06-10-2016, 10:49 AM
EXPERIMENTAL STUDY ON CORROSION PREVENTION ON COATED REBARS IN R.C SLABS AND FIBER R.C SLABS
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ABSTRACT
All materials or products, plants, constructions and building made of structural elements are subjected to physical wear during use. Corrosion is a multi- billion dollar problem. Corrosion causes deterioration of material and leads to destruction of structures ultimately affects the environment.
Corrosion is a chemical or electrochemical phenomenon which can attack any metal or substances through reaction by the surrounding environment. The importance of corrosion studies is two folds. The first is economic, including the reduction of material losses. The second is conservation, applied primarily to metal resources, the world’s supply of which is limited.
The project is aimed at preventing corrosion that is minimising the rate of corrosion using polypropylene fibre and epoxy coating.
In this work an attempt is planned to study the effects of fibres in concrete and to study the coating provided to the reinforcement. To accelerate the corrosion for a short term process of impressed current is induced.
INTRODUCTION
1.1 GENERAL
Concrete is widely and commonly used man made construction material in the world. It is obtained by mixing cementitious material, water and aggregate in required proportions. The mixture when placed forms and allowed to cure hardness into a rock like mass known as concrete. It has high compressive strength and low tensile strength. To develop the tensile stresses the concrete is strengthened by the steel bars called reinforced cement concrete.
The reinforced concrete is used throughout the world to build infrastructure and building. Today, the large numbers of civil infrastructures around the world in a state of serious deterioration due to carbonation, chloride attack, etc.
Corrosion of reinforcement is the principal cause of deterioration of structural concrete and a major economic cost for maintenance of national infrastructures. The effect of this deterioration on residual capacity is therefore a matter of concern to those charged with ensuring safe operation of concrete structures. It is clear, however, that many reinforced concrete structures remain in service once reinforcement has started to corrode and cover concrete over the bars has began to spall, there is extensive evidence that modest amounts of corrosion do not pose an appreciable threat to structural stability. It is essential that responsible engineers have at their disposal the means to verify that the affected structures retain an acceptable margin of safety. Corrosion may affect residual capacity through several mechanisms, including loss of bar section, loss of concrete section as a result of longitudinal cracking and spalling and a reduction in the interaction, or bond, between reinforcement and concrete.
Steel reinforcement is very effectively protected from corrosion by good quality of concrete, adequate thickness of cover and high alkalinity of the concrete. But due to various factors, the passive state of steel is lost and it beings to corrode
1.2 CORROSION
Corrosion is defined as the destruction of material due to chemical reaction with the environment, and also the loss of steel due to the formation of rust. The corrosion of steel reinforcement is the depassivation of steel with reduction in concrete alkalinity through carbonation. Most of the materials undergo corrosion on exposure to natural environments (air, water and soil) or artificial environments (gases, liquids and moisture)
1.3 CORROSION PROCECSS AND MECHANISM
1.3.1 General
Steel reinforcement is very effectively protected from corrosion by good quality of concrete, adequate thickness of cover and high alkalinity of the concrete. But due to various factors, the passive state of steel is lost and it begins to corrode.
1.3.2 Corrosion Process
The corrosion process (anodic reaction) of the metal dissolving as ions generates some electrons, as shown in the simple model on the left, that are consumed by a secondary process (cathodic reaction). These two processes have to balance their charges.
The sites hosting these two processes can be located close to each other on the metal's surface, or far apart depending on the circumstances. This simple observation has a major impact in many aspects of corrosion prevention and control, for designing new corrosion monitoring techniques to avoiding the most insidious or localized forms of corrosion.
1.3.3 Stages of Corrosion
In general four stages of corrosion of steel in concrete are
Passivity state
State of pitting corrosion
state of general corrosion
state of active, low potential corrosion
1.4 CORROSION MECHANISM
Corrosion of steel in concrete is initiated and maintained generally by two mechanisms.
i. Presence of depassivating ions which is particularly chlorides in large enough amounts to destroy passivating films located.
ii. Reduction in alkalinity of concrete (pH value around9.5) due to the effect of atmospheric carbon-di-oxide.
