Portland slag cement (PSC)

Portland slag cement (PSC)

PSC can be used for all construction jobs in place of ordinary portland cement, but its special properties render its adoption highly desirable for marine structures, for municipal works such as sewers, for structures involving large masses of concrete such as dams, retaining walls, and bridge abutments, and for structures exposed to sulphate bearing soils such as foundations and roads. PSC is the commonly used cement in the eastern region of the country.

Portland Pozzolana cement

Portland Pozzolana cement

On the basis of pozzolana added to PPC. It is divided into two parts. Flyash based Portland pozzolana cement and calcined clay based Portland pozzolana cement. PPC is maufactured either by grinding intimately together Portland cement clinker, gypsum and a pozzolana such as flyash, or by intimately and uniformly blending portland cement and fine pozzolana. The physical requirements of OPC and PPC are similar. Increased impermeability, lower heat of hydration, lower plastic shrinkage, reduced alkali- aggregate expansion, and improved resistance to aggressive chemical agents and corrosion are some of the major benefits to be derived from the use of PPC. In mass concrete construction, PPC concretes have shown rather better behaviour in request of cracking than OPC concretes because of lower heat of hydration. PPC ensures durability in addition to strength. 

Hydrophobic cement

Hydrophobic cement

Hydrophobic cement is made by adding water- repellent chemicals to OPC clinker at the grinding stage. This type of cement can be stored under humid and damp conditions for prolonged periods without deteriorating or forming lumps. The film formed around the cement grain breaks down when it is mixed vigorously with aggregates.

Low heat portland cement

Low heat portland cement

Low heat portland cement has its chemical constituents proportioned in such a way that the heat liberated due to hydration is reduced, making the cement particularly suitable for use in massive structures, such as dams, bridge abutments, and thick retaining walls.


Its rate of gain of strength is slower than that of ordinary portland cement, hence its requires longer curing period. Low heat portland cement conforming to IS: 12600 shall be used with adequate precautions with regard to removal of form-work, etc.

Sulphate resisting portland cement

Sulphate resisting portland cement

SRPC counters what is commonly known as " sulphate attack". Soluble sulphate salts when present  in groundwater or in the soil penetrate through the pores of the hardened concrete and chemically react with the tricalcium aluminate (C3A) constituent of the cement. The reaction product, occupies a volume, which is more than the reacting compounds. This creates internal pressure resulting in the cracking of the concrete, which eventually disintegrates. This is called "sulphate attack".


SRPC has low C3A content which helps in reducing sulphate attack. The use of  SRPC is strongly recommended for structures in marshy lands, creek areas, coastal areas, in seawater, and in all areas where soluble sulphate salts are present beyond tolerable limits. Whenever sulphate concentration measured in terms of SO3 content exceeds 300 ppm in ground water, or 0.2 percent in the soil, the use of SRPC is recommended. When chlorides are present besides sulphate beyond permissible limits, it would be preferable to use Blended cement instead of SRPC.

Portland cement with mineral admixtures

Portland cement with mineral admixtures

IS:456-2000 also permits use of other combinations of Portland cement with mineral admixtures of quality conforming to :

1. Low alkali Portland cement
2. Blended cements
3. Portland blast- furnace slag cement (PBSC)
4. Oil well cement
5. White cement
6. High alumina cement

Low alkali portland cement

Low alkali portland cement

It is used in regions where the aggregates have reactive silica or carbonates, alkalies in the cement are restricted to 0.6 percent of soda equivalent with a view to avoid the occurrence of alkali- aggregate reaction, leading to possible cracking and destruction of concrete.

Blended cements

Blended cements

Blended cement is a hydraulic cementitious product, similar to ordinary Portland cement, but due to the addition of blending material it has certain improved properties compared to OPC. The use of blended cements improves the properties of both, fresh and hardened concrete. These can be as a result of the extended hydration of the cement- pozzolana mixture, reduced water demand, and due to improved cohesion of paste. Another important benefit is the improvement in durability resulting from the lower permeability and improved micro structure of the concrete.

