Influence of Bleeding

Journal of Advanced Concrete Technology Vol. 2, No. 2, 187-199, June 2004 / Copyright 2004 Japan Concrete Institute 187

Influence of Bleeding on Minute Properties and Steel Corrosion in Concrete Melito A. Baccay1, Takahiro Nishida2, Nobuaki Otsuki3, Junpei Hamamoto4 and Kyoku Chin5

Received 19 November 2003, accepted 2 February 2004

Abstract This paper presents the results of laboratory and field investigations on the influence of bleeding on minute properties and steel corrosion in concrete. Test methods such as minute compressive strength test, minute tensile strength test and minute diffusion test were performed in the laboratory to assess the effect of bleeding on minute properties of concrete. In addition, electrochemical investigations were conducted both in the laboratory and in the field to determine the in-fluence of bleeding on the rate of steel corrosion in concrete. The various test results indicate a strong agreement be-tween the laboratory experiment and the field investigations. The upper layer of concrete affected by bleeding exhibited weaker strength, higher permeability, lower concrete resistance, and higher oxygen permeability. Consequently, a higher macrocell corrosion rate than the microcell corrosion rate prevailed in both the vertical and horizontal steel bar and the corrosion rate was enhanced at elevated temperatures (20-40oC).

1. Introduction

The premature deterioration of reinforced concrete member has become a major concern in many countries throughout the world. According to the technical report of the Concrete Society based in London, while con-crete has proved to be an essentially durable material it has not always been completely durable. Its perform-ance depends not only on the exposure conditions but also on the concrete quality, which can vary widely. One of the factors mentioned that affects the quality of concrete is bleeding. This phenomenon occurs as a re-sult of adding a large amount of water into the concrete mix to increase its workability. Bleeding of concrete is caused by the segregation of water from the cement paste. In the case of high segregation, suspended parti-cles precipitate depending on their fineness and specific gravity and the mix proportions of the concrete (Yo-nezawa 1988). As a consequence, concrete is weaker and less durable at or near its top layer compared to other parts. A study done by Wainwright and Ait-Aider

(1995 cited Kasai 1979; Giaccio et al. 1986) confirmed that bleeding results in variations in the effective water content throughout the depth of concrete, producing corresponding changes in concrete properties. Bleeding tends to create water pockets resulting in an area with high porosity and voids underneath the steel. A study done by Bentur et al. (1997) stated that the voids and porous area produced by bleeding presumably promote the corrosion of steel reinforcements causing changes in the chemical environment of the steel that result in a reduced bond between the steel and the concrete. Un-fortunately, scant data on the influence of bleeding on the properties of concrete (minute properties) and the corrosion of steel embedded in concrete is available. The term minute properties refers to the properties of concrete in the minute region at any concrete phase such as the bulk mortar matrix phase, coarse aggregate phase, and interfacial transition zone (ITZ) between the coarse aggregate and the bulk mortar matrix phase. The term ITZ refers to a region located at the vicinity of aggre-gate particles in concrete. This region is considered to be the weakest link in concrete (Mehta and Monteiro, 1993) and its properties relate to the properties of con-crete (strength, diffusion, modulus of elasticity etc.). Therefore, by conducting in-depth investigation on the influence of bleeding on the minute properties of con-crete we should be able to better understand the com-plex process and role of bleeding in the mechanism of steel corrosion in concrete. Given this background, the following are the objectives of this research: (1) To de-termine the influence of bleeding on the minute proper-ties of concrete; (2) To determine the rate of steel corro-sion in concrete affected by bleeding; and (3) To deter-mine the temperature dependency of steel corrosion in concrete affected by bleeding.

1Doctoral Student, Department of International De-velopment Engineering , Tokyo Institute of Technology, Japan E-mail: [email protected] 2Research Associate, Department of International De-velopment Engineering, Tokyo Institute of Technology, Japan 3Professor, Department of International Development Engineering, Tokyo Institute of Technology, Japan 4Civil Engineer, Toa Corporation, Japan 5Masters Student, Department of International De-velopment Engineering, Tokyo Institute of Technology, Japan

188 M. A. Baccay, T. Nishida, N. Otsuki, J. Hamamoto and K. Chin / Journal of Advanced Concrete Technology Vol. 2, No. 2, 187-199, 2004

2. Experimental procedure

2.1 Materials and mix proportions Ordinary Portland Cement (OPC) was used in the con-crete mix. The physical and chemical compositions of the OPC are shown in Table 1. The aggregates used were natural river sand and crushed coarse aggregates. Specific gravity, water absorption (%) and fineness modulus of the sand were 2.59, 2.08 and 2.51, respec-tively. On the other hand, the specific gravity, water absorption (%) and fineness modulus of the crushed coarse aggregates were 2.61, 0.88, and 6.85, respec-tively. Deformed 16-mm diameter steel bars (SD 345) conforming to the Japanese Industrial Standard (JIS) were used. The iron, carbon, silicon, manganese, phos-phorus and sulfur contents were 98.25%, 0.22%, 0.3%, 1.21%, 0.012%, and 0.009%, respectively. Air-entraining agent and air-entraining water reducing agent (JIS 6204) were used as admixtures. Except for the control specimen, each of the concrete specimens was admixed with a total amount of 10 kg/m3 of sodium chloride. The mixture proportions of the concrete specimens are summarized in Table 2.

2.2 Specimen layout A total of seven (7) 30 cm x 15 cm x 150 cm reinforced concrete specimens were prepared for the laboratory investigation. Figure 1(a) shows the detail layout of the specimen and the steel bars embedded in it. The steel bars used in the said specimen are composed of vertical and horizontal steel bars (Fig. 1(b)) formed by attaching divided steel bars together using epoxy resin as the ad-hesive material. In order to measure in detail the mac-

rocell and microcell corrosion current density in the steel, each segment of the steel bars was soldered with lead wires serving as contact points for the electrical connection Miyazato et al. (2001). In this study, the concrete specimens were cured for 56 days at 20oC temperature and 60 % relative humidity. After the cur-ing period, the specimens were transferred in a con-trolled environmental chamber where they were ex-posed to different temperature conditions (20oC, 30oC, and 40oC) at 55 % constant relative humidity.

