By
Kaushal Kishore
Materials Engineer
Roorkee

INTRODUCTION:
SCREENERS companies near Dehradun and elsewhere have set up highly sophisticated as well as, eco-friendly screening and washing plants for the production of uncrushed (Shingle) coarse aggregates and coarse sand direct from river bed. These plants are producing and supplying uncrushed (Shingle) aggregates of sizes 40 mm, 20 mm, 12.5 mm and river coarse sand, which complies to the specifications of
IS : 383-19702.

Our construction sites, particularly Govt. Departments hesitates in the use of uncrushed coarse aggregate as so far they are being supplied to them direct from river bed or by manual sieving without washing them with water. Thus neither they are clean nor properly graded. This draw back is not with the uncrushed aggregates produces and supplied from SCREENERS modern plants with regular quality control. In this booklet the readers will find that when quality uncrushed aggregates are available not only economically but locally, our construction sites particularly Govt. Departments should not hesitate in the use of uncrushed aggregates from the river bed and save our environment, as crusher generate pollution. Further in all the Civil Engineering Codes uncrushed aggregates from river bed has been specified to be used in our all Civil Engineering Construction.
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By
Er. Kaushal Kishore ,
Materials Engineer, Roorkee

Concrete Strength
Cement like water, aggregates and some times admixtures is one of the ingredient of concrete. The mixing of these materials in specified proportions produces concrete. Accordingly cement alone is not a building material, it is the concrete which is a building material. For a given cement and acceptable aggregates, the strength that may be developed by a workable, properly placed mixture of cement, aggregates, and water (under same mixing, curing and testing conditions) is influenced by the :

a) Ratio of cement to mixing water
b) Ratio of cement to aggregates, the strength of the mortar, the bond between the mortar and the coarse aggregate.
c) Grading, surface texture, shape, strength, and stiffness of aggregate particles.
d) Maximum size of aggregate.

Strength of concrete is directly related to the structure of the hydrated cement paste. Air in concrete produces voids. Excess of water in concrete evaporate leave the voids in the concrete. Consequently, as the W/C ratio increases, the porosity of the cement paste in the concrete also increases. As the porosity increases, the compressive strength of the concrete decreases.

STRENGTH OF CEMENT V/S STRENGTH OF CONCRETE
It is not possible to design a concrete mix of high strength with cement of low strength. The variation in strength of cement is due largely to the lack of uniformity in the raw materials used in its manufacture, not only between different source of supply, but also with in a quarry. Further, differences in details of the process of manufacture and above all, the variation in the ash content of coal used to fire the kilin, contribute to the variation in the properties of commercial cements. This is not to deny that the modern manufacturing of cement is a highly sophisticated process.

Upto 1975, the mass production of cement in India was only OPC-33 Grade. It was found difficulty in obtaining high strength concrete with this cement. The consumer has been normally finding it difficult to get consistent and ensured supply of high strength cement for prestressed concrete and certain items of precast concrete. For these special requirements BIS published IS:8112, Specification for OPC-43 Grade cement. Now, the varieties of cement manufactured in India are:

1. Ordinary Portland Cement (Grade OPC-33, OPC-43 and OPC-53. OPC-33 Grade almost vanished from Indian market)
2. Portland Pozzolana Cement (PPC)
3. Sulphate Resistance Cement (SRC)

Test results of different brand of cement minimum to maximum compressive strength are given in the table-1.

Due to variation of cement strength, the concrete made from these cement will also have variable strength. For a correct approach in the Concrete Mix Design, if the facilities at site are available, with the given set of materials, requirements and site conditions own W/C ratio v/s compressive strength of concrete curve should be developed at site itself.

It is often observed that cement bags marked as OPC-43 Grade may really be containing cement of much higher grade. PPC cement as per IS Code is only of 33 Grade. Where as on bags it is marked as 43 MPa or 53 MPa. Site cement samples should be tested for its actual strength and other properties. There are instances where higher grade cement is being used even for low strength concrete, as mortar or even for plastering. This can lead to unnecessary cracking of concrete/surfaces.