Workablility of Concrete as per IS 456: 2000

Workablility of Concrete as per IS 456: 2000

The concrete mix needs to be designed for adequate workability so that it could be being mixed, transported, laid down and compacted as efficiently as possible. Depending upon the placing conditions, IS:456-2000 has recommended different ranges of workability and these are given in Clause no 7.1.

High alumina cement

High alumina cement

High alumina cement conforming to IS 6452 or supersulpahted cement conforming to IS 6909 may be used only under special circumstances with the prior approval of the engineer-in- charge.

Some of major producers of cement in the country are

Some of major producers of cement in the country are

J.K. Cements
Vikram Cements
ACC Cements
Gujrat Ambuja
Birla Cements

Combined Pile - Raft foundation effective for Tall Buildings

Combined Pile - Raft foundation effective for Tall Buildings

High rise buildings are usually founded on piled foundation, which is subjected to a combination of vertical, lateral and overturning forces. 

 

Combined pile- raft foundations 

• can be particularly effective form of foundations for tall buildings because the raft is able to provide a reasonable measure of both stiffness and load resistance.
• It is possible to reduce no of piles in Piled raft foundation. 
• Another feature is that the pressure applied from the raft on to the soil can increase the lateral stress between the underlying piles and the soil, and can increase the ultimate load capacity of a pile as compared to free- standing piles.

Following points should be consider while designing high- rise buildings:

• Ultimate capacity of the Foundation under vertical, lateral and moment loading combinations.
• Influence of the cyclic nature of the wind
• Earthquake 
• Overall settlements
• Differential settlements
• There should be good cooperation and interaction between the geotechnical designers and the structural designers. 

(Ref: C&CR magazine)

Reinforcing Steel

Reinforcing Steel

As per Table No. 22 of IS:456-2000, we can use following types of reinforcing bars in RCC:-

a) Mild Steel bars                                                     conforming to Grade-I of IS: 432 (Part 1)

b) Medium Tensile Steel                                            conforming to IS: 432 (Part 1)

c) High Yield Strength deformed bars                      conforming to IS: 1786 (Grade Fe 415 & Fe 500)

Mild steel reinforcing bars are plain, whereas Medium Tensile & High Yield strength bars are Ribbed/ Deformed/ Tor. In modern construction, usually High Yield Strength deformed bars are being used. The High Yield Strength deformed bars manufactures by two processes namely:-

a) Cold twisting

b) Thermo-Mechanical Treatment

The high yield strength was earlier being achieved by raising carbon content and manganese and to a great extent by cold twisting. In larger diameter bars, it is not possible to achieve high yield strength combined with elongation, weldability and bendability by this method. THus, to get all the above properties, technologically advanced technique, namely " Thermo-Mechanical Treatment" is used to achieve higher strength values with low carbon content, in larger diameter bars also.

Properties of water used for concreting as per IS 456:2000

Water reacts chemically with cement, the reaction being called hydration of cement, and brings about the setting and hardening of cement. Water also lubricates the mix and gives it the workability required to place and compact it properly. Water used for mixing in concrete should be free from oil, acids and alkali's, salts, sugars, organic materials, or any other substances that may be deleterious to concrete. Generally, it should be of potable quality. The pH value of water shall not be less than 6.

Seawater is not recommended for reinforced and prestressed concrete because of harmful salts present in it, but can be used only under unavoidable circumstances for plain concrete. It is well known that the chlorides and sulphate contents of water have a major influence on the durability of concrete. The permissible limits for solids in water are given in Table 1 of IS:456-2000 (clause no.5.4). Water used for curing should not produce any stain or unsightly deposit on the concrete surface.

Classification and Properties of Aggregates as per IS 456:2000

Classification and Properties of Aggregates as per  IS 456:2000

Aggregates constitute nearly 70 to 75% of the total volume of concrete and are essentially inert in nature. Aggregates have two prime functions, namely, to provide concrete with a rigid skeletal structure and reduce the void space to be filled by the cement paste. Most natural rocks, whether massive or broken down by nature, are suitable for making concrete. 

Aggregates are commonly classified into two sizes, fine and coarse:; the dividing line being the 4.75-mm IS sieve. Aggregates can also be classified in tow more ways. 

