21

CHAPTER 2

LITERATURE REVIEW

2.1 GENERAL

A review of literature focusing on the studies related to the

improvement in durability characteristics of fly ash blended reinforced

cement concrete with corrosion inhibitors is presented in this chapter.

2.2 UTILIZATION OF FLY ASH AS ADMIXTURE

Ha et al (2005) have investigated the influence of mineral

admixture, namely fly ash (FA) on the corrosion performance of steel in

mortar and concrete by some accelerated short-term techniques in sodium

chloride solutions. The various techniques adopted for determination of

durability enhancements were weight loss method, open circuit potential

(OCP) measurements, anodic polarization technique and impressed voltage

technique. Apart from these macrocell corrosion studies, pH measurements

and estimation of free chloride content were also performed.

In weight loss method, steel rods after thoroughly cleaned with

hydrochloric acid and washed with double distilled water were centrally

placed in concrete cylinders cast with OPC and OPC containing various fly

ash replacement levels. After 24 hours, the specimens were demoulded and

cured for 28 days and then immersed in 3% NaCl solution. After 15 days the

specimens were subjected to drying in open air at room temperature for 15

days. Each wetting and drying cycle thus consisted of 30 days and the

22

specimens were ultimately subjected to 6 complete cycles (180 days) of test

period. They had reported that for the OPC system the corrosion rate was

0.0024 mmpy and upto 30% fly ash replacement levels the corrosion rate

were 0.0024 mmpy and 0.0025 mmpy and so on. But for 40% and 50% fly

ash replacement levels, the corrosion rates drastically increased to 0.0055

mmpy 0.0073 mmpy respectively.

Anodic polarization test was performed and the anodic current

measured for OPC system were found to be 0.40 and 1.00 mA respectively at

+300 and +600 mV vs. SCE. For fly ash systems upto 30% replacement levels

the anodic current was lesser than OPC indicating the superior performance of

the system with better corrosion resistance properties. Above the 30% level,

the passivity got destroyed and as a result large anodic currents in the range

0.44-1.26 mA at +300mV vs. SCE and 1.09-2.5 mA at +600 mV vs. SCE

were measured. The anodic polarization data has thus confirmed that the

integrity of passivity was maintained by the incorporation of fly ash upto 30%

replacement level.

Similarly the impressed voltage technique data for OPC and various

fly ash admixed concretes reveal that 10%, 20% and 30% fly ash replacement

levels have decreased the permeability of concrete and have showed better

improvement in time to cracking indicating better corrosion resistance

properties. On the other hand, beyond 30% levels, there was earlier cracking

of concrete indicating the inferior properties at these levels.

Macrocell corrosion studies conducted by measuring the

electrochemical characteristics of half-cell potentials against SCE with time

indicated that the passive behaviour was preserved upto 6 cycles of exposure

by fly ash systems upto 30% replacement levels whereas for 40% and more

severe corrosion of the anode was indicated agreeing with other tests adopted

earlier in the investigation.

23

The average pH value for plain cement concrete was found to be

13.0 and for various fly ash replacement levels it was found to vary from 13.0

to 11.0 Upto 30% fly ash replacement levels there was not much variation in

pH value but there was a drastic reduction in pH value indicating a steep

reduction in alkalinity beyond 30%. Similarly free chloride contents estimated

upto 20% replacement levels, the penetration of chloride ion was less and at

30% level it is comparable with OPC. Beyond 30% replacement levels, the

chloride ion penetration also increased.

From the above investigations, the following conclusions were

drawn.

The replacement of fly ash upto 30% level improved the corrosion

resistance properties of steel in concrete, improved the permeability

characteristics, delayed corrosion initiation time and decreased the corrosion

rate.

2.2.1 Effect of fly ash on workability

Mora et al (1993) examined the workability of fly ash mixes and

reported that there is a gradual increase of water volume in control mortar and

in fly ash replacing mortar from 150 to 225 ml imply greater flow table spread

(FTS) i.e. workability is increased when part of cement is replaced by fly ash

and coarser fractions gave less FTS values than finer ones. FTS increases as

do specific surface and decreases with mean diameter and for finest fraction

FTS were smaller than expected.

Joshi and Lohtia (1997) reported the work of Brown on the

workability of four concretes of different water-cement ratios in which ash

was substituted for cement on an equal volume basis and found that the

workability increased with the increased ash substitution. The changes were

24

found to depend upon the level of ash substitution and on water content. An

empirical estimate which indicates that for each 10% of ash substituted for

cement, the compacting factor changed to the same degree as it would be

increasing the water content of the mix by 3 to 4 percent .In another series of

experiments, Brown determined the effects of ash substitution for equal

volumes of aggregate or sand in one concrete, keeping all other mix

proportions constant. The test concrete was modified by replacing 10, 20 or

40% of the volume of sand by ash or 10, 20 or 40% of the volume of the total

aggregate by ash. The replacement of 40% of the total aggregate gave a mix

that was unworkable.

