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CHAPTER 5 SUMMARY AND CONCLUSIONS
5.1
SUMMARY Plain concrete is strong in compression but is weak in tension. The
reinforcement with continuous reinforcing bars is providing a solution to withstand tensile stresses. Even though the addition of admixtures significantly increases the strength of concrete, the development of micro cracks must be controlled to produce concrete with homogeneous tensile properties. The introduction of waterproofing admixtures is brought in as a solution to develop concrete to reduce permeability, corrosion, shrinkage, chemical resistance and enhanced to compressive, tensile and flexural strength, and workability. This thesis deals with the changes due to the addition of waterproofing admixtures on the workability, strength and durability of concrete. The workability studies have been carried out by using various w/c ratios such as 0.55, 0.60 and 0.70. Five different chemical admixtures were chosen and they were added in three different dosages (0.12 to 0.59% by weight of cement) mixed thoroughly with the M20 designed concrete mix, cured for 7 days and 28 days and then tested. The specimens were tested for various strength properties such as compressive strength, tensile strength, flexural strength and modulus of elasticity with and without admixtures and the results were compared. The durability of concrete was studied by conducting water permeability test by steady state flow method in 7 and 28
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days cured concrete specimens. The resistance to chemical attack of concrete using waterproofing admixtures was studied by conducting acid and chloride test by immersing the concrete specimens in chemicals for 90 days and they have been tested for compressive strength for finding the strength reduction after immersion. The effect of admixtures on the corrosion behaviour of reinforced concrete was studied by conducting accelerated corrosion test on 28 days cured concrete specimen embedded with steel bar. Since the pores present in the concrete are blocked due to the addition of admixtures, it reduces the corrosion and improves the life of RCC structures. The influence of waterproofing admixtures on the chloride ion penetration characteristics of concrete was studied by conducting rapid chloride penetration test and the results are compared with conventional concrete. The contribution of waterproofing admixtures on the shrinkage characteristics of concrete with various admixtures and various curing periods such as 28 days, 60 days and 90 days were also studied by conducting drying shrinkage test. Addition of waterproofing admixtures reduces the mid span and 1/3rd span deflections in the flexural strength test and thus increases the flexural stiffness. 5.2
CONCLUSIONS Based on these experimental investigations, important conclusions
have been arrived at as follows: 1.
The addition of waterproofing admixtures modifies the workability of concrete without change in the w/c ratio. The lubricating actions may be due to the surfactants present in the admixtures. The increase in the slump values by 2.5 times compared to that of conventional concrete after addition of the above admixtures indicated that the cohesiveness of the cement concrete mix has increased. This also enhances the pump-able characteristics of concrete.
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2.
Waterproofing admixtures result in a better dispersion of cement particles during mixing resulting in a dense concrete than the conventional cement concrete. This increased the mechanical properties such as compression, tension and flexure even after 7 days and 28 days of hydration. Since the strength enhancement is more due to the addition of admixtures, it facilitates the removal of form work at early ages.
3.
It has been found that during the flexural strength test, the mid span deflection and 1/3rd span deflection of cement concrete with waterproofing admixture is considerably reduced than the conventional concrete. This may be due to the dense network in the cement concrete due to better compaction by the addition of admixtures.
4.
The addition of admixtures reduces the co-efficient of permeability value as 7 to 21% for 7 days and 12 to 23% for 28 days depending upon the type of admixture used and its dosage added. This may be because of the reduction of pores present in the concrete due to better compaction by the addition of admixture. Out of all the admixtures used, napthalene based admixtures shows better performance than other admixtures in the permeability studies.
5.
The incorporation of waterproofing admixtures in concrete reduces the corrosion behavior of steel in reinforced cement concrete structures. The pores present in the concrete are blocked by the action of admixture particles which increases the impermeability characteristics of concrete. By the addition of different types of admixtures, the percentage loss of weight
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of corroded bar embedded with concrete is reduced from 12% to 29.69% depending upon the various dosages. While comparing the performance of different admixtures used, polymer based admixtures shows good resistance in the corrosion behavior of steel in RCC elements. 6.
Since the addition of admixtures reduces the pores present in the concrete by the possibility of good workability and well compaction, the electric charge passed (in Coloumbs) through concrete pores also reduced with respect to the dosage of admixtures in the rapid chloride penetration test. By the addition of these admixtures, it reduces the chloride ion penetrability rate by 1.5 to 2.7 times than the conventional concrete depending upon the type of admixtures used. Out of all five admixtures used in this study, strearate based admixtures conventional
shows
better
concrete
performance
shows
the
in
moderate
RCPT.The range
of
penetration and the concrete with waterproofing admixture produced low and very low range of penetration. 7.
It is deduced that the percentage loss of weight in the acid and chloride test depends upon the dosage of waterproofing admixtures. The percentage loss of weight of specimen is reduced from 0.74 to 0.47% in the acid test and 0.69 to 0.42 % in the chloride test. The addition of strearate based admixture gives the best performance than other chemicals with its maximum specified dosage in the acid and chloride test.
8.
The addition of waterproofing admixtures also increases the modulus of elasticity of concrete in 7 days and 28 days with
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respect to dosage. This is due to dense concrete with less pores obtained by well compaction which increased the strength of concrete. The 28 days modulus of elasticity of concrete is verified with regular equation using the 28 days compressive strength. It has been found that it is acceptable with less error while comparing the concrete with and without admixtures. The better performance is obtained by the addition of stearate based admixtures in 7 days and 28 days concrete. 9.
The shrinkage strain of concrete after the addition of waterproofing admixtures is found to be lesser than the conventional concrete.The percentage reduction is varying from 3.3 to 37.5% for various curing periods of 28 days,60 days and 90 days. This reduction in shrinkage strain depends upon the admixture used, dosage and also the curing period. Out of all admixtures, stearate based admixture shows better performance in 28 days shrinkage strain and melamine shows the better performance in 60 and 90 days. This may be because the diffusion of water to the surface of concrete is less due to the presence of the waterproofing admixtures.
10. As the waterproofing admixtures enhance concrete properties in terms of strength and durability, the end product is also suitable for aggressive environment. 11. The dosage of admixture usage is limited from 0.12% to 0.59%. Within this range, the increasing of admixture-cement ratio improves the various properties of concrete. 12. While considering the individual efficiency of various chemical admixtures used in this research work, Melamine
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based admixture gives better workability property. Stearate based admixtures enhancing the overall strength properties, chemical
resistance
and
also
shrinkage
properties.
Naphthalene based admixtures gives better performance in the permeability characteristics of concrete in 28 days which is 22.85% less than the permeability of conventional concrete. The corrosion resistance of concrete is also encouraged by the addition of polymer based admixtures which is 28.78% more than that of the conventional concrete. Ligno based admixtures shows relatively less performance while compared with other admixtures. 5.3
SCOPE FOR FURTHER RESEARCH WORK 1.
Effect on strength and durability of concrete due to addition of combination of two or more admixtures with different specifications added to the concrete can be studied.
2.
Waterproofing admixture in powder form gives better performance in permeability in this research work. So in future to get the effect on performance in permeability and other strength properties, combination of liquid and solid admixtures of different specifications added in a particular ratio with concrete can be studied.
3.
In this research work only water curing is used. In future, concrete with waterproofing admixtures with various type of curing can be studied.
4.
In this research work only M20 designed concrete of mix has been used. In future different concrete mixes like M25, M30 etc., may be taken and the results can be analyzed.