For the field investigation, a test was performed on the concrete wall (cast-in-place) of a 37-year old 5-storey building. The building is located in the Mi-dorigaoka area of Meguro-ku, Tokyo. Details of the measuring points are shown in Fig. 2. The total area covered was approximately 0.24 m2. Cold-joints were found on the surface of the wall by visual inspection (Fig. 2). As shown in the picture the quality of the con-crete in the upper layer is porous compared to the con-crete found at the lower layer. In terms of compressive strength, the concrete in the upper layer is 15% weaker compared to the strength of the concrete in the lower layer. On the other hand, the results of the carbonation test reveal that the concrete in the upper layer and the lower layer has carbonated to a depth of 43 mm and 35 mm, respectively. Judging from the above-results, we can conclude that the said member has suffered from segregation due to bleeding. To further investigate the effect of bleeding, cored samples were taken and studied in the laboratory to determine their minute properties. In addition, electrochemical investigations to determine the rate of steel corrosion in that part of the building were conducted in February, May, and August of 2002, with average temperatures of 10.2oC, 22.2oC, and 31.4oC, respectively. The results are presented in the following sections.

3. Measurement methods

The measurement items considered in the investigation to determine the influence of bleeding on minute prop-erties and steel corrosion in concrete are briefly dis-cussed below. In this study, the test methods (minute compressive strength test, minute tensile strength test and minute diffusion test) used to assess the minute properties of concrete were based from the measure-

Table 1 Physical and chemical compositions of cement.

Item OPC Specific Gravity 3.16 Blaine Fineness, cm2/g 3270.00 Loss on Ignition 0.90 SiO2 % 21.80 Al2O3 % 5.10 CaO % 63.80 MgO % 1.70 SO3 % 2.00 Fe2O3 % 3.00

Table 2 Mixture proportions of concrete.

Aggregates (kg/m3)

W/C

Water (kg/m3)

Cement(kg/m3)

Sand

(kg/m3) small large

AE*1

(g/m3)

AEWRA*2

(g/m3)

Slump Flow*3

cm

Air Con-tent (%)

Bleed-

ing (%)*4

175 318 854 518 518 2226 3180 16.90 3.70 0.62 225 409 760 462 462 2863 4090 18.50 8.00 1.08

0.55

275 500 668 405 405 3500 5000 69.0 0.80 6.67 W, C, G, S refers to water, cement, gravel and sand, respectively. *1 AE- Air-entraining Admixture *2AEWRA- Air-entraining Water-Reducing Admixture *3 Slump Flow *4JIS A1123

M. A. Baccay, T. Nishida, N. Otsuki, J. Hamamoto and K. Chin / Journal of Advanced Concrete Technology Vol. 2, No. 2, 187-199, 2004 189

ment methods developed by Yodsudjai (2003). On the other hand, concrete resistance test, oxygen permeabil-ity test, gap measurements, and macrocell/microcell corrosion measurements were conducted to determine the influence of bleeding on the corrosion of steel. (1) Minute compressive strength test The minute compressive strength of the concrete speci-men was determined using a Universal Testing Machine (UTM). The procedure was done by placing the test piece (Fig. 3(a)) at the center of the loading board as shown in Fig. 3(b), followed by application of a uniax-ial compression load at a rate of 1.0 mm/min. The min-ute compressive strength was calculated using Eq. 1

AP

c = (1)

where c = compressive strength (MPa), P = load (N), A = cross-sectional area of the test-piece (mm2).

The preparation of the test-piece for the minute com-pressive strength test is briefly described below. Using a concrete saw, representative samples containing mortar

aggregate matrix were taken from each layer (upper, middle and bottom) of the 30 cm x 15 cm x 150 cm concrete specimen. Concrete bars measuring 15 mm x 15 mm x 50 mm were cut from each of the sample specimens using a diamond cutter. To reduce their sizes, the bars were cut crosswise into 15 mm x 15 mm x 3 mm specimens using a Low Speed Saw (ISOMETTM). The 15 mm x 15 mm x 3 mm cut specimens were set on a glass plate using an electron wax and then cut into cubes (3mm x 3mm x 3 mm). To detach and get the fi-nal test-piece (3 mm x 3mm x 3mm), the glass plate was subjected to gentle heating using a lighted candle. Fi-nally, the surfaces of the cube specimens were cleaned with acetone to ensure the complete removal of any adhering electron wax.

(2) Minute tensile strength test The tensile strength of the concrete specimen (Fig. 4(a)) was measured using a minute tensile device originally developed at Otsukis laboratory. Testing was performed by setting the specimens on the test-piece holders, and

150

cm30 cm 15 cm

Ver

tical

ste

el b

ar

Hor

izon

tal s

teel

bar

Epoxy resin

Concrete

Upper

Middle

Lower

Epoxy

Concrete

Upper

Middle

Lower

150

cm30 cm 15 cm

Ver

tical

ste

el b

ar

Hor

izon

tal s

teel

bar

Epoxy resin

Concrete

Upper

Middle

Lower

Epoxy

Concrete

Upper

Middle

Lower

Fig. 1 (a) Detail layout of concrete specimen.