Malhotra and Berry (1986) reported the work of Owen that with the

use of fly ash, containing large fraction of particles coarser than 45µm or a fly

ash with high amount of unburnt carbon exhibiting loss on ignition more than

1 %, increasing water demand is observed. Water demand is increased to

maintain the desired level of fluidity.

2.2.2 Effect of fly ash on segregation and bleeding

Malhotra and Berry (1986) reported in his compilation that concrete

using fly ash reduced segregation and bleeding more satisfactory than plain

concrete when placed by pumping. He further reported the work of Johnston

that the use of fly ash particularly in the harsh mixes, which are deficient in

fines, would resolve the problem of excessive bleeding by increasing the

overall paste volume by the addition of fly ash in concrete as mineral

admixture.

From the study on high volume fly ash concrete mixes, Joshi and

Lohtia (1997) reported that the fly ash concrete mixes were more cohesive

than control mixes. During the slump test, the fly ash concrete mixes subsided

25

more slowly and gradually than the control mixes which exhibited abrupt fall

or subsidence.

2.2.3 Effect of fly ash on time of setting

Tarun and Singh (1997) studied the effects of various sources of

class C fly ashes on the setting and hardening characteristics of concrete, and

concluded that the addition of fly ash up to a certain level (typical upto about

60% replacement) caused significant delay in the times of initial and final

setting of concrete and beyond which a reverse trend was observed. The times

of setting varied greatly from fly ash to fly ash.

2.2.4 Effect of fly ash on air entrainment

Joshi and Lothia (1997) reported that the problem of erratic air

entrainment is encountered even with the ashes with carbon content less than

0.5%. Thus not only the amount but the form in which carbon is present in fly

ash possibly affects the air entraining admixture demand in fly ash concrete.

Gebler and Klieger (1983) reported that concretes made with class

C fly ash generally require less air entraining agent (AEA) than those made

with class F fly ash. They reported that for 6% air content in control concrete,

the air content in control concrete, the air entraining agent (AEA) demand

varied from 126 to 173% for fly ashes having more than 10% CaO, where as

it was in control for the range 177 to 553% for fly ashes containing less than

10% CaO. They further suggested that increase in both total alkalis and SO3

contents in fly ash affect the air entrainment favourably. A concrete

containing class F fly ash that has relative high CaO content and less organic

matter or carbon tends to be less vulnerable to loss of air.

26

Malhotra and Berry (1986) reported the study carried by Bamforth

on fly ash concrete and slag concrete for use in large size foundation. It is

observed that with an increase in the quantity of cement replaced by fly ash,

the rate of heat release is slowed down and as a result the maximum

temperature reached at any point in the concrete mass is lower than the

concrete containing no fly ash.

Joshi and Lohtia (1997) also reported the work of Korac and

Ukraincil for fly ash containing high calcium (22.93% CaO) from coal, from

in-situ measurements of temperature rise. The concrete made with cement

containing 50% fly ash showed less temperature rise than the concrete with

the commercially available cement containing 5% pozzolan and 15% slag.

2.2.5 Effect of fly ash on compressive strength

Lame et al (1998) investigated the effect of replacing cement

(0 to 55%) by fly ash taking three series of concrete mixes with

water-cementitious material ratio (W/C) of 0.3, 0.4 and 0.5 respectively. They

concluded that fly ash contributed little to strength at early ages. At 3 days,

compared to Portland cement concrete, the cube compressive strength was

reduced by 16% in average for a 15% fly ash replacement, and by 66% for

55% fly ash replacement. At 28 days, the strength of 15% fly ash mixes was

only slightly lower (4% in average) than Portland cement mixes, although

55% fly ash replacement still resulted in a 44% strength reduction. At the later

ages, the contribution of fly ash to compressive strength development became

significant.

In a laboratory study, Joshi and Lohtia (1997) tested a large number

of fly ash concrete mixes made by using three different fly ashes containing

about 10% CaO. The replacement level was between 40 to 60% by weight of

cement. The results indicated that with fly ash replacement level up to 50% by

27

weight of cement, concrete with 28 days strength ranging from 40 to 60 MPa

can be produced. Swamy and Mahmud (1986) also reported that concrete

containing 50% low calcium bituminous fly ash as cement replacement and

using a superplastisizer is capable of developing 60 MPa compressive

strength at 28 days and strength of 20 to 30 MPa at 3 days.

