MORTAR MIX – DO YOU KNOW THIS? 5 WHYs YOU SHOULD KNOW

Why to Use Clean Sand for Mortar?

Contaminated sand will not bind with cement, hence is weakening the mortar. Also sand with high percentage of clay or silt will weaken the mortar, because the clay or silt contains too many fines that needs to be covered by cement for proper binding.

Why Use Fresh Cement for Mortar?

The more is the storing time period of cement, the less strength will it remains. Cement which has been stored for about 6 months will show 30% less strength as compared to fresh cement. Strength of mortar is very important for a good masonry work as it influences the overall quality of the building.

Why Mix Dry Ingredients (i.e. sand & cement) First Before Adding Water?

Wet sand particles have the tendency to stick together and are therefore hindering that cement can cover them. This results in a non-uniform mix that is reducing the mortar quality, because each sand particle should ideally be fully covered with cement.

Further, adding water together with sand and cement I one go makes mixing the mortar extremely difficult for the laborers.

Why to Protect Mortar Mixing Place from Wind, Rain & Sunshine?

Wind and sunshine is entraining the water from the mortar and is accelerating the hardening process before it is being used. This makes the mortar useless for any purpose. Rain adds water and mortar becomes unusable too.

Why Not Use or Re-use Mortar That Has Already Set?

In hardened mortar, the hydration process of the cement has started and re-mixing of it is destroying the bond between cement and sand. This bond cannot regain strength again by simply adding fresh water to the mortar.

As cement mortar sets relatively quickly (approx. 30 minutes), it should never mixed in huge quantities.

HOW TO STORE STEEL ON SITE?

Steels should be stored in such a way, so as to avoid distortion and to prevent deterioration and corrosion. Steels of different classes should be stored separately. Follow the guidelines given below while storing steel on site.

Storing of Steel Reinforcement

• Store bars of different classes, sizes and lengths separately to facilitate issues in such sizes and lengths so as to minimize wastage in cutting from standard lengths.
• Paint the ends of bars of each class in distinct separate colors.
• Coat steel reinforcement with cement wash before stacking to prevent scaling and rusting.
• If reinforcement bars have to be stored for a long period, then stack it above ground level by at least 150 mm.

Storing of Structural Steel

• Assign separate areas for storing structural steel of different classes, sizes and lengths.
• Store it above ground level by at least 150 mm upon platforms, skids or any other suitable supports to avoid distortion of sections.
• In coastal areas or in case of long storage, apply protective coating of primer to prevent scaling and rusting

Specification For Burnt Clay Fly Ash Building Bricks

General Quality

• Bricks shall be free from cracks and flaws and nodules of free lime.
• Bricks shall have smooth rectangular faces with sharp corners
• Bricks shall be uniform in colour
• The proportion of soil and fly ash shall be as per IS-2117-1991.
• Fly ash used to manufacture of bricks shall conform to grade 1 or grade 2 as per IS-3812-1981.
• Frog size of a hand moulded brick having height of 90 mm or 70 mm shall be as per the figure shown below.

 Dimensional Specification

Dimension for Modular Bricks
Length (L), mm	Width (W), mm	Height (H), mm
190	90	90
190	90	40
Dimension for Non-Modular Bricks
Length (L), mm	Width (W), mm	Height (H), mm
230	110	70
230	110	30

Physical Requirements

Compressive Strength	Minimum average compressive strength shall be as per various class of group (See table below)Compressive strength of any individual brick shall not be less than the minimum compressive strength for the corresponding class of brick. (See table below)
Water Absorption	Shall not be more than 20% by weight up to class 12.5 and 15% by weight for higher classes.
Efflorescence	Shall not be more than ‘moderate’ up to class 12.5 and ‘slight’ for higher class.

 Classes of Burnt Clay Fly Ash Bricks

Class Designation	Average Compressive Strength
	N/mm2	Kgf/cm2, (approx)
30	30	300
25	25	250
20	20	200
17.5	17.5	175
15	15	150
12.5	12.5	125
10	10	100
7.5	7.5	75
5	5	50
3.5	3.5	35

 

SPECIFIC GRAVITY TEST ON BITUMEN

Introduction

Specific gravity is defined as the ratio of the mass of a given volume of the bituminous material to the mass of an equal volume of water, the temperature of both being specified as 270C.

