Behavior of carbon steel in simulated concrete pore

solutions of air-entrained and ordinary concrete

Adriana de Araujo, Zehbour Panossian, David R. das Neves

Filho, Alberto D. Siqueira

Junior

IPT – Institute for Technological ResearchCorrosion and Protection LaboratorySão Paulo/Brazil

• introduction: overview of the results obtained in field inspections that motivated the present study;

o objectives of the study;

o methodology;

o results;

o conclusions.

Outline

Recently, the air-entrained concrete has been extensively used as structural walls

in Brazilian popular housing units.

One of the biggest unit constructions, 18 thousand houses.

Some houses and their construction system

House formwork Reinforcement and concrete placement

ACI concrete terminology: causes the development of a system of microscopic air bubbles in concrete, usually to increase the concrete workability and frost resistance.

The concrete is delivered to the site (in a truck agitator). The air-entraining admixture is incorporated at that point.

Carbon steel weld wire mesh is used as reinforcement

(Ø 3.6 mm - 150 mm x 150 mm)

In the housing units: This admixture is used to increase the concrete workability (self-compacting concrete) and thermal and acustıc comfort of the houses.

Corroded bars (outdoor storage)

• urban atmosphere (mostly in tropical regions);

• built by different constructers (less than 3 years ago).

Visual inspection (concrete and bars surface)

Potential and electric resistivity

Concrete coverChloride ions profile Carbonation front

Some houses (6 units) were inspected, all were:

The usual tests were performed during the inspection:

Insıgnıfıcant chloride ions

content

Variations in concrete cover and porosoty

In all the units inspected were detected:

High carbonation front (it reaches the steel in some

walls)

Intense reddish

stains and corrosion

products on the surface of the bars

They were present in carbonated and integrity concrete!

high presence of air bubbles was visualized (by naked eyes and SEM images) in:

Concerning the porosity:

Soylev, T.A.; Francois, R. (2003): the porosity and the density variation of the concrete enhance the corrosion of steel embedded due the lack of chemical and physical protection supplied. Jingak, N. et al (2005): the corrosion initiated preferentially at an air void along the steel-concrete interface. Glass, G.K. et al (2000): the absence of calcium hydroxide at the location of the entrapped air voids in steel-concrete interface is a dominant effect in initiate a corrosion process by chlorides. Page, C. L. (1975): local heterogeneities in the zone adjacent to steel are liable to influence its corrosion behavior in the presence of aggressive agents. Lambert et al (1991): the interface macro-defect disrupts the integrity of the cement hydration-product layer.

Concrete-steel interface

Concrete bulk and at the surface

The entrapped air voids disrupt the integrity of the

buffering layer of cement hydration

products. This may result in a missing

passivation for parts of the steel and the premature corrosion in the defect areas

(crevices).

The literature reports the interface defects and its impact on the corrosion:

Dispersed areas

Wire ties

Dispersed areas

Concerning the corrosion in the integrity concrete: The stains and the corrosion products were visualized (naked eyes) in dispersed areas of the bars’ surface and in wire ties and weld areas.

Ecorr: mostly more positive than -250 mVCSE, most potential gradients were less than 200 mVCSE;

GONZÁLEZ et al. (2004) low levels of wetness can confuse active and passive based only in Ecorr measurement. The most value Ecorr will be that one obtained when the concrete resistivity is less than 20 kΩ.cm. The electric resistivity between 100-50 kΩ.cm indicate low risk of corrosion, between 50-20 kΩ· cm there is a high risk and less than 20 kΩ· cm very high risk. ASTM C876 (2009) not considers normally applied for carbonated concrete the criteria (i.e.: 90 % probability of corrosion for Ecorr values ≥ -350 mV, CSE). LIVECON D3.1 (2003) considers significant potential gradient ≥ 200 mV (in 1 m). The gradient has more weight then the absolute potential value. BROOMFIEL (1991) considers that the Ecorr measured by a reference electrode is not the true value, but a mixed potential of an unknown area of the rebars. GONZÁLZ et al. (1979) considering the Nernst formula, the concrete pH reduction (~7-8) by carbonated reactions mean a 250 mV to -300 mV increase. ELSENER et al. (1990) when surface potentials are taken, they are essentially remote from the reinforcement due the concrete cover. The potential thereby measured are, in fact, mixed potentials. It can not be interpreted in a straightforward manner.