Current flows in steel from anode to cathode in the presence of oxygen and water and results in the protection of hydroxyl ions at the cathode. As these migrate to the anode they react with ferrous ion and hydrous ion oxide called black rust. The red rust is responsible for the cracking of concrete because its volume is four times larger than that of steel, while the black rust volume is only twice as large as steel.
The following equations, illustrate the corrosion of iron resulting in rust formation
At the anode, oxidation of iron occurs:
Fe(s) ---> Fe2+ + 2e-
At the cathode, reduction of atmospheric oxygen with water occurs:
½ O2 (g) + H2O + 2e- --->2 OH-
Anodic and Cathodic reaction products combine:
Fe2+ + 2 OH- ---> Fe (OH) 2
Subsequent oxidation reaction results in the formation of rust:
Fe (ΟΗ) 2 + Ο2 (g) + Η2Ο---> Fe2Ο3•2Η2Ο
Fe(s) ---> Fe2+ + 2e-
½ O2 (g) + H2O + 2e- --->2 OH-
Fe2+ + 2 OH- ---> Fe (OH) 2
Fe (ΟΗ) 2 + Ο2 (g) + Η2Ο---> Fe2Ο3•2Η2Ο
1.5 FACTORS INFLUENCING CORROSION
a.) Concrete quality
As concrete is in the immediate neighbourhood of rebars, it plays a vital role in the corrosion prevention of rebars. The strength and density of concrete mainly depends on the water cement ratio. At water cement ratio is above 0.45, the voids within the concrete are found to constitute continuous capillary systems, which provide access to corrosive agents. Inadequate curing makes the concrete more permeable. Based on experience and experimental results, the codes of practice have stipulated limits for water cement ratios for different exposure conditions. The strength, density and water sealing properties of concrete to maintain is alkalinity over long periods. The aggregates used also affected the corrosion of rebars and hence aggregates having good grain size distribution are to be used to obtain dense concrete. The coarse aggregates do not affect permeability to a great extent while the fine aggregate and cement affect permeability to a great extent. Fresh concrete must be compacted properly so that there are no air voids or honey combs.
b.) chloride content:
Chlorides, which cause depassive of rebars, can be present in concrete either from constituents of the concrete or as a result of penetration from the environment. The presence of chloride ions can cause depassivation of the rebar even when the associated pore solution has high pH value and corrosion is often in the form of intense localised attack. A distinction is to be made between initial chlorides (present in chloride) and the acquired chlorides (diffusing into the concrete from environment).
As long as the pH value of the surrounding concrete is more than 11, the rebar remains in the passive state. Once the pH value is lowered to a value below 11 either by carbonation of concrete or by the ingress of chloride ions. Then the rebar loses its passive state and corrosion sets in.
c.) Resistivity of concrete:
This is one of the important environment parameters that influence corrosion. From previous investigations it is found that the resistance decreases with increases in quantity of water added. As the resistivity of concrete decreases, the rebar is more susceptible to corrosion. Hence the water cement ratio should be kept as low as possible.
d.) Cover thickness:
Providing minimum cover made of good quality of concrete is essential to impart adequate protection to the reinforcing steel. The codes of practice recommend more cover for corrosive environments. These values are fixed based on experiments or previous experience.
However the percentage or level of corrosion to induce cracking varies depending upon cover thickness and diameter of the bar. Hence suitable guidelines have to be formulated to determine the cover based on exposure conditions and the diameter of the bar.Table1.1shows the recommended cover thickness as per code. And table 1.2 gives different exposure conditions as defined in the code
e.) Sea water attack (environment):
The marine environment is characterised by wave action, which imposes shock load and causes the corrosion of the concrete surface by abrasion and cavitations. Concrete is exposed to the aggressive constituents of sea water and subjected to repeated freeze-thaw and wet-dry cycles. Thus, the deterioration of concrete in such an environment is both chemical and physical in nature and the type of the attack may be demarcated into three zones depending on the tidal lines.
