SIMPLE GATE DESIGN DRAWING

GUIDELINES ON STORING OF CEMENT ON SITE

In large work or in major construction works, cement is generally stored at site. The cement must be stored in such a manner so that, it can be easily accessible for proper inspection. The building in which cement is stored should be water tight in order to prevent dampness.

The guidelines given below should be observed while storing the cement.

1.      Cement should not be stored for a long period. During rainy season, the storage time period of cement should be as minimal as possible.

2.      Dampness in godowns must be avoided.

3.      Cement should not be piled against the wall. A minimum space of 30 cm all –round should be left between the exterior walls and the tacks. The distance between two consecutive stacks should be the minimum to reduce circulation of air.

4.      Cement should not be piled directly on the floor; instead it should be piled off the floor on wooden planks so as to be clear of the floor by at least 10 to 20 cm.

5.      There should not be more than 15 bags in one pile. This is done to avoid lumping under pressure.

6.      Cement bags should be arranged that they can be used on the principle of “first come first served”.

7.      If more than 7 bags of cement are to be stored in a pile, then it can be arranged in header and stretcher fashion or alternatively lengthwise and crosswise so as to tie the piles together and to avoid the danger of toppling over.

8.      When cement bags is to be stored for a long period or during rainy season, the stack should be enclosed completely by polythene sheet, tarpaulin or any other suitable water proofing material.

9.      If different brands of cement are meant to be used on one work, they should be stacked separately.

©Ishu Mainali

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

How to check quality of cement on site

FIELD TESTS ON CEMENT

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 150 mm apart and it is loaded with a weight of 340 N. the block should not show any sign of failure.

-The briquettes of a lean mortar (1:6) are made. The size of briquette may be about 75 mm ×25 mm ×12 mm. They are immersed in water for a period of 3 days after drying. If cement is of sound quality such briquettes will not be broken easily.

©Ishu Mainali

ADVANTAGES OF PPC (PORTLAND POZZOLANA CEMENT)

ADVANTAGES OF PPC IN FRESH CONCRETE

Portland pozzolana cement (PPC) has following advantages when concrete is in its fresh state.

1. WORKABILITY

Portland pozzolana cement has spherical cement particles and they have higher fineness value. Due to the spherical shape concrete move more freely and more fineness of particles allows better filling of the pores. This type of cement also gives better cohesiveness to concrete. PPC cement also reduces the rate of slump loss of concrete as compared to concrete made with ordinary cement, particularly in hot weather condition.

2. BLEEDING

Bleeding is a type of segregation in which some of the water in the concrete mix tens to rise to the surface of fresh concrete. As a result of bleeding, the top surface becomes too wet and concrete will become porous, weak and non durable. PPC cement reduces bleeding by providing greater fines volume and lower water content for a given workability. This also helps to block bleed water channels.

3. PUMPABILITY

PPC cement helps to produce more cohesive concrete and is less prone to segregation & bleeding. The spherical shape of particles serves to increase workability and pumpability by decreasing friction between aggregate particles and between concrete & pump line.

4. SETTING TIME & FINISHABILITY

PPC cement slightly prolongs the setting time of concrete which helps the mason for good finishing of concrete or cement mortar. The cohesiveness of concrete mix helps for better finishing of concrete.

ADVANTAGES OF PPC IN HARDENED CONCRETE

Portland pozzolana cement (PPC) has following advantages when concrete is in its hardened state.

1. COMPRESSIVE STRENGTH & RATE OF STRENGTH GAIN

The strength & rate of strength gain of concrete made with PPC will be equivalent to ordinary concrete at 28 days. The silicate formation of PPC continues even after the rate of hydration of ordinary cement slows down. This results in increased strength gain at later ages. This higher rate of strength gain will continue with time and result in higher later age strength.

2. MODULUS OF ELASTICITY

The modulus of elasticity of PPC concrete is somewhat lower at early ages and little higher at later ages than ordinary concrete.

3. BOND OF CONCRETE TO STEEL

The bond or adhesion of concrete to steel is dependent on the contact area of steel with concrete, the depth of reinforcement & density of concrete. PPC being finer in nature usually increases paste volume & reduce bleeding thus the contact will be increased, resulting into improved bond with steel.

4. HEAT OF HYDRATION

The hydration of PPC is a slower process than hydration of ordinary cement, resulting into slower heat generation and lower internal stresses in concrete. Thus PPC becomes ideal cement for mass concreting like dams, retaining walls, large foundation etc.

5. REDUCED SHRINKAGE

PPC in concrete helps to reduce drying shrinkage & plastic shrinkage. Drying shrinkage is reduced because of lower internal concrete stresses & slower heat generation. Plastic shrinkage is also reduced considerably because concrete bleeds less at a given slump or workability by using PPC.

6. PERMEABILITY

If concrete has interconnecting void spaces, then the concrete becomes permeable. In PPC concrete, the lime [Ca (OH)₂] liberated during initial hydration is consumed by reactive silica & forms an insoluble cementitious compound instead of leaching on the concrete surface. This helps in reducing void spaces & also blocks capillary channels & subsequently reduces permeability of concrete.

©Ishu Mainali

Chemical Composition of Clinker

                                                                                                             

The Cement clinkers (which are formed when calcareous and argillaceous raw materials are mixed and burned in rotary kilns) consist of following major compounds:

Compound	Common Proportion	Cement chemist notation
Tricalcium Silicate (3CaOSiO2)	40-65%	C3S
Dicalcium Silicate (2CaOSiO2)	20-35%	C2S
Tricalcium Aluminate (3CaO Al2O3)	5-15%	C3A
Tetracalcium Aluminium Ferrite (4CaO Al2O3 Fe2O3)	8-18%	C4AF
Gypsum (CaSO4)	3.0-4.0%
Other Constituents	0-5.0%

Tricalcium Silicate / (C₃S) / {Alite}

·         Generate heat more rapid

·         Hydrate more rapidly

·         Possess less resistance to chemical attack

·         Develop early strength

Dicalcium Silicate / (C₂S) / {Belite}

·         Imparts ultimate strength of the cement

·         Offers more resistance to chemical attack

·         Hardens more slowly

·         Less heat of hydration

·         Largely responsible for increase in strength beyond 7 days

Tricalcium Aluminate / (C₃A) / {Celite}

·         Weak against sulphate attack

·         Reacts fastly generating a large amount of heat

·         Does not contribute to develop strength

·         Causes initial setting of cement

·         It is the first compound which reacts with water when mixed with cement.

Tetracalcium Aluminium Ferrite / (C₄AF) / {Felite}

·         Poor cementing value

·         Reacts slowly generating small amount of heat

·         Comparatively inactive

·         Most Portland cement colour effect are due to C₄AF

·          

It has been observed that most of the strength developing properties of cement are controlled by C₃S and C₂S (the sum of their percentage varies from 70 to 80 percentage)

High percentage of C₃S and low C₂S result in

·         Rapid hardening

·         High early strength

·         High early strength with high heat generation

·         Less resistance to chemical attack

Low percentage of C₃S and high percentage of C₂S result in

·         Slow hardening

·         Much more ultimate strength with less heat generation

·         Greater resistance to chemical attack.