5 cm 10 cm10 cm8 cm

60 cm

60 cm

8 cm

8 cm

Lead

wire

5 cm

Lead wire

Each divided steel bar was connected using an epoxy resin

Concrete cover: 5 cmSteel bars: Divided D16, SD345Upper and bottom surfaces of con-cretewere coated with epoxy resin

5 1010 cm8

60

8

5 cm5 cm5 cm

Lead


Horizontal steel barVertical steel bar

Upper and bottom surfaces of con--

5 cm 10 cm10 cm8 cm

60 cm

60 cm

8 cm

8 cm

Lead

wire

5 cm

Lead wire


Concrete cover: 5 cmSteel bars: Divided D16, SD345Upper and bottom surfaces of con-cretewere coated with epoxy resin

5 1010 cm8

60

8

5 cm5 cm5 cm

Lead


Horizontal steel barVertical steel bar

Upper and bottom surfaces of con--

Fig. 1 (b) Detail of connection of divided steel bars (ver-tical and horizontal steel bar).

Fig. 2 Field-testing on existing concrete structure.

10 cm 10 cm 10 cm

10 cm

10 cm

10 cm

30 cm

80 cm


loads were then progressively added to the loading tray until failure occurred (Fig. 4(b)). Since each loading tray was connected to a rope frictional resistance be-tween the rope and pulley occurred as measurement was performed. Therefore, to factor in the effect of frictional resistance, the product of a constant of 0.77 and the ap-plied load had to be calculated to obtain the actual load. The 0.77 constant was derived based on a series of ex-periments done to calibrate the device. The minute ten-sile strength was calculated using Eq. 2

=AP

t (2)

where t = tensile strength (Pa) , P = load (N), = con-stant coefficient (0.77).

The procedure for preparing the test-specimens for the minute tensile strength test was almost the same as that for the minute compressive strength test, except for the different size of the final test-pieces. The size of the final test-pieces was 15 mm x 4 mm x 1 mm. These test pieces were obtained from the 15 mm x 15 mm x 1 mm cut specimens after selecting the best position of the interfacial transition zone between the coarse aggregate and the mortar matrix.

(3) Minute diffusion test To perform the minute diffusion test, the test specimen (5mm x 5mm x 0.5 mm) as shown in Fig. 5(a) was en-cased with rubber and fixed at the center of the acrylic cylinder cell. A thin rubber sheet was used to seal the cylinder. As shown in Fig. 5(b), one side of the acrylic cylinder cell was filled with 3 % NaCl solution and on

the other side with Ca(OH)2. The chloride ion concen-tration at the saturated Ca(OH)2 side was monitored everyday using an ion chromatography device. Since the gradient of the chloride ion concentration is known, the flux of chloride ion can be calculated using Eq. 3

=A

VQJ cell (3)

where J = ion flux through a unit area per unit of time,

Fig. 3 (a) Test-piece for minute compressive strength test (3 mm x 3 mm x 3 mm).

Fig. 3 (b) Minute compressive strength test.

Fig. 4 (a) Test-piece for minute tensile strength test (15 mm x 4 mm x 1 mm).

Fig. 4 (b) Minute tensile strength test.

Fig. 5 (a) Testpiece for minute diffusion test.

3% NaCl solution

Saturated Ca(OH)2 solution

Specimen3% NaCl solution

Saturated Ca(OH)2 solution

Specimen

Fig. 5 (b) Minute diffusion test.


Q = penetration speed, Vcell = solution volume in the saturated Ca(OH)2 solution, A = cross-sectional area of the test piece.

The diffusion coefficient is calculated using Ficks first law of diffusion as shown in Eq. 4

( )

= xC

JDcl

clcl

(4)

where Dcl = chloride ion diffusivity ( cm2/s), Jcl = ion flux of the chloride ion (mol-cm/L/s), Ccl = concentra-tion of the chloride ion (mol/L), x = thickness of the test piece (cm). Except for the size of the final test-piece, the method of preparation of the test specimen for the minute diffusion test is almost the same as for the minute compressive strength test and the minute tensile strength test. This is done by cutting a 5 mm x 5mm x 4 mm specimen from the 15 mm x 15 mm x 4 mm cut specimens after care-fully selecting the best position of the ITZ containing only the mortar matrix phase. Subsequently, the 5 mm x 5 mm x 4 mm test-piece was set in a plastic mould con-taining epoxy resin. After the epoxy resin hardened, both sides of the test-piece were polished to a thickness of 0.5 mm using a grinder and a polisher (META-SERV2000). Thus, the size of the final test-piece was 5 mm x 5 mm x 0.5 mm.

(4) Concrete resistance and oxygen permeability The concrete resistance in the reinforced concrete specimen was measured through the used of a portable corrosion monitor. Concrete resistance was simultane-ously obtained as output data when the test measure-ment for the microcell corrosion current density was performed. Details of the microcell corrosion current density test are discussed in the following section. The measurement layout is shown in Fig. 6. where CE = Counter Electrode; WE = Working Elec-trode; RE = Reference Electrode

On the other hand, the measurement set-up for the oxygen permeability test was the same as in the micro-

cell corrosion current density test, except that no fre-quency response analyzer (FRA) was used. As shown in Fig. 6, the electrochemical measurement system con-sisted of a working electrode (reinforcing steel bar with lead wire attached to it), counter electrode (steel plate) and a reference electrode (Ag/AgCl). The limiting cur-rent density was measured by a potentiostat. Oxygen permeability in the concrete specimens was calculated using Eq. 5 (Nagataki et al. 1996):

=nFi

dtdQ lim (5)

where dQ/dt = oxygen permeability (mol/cm2/sec), ilim = limiting cathodic current density (A/cm2); F = Faradays constant (96,500 coulombs/mole); and n = 4.

(5) Gap measurements In order to determine the relationship between the total amount of corrosion and the gap formed at the steel concrete interface, at the end of the electrochemical investigation, the 30 cm x 15 cm x 150 cm specimens were cut by layer (upper, middle and lower) along the longitudinal axis using a concrete saw. Cut sections of the reinforced concrete specimens (obtained from the upper, middle and lower layer of concrete) were taken pictures using a digital microscope. The pictures were then transferred and stored in a computer. Through the use of a computer software, the area of gaps formed between the steel and the concrete interface were ob-tained. The process of determining the area of gap is very convenient because the computer software does the analysis based on the color distribution in the picture and then plots the corresponding area, for instance, the area of the color formed at the gap between the steel and the concrete.

