J. Inst. Eng. India Ser. A (February–April 2012) 93(1):73–78 DOI 10.1007/s40030-012-0009-4

CASE STUDY

Effect of Waterproofing Admixtures on the Flexural Strength and Corrosion Resistance of Concrete A. Geetha • P. Perumal

Received: 17 February 2011 / Accepted: 12 January 2012 / Published online: 6 April 2012 Ó The Institution of Engineers (India) 2012

Abstract This paper deals about the flexural strength and corrosion behaviour of concrete using waterproofing admixtures. The effect of waterproofing admixtures on the corrosion behaviour of RCC specimen has been studied by conducting accelerated corrosion test. To identify the effect of corrosion in pull out strength, corrosion process was induced by means of accelerated corrosion procedure. To accelerate the reinforcement corrosion, direct electric current was impressed on the rebar embedded in the specimen using a DC power supply system that has a facility to adjust voltage. The addition of waterproofing admixtures also shows the improvement in the flexural strength of concrete has been studied by conducting flexural strength tests on the concrete prism specimen of size 100 9 100 9 500 mm with and without admixtures for various dosages and various curing periods of 7 and 28 days. The results showed that the presence of waterproofing admixtures always improves the corrosion resistance and thus increases the strength of concrete due to the hydrophobic action of waterproofing admixtures. Keywords Admixtures  Concrete  Durability  Corrosion  Flexural strength  Waterproofing

Introduction Insufficient durability of concrete structures has become a serious problem. One of the most important parameters influencing the durability of concrete is its permeability. The permeability of concrete determines the ease with which gases, liquids and dissolved deleterious substances such as carbon-di-oxide or oxygen or chloride ions penetrate the concrete. If the corrosion process has started, the rate of corrosion is still dependent on the supply of oxygen. The permeability of concrete is a major factor affecting the service life of reinforced components. Addition of waterproofing admixtures reduces the permeability of concrete and thus protects reinforcement for corrosion. The experimental work also has been carried out on concrete to study the effect of waterproofing admixtures in the permeability, various strength like compressive strength and split tensile strength, workability and chemical resistance of concrete. Chemical resistance studies on concrete using waterproofing admixture are studied by conducting acid test and chloride test. In this paper, the flexural strength and corrosion behaviour of concrete were studied using waterproofing admixtures. Mode of Action

A. Geetha (&), Non-member Rural Development, Kalakkadu 627107, Thirunelveli District, Tamil Nadu, India e-mail: [email protected] P. Perumal, Fellow Department of Civil Engineering, Government College of Engineering, Salem 636011, Tamil Nadu, India e-mail: [email protected]

Ordinary concrete even of high quality contains capillaries and micro cracks. This allows water to pass through its structure by an action similar to tree drawing water to its canopy. Concrete absorbs water because surface tension in capillary pores in the hydrated cement paste ‘‘pulls in’’ water by capillary suction. It is called capillary absorption. Integral waterproofing admixture increases resistance to capillary absorption. When they are mixed with concrete, it has two distinct waterproofing actions [1]. The first is the

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Since the reaction releases electron, these electrons are simultaneously accepted in cathode, where oxygen reduction occurs. This is called cathodic reaction. 1=2O

2

þ H2 O þ 2e ! 2OH ! to anode

Thus, one can see that oxygen (O2) and water (H2O) are required to the cathode reaction, of the overall corrosion process. When concrete is dry oxygen is able to diffuse and reach the steel and when concrete is wet, water is able to reach the steel. Corrosion is usually accompanied by the formation of solid corrosion debris from the reactions between anodic and cathodic products [5].

