material such as grease or cellulose paste, it undergoes multiple reflections at the boundaries of the different material phases within the concrete. A complex system of stress waves develops, which include both longitudinal and shear waves, and propagates through the concrete. The first waves to reach the receiving transducer are the longitudinal waves, which are converted into an electrical signal by a second transducer. Electronic timing circuits enable the transit time T of the pulse to be measured.

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By
KAUSHAL KISHORE
Materials Engineer, Roorkee

The examination and compression testing of cores cut from hardened concrete is a well – established method, enabling visual inspection of the interior regions of a member to be coupled with strength estimation. Other properties which can be measured is also given in this paper.

IS: 456-2000 specified that the points from which cores are to be taken and the number of cores required shall be at the discretin of the engineer-in-charge and shall be representative of the whole of concrete concerned in no case, however, shall fewer than three cores be tested. Core shall be prepared and tested as described in IS: 516.

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By
KAUSHAL KISHORE
Materials Engineer, Roorkee

Concrete mix design is the process of choosing suitable ingredient of concrete and determining their relative quantities with the object of producing as economically as possible concrete of certain minimum properties, notable workability, strength and durability. It should be explained that an exact determination of mix proportions by means of table or computer data is generally not possible. The materials used are essentially variable and many of their properties cannot be assessed truly quantitatively. A Laboratory trial mix does not provide the final answer even when the moisture condition of aggregates are taken into account. Only a mix made and used on the site can guarantee that all properties of the concrete are satisfactory in every detail for the particular job in hand. In fact mix selection requires a knowledge of the properties of concrete and experimental data, and above all the experience of the expert who conduct the mix design. The selection of mix proportions is an art as much as a science. It is not enough to select a suitable concrete mix; it is also necessary to ensure a proper execution of all the operation involved in concreting. It cannot be stated too strongly that, competently used, concrete is a very successful construction material but, in the literal service of the word, concrete is not fool proof. The mix proportions once chosen, cannot expected to remain entirely immutable because the properties of the ingredients (cement, sand, aggregate, water and admixture) may vary from time to time.
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By
Er. KAUSHAL KISHORE
Materials Engineer, Roorkee

Ferrocement as a construction material has now gained acceptance in different applications, namely : housing, agriculture, marine, water supply, sanitation, water proofing treatment etc. Numerous studies published have built up confidence in the material resulting its wider application,

Traditional methods of roof water proofing by lime concrete and mud phuska with thin burnt clay tiles are very cumbersome, time consuming involved high labor cost and also due to non availability of traditional skills and good materials these methods of water proofing are now not very popular. Though bitumen felts are also provided for water proofing their life is less than five years and need frequent replacement.

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By
KAUSHAL KISHORE
Materials Engineer, Roorkee

Fly ash or pulverished fuel ash (pfa) is a finely divided powder thrown out as a waste material at the thermal power plants using pulverized coal for raising steam in the boilers. In the building industry, the use of fly ash a part replacement of cement in mortar and concrete at the construction site has been made all over the world including India and is well known. The important building materials which can be produced from fly ash are:

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By
KAUSHAL KISHORE
Materials Engineer, Roorkee

The problem of Alkali-silica reaction was believed to be non-existent in India till 1983, when its occurrence was diagnosed in two concrete dams. This paper describes this problem with respect to Indian aggregates and cement. A rapid method of test for alkali-aggregate reaction is investigated and described in the paper.

INTRODUCTION
The most common causes of deterioration in structural concrete with steel reinforcement in it are

• carbonation and chloride penetration leading to corrosion of steel resulting cracking and spelling of the concrete cover.
• inadequate cover to reinforcing steel Less common causes of deterioration in clude,
• freezing and thawing
• sulphate attack
• alkali-aggregate reaction.

There are three types of alkali-aggregate reactions, namely the alkali-silica, alkali-silicate and akali-carbonate reactions. Deterioration due to the alkali-silica reaction is more common and this paper refers to this aspect.
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By
Er. KAUSHAL KISHORE
Materials Engineer, Roorkee

NEED FOR CURING

The necessity for curing arises from the fact that hydration of cement can take place only in water-filled capillaries. That is why a loss of water by evaporation from the capillaries must be prevented. Evaporation of water from concrete, soon after placing depends on the temperature and relatively humidity of the surrounding air and on the velocity of wind over the surface of the concrete. Curing is essential in the production of concrete to have the desired properties. The strength and durability of concrete will be fully developed only if it is properly cured. The amount of mixing water in the concrete at the time of placement is normally more than required for hydration & that must be retained for curing. However, excessive loss of water by evaporation may reduce the amount of retained water below what necessary for development of desired properties. The potentially harmful effects of evaporation shall be prevented either by applying water or preventing excessive evaporation.
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By
Kaushal Kishore
Materials Engineer, Roorkee

A mix of M-80 Grade suitable for pumped concrete is to be designed with the following materials and detail.

1. OPC 53 Grade, 7-day strength 52.5 N/mm², Spgr 3.15

2. Silica Fume Specific Gravity 2.20

3. Standard deviation for the mix 5.0 N/mm²

4. Grading and properties of river sand and 12.5 mm crushed aggregate are given in Table-1

5. Superplasticizer based on modified Polycarboxylate, specific gravity 1.06, liquid pH 6.0. With the given set of materials, it was found that at a dosages of 2.5 % bwc it gives a reduction of 30% of water for the required slump of 100 mm after one hour at the average day site temperature of 37 degree C.
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