The measurement results were not in agreement with the results obtained from the visual

examination. The small diameter bars embedded in high depth must have affected the

measurements, as well as the surface wetness variation and its impregnation with paint product. …the literature mentions restrictions in the measurements:

In all the units inspected:

Resistivity: usually above 50 KΩ· cm.

The present study complements the presented results by:

characterization of air-entrained concrete;

Evaluation of the behavior of steel bars immersed in solutions that simulate the water in the pores of the air-entrained concrete.

Objectives

methodology

Concrete bulk analysis: calculation of quantity and diameter of the air bubbles in petrographic samples prepared according to ASTM C856 (2011) and observed in SEM (associated with Scandium program);

Physical and mechanical tests: performed according to Brazilian standards.

2 air-entrained concretes - AC1 and AC2 (specimens were casted in the field, in 2

different units)

1 ordinary concrete (reference) – OC (specimens were casted in the laboratory)

The pore solutions were obtained from the mixtures prepared with distilled water and the

powder obtained by grinding the concrete specimens (AC1, AC2 and OC).

3 different conditions of the bar were defined(deformed, Ø 3.6 mm):

• blasted (BS),

• corroded - open air exposition

(CS);• chromate treated galvanized

(GS).

Cupper wire - electric contact

Sealant over epoxy paint in the bars extremity

o-ring wrapped around the bar (middle of the exposed length)

1 bar to simulate crevice corrosion

3 replicate bar +

Experimental set-up (bar immersed in the supernatant

of the solution)

Cylindrical concrete specimens were used (Ø 5 cm x 10 cm).

They were cured in laboratory atmosphere. 1 blasted bar + 2 corrode bars

To validate the results in pore solution, tests on air-entrained concrete specimens

(AC1 and AC2) were also conducted

Bar embedded in a concrete specimen

Potentiostat (Solartron), using three-electrode cell (saturated calomel

electrode, SCE). The Stern-Geary equation was applied. IR drop compensation for concrete specimens measures.

After 1 day and 10 days of the bars immersed in the solutions (AC1, AC2 and OC) and after 28 days of the concrete specimens cure (AC1 and AC2), the bars were assessed by means of corrosion potential (Ecorr) and linear polarization resistance measurements of instantaneous corrosion current density (icorr) and

visual (naked eyes) examination. Ecorr was monitoring during in the immersion tests.

… the usual electrochemical procedure for concrete studies was followed:

Results – characterization

of the concretes

MaterialsAC1 AC2

Ratio Features Ratio Features

Da

ta

su

pli

er

s

Water 185 l W/C 0.66 185 l W/C 0.71

Admixture 0.5 l

Air-entraining based on pitch

soap, 0.18 % by weight of blinder

0.5 l

Air-entraining based on pitch soap, 0.19 % by

weight of blinder

Fine aggregates 943 kg - 839 kg -

Coarse aggregates

725 kg - 796 kg -

Portland cement 280 kg

CP IV 32 (pozzolanic-

modified cement)260 kg

CP II E 40 (slag-modified

cement)

Fiber 0.3 kgPolypropylene

microfiber0.2 kg

Polypropylene Microfiber

Te

st

s

re

su

lts

Compressive strengh 6.5 MPa 6.2 MPa

Voids content 37.5 % 36.8 %

Density 1730 kg/m³ 1650 kg/m³

Water absorption 22.7 % 22.4 %The tests showed the inferior quality of the air-entrained

concretes. Both do not fulfill the Brazilian standard (design of structural concrete). The poor quality must affect the performance