Consequently, cracking due to reinforcement corrosion and or freeze thaw cycles is the main deterioration phenomena in this zone. In the tidal zone, the structure is subjected to alternate wet and dry cycles, freeze-thaw cycles, impact of waves and floating ice, abrasion by sand and gravel and reinforcement corrosion. The lower zone submerged in water, is in a relatively stable environment , where freeze –thaw action and reinforcement corrosion does not occur. Here the predominant deleterious action is chemical attack, which causes strength retrogression. Concrete density, cement type, and content play a pivotal role in the resistance of concrete to sea water. Concrete made with calcium aluminates, super sulphate cements, and also those containing supplementary cementing materials, resists sea water fairly well such improved resistance as compared to ordinary Portland cement (opc) stems from reduced free lime content in such concrete. In case of reinforced concrete, the absorption of salt establishes anodic and cathodic areas: the resulting electrolytic action leads to an accumulation of corrosion products in the steel with a consequent rupture of the surrounding concrete. Hence, it is essential to provide sufficient cover to reinforcement adopting low w/c ratio is essential to get a less permeability. A well- compacted concrete, good workmanship, especially in the construction joints and type of cement is of vital role.
1.6 CAUSES OF CORROSION
1. Surface heterogeneity
2. Impurities, grains, and grain boundaries, cut edges
3. Environmental variations
4. Industrial : H2S,NH3,SO2
5. Marine : salt
6. Urban
7. Rural
The copper earth mats can causes galvanic corrosion. Copper in direct electrical contact with the tower steel increases the rate of the corrosion. 500µm/year and even higher have been measured. Steel stays on wood poles have broken due to galvanic corrosion after only 10 years in service. The corrosion rate on steel towers caused by galvanic action is much smaller than that of the stays. The tower has a much larger area exposed to the soil, and corrosion proceeds more evenly on this large area.
1.7 PREVENTION OF CORROSION
To minimize the changes of development of corrosion of steel in concrete the following preventive measures may be taken
1. Avoiding heavily congested reinforcement especially at the intersection of beams and columns
2. Avoiding the steel to come into contact with bricks, soil, wood and other porous non alkaline materials.
3. Avoiding the use of materials which accelerates the process of corrosion like aggregate with salt contents etc.,
4. Cleaning the reinforcement with brush to remove the rust scales before placing of concrete.
5. Maintain the high degree of workmanship.
6. Proper structural design with due provision of cover.
7. Providing surface coating with paints, tars, asphalts etc.
8. Use of high quality and impermeable concrete.
9. Using water cement ratio
10. Using suitable corrosion inhibitors as chemical admixtures.
Among the various preventive measures, adding suitable corrosion inhibitors. As chemical admixture during concrete preparation is the easiest and also the best method for new construction.
1.8 COATING TO REINFORCEMENT
1.8.1 General
In this method of prevention of corrosion of rebars, anti-corrosive coating is applied on the rebar surface before concreting is done. This coating protects steel from corrosive agents and thus prevents corrosion. This type of treatment is necessary for rebars when chloride contraction is normally high.
Requirements of good coating are as follows:
It should be alkali and chloride resist
It should provide adequate bond between steel and concrete.
It should have proper adhesion to steel.
It should be capable of withstanding handling stresses
1.8.2 Epoxy Coating Advantages
Anti corrosive
Two component product – easy to mix and use
Timesaving – touch dry after 30 to 45 min
Excellent adhesion – exhibits excellent bond strength in cementitious repairs.
1.8.3 Application
1. The reinforcement is cleaned with the rust clear solution in order to remove the loose rust particles present on the reinforcement.
2. After application of rust clear solution the reinforcement is washed with water and wiped out.
3. The application of nitro epoxy coating must take place as soon as possible to a dry steel surface after completion of the preparation work but always within 3 hours.
4. And allow the first to dry fully in case of unbroken coating a second application should be made as soon as the first coat is fully dry
5. The primed coating should not be left exposed to the elements for longer than necessary before over coating. Epoxy coating will,
however protect steel under clean interior exposed conditions for a period of several months
6. In exterior environments, a maximum interval of 14 days will be tolerated.
LITERATURE REVIEW
2.1 STUDIES OF CONCRETE & REBAR PROPERTIES
Dhir R.K, Jones M.R and McCathy.M.J has presented the paper on “chloride-induced reinforcement corrosion”,magazine of concrete research.
Dugarte M., Sagues A.A., powers R.G., Lasa I.R. (2007)
The polarization performance of two types of commercial galvanic point anodes for protection of rebar around patch repairs is evaluated. Experimental include measurement of the polarization history of the anode under galvanostatic load simulating various aging regimes. Additionally, the anodes were evaluated in reinforced concrete slabs with residual chloride contamination and in field installations. Preliminary results indicate that only modest performance may be achieved with typical expected anode placement spacing in commonly encountered applications.