Langley et al (1989) have reported the results of the investigations

carried out to determine the effects of incorporating high volumes of ASTM

class F fly ash on concrete with 56% replacement level of cementitious

materials by fly ash. Tests results indicated that fly ash concretes show

substantial increase in compressive strength, split tensile strength and flexural

strength from the ages of 28 to 365 days. Haque et al (1988) reported that for

concrete mixes with 40 to 75% bituminous fly ash replacing cement, the

increase in flexural strength was slightly less than the increase in compressive

strength between 28 days and 91 days of curing.

2.2.6 Effect of fly ash on elastic properties

Lane and Best (1982) and Ghosh and Timusk (1981) have reported

that the effect of fly ash addition on the elastic properties is almost the same

as on compressive strength. The modulus of elasticity like compressive

strength is lower at early strength and higher at ultimate strength when

compared with concrete without fly ash.

Ghosh and Timusk (1981) also reported that for all strength levels

the modulus of elasticity of fly ash concrete was generally equivalent to that

of the corresponding reference concrete. They also found by ACI formula

Ec = 0.43 W (fc)3/4

MPa, where W is the unit weight of concrete in kg/m3 and

fc is compressive strength in MPa.

28

Langley et al (1989) found that at 28 days the modulus of elasticity

of concretes made with 50% fly ash constituting the cementitious material

varied between 27.9 and 36.1 GPa compared to 31.5 – 36.8 GPa for control

concrete mixes. However at 365 days, fly ash concrete mixes exhibited

significant increase in modulus of elasticity as compared to control concrete

mixes.

2.2.7 Effect of fly ash on permeability

Naik et al (1994) have evaluated the influence of the addition of a

class C fly ash on concrete permeability by replacing cement with fly ash in

the range of 0-70% by weight in concrete mixtures. On the test results for air

permeability, they have concluded that at lower ages upto 28 days, the high

volume fly ash concrete showed higher levels of ingress of air relative to the

plain Portland cement concrete. When curing was extended upto 91 days, the

50% fly ash concrete showed maximum permeability and out-performed the

reference concrete without fly ash. This is due to the pozzolanic contribution

of fly ash in concrete. This may be primarily due to the presence of Ca(OH)2

hydrated lime in concrete.

On water permeability test results, they have reported that concrete

water permeability decreased with age. All the three concrete mixtures

showed fair resistance to water permeability upto the ages of 14-40 days.

At 91-day age, the high volume (50%) fly ash concrete exhibited lower water

permeability to that of plain Portland cement concrete. This is probably due to

increased pore grain refinements of fly ash concrete system (upto 50% cement

replacement) that occurred due to pozzolanic reaction of fly ash. They

observed that chloride permeability decreased with age. The 50% fly ash

concrete showed the lowest permeability to chloride ions amongst all the

mixture tested. The concrete mixtures with 50 and 70% cement replacements,

29

with fly ash were superior to the no-fly ash concrete at 91 days with respect to

choride-ion permeability.

Malhotra and Berry (1986) reported the work of Kanitakis on

permeability of concreted with and without a low-calcium fly ash

(CaO, 2.0%). Absorption measurements are made at 7, 17, 28 and 56 days of

curing. It was found that at early ages, fly ash concrete behaves as a lean-mix

concrete and is thus permeable. At later ages, permeability is reduced as the

pozzolanic action proceeds.

Kasai et al (1983) studied the air/gas permeability of mortar made

with blended cements containing fly ash and blast furnace slag and concluded

that at early ages upto 7 days, blended cement mortars exhibited more

permeability than plain cement mortars. However, with increased curing age,

the permeability of blended cement mortars decreased.

2.2.8 Effect of fly ash on resistance to corrosion of reinforcing steel in

concrete

Mohammed et al (1988) reported corrosion resisting characteristics

of the concrete mixture, in which fly ash was used as an admixture (an equal

quantity of sand replaced). The corrosion rates of the reinforcing bars in plain

and fly ash concrete specimens after about 4 years of immersion in the salt

solution were noted. They have concluded that the corrosion rates of

reinforcing bars in plain concrete specimens were higher than that of fly ash

concrete. The corrosion rates of reinforcing bars in plain concrete specimens

were about 13 to 19 times the corrosion rates of those in fly ash (30% sand

replaced) concrete. Further, the data indicated that the corrosion rate increased

with the increasing water-cement ratio in both plain and sand replaced fly ash

concrete.