In low grade OPC, the gain in strength will continue beyond 28th day. Due to early strength gain of higher grade of OPC the concrete strength do not increase much beyond 28th day. The heat of hydration of higher grade OPC being higher, the chances of micro-cracking of concrete is much greater. Thus during initial setting period of concrete, the higher head of hydration can lead to damaging micro-cracking with in the concrete which may not be visible at surface. The situation can be worse when we tend to increase the quantity of the cement in concrete with a belief that such increase are better for both strength and durability of concrete.

Table-1 : Compressive Strength of different Grade of Cement:

The variation in cement strength is given below : (in N/mm2)
OPC-43 Grade : 7-days from 24.7 to 57.7 and 28-days from 40.0 to 74.5
OPC-53 Grade : 7-days from 37.8 to 57.8 and 28-days from 52.5 to 73.9
PPC : 7-days from 25.5 to 52.6 and 28-days from 48.0 to 67.0
Note: Compressive Strength (CS) of cement given are average values.

We at engineeringcivil.com are thankful to Sir Kaushal Kishore for submitting this research paper and helping all civil engineers understand the concept of Concrete Strength and factors affecting it.

By
Er. Kaushal Kishore ,
Materials Engineer, Roorkee

In about 80% of our construction sites, the water in the concrete mixer is added in a very crude manner either direct from a hoze pipe or by some container without any proper measured quantity. Thus no consideration is given to maintain free Water/Cement ration to its correct specified value resulting production of poor quality of concrete. The addition of mixing water in the concrete mixer with these crude methods always add more water then actually required. This excess water in due course evaporated leaving voids and increasing the porosity of the concrete. Such concrete will have lower strength and also will be not durable.

Therefore it is very important to maintain free W/C ration to its correct value in all the batches of concrete. Free W/C ratio means mixing water added to saturated and surface dry aggregates ie, if the site aggregates are dry extra water is to be added in the mixing water as per the absorption of aggregate, and if the site aggregates contains surface water, this surface water is to be deducted from the mixing water. The weight of aggregates should also be adjusted accordingly. A Concrete Mix Design is reported in standard moisture condition of aggregates and this is saturated and surface dry aggregates. If aggregates are being taken by volume bulking of sand should be taken into consideration.

To solve the construction sites mixing water problems, a simple graduated transparent plastic jar of least count 0.5 ltr, as per drawing should be supplied along with the mixer or may be fabricated at site. This Jar be installed at site near concrete mixer as shown in the drawing. The water may be filled in the jar to the quantity of required gauging water. While mixer is running the measured water in the jar slowly drain in the mixer drum through rubber hoze by opening the valve. If ADMIXTURES are to be used and required to be mixed with the gauging water, this may be mixed with water of the jar.

We at engineeringcivil.com thankful to Sir Kaushal Kishore for publishing his paper on “Water Measuring Jar for Concrete Mixer”.

We at engineeringcivil.com are pleased to announce that our website supporter and regular contributor, Sir. Kaushal Kishore have been awarded “NATIONAL (N) ICI-JA LIFE TIME ACHIEVEMENT AWARD – 2011″ for his valuable contributions to the field of civil engineering.

We congratulate Sir. Kaushal Kishore and hope his work in civil engineering field gets more and more recognition and we here at engineeringcivil.com continue to get his support and knowledge which will be of tremendous use to the upcoming engineers.

Paper of Efflorescence in Bricks and Efflorescence and Leaching in Concrete by Sir. Kaushal Kishore
Materials Engineer, Roorkee

Efflorescence is the usual terms for deposit of soluble salts, formed in or near the surface of a porous material, as a result of evaporation of water in which they have been dissolved.