(6) Macrocell and microcell corrosion The macrocell and microcell corrosion current density flowing in the steel were measured using a corrosion monitor and an ammeter, respectively. Macrocell corro-sion refers to a corrosion cell in the reinforcing steel in which the anode and the cathode are physically sepa-rated along the length of the reinforcement. Microcell corrosion refers to a corrosion cell in the reinforcing steel in which the anode and the cathode are in the same physical location within the reinforcement.

In order to obtain the microcell corrosion current density, the polarization resistance in the steel element was measured using AC impedance with a frequency response analyzer (FRA) (Fig. 6). The supplied voltage was 50 mV with an amplitude ranging from 0.05 Hz to 5000 Hz. The polarization resistance reading obtained from the device was substituted in Eq. 6

ipimicro SR

KI = (6)

where: Imicro = microcell corrosion current density (A/cm2), K = constant (= 0.0209V) (Tsuru et al. 1979),

i

Concrete Epoxy resin Divided steel bar

WE

REFRA Potentiostat

CE

ii


WE

REFRA Potentiostat

CE

Fig. 6 Measurement set-up of microcell corrosion current density test.


Rpi = polarization resistance (), Si = surface area of component i (cm2).

On the other hand, the macrocell current density in the steel was measured using an ammeter. Figure 7 shows the diagram of the measurement method. As an example, the macrocell current flowing in the middle bar (Fig.7) can be calculated by substituting in Eq.7 the readings obtained from the ammeter and dividing them with the surface area of the steel.

i

iiiimacro S

III 1,,1-

- += (7)

where Imacro = macrocell corrosion current density (A/cm2), Si = surface area of steel element i (cm2), Ii-1, i = current flowing from component i-1 to steel element i (A), Ii,i+1 = current flowing from steel element i to steel element i+1 (A).

For sign convention purposes, the anodic current density is denoted as positive (+) and the cathodic cur-rent density as negative (-).

The total current density was obtained by getting the sum of the microcell and macrocell corrosion current density. The total corrosion rate per year was calculated using the conversion factor (100 A/cm2 current density = 1.16 mm/year corrosion rate per year), Miyazato et al. (2001).

In the field investigation, the process for measuring the microcell corrosion current density was the same as in the laboratory. However, in the case of macrocell corrosion current density, measurement was slightly different since the steel bars embedded in the concrete were made of straight bars, unlike in the laboratory test, where divided steel bars were used. Therefore, to de-termine the macrocell corrosion current density in ex-isting concrete structures, the measurement method de-veloped by Miyazato et al. (2001) was adopted (Fig. 8). The electric circuit model was composed of polarization resistance, concrete resistance and potential derived from non-destructive testing. Potentials at some points in the circuit model mentioned above were obtained through the use of a corrosion monitor. However, it was

necessary to perform calculations for the potentials (V) at the junction because these could not be obtained di-rectly from the corrosion monitor. Calculation was done by deriving the equivalent equation from the circuit model using Ohms Law and Kirchoffs Law. The mac-rocell corrosion density was then obtained by substitut-ing the calculated value of the potential and the resis-tance in the equivalent equation.

3. Results and discussions

3.1 Minute compressive strength Figure 9 shows the distribution of the minute compres-sive strength of the concrete specimens. The minute compressive strength of the mortar aggregate matrix taken near the upper layer is apparently almost 50% weaker compared to the minute compressive strength of the mortar aggregate matrix taken in the bottom layer. On the other hand, as bleed water moves horizontally from inside the concrete towards the formwork due to the consolidation of the fresh concrete, it was found out that the minute compressive strength of the hardened concrete along the horizontal direction (distance from the center) is 36% weaker as compared to the minute compressive strength of the specimens taken in the inner layer (Fig. 10). The same trend was also observed in the

i-1 i i+1

A

Ii-1,i

A

Ii,i+1


Ammeter

i-1 i i+1

A

Ii-1,i

A

Ii,i+1

Epoxy Divided

Ammeter

i-1i-1 ii i+1

A

Ii-1,i

A

Ii,i+1


Ammeter

i-1 i+1

A

Ii-1,i

A

Ii,i+1


Ammeter

i-1 ii i+1

A

Ii-1,i

A

Ii,i+1

Epoxy Divided

Ammeter

Fig. 7 Measurement set-up of macrocell corrosion cur-rent density test.

Rp = Polarization resistance, V=Potential Rs = Concrete resistance

Concrete

Electric double layer

Steel bar

RSR

RpRp

RSRS RSRS RSRS RSRS

RpRp RpRp RpRp RpRp RpRp

V V V V V V


Concrete


Steel bar

RSR


Concrete


Steel bar

RSR

RpRp

RSRS RSRS RSRS RSRS

RpRpRp

RSRS RSRS RSRS RSRS

RpRp RpRp RpRp RpRp RpRp

V V V V V V

Fig. 8 Electric circuit model.

0

20

40

60

80

100

120

140

160

0 20 40 60 80Minute compressive strength (MPa)

Ver

tical

dis

tanc

e (c

m)

0.62 (% Bleeding)1.08 (% Bleeding)6.67 (% Bleeding)

Fig. 9 Minute compressive strength distribution along vertical direction (specimen).

Verti

cal d

ista

nce

(cm

)


field investigation. Figure 11 shows the distribution of the minute compressive strength obtained from the field test. The result quantitatively confirmed that concrete affected by bleeding generally exhibits lower minute compressive strength. The more water content in the mix the lesser the strength of the concrete. A noteworthy finding of this study was the quantitative demonstration that concrete affected by bleeding has lower minute compressive strength.