Apparatus

• Specific gravity bottle of 50 ml capacity, ordinary capillary type with 6 mm diameter neck or wide mouthed capillary type bottle with 25 mm diameter neck
• Balance having least count of 1g

Procedure

1. The specific gravity bottle is cleaned, dried and weighed along with the stopper.
2. It is filled with fresh distilled water, stopper placed and the same is kept in water container for at least half an hour at temperature 270C.
3. The bottle is then removed and cleaned from outside. The specific gravity bottle containing distilled water is now weighed.
4. The bituminous material is heated to a pouring temperature and is poured in the above empty bottle taking all the precautions that it is clean and dry before filling sample materials. The material is filled up to the half taking care to prevent entry of air bubbles.
5. To permit an escape of air bubbles, the sample bottle is allowed to stand for half an hour at suitable temperature cooled to 270C and then weighed.
6. The remaining space in the specific gravity bottle is filled with distilled water at 270C , stopper placed and is placed in water container at 270C.
7. The bottle containing bituminous material and containing water is removed, cleaned from outside and is again weighed.

Calculation

The specific gravity of the material is calculated as follows:

Specific gravity = weight of bituminous material/weight of equal volume of water

= (c-a)/[(b-a)-(d-c)]

Where,

a = weight of specific gravity bottle, g

b = weight of specific gravity bottle filled with distilled water, g

c = weight of the specific gravity bottle about half filled with bituminous material, g

d = weight of the specific gravity bottle about half filled with material and the rest with distilled water, g

Results

At least three measurements should be made for determining value of the specific gravity

Precautions

• It is necessary that all precautions are taken in making the specific gravity bottles thoroughly cleaned and dried in the first weighting.
• The surface of the specific gravity bottle should be cleaned dry after filling with water, before weighing.
• The test temperature should be firmly adhered to.
• Inaccurate balance would never give reproducible results.

HOW TO CHECK QUALITY OF CEMENT ON SITE?

It is necessary to check the quality of cement on site at the time of preliminary inspection. It is not possible to check all the engineering qualities of cement on site but there exist some field test which gives us a rough idea of quality of cement. While on site we can perform these field tests to judge the quality of cement. These field tests are as follows:

1. Date of packing
2. Colour
3. Rubbing
4. Hand Insertion
5. Float Test
6. Smell Test
7. Presence of lumps
8. Shape Test
9. Strength Test
   

1. Date of Packing

Date of manufacture should be seen on the bag. It is important because the strength of cement reduces with age.

2. Colour

The cement should be uniform in colour. In general the colour of cement is grey with a light greenish shade. The colour of cement gives an indication of excess lime or clay and the degree of burning.

3. Rubbing

Take a pinch of cement between fingers and rub it. It should feel smooth while rubbing. If it is rough, that means adulteration with sand.

4. Hand Insertion

Thrust your hand into the cement bag and it should give cool feeling. It indicates that no hydration reaction is taking place in the bag.

5. Float test

Throw a small quantity of cement in a bucket of water. It should sink and should not float on the surface.

6. Smell Test

Take a pinch of cement and smell it. If the cement contains too much of pounded clay and silt as an adulterant, the paste will give an earthy smell.

7. Presence of Lumps

Open the bag and see that lumps should not be present in the bag. It will ensure that no setting has taken place.

8. Shape Test

Take 100g of cement and make a stiff paste. Prepare a cake with sharp edges and put on the glass plate. Immerse this plate in water. Observe that the shape shouldn’t get disturbed while settling. It should be able to set and attain strength. Cement is capable of setting under water also and that is why it is also called ‘Hydraulic Cement’.

9. Strength Test

A block of cement 25 mm*25 mm and 200 mm long is prepared and it is immersed for 7 days in water. It is then placed on supports 15000 mm apart and it is loaded with a weight of 340 N. the block should not show any sign of failure.