Fig. 1 Contact angles of water droplet over concrete surface

Fe2þ þ 2OH ! FeðOHÞ2 ðFerrous hydroxideÞ reaction of hydrophobic component with concrete mix, which fundamentally changing its surface tension, producing a concrete, which is inherently water repellent and non-absorptive throughout its entire mass. It is meant an increase in the contact angle between the walls of the capillary pores and water, so that water is ‘‘pushed out’’ of the pores. Figure 1 shows the increase in contact angle and ‘‘pull in’’ capillary suction. In the second action, polymer globules moving with the bleed water during hydration collect in the capillaries When the hardened concrete is subjected to water pressure, these globules are compacted together to form a physical ‘‘plug’’ blocking the capillaries and prevent water entry. Chemicals such as stearates (RCOOH) are reacted with calcium hydroxide (Ca (OH)2) and formed insoluble calcium stearate (Ca?COOR-). It coats the capillary pores. CaðOHÞ2 þ RCOOH ! Caþ COOR þ H2 O

ð1Þ

Theoretically, the effect of high contact angle produced by the use of the waterproofing admixture means that 1–4 m head of water would be required to penetrate the surface through the largest capillaries. The rain water has pressure head in centimeters. Corrosion Mechanism The reinforced steel in concrete is in highly alkaline environment due to the formation of calcium hydroxide, formed by the hydration of cement. At this environment of higher alkalinity, reinforcement is protected by the passivate layer of ferric oxide and it is not initiate any corrosion [2]. If the passivate layer is destroyed by any corrosion influencing factor, then the ferric oxide is reduced into ferrous oxide and it initiates corrosion [3]. If the steel is active (more negative potential), the solid steel surface dissolves and goes into the solution as ferrous ions. This is called anodic reaction [4]. FeðsolidÞ ! Feþþ ðionsÞ þ 2e ! to cathode

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ð3Þ

ð2Þ

ð4Þ

Pure Ferrous hydroxide is white but the material initially produced by corrosion is normally a greenish colour due to partial oxidation in air. H2 O þ 1=2O2 FeðOHÞ3 ðFerric hydroxideÞ

ð5Þ

Waterproofing admixtures may consist of pore filling or water repellent materials. The chief materials in the pore filling class are chemically active pore fillers [6]. These waterproofing admixtures fill the natural pores within the concrete or mortar to reduce porosity and improve water tightness. They increase the resistance of concrete to water penetration by creating a hydrophobic coating within the pores. The hydrophobic coating forces the water to be pushed out of the pore by surface tension.

Experimental Programme Materials Cement, Fine and Coarse Aggregates Ordinary Portland cement of 53 grade is used in this study. Natural river sand was used as fine aggregate. Crushed granite stones obtained from local quarries were used as coarse aggregate. The maximum size of coarse aggregate used was 20 mm. The results are furnished in Table 1. Water Potable water free from salt was used for casting and curing of concrete as per IS: 456-2000 recommendations [8]. Admixtures According to IS 2645-1975 integral waterproofing chemicals are used. The properties of chemicals used and their dosages are given in Table 2 [7].

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Table 1 Properties of cement, fine aggregates and coarse aggregates S.No

Tests

Cement 3.15

Fine aggregate

Coarse aggregates

2.6

2.5

1

Specific gravity

2

Fineness

2%

–

–

3

Fineness modulus

–

2.2

5.73

4

Standard consistency

31 %

–

–

5

Compressive strength 34 N/mm2

7 days 6

28 days Setting time

54 N/mm

Initial

30 min

Final

585 min

POWER SUPPLY

Rebar PVC Pipe

–

2

Non metalic container 3.5 %Na Cl Solution Cube Speciman

–

Stainless Steel plate (cylinder)

M seal

Concrete

Fig. 2 Schematic representation of the electrochemical system

The grade of concrete used was M 20 (1:1.55:3.1) designed mix. Accelerated Corrosion Test To identify the effect of corrosion in pull out strength, corrosion process was induced by means of accelerated corrosion procedure. To accelerate the reinforcement corrosion, direct electric current was impressed on the rebar embedded in the specimen using a DC power supply system that has a facility to adjust voltage. Figure 2 shows schematic representation of the electrochemical system used. An electric wire was soldered the reinforcement to impress the electric current. The stainless steel annular cylinder was placed in the tank to act as a cathode. Power supply with adjustable voltage was chosen for the above corrosion process. The positive terminal was connected to the rebar and the negative terminal was connected to the stainless steel cylinder. The stainless steel cylinder that served as the cathode consumed the excess electrons given by the reinforcement bar during the corrosion process. To ensure that the bond zone only gets corroded, the entire length except the bonded portion was covered with PVC tube and the PVC ends were properly sealed.