3.2 Tensile strength Figure 12 shows the trend of the minute tensile strength in the interfacial transition zone between the coarse ag-gregate and the mortar matrix. The experiment results show that the tensile strength of the mortar aggregate matrix taken above the coarse aggregate is 34% higher than the strength of the mortar aggregate matrix taken underneath it. The tensile strength of both of the ITZs located above or below the aggregates surface decreases depending on the water cement ratio of concrete. This clearly suggests that the aggregate-cement paste interfa-cial bond strength in the concrete matrix was signifi-cantly influenced by bleeding. As a consequence, the matrix structure around the coarse aggregate was not uniform, with a thicker water film forming underneath the coarse aggregate surfaces while a thinner film de-

veloped above it. The same trend was also reported in research conducted by Bentur et al. (1996). The forma-tion of a thicker film of water underneath the aggregate surface occurs due to the poor condition of the concrete matrix. This is the reason why a concrete affected by bleeding is more likely to fail in tension due to the rela-tively low tensile strength of its constituent material such as the weak bond between the mortar and the ag-gregate. It is noteworthy that through this study the lower minute tensile strength of concrete affected by bleeding was quantitatively established.

3.3 Chloride ion diffusion Diffusion is the process by which liquid, gas, or ions move through a porous material due to the presence of a concentration gradient (Kropp and Basheer 2000). Alexander et al. (2001) reported that diffusion rates are dependent on temperature, moisture content of the con-crete, type of the diffusant, and the inherent diffusibility of the material. Based on the study by Kropp and Basheer (2000), they emphasized that the movement of gases, liquids, ions through concrete is important be-cause of their interaction with concrete constituents or the pore water, thus it can alter the integrity of concrete directly and indirectly leading to the deterioration of structures. Therefore, diffusivity of chloride through concrete depends on the quality of the microstructure of the concrete and mortar. In this study, a time-constant diffusion was considered. Figure 13 shows the typical variation in the distribution of the chloride ion diffusion coefficient in the bottom layer and upper layer of con-crete affected by bleeding. As shown, the chloride ion coefficient on the upper layer is higher by 59% com-pared to the chloride ion diffusion coefficient in the bottom layer. From the chloride ion distribution trend, it is predicted that the ions may reach the steel reinforce-ment near the surface layer in a very short period of time as shown in Table 3. The predicted results were assumed to be dependent on the actual surface chloride concentration. If this is the case, the initiation of corro-sion in reinforced concrete will be hastened. The same trend was also noted in the chloride ion diffusion test

0

20

40

60

80

0 20 40 60 80

Horizontal distance from wall surface (mm)

Min

ute

com

pres

sive

st

reng

th (M

Pa)

Upper (145 cm from base)Middle (75 cm from base)Bottom (5 cm from base)

Fig. 10 Minute compressive strength distribution along horizontal direction (specimen).

0

20

40

60

80

70 80 90 100 110

Minute compressive strength (MPa)

Ver

tical

dis

tanc

e (c

m)

Fig. 11 Minute compressive strength distribution along vertical direction (existing concrete member).

0

75

150

0.00 0.50 1.00 1.50 2.00

Minute tensile strength (MPa) (Bleeding ratio =6.67%)

Ver

tical

dis

tanc

e (c

m)

Above coarseaggregateBelow coarseaggregate

Fig. 12 Minute tensile strength distribution (specimen).

Verti

cal d

ista

nce

(cm

)

Verti

cal d

ista

nce

(cm

)


done on existing concrete structures (Fig. 14). It was quantitatively confirmed through the minute diffusion test that a concrete member that suffered from bleeding can be easily penetrated by chloride ions due to its po-rous condition at or near the upper layer of concrete.

3.4 Concrete resistance and oxygen permeabil-ity Figure 15 shows the influence of bleeding on concrete resistance. The results show that there is a gradual de-crease in the concrete resistance of concrete from the bottom layer towards the upper layer. Concrete speci-mens affected by bleeding generally exhibited lower resistance (40-53%) in the upper layer compared to the resistance of concrete measured in the bottom layer. This condition may be due to the less dense and more permeable condition of the concrete in the upper layer as attested by the results obtained in the minute com-pressive strength test and the minute diffusion test.

As expected, higher oxygen permeability ensued due

to the poor quality of concrete in the upper layer. The rate of oxygen permeability in the upper layer was 38% higher compared to the rate of oxygen permeability in the bottom layer (Fig. 16). The same reason may be attributed to the porous and permeable quality of con-crete in the upper layer. This observation is in agree-ment with the study done by Nolan et al. (1995) stating that bleed water in normal concrete during compaction tends to move upwards and towards the formwork where it stays, causing the formation of a more porous and permeable surface. As this phenomenon occurs, the rate of oxygen permeability in concrete becomes higher.

3.5 Measurements of gaps The relationship between the area of gaps and the total amount of corrosion rate in the horizontal bar is shown both in Fig. 17 and Fig. 18. One can see that the greater the area of the formed gaps the higher the amount of generated corrosion.

3.6 Macrocell and microcell corrosion Based on the results of the electrochemical investigation, it was found that the rate of corrosion obtained from the control specimens during the entire period of the inves-tigation is negligible or almost zero. This therefore sug-gests that even if concrete is affected by bleeding, the

0

75

150

0 5 10 15 20 25

Cl- ion diffusion coefficient (10-9cm2/s)

Ver

tical

dis

tanc

e (c

m)

0.62 (% Bleeding)1.08 (% Bleeding)

6.67 (% Bleeding)

Fig. 13 Minute chloride ion diffusion coefficient(W/C = 0.55, specimen).

0

20

40

60

80

6.00 7.00 8.00

Cl- ion diffussion coefficient (10-9 2/s)

Ver

tical

dis

tanc

e (c

m

Fig. 14 Minute chloride ion diffusion coefficient (existing concrete member).

Table 3 Chloride ion diffusion prediction results.

