A CONDITION SURVEY OF CONCRETE ELEMENTS WITH CORROSION INHIBITORS EXPOSED 12 YEARS FOR SEA WATER IN TIDAL ZONE

T.A. Østnor1, H. Justnes1 and W. Franke2 1SINTEF Building and Infrastructure, Trondheim, Norway

2Yara International, Porsgrunn, Norway

ABSTRACT

Three wall elements have been stored in the tidal zone in the Trondheim fjord, Norway (average yearly temperature +5°C) for 12 years. Two concrete elements were added either calcium nitrite or calcium nitrate as corrosion inhibitors, while the third element without inhibitor served as reference. The wall elements with calcium nitrate and the reference appeared to be in pristine state by the naked eye, while the element with nitrite had spalled off the surface skin. It was then decided to survey the microstructure of the binder for all the 3 concrete elements.

The compressive strength of the concrete with nitrite was higher than concrete with nitrate, which again was higher than the reference. On the other hand, the compressive strength of both concrete with nitrite and without seemed to have a significant drop in strength from 1 year to 12 years (only one core), while the strength of concrete with nitrate was rather constant in the same period.

The calculated apparent diffusion coefficient of chlorides was about double for the concrete with nitrite compared to the two others. The binder near the surface was substantially cracked in the concrete with nitrite, and the sulphate ingress from sea water was higher than for the two other concretes. Evidence was found for thaumasite in the concrete with nitrite, but not in the others, in spite of the recipe was the same for all wall elements containing limestone filler.

Key-words: corrosion inhibitor, nitrate, nitrate, microstructure, sulphate, sea water

INTRODUCTION

The phase changes in concrete exposed to sea water are complex due to the presence of a variety of ions in the sea water such as sodium, chloride, magnesium, sulfate, calcium and carbonate ions. These ions can affect the phase assemblage of the concrete in different ways [1–3]. Most studies on the effect of sea water on concrete focus on the ingress of chloride ions in concrete as chlorides pose a threat to the reinforcement by causing pitting corrosion. The chloride ingress from sea water can, however, be influenced by the other ions present in the sea water either by changing the chemistry and/or porosity or even by causing scaling. The concrete wall elements in question were cast in 2002 and are described in detail by Justnes [4]. In a previous study [5, 6] the phase changes in a concrete wall element exposed in the tidal zone of the Trondheim fjord for 10 years were investigated and it was shown that the chloride level near the exposed surface is low (higher further in) as the chloride peak is preceded by a magnesium enriched zone followed by a sulfate enriched zone. In addition, these zones showed signs of carbonation. This zoned attack has also been observed by Marchand et al. [7], Chabrelie et al. [8] and Jakobsen [9] on marine exposed concrete.

EXPERIMENTS, RESULTS AND DISCUSSION

The appearance of the wall elements and their placement are shown in Fig. 1. Note that at high tide, the wall elements are totally submerged and at low tide they are out of the water as shown in the photo. The boxes on top are remains of instrumentation of embedded reinforcement nets [4].

Fig. 1 – The appearance and placement of the three concrete wall elements; 1) with nitrite, 2) with nitrate and 3) reference. Image to the right is a close-up of the skin spalling of wall element 1.

The concrete recipes were the same for all the walls as shown in Table 1, with the exception of being without inhibitor (reference), added 4 % calcium nitrate, Ca(NO3)2, of cement mass, and equimolar dosage of calcium nitrite Ca(NO2)2, as corrosion inhibitors.

Compressive strength The specimen was prepared by sawing and grinding of the end surfaces of a piece cut from the drilled out core. Determination of density and compressive strength after 12 years was performed according to EN 12390 (2014-06-30) after 2 days of water immersion. The size of the specimen was for wall element 1; 85 x 94 mm, wall element 2; 86 x 94 and wall element 3; 91 x 94 mm and it was about 40 mm from the surface. The results are compared to the 1 year compressive strengths of 100 mm cubes in Table 2. Note that the densities of the concrete after 12 years are lower than at 1 year. This can be because that the specimens after 12 years are taken from a cast wall element, while the 1 year specimens are cast cubes with presumably better compaction. This cannot explain the reduction in strength from 1 year to 12 years for the reference and the nitrite sample since the strength of the nitrate sample is relatively constant over the period.

1)

2)

3)

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It should be noted that the strength after 12 years is measured on only 1 sample and you can always measure lower strengths than reality due to flaws, but never higher. Hence, to verify the reduction in strength, 2-3 more cores should be drilled from the walls to get some kind of standard deviation of the strengths. The strength of the nitrite sample is still higher than the others after 12 years, which can be explained by its 3.4% higher density. Nitrite is supposed to activate the C4AF phase in the cement to a greater extent than calcium nitrate. The nitrate sample has higher strength than reference in spite of same density. Calcium nitrate has been shown to lead to higher long term strength than reference also for cast and well compacted specimens.