30

Saadoun et al (1993) reported the corrosion resisting characteristics

of reinforcement of four plain and 36 fly ash blended cement concretes. Three

fly ash of bituminous, sub-bituminous and lignite origin have been used in

conjunction with four plain cements having C3A contents of 2%, 9%, 11%

and 14%. The 36 blended cements were formulated such that each of the four

blended cements had 10% 20% and 30% cement replacements by each of

three fly ashes. They have concluded that for the type V 2% blended cement,

with the fly ash 3 and 30% replacement level, the corrosion protection

performance of fly ash blended cement concrete in terms of corrosion-

initiation time is over three fold better compared to that of plain cement

concrete. Also for the three type 1 cements, for the same fly ash 3 and 30%

replacement level, the average performance of fly ash bended cement concrete

is 2 times superior to that of the corresponding plain cement concretes.

Andrade (1986) tested concrete mixes with and without fly ash for

corrosion using polarization resistance technique. The addition of fly ash

promoted the corrosion of steel in mortars but had no effect on concrete

specimens. The decrease in the alkalinity due to introduction off fly ash was

reported to have a major effect in promoting corrosion in the fly ash mortar mixes.

Civjan et al (2005) have conducted a long-term corrosion study to

determine the effectiveness of inhibiting admixtures like calcium nitrite (CN),

silica fume (SF), fly ash (FA), ground granulated blast furnace slag (BFS) and

disodium tetrapropenyl succinate (DSS) in reducing corrosion of reinforcing

steel in concrete. Fourteen concrete mixtures were tested. Mixtures included a

control, single admixtures, double combinations and triple combinations.

Specimens were cast in replicates of three with two non-cracked and one pre-

cracked for each mixture. Cracks were formed using stainless steel metal shims of

0.3mm thick. The top surface of each specimen was exposed to chlorides, while

the sides were sealed and the bottom surface was open to the air.

31

Four evaluation methods were used to record the amount of

corrosion activity in the specimens; visual inspections, half-cell potential

readings, macrocell corrosion readings and destructive evaluations

(“autopsies”). The first of these simply involved periodically examining the

specimens for any changes in appearance, including rust or precipitate on the

surface and measuring the width of any cracks that developed.

Copper-copper sulphate half-cell potential readings were used to

evaluate corrosion activity. The magnitude of the electrical half-cell potential

was considered to be an indicator of whether or not there is active reinforcing

steel corrosion.

Macrocell corrosion current readings obtained by measuring the

voltage across a resistor between each set of top and bottom reinforcing bars

were converted into iron loss data by dividing the voltage from each reading

by the value of the resistor (10 ) and then multiplying this result by the

average number of hours at that reading.

The specimens were destructively evaluated for visual assessment

of rusting on the surface of the reinforcing bars at the conclusion of testing.

It was reported that a moderately lower compressive strengths were

observed in mixtures with CN/DSS and only DSS. But increase in

compressive strength by about 15% in comparison with control was noted for

mixtures that included BFS as well as in mixtures with multiple mineral

admixtures. Mixtures with only SF and SF in combination with CN showed a

greater tendency for micro-cracking than other mixtures. The potential for

early micro-cracking in SF appeared to be minimized when FA or BFS was

also included in the mixture proportions.

32

For optimal protection against corrosion, in structural concrete, a

triple combination of CN, SF and FA or a double combination of CN and BFS

all at moderate dosages was recommended. The above two recommended

mixtures also resulted in higher compressive strengths than in the control

concrete indicating an overall improvement in material performance and

quality.

Kayali et al (2005) have investigated on high strength reinforced

silica fume-cement concrete slabs with a compressive strength of 70 MPa for

chloride diffusion and corrosion activity after partial immersion in a 2%

chloride solution. They have also investigated similar slabs with 32 MPa

conventional concrete in the same environment. The medium-strength

concrete (MS) and high strength concrete (HS) consisted of crushed granite as

coarse aggregates and river sand as fines. HS concrete contained silica fume

as 10% of the cement mass. A sodium polynaphthalene sulphonate super

plasticizer was also added to produce HS concrete. The dimensions of the

slabs cast were 470mm x 470mm x 150mm. Profiles of chloride ion

penetration for the concrete slabs have been determined over a period of 390

days. The following conclusions were drawn.

Reinforced concrete slabs, where the concrete was of conventional

ingredients and medium strength, displayed a low to moderate tendency to

reinforcement corrosion when partially submerged in chloride solution. This

tendency was associated with a high chloride ion concentration occurring at

the steel level.

The chloride concentration that has been recognized previously as

the threshold value for initiating reinforcement corrosion appeared to be

conservative. A concentration value of about 1% by mass of cement seemed

to be more consistent with other corrosion measurements.

33

Reinforced concrete slabs whose concrete included silica fume as

10% by mass of cement and whose strength was around 70 MPa showed

extremely low values of corrosion current density and half-cell potentials.