Specimen Position

Upper (145 cm)

Middle (75 cm)

Bottom ( 5 cm)

Diffusion Co-efficient of Cl-

(10-8 cm2/s) 20.80 10.00 8.50

Period (years) 3.47 7.22 8.47 Period: Number of years for the chloride ion to reach the steel bar

0

75

150

0 1000 2000

Concrete resistance (Ohm)

Ver

tical

dis

tanc

e (c

m)


Fig. 15 Concrete resistance distribution in concrete (specimen).

Verti

cal d

ista

nce

(cm

)

Verti

cal d

ista

nce

(cm

)

Verti

cal d

ista

nce

(cm

)


time before corrosion occurs will be somewhat longer as long that the concrete is not attacked or contaminated with chloride and is not carbonated. On the other hand, since the same amount of NaCl (10 kg/m3) was admixed to the concrete specimens it was clear that bleeding has a great influence on the rate of macrocell and microcell corrosion in concrete specimens. The influence of bleeding on the rate of macrocell and microcell corro-

sion in a chloride contaminated concrete is shown in Fig. 19. The specimen that bled the most generally exhibited higher macrocell corrosion rates. In these graphs, it is interesting to note that the corrosion rate in the upper layer is higher than in the bottom layer. This trend may be attributed to the high oxygen permeability and lower concrete resistance in that section of the concrete. Ac-cording to Tarek et al. (2002) the concentration corro-sion cell formed on the vertical steel bar occurs due to the concentration difference between the upper and lower layer of the concrete. The porous and weaker strength of concrete in the surface layer causes rapid diffusion of a large amount of moisture and easy access of oxygen, resulting in an accelerated rate of corrosion in the steel. This explains why the upper layer has greater rates of corrosion. Conditions such as this nor-mally occur in concrete that suffered excessive bleeding as confirmed by the results obtained in the concrete re-sistance test and oxygen permeability test.

On the other hand, the corrosion rate distribution on the horizontal steel bar as shown in Fig. 20 indicates a higher rate of corrosion underneath the steel bar. It is also interesting to note in this figure that the macrocell corrosion rate on the horizontal steel bars, particularly

0

75

150

0 2 4 6Rate of oxygen permeability

(10-11 mol/cm2/s)

Ver

tical

dis

tanc

e (c

m)


Fig. 16 Rate of oxygen permeability distribution in con-crete (specimen).

Steel bar (D16)

Concrete Gap

Steel bar (D16)

Concrete Gap

Fig. 17 Gap between steel and concrete (specimen).

0

0.005

0.01

0.015

0.02

0.025

0.03

0 2 4 6 8

Area of gap (mm2)

Tota

l cor

rosi

on ra

te (m

m/y

ear)

0.62 (% Bleeding)

1.08 (% Bleeding)

6.67 (% Bleeding)

Fig. 18 Total corrosion rate vs. area of gap (specimen).

0

50

100

150

0 0.005 0.01 0.015Microcell corrosion rate

(mm/year)

Ver

tical

dis

tanc

e (c

m)


0

50

100

150

0 0.005 0.01 0.015Macrocell corrosion rate

(mm/year)

Ver

tical

dis

tanc

e (c

m)


0

50

100

150

0 0.005 0.01 0.015

Total corrosion rate (mm/year)

Ver

tical

dist

ance

(cm

)


Fig. 19 Influence of bleeding on rate of corrosion in vertical steel bar (specimen).

Verti

cal d

ista

nce

(cm

)

Tota

l cor

rosi

on ra

te (m

m/y

ear)

Verti

cal d

ista

nce

(cm

)

Verti

cal d

ista

nce

(cm

)

Verti

cal d

ista

nce

(cm

)


the upper steel bar, is zero in all cases. Based on the results obtained in the electrochemical investigation, the reason for this is that the upper steel bar behaved as cathode while that of the lower steel bar behaved as anode. Corrosion in the anodic area is more active than in the cathodic area. That is why a higher rate of corro-sion occurred at the anodic area. This condition may be attributed to excessive bleeding in the concrete mix. As a result of bleeding, water pockets trapped under the coarse aggregates and underneath the steel bars caused the formation of gaps between the steel and concrete allowing corrosion cells to develop on the horizontal bar. Just like in the vertical steel bar, macrocell corrosion generally prevailed in the horizontal steel bar. The same trend was also observed in the macrocell and microcell corrosion distribution obtained in the field test (Fig. 21).

The prevalence of macrocell corrosion over microcell corrosion confirms laboratory findings. It must be noted that the high incidence of macrocell corrosion in con-crete poses a serious threat to the structure. According to a study done by Jaggi et al. (2001), macrocell corro-sion between actively corroding areas of rebars and large passive areas is of great concern because it results in very high anodic current densities. The resulting local loss in cross section has dangerous implications for the structural safety especially if the corroded rebars are located in a zone of high tensile or shear stresses.

4. Temperature dependency of corrosion in concrete affected by bleeding

The corrosion of steel is an electrochemical reaction.

Upper steel

Lower steel

Percentage bleeding0.62% (Upper steel)0.62% (Lower steel)1.08% (Upper steel)1.08% (Lower steel)6.67% (Upper steel)6.67% (Lower steel)

Horizontal steel bar

Lead wire

Epoxy resin

0

50

100

150

0 0.01 0.02 0.030

50

100

150

0 0.01 0.02 0.03

0

50

100

150

0 0.01 0.02 0.03

Ver

tical

dis

tanc

e (c

m)

Ver

tical

dis

tanc

e (c

m)

Microcell corrosion rate (mm/year)

Macrocell corrosion rate (mm/year)


Ver

tical

dis

tanc

e (c

m)

Upper steel

Lower steel



Lead wire

Epoxy resin Upper steel

Lower steel



Lead wire

Epoxy resin

0

50

100

150

0 0.01 0.02 0.030

50

100

150

0 0.01 0.02 0.03

0

50

100

150

0 0.01 0.02 0.03

Ver

tical

dis

tanc

e (c

m)

Ver

tical

dis

tanc

e (c

m)




Ver

tical

dis

tanc

e (c

m)

0

50

100

150

0 0.01 0.02 0.030

50

100

150

0 0.01 0.02 0.03

0

50

100

150

0 0.01 0.02 0.03

Ver

tical

dis

tanc

e (c

m)

Ver

tical

dis

tanc

e (c

m)




Ver

tical

dis

tanc

e (c

m)

Fig. 20 Influence of bleeding on rate of corrosion in horizontal steel bar (specimen).

