CONSTRUCTION TIPS

Cost of Construction

Share of Major Items

As per a rough estimate the ratio of expenditure on Materials, labour and other items worksout to about 60:30:10 or 60 per cent, 30 per cent and 10 per cent. Using the table givenbelow you can estimate the cost of different items in your total investments.


Note : Above estimates are tentative and may vary from place to place

Steel Rods or Rebars

Rebar is a short form of reinforcing steel rods. It is the mostimportant component ofRCC structure, since concretecannot take tensi le loads, steel rebars arecast into it to carry the tensile loads. There are many gradesof rebars in the market but nowadays Fe 415, Fe 500, Fe500D and550 grades of TMT rebars are most commonly used.

For any steel rebar, weight per running metre is equal to d2/162Kg, where d is diameter of the bar in mm. For example, 10mm diameter bar will weigh 10*10/162=0.617 Kg/m. In India, quality and properties of steel rebars are governed by thecods IS : 1786.

Always use best quality steel for your house.

Steel Reinforcement :

Plain concrete without reinforcement is like a bodywithout bones. Steel reinforcement is one of the bestforms of reinforcement to take care of all kinds ofstresses generated in a structure. To make the structurestrong and long lasting, the steel rods must be of goodquality and strength. It is advised that the same type andgrade of steel should be used in structural member.

Ask your engineer to do the reinforcement detailing for earthquake resistant constructions,if your house is located in seismic zone III, IV or V. Special rebars which have better UTS/YSratio can be used in such constructions. AF Star &MS Life Fe 500 / 550 D Steel TATA Tiscon rebar is one suchrebar available.

Bar Bending Schedule :

You should ensure that bar bending schedule of steelreinforcement table prepared in advance before cuttingand bending of rebars.

This schedule contains all the details of shape, size anddimension of rebars to be cut. A good bar bendingschedule will help you in buying the rebars in mosteconomical quantity and minimize steel wastage.

Standard Steel Reinforcement

It is necessary to understand the mechanism of corrosion of steel inside in the concrete andthe damages it causes the service life of concrete structures. Corrosion of steel is an Electrochemical process. The electro- chemical process starts when there is a potential differencecaused due to the difference in concentration of dissolved ions such as alkaline, chloridesand oxygen, in the vicinity of steel. Due to the potential difference some parts of the metalbecome anodic and other parts become cathodic. Dissolution or pitting of iron takes place onthe anodic parts and reduction of oxygen takes place on the cathodic parts. Rust appears onthe anodic part as iron (ferrous) gets converted to ferrous oxide or ferrous hydroxide.

The concrete act as electrolyte and theelectrochemical process takes place as shown in Fig3 Fig3 Electro Chemical Process ofCorrosion

This results in loss of bond between steel andconcrete which ultimately weaken the structure. Thechemical conversion of iron (Fe) to iron oxide (Fe O )and later to iron hydroxide (Fe (OH) ) causes anincrease in volume, over 6 times its original volume as shown in Fig 5.4 , within the concrete mass. Figure 5.4 Increase in Volume of Reinforcement asCorrosion process progresses.

This growth create tensile stress within the concretemass surrounding the reinforcement steel. Asconcrete is weak in taking tensile stress/exterior forces it cracks and spilling of concrete takes place.

Figure No5 : shows of concrete due tocorrosion of steel reinforcement.
Figure No5 : Cracks in concrete due to corrosion ofsteel reinforcement

CAUSES OF CORROSION OF STEEL

(i) Porosity in Concrete :

There can be micro or macro pores present inconcrete. Micro pores are in form of capillary pores inthe cement gel. Capillary pores in concrete can be as large as 5 μ in diameter. The number and size mainlydepend onwater to cement ratio (W/C) used in theconcrete.

The relation between the age at which capillary pores get blocked or concrete becomes impermeable. Water to cement ratio age at which capillary pores become blocked The abovetable shows that as water to cement ratio increases the concrete remains permeable forlonger period thereby permitting ingress of moisture and chemicals to penetrate it, leading tocorrosion of reinforcement and creating other durability problems. This water cement ratiocan be controlled in the sites where mix design is applied. Proper adoptions of mix design,site laboratory as well as trained Engineers are compulsory for regular testing of concretematerial concrete mix design and its implementation. If we use Corrosion proof steel we canavoid durability problems by the corrosion of steel even at higher water to cement ratio.

(ii).Chloride :

Chlorides present in concrete and in theenvironment around the concrete structure can have harmful effect on steel reinforcement in theconcrete. The presence of chlorides near thereinforcement is extremely dangerous. If thechloride to hydroxide ratio near the steel reinforcement drops below 0.3 corrosion ofordinary Tor / TMT bars are inevitable.Figure 6 shows of concrete ceiling due tothe corrosion of reinforcement by moisture andchloride attack. Figure No.6: spilling of concrete ceiling due to corrosion of reinforcement, by the attack ofmoisture and chloride.

shows deteriorated jetty structure with corrosive reinforcement due to chloride and sulphate attack from sea water. So corrosion proof steel reinforcement is required in sulphateand chloride environment.

Figure.7 : Corroded Reinforcement and Deteriorated Concrete in a structure.

iii). Sulphate

Sulphates are generally found in ground waterand subsoil. Seawater also contains largequantity of sulphates. Sulphate can be naturallyoccurring or industrial waste. Sulphate reacts withcement forming an expensive ettringite. This willcreate concrete swelling and consequentcracking. These cracks will guide again and again easy penetration of the moist and other aggressive ions such as chlorides and sulphate and corrosion and disintegration of concrete isinevitable.

shows deteriorated jetty structure with corrosive reinforcement due to chloride and sulphate attack from sea water. So corrosion proof steel reinforcement is required in sulphateand chloride environment.

Figure.7 : Corroded Reinforcement and Deteriorated Concrete in a structure.

iv) Carbon dioxide :

As carbon dioxide is harmful to human beings, it isharmful to ordinary steel reinforcement. Concreteis alkaline in nature with PH of around 13.5 and when the PH of concrete drops below 10 concreteis said to be carbonated and ordinary steelreinforcement inside the concrete is not at allprotected. Carbon dioxide in the air reacts withwater forms carbonic acid. When this carbonic acidtouch on the concrete structure, concrete getscarbonated or in other words turns acidic. Thischemical reaction starts at the surface andgenerally goes within the concrete mass and isgenerally measured as depth of carbonation andtable No 2 shows estimated 20 years depth ofcarbonation for different grades of concrete.


Concrete are around 20N/Sq mm and the average cover is around 25 mm. From the abovetable Sr No 4, it is clear that within 20 years, carbonation reaches beyond the cover area andthereby corrosion of standard steel reinforcement starts within 20 years of its service life. Fig6 shows corrosion of steel and spilling of concrete due to carbonation beyond cover area. Souse of corrosion proof steel reinforcement increase the life of concrete structure.