These values remained very low even after long exposure to chloride ion

solution.

The performance of high-strength concrete of the type investigated

was believed to be excellent as far as resisting chloride-initiated corrosion is

concerned.

A good correlation was noted between corrosion potential and

corrosion current density values for medium-strength concrete.

Hansson et al (2006) have undertaken an experimental study to

determine the influence of concrete type and properties on the relative

microcell and macrocell corrosion rates. They have defined microcell

corrosion as the situation where active corrosion and the corresponding half-

cell reaction take place at adjacent parts of the same metal and macrocell

corrosion as the situation where an actively corroding bar is coupled to

another bar which is passive, either because of its different composition or

because of different environment.

The specimens cast were prisms of size 279mm x 152mm x 114mm

with three reinforcing bars (rebars) embedded: one at the top and two at the

bottom, all with a cover depth of 25mm. Three different concrete mixtures

were adopted. One concrete mixture consisted of ordinary Portland cement

concrete and the other two were high performance concrete mixtures in which

one of them consisted of silica fume and 25% blast furnace slag and the other

consisted of silica fume and 25% fly ash of class C. The specimens were

34

cured initially and then stored outdoors for 5 months prior to preparation for

exposure to chlorides.

The prism specimens were prepared for corrosion measurements as

follows:

The vertical surfaces were coated with epoxy resin to prevent

access of oxygen from those surfaces; A ponding well was mounted on the

top surface; and the two bottom bars were connected together and then

connected to the top bar through a 100 resistor.

The ponding well was filled with a 3% NaCl solution and the

specimens were alternately exposed to 2-week periods with solution then 2-

weeks without solution. The voltage drop across the resistor was monitored

daily allowing the macrocell corrosion current between the top (anode) bar

and the bottom (cathode) bar to be determined using Ohm’s law.

After 180 weeks of macrocell measurements, the microcell

corrosion rate of the top bar was determined by the linear polarization

resistance (LPR) technique using saturated calomel reference electrode and a

stainless steel counter electrode immersed in the ponding solution. Thereafter,

the top bar was disconnected from the bottom bars and after being allowed to

stabilize for a week, the microcell corrosion rate was measured again.

For the OPCC prisms, the microcell corrosion rates were observed

to be approximately 2 times greater than that of the macrocell corrosion rates

and on the other hand, the macrocell corrosion rates of steel in HPC are three

to four times lower than that in OPCC and their microcell current densities are

only about one order of magnitude lower than that in OPCC. This was

attributed due to the fact that the chloride level at the top reinforcing bar is

35

lower in HPCs than in OPCC because of the difference in chloride diffusion

rates.

The behaviour of HPC containing 25% fly ash and HPC containing

25% slag were reported to be very similar and so there is no apparent

advantage to the use of one or the other.

2.3 SIGNIFICANCE OF CORROSION INHIBITORS

Ormellese et al (2006) have investigated the effectiveness of three

organic commercial inhibitors in preventing carbon steel chlorides induced

corrosion in concrete. Their report illustrated the results of three years

research on the inhibitive effectiveness of organic commercial corrosion

inhibitors. The effectiveness of three organic compound inhibitor admixtures

considered for study were amine-esters, amino-alcohols and alkanol-amines

based. One nitrite based inhibitor was also considered for comparison

purposes. Potentiostatic tests were carried out with a potentiostat. Ten

identical carbon steel specimens were polarized at 0 mV SCE in the same cell,

connecting them to the potentiostat. Specimens were faced to a saturated

calomel reference electrode (SCE) placed in the centre of the cell. An

activated titanium wire-net on the bottom of the cell acted as a counter-

electrode. After 48 hours of passivation in free chlorides conditions, 0.2%

chlorides were added every 48 hours upto 3%. The current flowing in each

specimen was measured and corrosion has been considered initiated when the

anodic current density flowing in the specimen rose above 5 A/m2.

Cathodic potentiodynamic tests were performed in alkaline solution

at potential scan rate of 20mV/min in the cathodic direction, starting from the

free corrosion potential until -1.2 V SCE. At the end of the test, rebars were

36

extracted from concrete specimens for visual inspection and weight loss

measurements.

Based on the above investigations, the following conclusions were

drawn.

All the tested inhibitors seem to be able to increase corrosion

initiation time in concrete subjected to accelerated chlorides penetration.

There was reduction in corrosion rate due to lower penetration rate and lower

corroded area. The comparison between the specific weight loss calculated by

integration in time of measured corrosion rate and the specific weight loss

measured at the end of the tests have shown good correlation between

experimental and calculated data.