0

10

20

30

40

50

60

70

0 0.002 0.0040

10

20

30

40

50

60

70

0 0.002 0.0040

10

20

30

40

50

60

70

0 0.002 0.004Microcell Corrosion Rate

(mm/year)Macrocell Corrosion Rate

(mm/year)Total Corrosion Rate

(mm/year)

Hei

ght (

cm)

10.222.231.4

Temperature10.222.231.4





Temperature


Macrocell corrosion rate(mm/year)

Total corrosion rate(mm/year)

Ver

tical

dis

tanc

e (c

m)

0

10

20

30

40

50

60

70

0 0.002 0.0040

10

20

30

40

50

60

70

0 0.002 0.0040

10

20

30

40

50

60

70




(mm/year)

Hei

ght (

cm)

10.222.231.4






Temperature




0

10

20

30

40

50

60

70

0 0.002 0.0040

10

20

30

40

50

60

70

0 0.002 0.0040

10

20

30

40

50

60

70




(mm/year)

Hei

ght (

cm)

10.222.231.4






Temperature




Ver

tical

dis

tanc

e (c

m)

Fig. 21 Microcell and macrocell corrosion rate distribution (existing concrete member).


Its rate is dependent on the ambient temperature, such that the rate is increased as the ambient temperature is increased (Zivica et al. 1997). Theoretical discussion on the influence of temperature on chemical reaction can be best illustrated by the Arrhenius equation, as shown in Eq. 8

=RT

EAk aexp (8)

where k = rate constant, A = frequency factor, Ea = ac-tivation energy, R = ideal gas constant, T = temperature.

If we take the natural logarithm of the above equation

=RTEAk alnln (9)

we get

ATR

Ek a ln1ln +

= (10)

a linear equation that relates the natural logarithm of k to the inverse of the temperature.

Thus, if one were to plot ln k vs. 1/T, it gives a line

with a slope equal to Ea/R and intercept equal to ln A. The relationship between the total corrosion rate and temperature is shown in Fig. 22, and its corresponding Arrhenius plot is presented in Fig. 23. It may be noted from the graph that the logarithm of the corrosion rate is linearly related to the reciprocal of the absolute tem-perature. As shown, the rate of corrosion in concrete is enhanced at elevated temperatures, which is consistent with the theoretical expectations that corrosion should increase with temperature. A study by Melchers (2002) stated that in general, when corrosion process is ki-netically controlled, the corrosion rate should double for every 10oC increase in temperature.

The relationship between the total corrosion rate in the steel bar and temperature in existing concrete struc-ture is shown in Fig. 24. Fig. 25 shows the correspond-ing Arrhenius plot. As shown, existing concrete mem-bers that suffered from excessive bleeding have a greater rate of corrosion at elevated temperatures. This trend agrees well with the empirical data obtained in the laboratory. As illustrated in the graph, measurements done during the summer (31.4 oC) show a higher rate of corrosion compared to measurements done during win-ter (10.2 oC) and spring (22.2 oC).

0

0.001

0.002

0.003

0.004

0 10 20 30 40

UpperMiddleLower

PositionUpperMiddleLower

Position

Temperature (oC)

Tota

l cor

rosi

on ra

te (m

m/y

ear)

0

0.001

0.002

0.003

0.004

0 10 20 30 40

UpperMiddleLower


Position

0

0.001

0.002

0.003

0.004

0 10 20 30 40

UpperMiddleLower


Position

Temperature (oC)

Tota

l cor

rosi

on ra

te (m

m/y

ear)

0

0.001

0.002

0.003

0.004

0 10 20 30 40

UpperMiddleLower


Position

Fig. 24 Total corrosion rate vs. temperature (existing concrete member).

-4.0

-3.5

-3.0

-2.5

-2.0

3.2 3.3 3.4 3.5 3.6

UpperMiddleLower

PositionUpper

MiddleLower

Position

Log

(tota

l cor

rosi

on ra

te)

-4.0

-3.5

-3.0

-2.5

-2.0

3.2 3.3 3.4 3.5 3.6

UpperMiddleLower

PositionUpper

MiddleLower

Position

1/Temperature x 1000 (1/K)

-4.0

-3.5

-3.0

-2.5

-2.0

3.2 3.3 3.4 3.5 3.6

UpperMiddleLower

PositionUpper

MiddleLower

Position

Log

(tota

l cor

rosi

on ra

te)

-4.0

-3.5

-3.0

-2.5

-2.0

3.2 3.3 3.4 3.5 3.6

UpperMiddleLower

PositionUpper

MiddleLower

Position


Fig. 25 Arrhenius plot (existing concrete member).

0

0.01

0.02

0.03

0.04

0.05

0.06

10 20 30 40 50

Temperature (oC)

0.62%1.68%6.67%

Bleeding Ratio0.62%1.08%6.67%

% Bleeding

Tota

l cor

rosi

on ra

te

(mm

/yea

r)

0

0.01

0.02

0.03

0.04

0.05

0.06

10 20 30 40 50

Temperature (oC)

0.62%1.68%6.67%


% Bleeding

Tota

l cor

rosi

on ra

te

(mm

/yea

r)

Fig. 22 Total corrosion rate vs. temperature (specimen).

- 2.4- 2.2- 2

- 1.8- 1.6

- 1. 4- 1.2- 1

3.1 3.2 3.3 3.4 3.5

Log

(tota

l cor

rosi

on ra

te)


0.62%1.68%6.67%


% Bleeding

- 2.4- 2.2- 2

- 1.8- 1.6

- 1. 4- 1.2- 1

3.1 3.2 3.3 3.4 3.5

Log

(tota

l cor

rosi

on ra

te)


0.62%1.68%6.67%


% Bleeding

Fig. 23 Arrhenius plot (specimen).