After the power supply was turned on the voltage was adjusted and fixed. The current was monitored and recorded at regular intervals. The expected amount of corrosion in terms of mass loss of the reinforcing bar due to corrosion can be estimated by the following Eq. CR ¼ ½ðW0  WÞ=W0   100

ð6Þ

where CR is the expected amount of rebar corrosion, W0 is the initial weight of the bonded length of rebar, W is the weight of the bonded length of rebar after removal of corrosion products Flexural Strength Test The extreme fibre stress calculated at the failure of specimen is called modulus of rupture. Flexural strength test was conducted as per recommendations IS: 516-1959. In flexural strength test, prisms of size 100 9 100 9 500 mm were cast. The tests were conducted on specimen for the curing periods of 7 and 28 days with and without admixture. The flexure test has been conducted in flexure testing machine. The rate of loading is 180 kg/min. The axis of the specimen is carefully aligned with the axis of the loading

Table 2 Properties of admixtures used and their dosages S.No Name of the admixture

Supply form

Colour

1

Naphthalene based

Powder

White

2

Polymer based

3

Melamine based

Dosage of admixture

Chloride content

Specific gravity @27 °C

Density (Kg/m3)

Nil

1.20–1.25

1225

1.6 kg/100 kg of 2.0 kg/100 kg of 2.4 kg/100 kg of cement cement cement

Liquid

Dark Nil yellow

1.30–1.35

1320

0.1 l/100 kg of cement

Liquid

Light \0.005 % brown

1.28–1.31

1300

0.22 l/100 kg of 0.33 l/100 kg of 0.40 l/100 kg of cement cement cement

D1

D2

0.2 l/100 kg of cement

D3

0.3 l/100 kg of cement

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device. Three dial gauges were being used for the investigation. One was placed at centre and the other two were placed at one-third distance from two supports of the beam respectively. The deflections were taken at the mid span and one-third span points [9].

Results and Discussions Accelerated Corrosion Test The results are tabulated in Table 3. The average percentage loss of weight has been found as 3.30 for specimens without admixtures. For the specimens with dosage1; the average percentage of loss of weight is estimated as

Table 3 Comparison table for % variation in loss of weight with and without admixture S.No

Specimen

1

Without admixture

3.30

2

Naphthalene Based D1

2.81

14.85

3 4

Naphthalene Based D2 Naphthalene Based D3

2.62 2.35

20.61 28.78

5

Polymer based D1

3.12

5.45

6

Polymer Based D2

2.85

13.64

7

Polymer Based D3

2.32

29.69

8

Melamine Based D1

3.17

5.94

9

Melamine Based D2

2.95

10.61

10

Melamine Based D3

2.61

20.91

Fig. 3 Comparison of deflection of specimen with and without admixtures 28 days/ polymer based

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% Loss of weight

% variation with and without admixture –

2.81. For dosage 2 of the admixtures, the percentage loss of weight has been estimated as 2.62 and for dosage 3, it is 2.35 % [10]. For example, the addition of 1.6 % of naphthalene based admixtures per 100 kg of cement, resulted 14.8 % decreasing the loss of weight while comparing with the specimen without admixture. For the addition of 2 % of naphthalene based admixtures per 100 kg of cement, 20.6 % decreasing the loss of weight and for the addition of 2.4 % of naphthalene based admixtures per 100 kg of cement, 28.78 % decreasing the loss of weight, were observed [11]. The addition of polymer based admixtures of 0.1 l per 100 kg of cement, resulted 5.46 % decreasing the loss of weight while comparing specimen without waterproofing admixture. For the addition of 0.2 l of polymer based admixtures per 100 kg of cement, 13.63 % decreasing the loss of weight and for the addition of 0.3 l of polymer based admixtures per 100 kg of cement, 29.69 % decreasing the loss of weight, were found [12]. Similarly for the other waterproofing admixture also, increasing the dosage decreases the % loss of weight. Result analysis shows that the polymer based admixture gives better performance than the other admixtures [13]. Flexural Test The addition of admixtures shows the good improvement in flexural strength also. The addition of polymer based admixtures increases the strength about 12.93 % for dosage1, 24.34 % for dosage2 and 32.89 % for dosage3 in 7 days and 8.99 % for D1, 16.93 % for D2 and 20.30 % for D3 in 28 days than the conventional concrete. In case of