Fedrizzi et al (2005) have studied the effectiveness of migrating

corrosion inhibitors and repair mortars against rebar corrosion. The studies

were conducted in concrete specimens made by ordinary Portland cement

with water-cement ratio equal to 0.6 and containing 1wt% of chlorides. Rust

free 10mm diameter bars were embedded into the concrete parallelepiped

blocks. The choice of the high w/c ratio was made to obtain a high-porosity

concrete that would promote an accelerated life test and simulate the worse

condition of a real structure. The addition of sodium chloride in the mix water

was made to simulate a real rehabilitation condition, where the front of

chlorides has achieved the rebars in sufficient quantities to break down the

passive layer.

Corrosion potentials of the rebars of each specimen have been

measured during the exposure time to study the effect of migrating inhibitor

on the potential of the reinforcing bars of the specimen which were subjected

to ponding by a sodium chloride solution besides the already present amount

of chlorides added in the concrete mix.

37

On the basis of the performance recorded from the investigations,

they have concluded that the alkanolamine inhibitor based treatment for

corroding reinforced concrete proved to possess low porosity, low

conductivity and low permeability to aggressive substances. The use of

alkanolamine-based migrating inhibitor as supplementary anticorrosion

system brought back the passivity for the rebars which were initially corroded

by the chlorides present in the concrete mix.

2.4 OTHER CORROSION STUDIES

Ozturan et al (2005) have experimentally investigated on steel

reinforcement corrosion, electrical resistivity and compressive strength of

concrete. The objective of their study was to investigate the relative

performance of a range of Portland and blended cement concretes exposed to

high chloride concentrations. The performance evaluation has been carried

out in terms of corrosion of embedded reinforcement and related properties

mentioned above under three different curing conditions namely,

uncontrolled, controlled and wet curing. Based on the test results, the effects

of the type of cement, water-cement ratio (w/c), age and curing conditions

have been discussed.

Five different cements from various sources, namely Portland

cement, 2 different Portland composite cements, composite cement and blast

furnace slag cement were used for their investigation. The coarse aggregate

was a crushed limestone with a maximum particle size of 20mm whereas the

fine aggregate was a mix of natural sand and crushed limestone sand. A

sulphonated naphthalene formaldehyde-based superplasticizer was used to get

a workable fresh concrete. The 28, 90 and 180 day compressive strengths for

the plain and blended cement concretes subjected to different curing

procedures were observed in the range of 32.5 MPa to 67.1 MPa and from

23.3 MPa to 69.7 MPa respectively depending upon w/c ratio, curing

38

condition and age of testing. It was observed that improving the quality of

curing was more effective on the later age compressive strength development

of the plain cement concretes with larger w/c ratio than the concretes with

lower w/c ratio. Also the observed results indicated that the increase in

compressive strength of concretes at later ages made with blended cements

was higher than that of concretes with Portland cement especially under

controlled and wet curing procedures. Both uncontrolled and controlled

curing procedures resulted in great differences with respect to wet curing in

terms of electrical resistivity and corrosion time of the concrete made with

plain and blended cements.

For a given curing condition, lowering w/c ratio of the mixes

increased the concrete resistivity and for a given w/c ratio, better curing

procedure yielded higher electrical resistivity for all concretes. The blended

cement concretes had greater electrical resistivity than the plain Portland

cement concretes for all w/c ratios and ages.

The accelerated corrosion test results indicated that the specimens

with blended cements had superior performance and mostly yielded longer

time to corrosion cracking at similar curing condition and testing age

compared to those with plain Portland cements. The corrosion resistance of

the blended cement concretes increased significantly with age while that of

the conventional concrete had a marginal increase. They also concluded that

wet curing was essential to achieve higher strength and durability

characteristics for both plain and especially blended cement concretes.

Vidal et al (2007) have studied the long-term corrosion process of

reinforced concrete beams. The reinforced concrete beams were stored in a

chloride environment for 17 years under service loading in order to be

representative of real structural conditions. At different stages they have

39

drawn cracking maps, measured total chloride contents and performed

mechanical tests.

Based on their test results they have arrived at certain major

conclusions. They have stated that the transversal cracks that appeared do not

influence significantly the corrosion process of tension reinforcing bars and

the service life of the structure. The chloride threshold at the reinforcement

depth, used as a single parameter to predict the end of the initiation period, is

a necessary but not a sufficient parameter to define service life. The bleeding

of concrete is an important cause of interface de-bonding which could lead to

an early corrosion propagation of the reinforcements located in the

compressive zone compared to tensile bars in the case of simply supported

beams.

During the propagation period, in spite of very controlled

environmental conditions, the corrosion distribution and evolution along all

the reinforcing bars is quite different and heterogeneous. This observation

concerns the location, the intensity and the corrosion rate of corroded areas on

a same re-bar and also between the two different reinforcement bars.