This experimental has been performed in chambers of relative humidity (R.H) controlled at 60% and 95% at room temperature. The basic test specimen is a prism 20cm x 20cm x 10cm, with a sacrificial anode placed against one of the external mortar faces of the anodes. Two embedding media were used: a speciality polymer modified cementitious repair mortar (proprietary mix) and an ordinary repair concrete For both anode types, the results to date to date project that only modest anode operating potentials may be achieved with typical expected anode placement spacing in commonly encounted applications. That expectation is supported by the limited depolarization values observed in the test slabs before disconnection of the active rebar. After disconnection of the rebars in the chloride greater depolarization was recorded for the C anodes but the area of steel served was small. Initial trends suggest that after 22 months again under regular service conditions the anodes polarized significantly when delivering current. Thus, operating potentials necessary for effective protection prevention action may not be achieved without using a high anode to the steel placement density. Field results were generally consistent with those from laboratory and test yards. Decreased performance with aging was noted for both makes of anode, and it was particularly severe in one of them. It is Emphasized that these findings and preliminary and confirmation of trends is pending on continuing evaluation.
T Charng and F.Lansing (1982) this report summarize a general review of causes of corrosion of metals and their alloys. The corrosion mechanism is explained using the concept of electro chemical reaction theory. The causes and methods of controlling of both physicochemical corrosion and biological corrosion are presented in detail. Factors which influence the rate of corrosion are also discussed.
Gonzalez, J.A et al have studied the corrosion mechanism of rebars and also the role of oxygen in the mechanism. They conducted that the beginning of corrosion, oxygen required and for this the oxygen in the pores of the concrete is consumed. The process continues due to cyclic catalytic mechanism similar to that in the atmospheric corrosion of steel in the presence of sulphur-di- oxide or chlorides.
Gonzalez,J.A et al., have studied the effect of chloride ions on the corrosion of rebars and have concluded that the presence of chloride ions in concrete reduces the life of RC structures to less than 10 years. Chloride ions increase the rate of corrosion and hence induce cracking of cover concrete due to corrosion.
Kamashwari.B, et al., this study showed the results of corrosion mechanism of steel reinforcement and its influence on the structural behaviour of concrete beams. Reinforcement concrete beam will be designed as per standard specification using M 20 grade of concrete and will be subjected to corrosion using chemical as well as electro chemical techniques. Condition of rebar will be periodically accessed through potential measurement. At the end of the period structural behaviour of beam will be studied. The investigation is expected to throw more light on the process of reinforcement corrosion and its effect on the flexure behaviour of concrete beams.
Lamya amleh and saeed mirza studied the influence of corrosion on the bond between steel and concrete and concluded that bond strength decreases rapidly with the increase in the corrosion level. The cohesion and adhesion at the bar surface is reduced to a great extent due to the longitudinal cracks that develop due to corrosion.
Makita M.,Mori,Y., and Katawaki.K carried out exposure specimen on the sea in Tokyo bay in 1972.the variables considered by them were water – cement ratio 0.4,0.55,0..75, cover thickness 20mm, 70mm, 120mm, 212mm, an water used were fresh water and seawater.The concrete specimens were subjected to exposure test on the sea at Tokyo bay, on a steel exposure table in the form of round pontoon floated on the sea. Some important conclusions made by them were.
1. With cover thickness of 20mm, degree of corrosion was dependent on the exposure condition.
2. With cover thickness of 70mm, specimens with 0.55 water –cement ratio had no corrosion. In specimens with 0.70 water- cement ratio had no corrosion. IN specimens with 0.70 water-cement ratio a sight degree of corrosion was noted even in specimen mixed with fresh water.