OCRatio Features

166 l W/C 0.55

2.11 kg

Polycarboxylate, 0.7% by weight of blinder

257 kg -

284 kg -

301 kg CP II E 32

- -

38.8 MPa

11.2 %

2280 kg/m³

4.8 %

Soylev e Francois (2003): the corrosion rate is a function of concrete quality and it increases as the class of concrete strength decreases

high presence of air bubbles with variation of size and distribution;

proximity of the air bubbles, some of them communicating;

trend of concentration of the air bubbles surrounding the aggregates in AC2.

OCAC2AC1

The AC1 and AC2 poor quality was confirmed by

microstructure analysis, being detected:

20 % 35 %

Ø 40 to 60 µm

10 %

Bubble content 18 % Bubble content 4.5 %

Bubble content 17 %

OCAR1 AR 2

Ø 120 to 140 µm

Amount

Dia

mete

r

Results – pores solutions

Solution Calcium (Ca2+)

Inorganic compounds (mg/l) pH Temperature

(oC)Chloride

(Cl-)Sulfate (SO4

2-)Initial

Final

Initial Final

PSAC1 0.21 ± 0.01 4.5 ± 0.2 12.0 ± 1.0 12.4

11.7 23 23

PSAC2 0.11 ± 0.01 6.7 ± 0.4 5.9 ± 0.2 12.2

11.7 22 19

PSOC 0.25 ± 0.01 2.4 ± 0,2 25.7 ±

0.612.2

11.7 21 21

Blasted surface - BS

(some corrosion areas)

Corroded surface - CS

Galvanized layer- GS

(bubbles and irregularities in the

layer and cracks at the surface)

In the 3 solutions, the chloride content is not significant. During the tests, the pH is found to be similar.

Pretest (SEM image plus EDS) showed the presence of cracks in the surface of GS bars and bubbles and irregularities in its layer.

BS - solution results:

> 90% probability of corrosion occurrence

> 90% probabi l i ty of no corrosion occurrence

Corrosion is uncertain

BS4 - PSAC2 Ecorr 5 day was

less than -300 mV

PSAC1, PSAC2 and PSOCMost values of Ecorr remained more positive than -300 mV,

except to

AS

TM

C8

76

(2

00

9)

PSAC1icorr ≤ 0.15

µA/cm2 PSAC2

icorr ≤ 0.10 µA/cm2

PSOCicorr ≤ 0.11

µA/cm2

PSAC1icorr ≤ 0.11 µA/cm2

PSAC2icorr ≤ 0.24 µA/cm2

PSOCicorr ≤ 0.25

µA/cm2

BS2

BS4

10

The corrosion levels were low* in all the

solution(icorr ≤ 0.5 µA/cm2) after

1 and 10 days of immersion.

1

BS2 29 µA/cm2

* RILEM TC 154 (2004)

BS3 1.6 µA/cm2

BS2 – PSAC2Ecorr 5-10 days was less than -300

mV

The exceptions were...

All bars presented

corrosion - spots under the

sealant and the o-ring.

PSAC1

BS – visual analysis: PSAC2

BS2 and BS4 presented severe

corrosion:initiated under the sealant

and propagated toward the free surface. BS1 and BS3 presented

corrosion under the sealant.

BS2 BS4

BS2 Ecorr max.: -498

mVSCE BS4

Ecorr max.: -329 mVSCE

BS4BS2

BS2 Ecorr max.: -282

mVSCE

BS4 Ecorr max.: -211

mVSCE

PSOC

Only BS4 presented

corrosion - under the sealant and

the o-ring.