The structural performance under service load (i.e. stiffness in

flexure) is affected by the corrosion of the tension reinforcement and not

significantly by the concrete cracking due to the corrosion of the steel bars

located in the compressive zone. The reduction of stiffness resulted from two

coupled effects: the steel cross-section reduction and the steel-concrete bond

loss due to steel corrosion between the bending cracks.

Also they have concluded that the limit state service life design

based on structural performance reduction in terms of serviceability showed

the propagation period of the corrosion process to be a consequent part of the

reinforced concrete service life.

40

Lachemi et al (2009) have investigated the corrosion of steel

reinforcement embedded in full-scale self-consolidating concrete (SCC)

beams when compared with normal concrete (NC). Beams of length 2340mm,

width 400mm and 363mm depth were cast with reinforcements and subjected

to accelerated corrosion test. The corrosion performance of normal concrete

and self-consolidating concrete beams were evaluated based on the results of

current measurement, half-cell potential tests, chloride ion content, mass loss

and bar diameter degradation. The investigation also included the effect of

admixture type and the size of specimen on corrosion performance.

SCC generally has a dense and less permeable microstructure

because of its superior resistance to bleeding and segregation. The production

of SCC usually involves the use of high range water reducer (HRWR),

superplasticizer and or supplementary cementing materials (SCM). SCM have

proven to increase the concrete corrosion resistance while HRWR help to

disperse cement particles in the mix and reduce overall concrete permeability.

Based on overall performance of the full-scale tested beams, SCC

mixture exhibited superior rebar corrosion protection compared to its NC

counterpart. The main disadvantage observed in the SCC beams was that the

SCC mixture showed non-uniform concrete properties along the length of the

full-scale concrete beams when casting occurred from one end, causing lesser

quality concrete at the far end due to improper compaction and distribution.

As a result, severe corrosion and spalling of concrete cover were observed at

corners located far away from the casting point. The results of half-cell

measurements, crack widths, chloride ion contents, rebar mass loss and rebar

diameter reduction confirmed these findings. Therefore, they have

recommended that while casting SCC beams, the casting point should be

moved along the beam length (particularly if the beam is long, shallow and

narrow) to ensure uniform compaction, especially at corners. Also they have

41

concluded that the type of admixture used in SCC mixes had no effect on

corrosion performance in terms of corrosion initiation, corrosion rate and

crack pattern widths. This is due to weaker, more porous layer of concrete in

SCC below the longitudinal bars in the large-scale beams.

Oh et al (2010) have explored the effects of non-uniform corrosion

on cracking behaviour of concrete cover in their investigation. They have

studied the effects of non-uniform corrosion distribution, cover-to-rebar,

diameter ratio and concrete compressive strength on the cracking pressure of

concrete cover.

Corrosion of steel bar in concrete cause expansion pressure and this

expansion pressure induce tensile cracking around the reinforcing bar. Since

chlorides are generally penetrated into concrete in one direction under actual

sea environments, the corrosion also start from the outermost part of the rebar

and thus the steel bar may not corrode uniformly in a cross section. Non-

uniform distribution of expansion pressure cause adverse effects on cover

cracking because higher pressure is concentrated at the outer region of rebar

toward concrete cover. The study also indicated that the pressure to cause

cracking of concrete cover due to corrosion expansion increases with an

increase of cover depth and is almost linearly proportional to the cover-to-

rebar diameter ratio.

Balouch et al (2010) have investigated on the surface corrosion of

steel fibre reinforced concrete. The corrosion of steel fibres affects its ability

to bridge the cracks thereby decreasing the strength of the concrete structures.

In this investigation, fibre reinforced concrete prisms were

subjected to cycles of salt fog for a week and drying for another week. All

fibres less than 1 mm embedded in concrete with high w/c ratio showed

corrosion spots at the surface. With decrease in w/c ratio, the surface

42

corrosion of the steel fibres also dropped. But decrease in w/c ratio beyond

0.5 did not produce any significant effect.

They had suggested that an enhanced mobility of the fibres in the

concrete matrix (higher workability and higher sand/gravel ratio) completed

by an adequate vibration process, especially by formwork vibration, help to

push the fibres away from the cast surfaces. Also the w/c ratio should be less

than or at the maximum equal to 0.5. If these two conditions could not be

fulfilled, rust resistant fibres like galvanized fibres are to be adopted.

Huang et al (2005) investigated the corrosion damage in three types

of concrete (C 25, C 45 and C 55) resulting from HCl with various contents.