3. With cover thickness of 12cm, no corrosion of steel reinforcement was noted.
4. The sped of penetration of chlorides into concrete varies with water-cement ratio, being greater in the case of 0.7 than in the case of 0.55 and 0.40
Sekar.A.S.S, Saraswathy .V and Parthiban.G.T., The results of a study of the effectiveness of sacrificial anodes in preventing the onset of pitting corrosion in chloride contaminated concrete using zinc overlay and a conductive coating in cathodically protected chloride contaminated slabs. Experimental tests were carried out on reinforced concrete slabs with steel embedded both in chloride free concrete and chloride contaminated concrete in order to compare the effects of sacrificial anodes may be more effective in preventing corrosion initiation(i.e. in providing cathodic prevention) than in controlling ongoing pitting corrosion(i.e. in guaranteeing cathodic protection). Monitoring criteria for this of prevention are also discussed.
Sugumar P has compared the weight loss of steel due to corrosion of Tiscon CRS, ordinary Fe 415 and coated Fe 415.He conducted that the loss in weight for coated rebar is about 30% less than that for uncoated rods and about 20% less than for CRS. The time enquired for CRS and coated Fe415 is 30 to 40% more than that for uncoated Fe415 rods.
Thangavel .K, et al., conducted experiments to study the influence of rebar coatings on the steel-concrete bond. Their conclusions are:
1. Coated steel rebar improved the bond strength when compared with plain mild steel rebar.
2. Galvanizing and epoxy coatings reduce the bond strength at higher thickness of coatings. On the other hand, the bon strength improves further at higher thickness of coating in the case of inhibited cement slurry coatings is for the increase in bond
3. strength is that the inhibited cement slurry coating is cement based and hence compatible with the surrounding concrete.
Wibenga,J.G. inspected 64 structures and got information about their age, specified concrete compaction, type of shuttering and concrete over. During inspection visual signs deterioration in splash zone were looked for. The cover as observed varied considerably between structures. The over values were 80mm to 90mm, although in some exceptional causes, some steel bars were situated very near to the concrete surface. Since the concrete is splash zone is always wet due to tidal movement and also due to the hygroscopic action of sea salts, the depth of cation was always smaller than the cove. Hence most of the structure showed no corrosion. He made the following conclusions
1. Corrosion of rebar was observed only in those structures having long exposure and in which the cover to steel was relatively small.
2. The corroded rears were examined and were found that the corrosion was due to chloride penetration.
3. The maximum depth of corrosion primarily depended on the exposure time and the porosity of the concrete.
Tangavel, k. et al conducted experiments to study he influence of rebar coatings on the steel – concrete bond. Their conclusions are While comparing wit plain mild steel rebars, the coated steel rebars has been improved the bond strength. Galvanizing and epoxy coatings reduce the bond strength at higher thickness of coatings. on the other hand , the bond strength improves further thickness of coatings in the case of inhibited cement slurry coatings is for the increase in bond strength is that the inhibited cement slurry coating is cement based and hence compatible with the surrounding concrete.
Trepanier used 2-inch diameter lollipop mortar specimens and also ASTM G-109 concrete slab specimens for evaluating four commercial concrete inhibitors. The lollipops were constantly submerged in 3.5% NaCl solution to approximately mid-height of the specimens, and the slabs were treated per ASTM G-109 procedures. Half cell potentials, linear polarization, and AC impedance methods were used to monitor the lollipops whereas the slabs were monitored using macro-cell current measurements. All four of the corrosion inhibitors delayed the initiation of corrosion to some degree. However, none of them completely prevented the corrosion from occurring. The performance of the inhibitors was indirectly proportional to the water/cement ratio. The effectiveness of the inhibitors increased as the w/c ratio decreased. The results from both the AC impedance and linear polarization measurements were comparable. After nearly one year of cyclic ponding only the control specimens of the ASTM G-109 slabs were actively corroding, which had initiated after 271 days. From this it was evident that a considerable amount of time was needed in order to obtain any results from this type of test. Though this form of testing is not quick to produce results, it is felt that the evaluation of corrosion inhibitors should be conducted using concrete specimens representative of real world conditions.
Nausha Asrar et al, noted that coating provides better corrosion protection to the rebars than the usual one. Also, its protective effect has been found dependent upon the type of cement.
Clear and Sohanghpurwala et al studied the corrosion characteristics of straight and bent epoxy coated reinforcing steel on a total of 40 small scale concrete slabs. Their results indicated that the epoxy coating on the straight and bent bars improves resistance to chloride induced corrosion.