EFFLORESCENCE IN BRICKS:
Usually sulphate of magnesium, calcium, sulphate and carbonate (and sometimes chloride and nitrates) of sodium and potassium are found in efflorescence. These salts may be traced to the brick itself, sand used in construction, the foundation soil, ground water, water used in the construction and loose earth left over in contact with brick work. Bricks with magnesium sulphate content higher than 0.05 percent should not be used in construction. Soluble salt content in sand (chloride and sulphate together) should not exceed 0.1 percent.

Water, if it finds access to brick work, moves along its pores by capillary action and carries with it dissolved salts. As the solution evaporates from the exposed surface of the brick work, the salts are left as deposit on the surface or on layers just below it. Disintegration or flaking of the brick surface is caused by the mechanical force exerted by salts as these crystallize just below the exposed surface. Magnesium sulphate, in particular, disintegrates bricks and pushes out plaster.

REMEDIES:
1. Well fired bricks should be used in construction.
2. Sand should be tested for its salt content.
3. Proper D.P.C. should be provided in the building.
4. Efflorescence on brick work traceable to salts in the materials can be removed by dry brushing and washing repeatedly. Such efflorescence may re-appear in dry season but usually are less in intensity. Finally these disappear as the salt content of the bricks is gradually leached out.

TESTING BRICKS FOR EFFLORESCENCE
Distilled water to be filled in a dish of suitable size. The dish should be made of glass, porcelain or glazed stone ware. Place the end of the bricks in the dish, the depth of immersion in water being 25 mm. Place the whole arrangements in a warm (for example, 20 to 30^oC) well ventilated room until all the water in the dish is absorbed by the specimen and the surface water evaporate. Cover the dish with suitable cover, so that excessive evaporation from the dish may not occur. When the water has been absorbed and bricks appear to be dry, place a similar quantity of water in the dish and allow it to evaporate as before. Examine the bricks for efflorescence after the second evaporation and report the results as:

(a) NIL – When there is not perceptible deposit of efflorescence.
(b) SLIGHT- Not more than 10% area of the brick covered with a thin deposit of salt.
(c) MODERATE- Covering upto 50% area of the brick.
(d) HEAVY- Covering 50% or more but unaccompanied by powdering or flacking of the brick surface.
(e) SERIOUS- When, there is a heavy deposit of salts accompanied by powdering and/or flacking of the exposed surfaces.

EFFLORESCENCE AND LEACHING IN CONCRETE:
When water percolates through poorly compacted concrete or through cracks or along badly made joints, the lime compounds with in the concrete leached out which leads to the formation of salt deposits on the surface of concrete, known as efflorescence. This caused primarily by calcium hydroxide Ca(OH)₂ one of the hydration products and slightly soluble in water, migrating to concrete surface through the capillary system. After evaporation, the solid Ca (OH)₂ reacts with the atmospheric carbon dioxide CO₂ to form calcium carbonate CaCO₃, a white deposit on the concrete surface.

Early efflorescence can be removed with a brush and water. Heavy deposits may require acid treatment of the surface of the concrete. The acid used is HCl diluted from the concentrated form in a ration of 1:20 or 1:10. The action of the acid stops when it has been used up by the reaction with lime, but the concrete should be washed in order to remove the salts which have been formed.

Efflorescence and leaching in concrete is harmful. In addition to blemish and ugly appearance, the process of carbonation of concrete is accelerated. In reinforced concrete, the chances of corrosion of steel are increased due to carbonation and higher permeability of concrete. It is therefore necessary the concrete making materials should be of good quality, mineral and chemical admixtures preferably be used, properly proportioned (preferable design mixes) as per required durability and grade of concrete. All the materials should be properly mixed, placed, compacted, finished and cured.

REFERENCES:
1. I.S. : 3495 (Part-III)- 1976 – Method of tests of burnt clay building brick) Part-III, determination of efflorescence (Second Revision)
2. A.M. Neville, Properties of Concrete, Fourth Edition.

We are thankful to Sir Kaushal Kishore for publishing his paper on Efflorescence. This paper will help civil engineers understand what is Efflorescence and how can they get rid of it