HOW TO CAP CONCRETE CYLINDERS? (4 METHODS OF CAPPING)

Capping of Concrete Cylinders

Capping is required to give a smooth surface for applying compressive load to concrete cylinders. Only cylindrical concrete test specimens are capped using appropriate method. When the ends of cylindrical test specimens are not plane within 0.05 mm, it is required to cap those test specimens. The capped surfaces should be at right angles to the axis of the specimens. After capping the specimen, check the plainness of cap by means of a straight edge and feeler gauge, making a minimum of three measurements on different diameters. Caps are made as thin as practicable and it should not get damaged when the specimen is tested.

We can use any one of the following 4 methods for capping the cylindrical concrete test specimen.

1. Neat cement capping
2. Sulphur capping
3. Gypsum plaster capping
4. Cement mortar capping

1. Neat Cement Capping

1. Prepare a stiff cement paste by mixing cement & water at a desired water/cement ratio.
2. Leave it for 2 to 4 hours in order to avoid the tendency of the cap to shrink.
3. Using a scoop, place some amount of stiff neat cement paste on top of the cylindrical test specimen.
4. Take a glass plate having thickness not less than 6.5 mm and diameter at least 25 mm larger than the diameter of the test specimen.
5. Using this glass plate, press down the stiff cement paste by giving the plate a rotary motion until it makes complete contact with the rim of the mould. Before doing this, apply a thin layer of oil or grease on the glass plate to avoid adhesion of the paste with the glass plate.
6. While giving a rotary motion to the plate, make sure that the plate remains parallel to the end surface at all times. After preparing the cap leave it for some time to become hard.
7. Repeat the same procedure to cap the other end of the cylindrical test specimen.

Note

• This type of capping is done after 4 hours of molding the concrete, so that the concrete completely settle down in the mould.
• Cement paste should have the desired strength so that the caps will not get damaged while applying load.

2. Sulphur Capping

1. Heat the sulphur compound (consisting of 2 to 3 parts of sulphur to 1 part of inert filler) at an appropriate temperature, until it reaches a pouring consistency.
2. Take a scoop of melted sulphur and pour it in the cap mould. Before doing this, apply a thin layer of oil or grease on the cap mould to avoid adhesion of the liquid sulphur with the cap mould.
3. Immediately after that, put one end of the cylindrical test specimen into the cap mould containing liquid sulphur and press it down. Hold it in this position until the sulphur becomes hard.
4. After that, tap the cap mould downward and remove the cylinder from the cap mould.
5. Repeat the same procedure to cap the other end of the cylindrical test specimen.

Note

• This type of capping is done after removing the concrete cylinder from the mould.

3. Gypsum Plaster Capping

1. Prepare a stiff plaster by mixing gypsum & water at a desired ratio. While preparing the paste mix it well enough. This will help gypsum to gain strength.
2. Using a scoop, place some amount of gypsum plaster on top of the cylindrical test specimen.
3. Take a glass plate having thickness not less than 6.5 mm and diameter at least 25 mm larger than the diameter of the test specimen.
4. Using this glass plate, press down the stiff gypsum plaster by giving the plate a rotary motion until it makes complete contact with the rim of the mould. Before doing this, apply a thin layer of oil or grease on the glass plate to avoid adhesion of the plaster with the glass plate.
5. While giving a rotary motion to the plate, make sure that the plate remains parallel to the end surface at all times. After preparing the cap leave it for 20 to 30 minutes to become hard.
6. Repeat the same procedure to cap the other end of the cylindrical test specimen.

Note

• This type of capping is done after removing the concrete cylinder from the mould.

4. Cement Mortar Capping

1. Prepare a mortar using cement similar to that used in the concrete and sand which passes 300 micron sieve and retained 150 micron sieve. The mortar should have a water/cement ratio not higher than that of the concrete of which the specimen is made.
2. If any free water has collected on the surface of the specimen, then remove it with a sponge, or blotting paper or other suitable absorbent material.
3. Prepare the cap by applying mortar firmly on the specimen and compact it with a trowel to a slightly convex surface above the edges of mould.
4. Then using a glass plate, press down the cap so formed, with a rotary motion until it makes complete contact with the rim of the mould. Apply a thin layer of oil or grease on the glass plate to avoid adhesion of mortar with the plate.
5. Left the glass plate in position until the specimen is removed from the mould.

Note

• This type of capping is done after 4 hours of molding the concrete, so that the concrete completely settle down in the mould.