Depending on the source, they could either be naturally occurring (gravel, sand, etc.) or synthetically manufactured (bloated clay aggregates, etc.). 

Further, depending on the bulk density, aggregates can either be normal weight, lightweight, or heavyweight. 

IS:456- 2000 specifies that aggregates shall comply with the requirements of IS 383. The nominal maximum size of coarse aggregates should not be greater than one- fourth of the minimum thickness of the member, provided that the concrete can be placed without difficulty so as to surround all reinforcement thoroughly and fill the corners of the form. 

                                The aggregates used to make concrete must be clean, dense, hard, durable, structurally sound, capable of developing good bond with cement, weather-resisting, and unaffected by water. The properties of the concrete depend upon the quality of the aggregates- their strength, water absorption, shape and texture, the maximum size of aggregate, etc. Impurities in aggregates are undesirable as they may hinder the hydration of cement and prevent adhesion of the aggregates with the cement paste, reducing strength and lowering durability. Sand containing an excess amount of silt or organic matter should be washed. Sand sometimes contains moisture, which cause a film of water on the surface of the particles, fluffing them apart. This is called bulking of sand, which should be taken into account while batching the mix. Storage of materials shall be as described in IS 4082. Storing on dusty, muddy, and grassy spots should be avoided. Aggregates should be placed in stockpiles in individual units not larger than a truckload and in suitable layers to prevent segregation.

Tor Steel acceptance criteria

TOR- STEEL:

The acceptance criteria can be broadly classified into three categories as under:-

1. Chemical composition
2. Weight
3. Physical properties.
   

1.Chemical composition:

The product analysis shall be as follows:

Constituent	Max. %        (Fe-415)	Max. %  (Fe-500)	Max. %     (Fe-550)	% age Variation limits
Carbon	0.30	0.30	0.30	0.02
Sulphur	0.060	0.055	0.055	0.005
Phosphorus	0.060	0.055	0.050	0.005
Sulpur + Phosphorous	0.11	0.105	0.10	0.010

Here, maximum carbon content permitted is only 0.25 %

 

2.Weight:

The weight per meter shall correspond to the following:

Nominal size                         in mm	Weight                     Kg/ meter	Acceptable range                   Kg/ meter
6	0.22	0.228-0.206
8	0.39	0.423-0.367
10	0.62	0.680-0.574
12	0.89	0.932-0.844
16	1.58	1.66-1.50
20	2.47	2.54-2.39
22	2.98	3.07-2.90
25	3.85	3.97-3.74
28	4.84	4.98-4.69
32	6.32	6.51-6.13

 

3.Physical properties:

The following shall be satisfied:

Test	Tor-40	Tor-50
0.2% proof stress	415 N/mm2 min.                     (4230 kg/cm2)	500 N/mm2 min.                           (5100 kg/cm2)
Tensile Strength	10% more than the actual but minimum 485 N/mm2	10% more than the actual
Max. elongation	14.5%	12.0%

 

Following routine test should be performed on samples selected and prepared as per CL.8.1 of  IS: 1786-1985:-  

• Tensile Test

• Bend Test
•  Rebend Test

Raw & finished steel products include:-

•  Semis- Ingots, Slabs, Blooms, Billets, Squares, etc.
• Structural- Channels, Beams (I-Sections, Angles, etc
• Rolled products- Bars, Rounds, Ribbed Torsteel Bars, Flats, etc.
• Flat product- Plates, Hot Rolled Sheets Skelp and Coils, Cold Rolled Sheets and Coils High Silicon Sheets.
• Track- materials- Crossing Sleeper Bars, Fish Plate, Bearing Plate Bars, Pressed Steel Sleepers, Heavy Rails, Light Rails, Wheel, Axles and Wheel Sets.
• Special items- Tinpulates- Electrolytic, Large Diameter Steel Tubes- ERW Spirally Welded Pipes, Galvanised Sheets, High Conductivity Rails, Crane Rails, etc.

Some of the major manufactures of steel products in the country are:-

• SAIL- Steel Authority of India Limited - a Government of India Undertaking.
• TISCO- Tata Iron & Steel company
• Vishakapatnam steel plant
• RATHI
• Bhilai
• Haldia