The test samples that were cured for 360 days were exposed in an aggressive

environment with 5%, 10%, 15% and 20% HCl content respectively for 24

hours. The mass loss, the dynamic modulus loss, the flexural strength and the

compressive strength were measured using a series of the etched samples.

The measured compressive strengths of the three types of concrete

exhibited similar degradation trend with growing HCl content. The strength

degradation was approximately described as an exponential function of HCl

content.

The investigators also concluded that their experiments

demonstrated that surface corrosion caused by HCl solution strongly affects

the flexural and compressive strengths and the elastic modulus of concrete

and that the effect degree is an increasing function of HCl content. The study

also revealed that the degradation of the flexural strength was more

remarkable for the high-strength concrete than for the normal-strength

concrete due to higher defect sensitivity in the high strength concrete than in

the normal strength concrete.

43

On the other hand, the loss of both the mass and the elastic modulus

caused by HCl corrosion was in reverse proportion to the strength grade of

concrete. Greater mass loss occurred in the normal strength concrete due to

higher chloride permeability.

Cabrera et al (1995) investigated the corrosion of embedded

reinforcement in two series of concrete samples made with and without

condensed silica fume (CSF) as partial replacement for the cement. Three

different curing regimes were used and samples were tested at three different

ages. Measurements of carbonation, electrical conductivity, strength, lime

content and chloride, oxygen and water vapour transport were carried out on

matching samples. The results have been analyzed using analysis of variance

and regression to show which aspects of the materials and methods used to

make the samples and which of the measured properties had significant

effects on the corrosion. The way in which these effects were modified by the

presence of the CSF was also analyzed.

Based on the test results, they have concluded that use of CSF in

concrete makes it significantly more sensitive to changes in curing. This

interaction was significant at the 0.1% level for all the corrosion

measurements. After exposure to chlorides all the samples made with CSF

concrete nevertheless had significantly lower corrosion than corresponding

samples without CSF.

The predictive models for corrosion in CSF concrete were very

different from those for OPC concrete. In particular, after exposure to

chlorides the very high significance of many transport properties and

compressive strength for OPC was not present for CSF. The electrical

conductivity and the chloride transport were the only two predictors which

were significant for all situations and the conductivity was more significant

than the chloride transport in three out of four.

44

Idriss et al (2001) made a comparative study on the effects of

corrosion resistance of six different cement mortar specimens under long-term

exposure to hydrogen sulphide. There are a number of apparent solutions such

as using sulphate resistant cement, silica fume cement, fibre mesh addition to

the cement and treatment of the concrete with linseed oil. They tested these

various treatments in the laboratory using impressed voltage tests and

electrochemical potential tests.

In impressed voltage tests, specimens made with 8% silica fume

cement replacements performed best and failed after 600 h of testing and

similar Portland cement mortar specimens with a water-cement ratio of 0.55

poorest failing after 165 minutes itself.. The other four treatments (Portland

cement, Portland cement with fibre mesh, Portland cement coated with

linseed oil and sulphate resisting cement) all with water-cement ratios of 0.45

were less effective in preventing corrosion than treatment SFC. The

electrochemical potential tests indicated that after 650 days of exposure to

hydrogen sulphide treatment SFC exhibited the best corrosion resistance.

2.5 CONCLUSION

The literature survey shows that many investigations were made

regarding the durability aspect of cement concrete by blending it with

different mineral admixtures and adding various dosages of inhibitors. Study

of literatures reveal that adding mineral admixtures to cement concrete by

partially replacing cement had reduced the porosity of the concrete and hence

reduction in its permeability and water absorption. This also had reduced the

ingress of harmful agents that penetrate through the concrete medium in

aggressive environments and corrode the steel bars. Though the rate of gain of

mechanical strengths were initially slow when cement is partially replaced

with mineral admixtures, around 90 to 180 days or so the blended concrete

exhibit greater strengths than normal concrete. Also the investigations reveal

45

that the durability of blended concrete is much better when compared to

normal concrete. This durability aspect was further enhanced by adding

nominal dosages of corrosion inhibitors.

Based on the above concluding facts arrived through the literatures

in enhancing the durability of concrete, the researcher proposed to utilize the

abundantly available fly ash from the nearby Mettur Thermal Power Plant and

blend it in various percentages in concrete by partially replacing cement to

arrive at an optimum content of replacement that would balance the

mechanical, micro structural and durability properties to their maximum

extent. Moreover literature survey revealed that organic inhibitors are

economical and at the same time effective in resisting corrosion of rebars in

concrete. Hence the researcher proposed to attempt four different organic

inhibitors that were commercially available to be added to fresh concrete and

to study the performances of the various dosages of organic inhibitors with

regard to fresh concrete properties, mechanical, micro structural and

durability properties.