WHAT IS NON-DESTRUCTIVE TESTING OF CONCRETE & VARIOUS NDT TEST METHODS?

Non-Destructive Testing of Concrete

 (NDT on Concrete)

Non destructive test is a method of testing existing concrete structures to assess the strength and durability of concrete structure. In the non destructive method of testing, without loading the specimen to failure (i.e. without destructing the concrete) we can measure strength of concrete. Now days this method has become a part of quality control process. This method of testing also helps us to investigate crack depth, micro cracks and deterioration of concrete.

Non destructive testing of concrete is a very simple method of testing but it requires skilled and experienced persons having some special knowledge to interpret and analyze test results.

Different Methods of Non-Destructive Testing of Concrete

Various non-destructive methods of testing concrete have been developed to analyze properties of hardened concrete, which are given below.

1. Surface Hardness Test

These are of indentation type, include the Williams testing pistol and impact hammers, and are used only for estimation of concrete strength.

2. Rebound Hammer Test

The rebound hammer test measures the elastic rebound of concrete and is primarily used for estimation of concrete strength and for comparative investigation.

3. Penetration and Pullout Techniques

These include the use of the simbi hammer, spit pins, the Windsor probe, and the pullout test. These measure the penetration and pullout resistance of concrete and are used for strength estimation, but they can also be used for comparative studies.

4. Dynamic or Vibration Tests

These include resonant frequency and mechanical sonic and ultrasonic pulse velocity methods. These are used to evaluate durability and uniformity of concrete and to estimate its strength and elastic properties.

5. Combined Methods

The combined methods involving ultrasonic pulse velocity and rebound hammer have been used to estimate strength of concrete.

6. Radioactive and Nuclear Methods

These include the X-ray and Gamma ray penetration tests for measurement of density and thickness of concrete. Also, the neutron scattering and neutron activation methods are used for moisture and cement content determination.

7. Magnetic and Electrical Methods

The magnetic methods are primarily concerned with determining cover of reinforcement in concrete, whereas the electrical methods, including microwave absorption techniques, have been used to measure moisture content and thickness of concrete.

8. Acoustic Emission Techniques

These have been used to study the initiation and growth of cracks in concrete.

ELECTRICAL RESISTIVITY TEST OF SOIL – GEOPHYSICAL METHOD OF SOIL EXPLORATION

Electrical Resistivity Test of Soil

This method depends on differences in the electrical resistance of different soil (and rock) types. The flow of current through a soil is mainly due to electrolytic action and therefore depends on the concentration of dissolved salts in the pores. The mineral particles of soil are poor conductors of current. The resistivity of soil, therefore, decreases as both water content and concentration of salts increase.

Dense clean sand above the water table, for example, would exhibit a high resistivity due to its low degree of saturation and virtual absence of dissolved salts. Saturated clay of high void ratio, on the other hand, would exhibit a low resistivity due to the relative abundance of pore water and the free ions in that water.

There are several methods by which the field resistivity measurements are made. The most popular of the methods is the Wenner Method.

Wenner Method

The Wenner arrangement consists of four equally spaced (A) electrodes driven approximately 20 cm into the ground as shown in the following figure.

In this method a dc current of known magnitude (I) is passed between the two outer (current) electrodes, thereby producing an electric field within the soil, whose pattern can be determined by the resistivities of the soils present within the field and the boundary conditions. By means of the inner electrodes the potential drop ‘E’ for the surface current flow lines is measured. The apparent resistivity ‘R’, is calculated using the following equation

 

Where,

A in centimeters,

E in volts,

I in amperes, and

R in ohm-cm

The apparent resistivity represents a weighted average of true resistivity to a depth Ain a large volume of soil, the soil close to the surface being more heavily weighted than the soil at greater depths. The presence of a stratum of low resistivity forces the current to flow closer to the surface resulting in a higher voltage drop and hence a higher value of apparent resistivity. The opposite is true if a stratum of low resistivity lies below a stratum of high resistivity.