Abdulkareem Mohammed Ali Alsamuraee studied those developing new reinforcement materials by using of different coating materials. Experimental works included examination of coating defect, coating Adhesion study, and the adhesion between concrete and coated bars, and different techniques are employed to assess the performance of the reinforcement embedded in concrete.
C.P. Atkins and J.D. Scantlebury studied that under normal circumstances steel reinforcement in concrete is in a passive condition due to the high pH environment provided by the hydration of cement. One of the causes of the breakdown of this passivity is attack by chloride ions . Currently common methods of finding the level of chlorides in a structure involve destructive techniques. Values for the activity coefficient of the chloride ion in pore water solution when added as sodium chloride has been produced from electromotive force data using a silver/silver chloride ion selective electrode.
CHAPTER-3
SCOPE AND OBJECTIVE
3.1 SCOPE OF THE PROJECT
From the literature review it is found that the RC slabs are corroded at a faster rate than any other elements. Various techniques have been adopted to prevent the RC slabs from corrosion.
In this project it is aimed to study the effect of coating given to the reinforcement and effect of adding fibre in the concrete.
3.2 OBJECTIVE OF THE PROJECT
To study the strength properties like compressive strength, split tensile strength of concrete with and without fibre.
To apply protective coatings to the reinforcement.
To cast RC slabs with coated and uncoated bars.
To test RC slabs for their durability behaviour by conducting accelerated corrosion test.
To compare the behaviour between coated and uncoated reinforced concrete slabs.
3.3 METHODOLOGY TO BE ADOPTED
Literature collection.
Material collection for experimental investigation.
Casting of slabs specimens and curing of specimens.
Test setup for slab specimens.
Testing of specimen.
Test results.
Comparison of results.
Conclusion.
CHAPTER-4
TESTS OF MATERIALS
4.1 SPECIFIC GRAVITY TEST OF CEMENT
4.1.1 Procedure:-
Weight of the empty dry specific gravity bottle(w1)
Fill the bottle with cement and weight the bottle (w2)
Dry the bottle and fill it with kerosene and weight (w4)
Pour some of the kerosene out and introduce a weighed quantity of cement (say 60g) in to the bottle. Roll the bottle to the top with kerosene and weight it (w3)
Fill the bottle with distilled water and weight (w5)
Calculate the specific gravity by using formula,
• Sc = (w2-w1)(w4-w1)/(w4-w1)-(w3-w2)(w5-w1)
4.2 SPECIFIC GRAVITY TEST OF FINE AGGREGATE
4.2.1 Procedure:-
Find the mass of empty pyconometer(w1)
Take about 200g to 400g of over dried soil .find mass of pyconometer+soil (w2)
Fill pyconometer to half its height with water to run of pyconometer (w3)
Empty pyconometer of fill water to fill run (w4)
4.2.2 Calculation:-
Empty weight of pyconometer, w1 = 655g
Pyconometer + sand weight, w2 =1535g
Pyconometer + sand + water, w3=2070g
Pyconometer + water weight, w4= 1525g
Specific gravity, G= (w2-w1)/ (w2-w1)-(w3-w4)
= (1535-655)/ (1535-655)-(2070-1525)
=880/(880-545) =2.626
4.3 SPECIFIC GRAVITY TEST OF COARSE AGGREGATE
4.3.1 Procedure:-
Find the weight of empty cylinder (w1) and measure the height (H)
Mark approximately 2/3rd height inside the cylinder (H1)
Fill up the aggregate up to that level
Find the weight of cylinder with aggregate (w2)
Pour water up to that mark and find the weight of cylinder (w3)
Remove all the contents in the cylinder
Pour water up to that level and find the weight (w4)
4.3.2 Calculation:-
Empty weight of cylinder, w1 = 2.365g
Cylinder + weight of 1/3rd of coarse aggregate, w2 = 3.895g
Weight of cylinder + CA + water, w3 =6.315g
Weight of cylinder + water, w4 =5.340g
Specific gravity, G = (w2-w1)/ (w2-w1)-(w3-w4)
= (3.895-2.365)/(3.895-2.365)-(6.315-5.340) =2.756
MIX DESIGN
5.1 GENERAL
The mix proportions shall be selected to ensure the workability of the fresh concrete and when concrete is hardened, it shall have the required strength, durability and surface finish
5.1.1 Procedure For Mix Design:-
STEP 1: TARGET MEAN STRENGTH OF CONCRETE:-
The target mean strength for specified characteristic cube strength is (fck)
fck’ = fck + ts
Where,
fck-characteristic compressive strength @ 28 days
t- risk factor from table 2
s - standard deviation from table 1 of IS 10262-1982.