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Table 4 Comparison of flexural test results obtained by equations and as per experiment S.No

Description

Flexural strength, N/mm2 7 Days as per experiment

28 Days as per experiment

28 Days as per equation using dosage/cement ratio versus flexural strength

% error

1

Conventional concrete

2.02

3.14

–

–

2

Naphthalene D1

2.51

3.57

3.52

1.33

3

Naphthalene D2

3.31

4.04

3.62

10.40

4

Naphthalene D3

3.37

4.52

3.73

1.76

5

Melamine D1

2.64

3.82

3.73

2.32

6

Melamine D2

2.82

3.85

4.02

4.31

7

Melamine D3

3.47

4.31

4.21

2.38

8

Polymer D1

2.32

3.45

3.52

2.27

9

Polymer D2

2.67

3.78

3.86

2.14

10

Polymer D3

3.01

3.94

4.14

4.78

Table 5 Proposed equations using dosage of admixture-cement ratio (D/C) and conventional strength for 28 days Type of WPA

Proposed equations Admixture dosage/cement ratio versus conventional strength of concrete

Naphthalene based

ffn = 3726(D/C)2 - 31.25 (D/C) ? ff

Melamine based

ffm = -1086.7 (D/C)2 ? 271.23 (D/C) ? ff

Polymer based

ffp = -28616 (D/C)2 ? 418.6 (D/C) ? ff

Fig. 5 Admixture–cement ratio (D/C) versus flexural strength–polymer based

Fig. 4 Admixture–cement ratio (D/C) versus Flexural strength– Melamine based

melamine based admixtures for 7 days the % increase in strength varying from 23.48 to 41.79 % and for 28 days the % increasing in strength varying from 17.80 to 27.14. Similarly the usage of naphthalene based admixtures increases the 7 days the strength as 19.52–40.01 % and in 28 days as 10.79–29.75 %. It reduces the mid and 1/3 portion deflection in the specimen while increases the dosage of chemicals. The

Fig. 6 Admixture–cement ratio (D/C) versus flexural strength– naphthalene based

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increasing factor varying from 1.29 to 1.94 for 7 days and 1.01 to 1.49 for 28 days. Figure 3 shows the load against deflection curve for the flexure test. Result analysis shows that the effects of addition of admixtures are more in case of 7 days specimen than the 28 days specimen. Relations between flexural strength and admixture parameters have been proposed which are given in Table 5. The Table 4 shows the comparison between the values obtained by using equations given in Table 5 and by experiment for 28 days. The values obtained using equations is reliable with experimental results with less error. The corresponding regression models are given in Figs. 4, 5, 6. Conclusions (1)

(2)

(3)

(4)

(5)

In the accelerated corrosion test addition of waterproofing admixtures reduces the loss of weight in the corroded bar and thus reduces the corrosion due to the hydrophobic action of waterproofing admixtures. Increasing the dosage of admixtures reduces the permeability of water into the specimen and reduces the corrosion since they blocked the pores or capillaries present in the concrete mass. It is concluded that the addition of admixtures increases the flexural strength with respect to the dosage. The effects of admixtures are more in case of 7 days specimen than 28 days specimen. Addition of admixture reduces the deflection in the middle and 1/3rd portion of the prism specimen in the flexure test and thus increase the strength. In the accelerated corrosion test polymer based chemical shows better performance and in the flexural

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strength test naphthalene based chemicals shows better results.

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