The method known as electrical sounding is used when the variation of resistivity with depth is required. This enables rough estimates to be made of the types and depths of strata. A series of readings are taken, the (equal) spacing of the electrodes being increased for each successive reading. However, the center of the four electrodes remains at a fixed point. As the spacing is increased, the apparent resistivity is influenced by a greater depth of soil. If the resistivity increases with the increasing electrode spacing, it can be concluded that an underlying stratum of higher resistivity is beginning to influence the readings. If increased separation produces decreasing resistivity, on the other hand, a lower resistivity is beginning to influence the readings.

Apparent resistivity is plotted against spacing, preferably, on log paper. Characteristic curves for a two layer structure are shown in the following figure.

For curve C1 the resistivity of layer 1 is lower than that of layer 2; for curve C2, layer 1 has a higher resistivity than that of layer 2. The curves become asymptotic to lines representing the true resistance R1 and R2 of the respective layers. Approximate layer thickness can be obtained by comparing the observed curves of resistivity versus electrode spacing with a set of standard curves. The procedure known as electrical profiling is used in the investigation of lateral variation of soil types. A series of readings is taken, the four electrodes being moved laterally as a unit for each successive reading; the electrode spacing remains constant for each reading of the series. Apparent resistivity is plotted against the center position of the four electrodes, to natural scale; such a plot can be used to locate the position of a soil of high or low resistivity. Contours of resistivity can be plotted over a given area. The electrical method of exploration has been found to be not as reliable as the seismic method as the apparent resistivity of a particular soil or rock can vary over a wide range of values. Representative values of resistivity are given in the following table.

HOW TO CALCULATE PILE LOAD CAPACITY? (STATIC ANALYSIS)

The ultimate bearing capacity of a pile is the maximum load which it can carry without failure or excessive settlement of the ground.

The bearing capacity of a pile depends primarily on 3 factors as given below,

1. Type of soil through which pile is embedded
2. Method of pile installation
3. Pile dimension (cross section & length of pile)

While calculating pile load capacity for cast in situ concrete piles, using static analysis, we need to use soil shear strength parameter and dimension of pile.

Load Carrying Capacity of Pile Using Static Analysis

The pile transfers the load into the soil in two ways. Firstly, through the tip-in compression, termed as “end-bearing” or “point-bearing”; secondly, by shear along the surface termed as “skin friction”.

Load carrying capacity of cast in-situ piles in cohesive soil

The ultimate load carrying capacity (Qu) of pile in cohesive soils is given by the formula given below, where the first term represents the end bearing resistance (Qb) and the second term gives the skin friction resistance (Qs).

Where,

Qu = Ultimate load capacity, kN

Ap = Cross-sectional area of pile tip, in m2

Nc = Bearing capacity factor, may be taken as 9

αi = Adhesion factor for the ith layer depending on the consistency of soil. It depends upon the undrained shear strength of soil and may be obtained from the figure given below.

ci = Average cohesion for the ith layer, in kN/m2

Asi = Surface area of pile shaft in the ith layer, in m2

A minimum factor of safety of 2.5 is used to arrive at the safe pile load capacity (Qsafe) from ultimate load capacity (Qu).

Qsafe = Qu/2.5

Load carrying capacity of cast in-situ piles in cohesion less soil

The ultimate load carrying capacity of pile, “Qu”, consists of two parts. One part is due to friction, called skin friction or shaft friction or side shear denoted as “Qs” and the other is due to end bearing at the base or tip of the pile toe, “Qb”.

The equation given below is used to calculate the ultimate load carrying capacity of pile.

Where,

Ap = cross-sectional area of pile base, m2

D = diameter of pile shaft, m

γ = effective unit weight of the soil at pile tip, kN/m3

Nγ= bearing capacity factor

Nq = bearing capacity factor

Φ = Angle of internal friction at pile tip

PD = Effective overburden pressure at pile tip, in kN/m2

Ki  = Coefficient of earth pressure applicable for the ith layer

PDi = Effective overburden pressure for the ith layer, in kN/m2

δi = Angle of wall friction between pile and soil for the ith layer

Asi = Surface area of pile shaft in the ith layer, in m2

The first term is the expression for the end bearing capacity of pile (Qb) and the second term is the expression for the skin friction capacity of pile (Qs).

A minimum factor of safety of 2.5 is used to arrive at the safe pile capacity (Qsafe) from ultimate load capacity (Qu).

Qsafe = Qu / 2.5