STEP 2: SELECTION OF WATER CEMENT RATIO:-
From Table 5 of IS 456, maximum water cement ratio = 0.45.
Based on experience, adopt water-cement ratio as 0.40.
0.40 < 0.45, hence O.K.
STEP 3: SELECTION OF WATER CONTENT:-
From Table 2, maximum water content is taken according to the size of the aggregates.
STEP 4: SELECTION OF WATER AND SAND CONTENT:-
For 20mm maximum size aggregate, sand conforming to grading zone II
STEP 5: DETERMINATION OF CEMENT CONTENT:-
Cement =water/ w/c ratio This cement content is adequate for mild exposure condition.
STEP6: DETERMINATION OF COARSE AGGREGATE & FINE
AGGREGATE CONTENT:-
With the quantities of water and cement per unit volume of concrete and the ratio of fine to total aggregate already determined, the total aggregate content per unit volume of concrete may be calculated from the following equations.
And V= [W+(C/Sc) + (1/p)*(fa/Sfa)]* 1/1000
V= [W+(C/Sc) + (1/1-p)*(ca/Sca)]* 1/1000
Where,
V – absolute volume of fresh concrete which is equal to gross volume (m3) minus the volume of entrapped air.
W –mass of water (kg) per m3 of concrete.
C –mass of cement (kg) per m3 of concrete.
Sc– specific gravity of cement.
P –ratio of fine aggregate to total aggregate by absolute volume.
fa, ca– total mass of FA and CA (kg) per m3
Sfa, Sca– specific gravities of saturated, surface dry FA and CA
5.1.2 Design Of Grade – M25
DATA:-
Characteristic compressive strength= 25 Mpa
required @ 28days (fck)
Maximum size of aggregate = 20 mm
Degree of workability = 0.9 compacting factor
(medium degree of workability slump 50-100)
table (IS 456:2000 pg.no:17 clause 7.1)
Degree of quality control = good
Type of exposure = mild
TEST DATA FOR MATERIALS:-
Specific gravity of cement = 3.15
i) specific gravity of coarse aggregate (20mm) = 2.76
ii) specific gravity of fine aggregate = 2.63
CALCULATION:-
STEP 1: TARGET MEAN STRENGTH OF CONCRETE:-
fck = 25 + (1.65*4) = 31.6 Mpa
STEP 2: SELECTION OF WATER CEMENT RATIO:-
The w/c ratio required for the target mean strength of 31.6 M pa is 0.45. This is lower than the maximum value of 0.5 prescribed for ‘mild’ exposure
Adopt w/c ratio of 0.45.
STEP 3: SELECTION OF WATER AND SAND CONTENT:-
For 20mm maximum size aggregate ,sand conforming to grading zone II,
Water content per cubic meter of concrete =186 kg
Sand content as percent of total aggregate by absolute volume =35%
STEP 4: DETERMINATION OF CEMENT CONTENT:-
w/c ratio =0.45
water =186
cement =186/0.45 = 413.33 ~ 413 kg/m3
This cement content is adequate for mild exposure condition
STEP5: DETERMINATION OF COARSE AGGREGATE & FINE AGGREGATE CONTENT:-
For the specified maximum size of aggregate of 20mm, the amount of entrapped air in the wet concrete is 12%
V=[W+(C/Sc)+(1/p)*(fa/Sfa)]* 1/1000
V=100-2= 98% ~ 0.98
Fa = 0.98 [186+ (413/3.15) + (1/0.35)*(fa/2.63)]* 1/1000
0.98 = [317.11+ 1.086 fa]* 1/1000
980-317.11= 1.086 fa
fa = 662.89/1.08 = 610 kg/m3.
Ca =(1-P)/P*fa*(Sca/Sfa)
= (1-0.35)/0.35*610*(2.76/2.63)
=1188.85 kg/m3.
The mix proportion then becomes
Water Cement Fine aggregate Coarse aggregate
186 413 610 1188.85
0.45 1 1.48 2.88