BS4 Ecorr max.: -202

mVSCE

BS4

CS - solution results:

> 90% probability of no corrosion occurrence

Corrosion is uncertain

PSAC1, PSAC2 and PSOC

Most values of Ecorr remained

more positives than -150 mV, except to

101

Although the values of Ecorr indicates the passive state of the bars in all the solutions, high corrosion levels were obtained

(higher than 3 µA/cm2)

CS2 – PSAC2Ecorr = -168.89 mV

The no change in the surface of the bars confirmed the passive state.

CS1 – PSAC2Ecorr = -174.22

mV

RILEM TC 154 (2004): the Rp values cannot be correctly determinate if the steel is passive.

GS - solution results:

The values of Ecorr were more negative than -600 mV in the beginning and went up later (passivation

process).

The values of Ecorr

remained more positive than -600 mV

PSAC1icorr ≤ 1.4 µA/cm2

PSAC2icorr ≤ 0.8 µA/cm2

PSAC1Icorr ≤ 3.7 µA/cm2 PSAC2icorr ≤ 2.1 µA/cm2

PSOCicorr ≤ 0.3 µA/cm2

10

In the solution PSAC1 and PSAC2, the corrosion levels were moderate

to high after 1 and 10 days of immersion .

In the PSOC, the corrosion levels were low after 1 and 10 days

1

PSAC1 and PSAC2

The values of Ecorr had a tendency to drop

(depassivation process)

PSOC

0 2 4 6 8 10 12 14 16 18 20-1200

-1100

-1000

-900

-800

-700

-600

-500

-400

-300

-200

PSAC1 PSAC2

PSOC3

Tempo (dias)

The solution tests for GS bars was repeated …

And the tendency of the bars be depassivated in PSAC1 and PSAC2

It also showed the tendency of the GS bars be passivated in PSOC

16 day

s

2 h

PSAC2

PSAC1

PSAC2

PSAC1 PSOC

PSOC

Localized corrosion takes place (dark spots and H2 bubbles at

the surface)

pH ~ 12.2pH ~ 11.5

White corrosion

products at the surface

2 h

The SEM images showed the formation of crystals on the surface of coating

at the PSOC. Similar crystals are found in other studies. They are formatted

during the zinc passivation.

BS1 - AC1

E corr = -520 mVSCE icorr = 0.8 µA/cm2 CS1 - AC1

E corr = +58 mVSCE icorr = 5.6 µA/cm2 CS2 – AC1

E corr = +54 mVSCE icorr = 8.9 µA/cm2

CS1 – AC2

E corr = -123 mVSCE icorr = 9.7 µA/cm2

BS1 – AC2

E corr = -160 mVSCE icorr = 3.2 µA/cm2

CS2 - AC2

E corr = +83 mVSCE icorr = 4.5 µA/cm2

The results obtained in the concrete specimens were in agreement with the pore solution tests: In the BS bars there was crevice corrosion under the sealant. For the CS bars the potential indicated a

passive state, being obtained a high corrosion level.

The characterization tests showed the low quality of the air-entrained concretes and the pore solution tests indicated the greatest susceptibility of steel corrosion in these concretes. The obtained results are in agreement with the inferior performance of the air-entrained concretes in the field in relation to the usually observed in a variety of ordinary concretes.

conclusions

The air-entrained concrete AC2 showed greater susceptibility to corrosion. This was clearly observed in the pore solution tests with blasted bar (BS) and confirmed by concrete specimen tests.

conclusions

The crevice corrosion was detected in the solutions. Thıs corrosion takes place under the sealant and the o-ring in the blasted bars (BS) in the 3 solutions. In the AC2 solution the corrosion propagated towards the free surface of the blasted bars.A tendency to depassivation of the galvanized bars (GS) was detected in the air-entrained concrete solutions. Localized corrosion was observed in the 3 solutions, being probably related to the surface defects.

It´s important to conduct detailed studies in the air-entrained concrete. These have to consider the bare and galvanized steel and a new system for protection of the bars extremities.

Thank youfor your attention!

aaraujo@ipt.br