5. Conclusion

The conclusions derived from the results of the labora-tory and field investigations that were performed can be summarized as follows 1. Bleeding significantly influences the minute proper-

ties of concrete. This is bolstered by the following results obtained in laboratory and field investigations, to wit: The minute compressive strength of concrete

taken near the upper layer particularly on the concrete specimen affected by high bleeding was weaker by almost 50% compared to the strength of concrete taken in the bottom layer. The minute compressive strength of concrete along the hori-zontal direction was also 36% weaker than that of concrete taken in the inner layer.

The tensile strength of the mortar aggregate ma-trix taken above the coarse aggregate was 34% higher than the mortar aggregate matrix taken underneath it.

The chloride ion diffusion coefficient in the up-per layer of the concrete was 59 % higher than the rate of chloride ion diffusion coefficient in the bottom layer.

Variations in the quality of the concrete (in the upper, middle and bottom layers), as a result of bleeding have great influence on concrete resis-tance, on the same order as oxygen permeability. Concrete resistance in the upper layer is 40-53% lower than the bottom layer. On the other hand, the rate of oxygen permeability at the upper layer is 38% higher than in the bottom layer.

In general, concrete affected by bleeding has weaker strength, higher permeability, lower concrete resistance and higher oxygen permeability at or near the surface layer.

2. Bleeding significantly influenced the macrocell and microcell corrosion rates in the reinforcing bars. Concentration difference in the upper layer of the concrete due to bleeding causes higher rates of corrosion in the vertical steel bars. Hence, due to the trapping of bleed water under the horizontal steel bars, gaps formed leading to the development of crevice corrosion cell. The greater the area of the formed gap the greater the rate of corrosion. Generally, macrocell corrosion prevailed over microcell corrosion both in vertical and horizontal steel bars.

3. The intensity of corrosion in concrete was generally enhanced at elevated temperatures. This was consistent with the theoretical expectation that corrosion should increase with temperature. Furthermore, the graphical relation between temperature and corrosion rate, using the data derived from the laboratory and field investigation is in full agreement with the Arrhenius equation. Thus we can state the principle that the logarithm of the corrosion rate is linearly related to the reciprocal of the absolute

temperature.

References Alexander, M. G., Mackechnie, J. R., Ballim Y. (2001).

Use of durability indexes to achieve durable cover concrete in reinforced concrete structures. in Materials Science of Concrete, Vol. VI, Mindness, S., and Skalny J. eds. The American Ceramic Society, 483-511.

Bentur, A., Diamond, S. and Berke, N. S., (1997). Modern Concrete Technology 6 Steel Corrosion in Concrete, Fundamentals and Civil Engineering Practice. E & FN Spon (Chapman & Hall).

Bentur, A. and Odler, I. (1996). Development and nature of interfacial microstructure in interfacial transition zone in concrete. Maso, J.C. ed., RILEM Technical Committee 108-ICC Report. Great Britain: E & FN Spon (Chapman & Hall), 8-44.

Concrete Society (1984). Repair of Concrete Damaged by Reinforcement Corrosion. London: The Concrete Society, Report 26.

Jggi, S., Bhni, H. and Elsener, B. (2001). Macrocell corrosion of steel in concrete Experiments and numerical modelling. Eurocorr 2001. Riva di Garda, Associazione Italia Mettalurgia, Milan.

Kropp, J. and Basheer, L. (2000). Assessment of the durability of concrete from its permeation properties: A review. P.A. M. Basheer Ed. Fifth CANMET/ACI International Conference on Durability of Concrete, Barcelona 65-79.

Melchers, R. E. (2002). Effect of temperature on the marine immersion corrosion of carbon steels. NACE International, 58(9), 768-774.

Mehta, P. K. and Monteiro, P. J. M. (1993). Concrete: Structure, Properties & Methods. 2nd ed. New Jersey: Prentice Hall.

Miyazato, S., Otsuki, N. and Kimura H. (2001). Estimation method of macrocell corrosion rate of rebar in existing concrete structures using non-destructive tests. East Asia-Pacific Conference (EASEC 8), 2, 531-542.

Nagataki, S., Otsuki N., Hisada, M. and Miyazato, S. (1996). The experimental study on corrosion mechanism of reinforced concrete at local repair part. JSCE Proceedings, 544(32), 109-119.

Nolan, E., Basheer, P. A. M. and Long, A. E. (1995). Effects of three durability enhancing products on some physical properties of near surface concrete. Construction Building Materials, 9(5), 267-272.

Mohammed, T. U., Otsuki, N., Hamada, H. and Yamaji, T. (2002). Chloride-induced corrosion of steel bars in concrete with presence of gap at steel-concrete interface. ACI Materials Journal, 99(2), 149-156.

Tsuru, T., Maeda, R. and Haruyama, S. (1979). Application of the alternating method to corrosion monitor in local corrosion. Technology of Corrosion Prevention , 28, 638-644.

Wainwright, P. J. and Ait-Aider, H. (1995). The


influence of cement source and slag additions, on the bleeding of concrete. Cement and Concrete Research, 25(7), 445-1456.

Yodsudjai, W. (2003). Evaluation of Strengths and Chloride Ion Diffusivity of Minute Regions in Concrete Using Newly Developed Methods. Thesis. Tokyo Institute of Technology

Yonezawa, T. (1988). Pore Solution Composition and

Chloride-Induced Corrosion of Steel in Concrete. Thesis. Victoria University of Manchester

ivica, V., Kraji, L., Bae, L. and Vargov, M. (1997). Significance of the ambient temperature and the steel material in the process of concrete reinforcement. Construction and Building Materials, 11(2), 99-103.

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Influence of Bleeding

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