Table 1 - The original concrete recipes of the wall elements for a 600 litre mix (w/c = 0.5)

Mix Reference Nitrate Nitrite Ordinary Portland cement 219.643 212.984 214.314 Limestone filler 26.357 25.558 25.718 0-2 mm fine sand 126.189 126.807 126.065 0-8 mm sand 505.744 506.734 510.693 8-11 mm crushed gravel 206.191 206.191 206.191 11-16 crushed gravel 206.191 206.191 206.191 Added water (total water)

102.672 (109.821)

89.243 (106.492)

83.022 (107.157)

~ 0.100 10% MicroAir to 5±1 vol % air 0.100 0.140 0.110

Scanflux CP30 plasticizer 1.318 1.278 1.286 Na-Gluconate retarder 0 0.106 0 33 % calcium nitrite 0 0 18.184 50 % calcium nitrate 0 17.039 0

Table 2 - Density and compressive strength on specimen

Test specimen from element 1 (Nitrite) 2 (Nitrate) 3 (Ref) Year 1 12 1 12 1 12 Height, mm - 85 - 86 - 91 Density, kg/m3 2457 2420 2397 2340 2412 2330

Compressive strength, MPa

Measured - 85.0 - 72.2 - 58.2 Recalculated* 70.6 64.3 50.1

Strength of cubes 85.4±2.4 73.4** 65.5±1.0 66.9** 65.2±0.3 52.1**

* Recalculated to strength for cylinder with height/diameter ratio = 0.9 ** Recalculated to strength for cube with a conversion factor of 1.04 (1 year samples was100 mm cube strength according to article by Harald Justnes; Corrosion inhibitors for reinforced concrete).

Chloride analysis The sample was ground in layers from the surface to powder and the total chloride content was determined. The analysis was performed with a potentiometric titration method according to SINTEF internal procedure KS14-05-04 571. The chloride contents are listed in Table 3 and plotted as a function of the distance from the surface in Fig. 2.

Table 3 - Chloride content in % Cl- of dry concrete weight

Element 1 (Nitrite) 2 (Nitrate) 3 (Ref) Depth from the surface [mm]

Chloride content, % Cl- of dry concrete weight

0 – 2 1.103 1.036 0.775 2 – 6 0.980 1.175 0.902 6 – 10 0.776 0.985 0.704 10 – 15 0.780 0.755 0.522 15 – 22 0.781 0.632 0.539 22 – 29 0.618 0.523 0.424 29 – 36 0.537 0.455 0.305 36 – 46 0.416 0.347 0.291 46 – 56 0.326 0.230 0.202 56 – 71 0.225 0.171 0.132 71 – 86 0.107 0.069 0.049

Figure 2 - Chloride profiles of concrete exposed to sea water in the tidal zone for 12 years.

Parameters obtained from curve fitting of the chloride ingress profiles according to mathematical solution of Fick’s 2nd law of diffusion; the chloride surface concentration, C0 (% of concrete mass), the diffusion coefficient, D (10-12 m2/s) and the regression factor, as well as the amount of chloride ingress (g Cl-/m2 surface), are presented in Table 4.

Table 4 - Parameters obtained from curve fitting of the chloride ingress profiles

Element 1 (Nitrite) 2 (Nitrate) 3 (Ref) C0 (%) 0.989 1.122 0.832 D (10-12 m2/s) 3.53 1.94 2.12 Ingress (g Cl-/m2) 1005.3 916.2 702.2 Regression factor (r2) 0.988 0.983 0.976

The lowest apparent diffusion coefficient (D) is found for the concrete with nitrate. The reference has only slighter higher (+9%) apparent diffusion coefficient than the nitrate sample, while D is significantly higher (+82%) for the nitrite sample relative to the nitrate sample. Hence, it seems like the structure of the concrete with added nitrite is more open for ion diffusion than the other samples, in the outer centimeters, in spite of higher strength of the interior concrete.

Nitrate analysis The nitrate analysis was performed on the powder from the ground cores. The powder was dissolved in hydrochloric acid. The amount of nitrate in layers from the surface is given in Table 5 and plotted as a function of distance from the surface in Fig. 3.

Table 5 - Nitrate content in % of dry concrete weight

Element 1 (Nitrite) 2 (Nitrate) Depth from the surface [mm] Nitrate content, % NO3 of dry concrete weight 0 – 2 0.062 0.372 2 – 6 0.041 0.236 6 – 10 0.033 0.214 10 – 15 0.036 0.222 15 – 22 0.067 0.235 71 – 86 0.087 0.440

The calculated nitrate content based on the concrete recipe and calcium nitrate dosage (Table 1) amounts to 0.476 % assuming 90% cement hydration. Only the inner part of the wall with nitrate corresponds to the expected value based on calculations. The calcium nitrite added was a 0.33% solution of a commercial product from WR Grace used as corrosion inhibitor. It contained already about 10% nitrate relative to nitrite. The inner part of the concrete added nitrite contains then about double the amount of expected nitrate, which indicates that some nitrite can have oxidized to nitrate over time. Such an oxidation of nitrite to nitrate was also found by Schiessl at al. [10].

Investigation of the concrete surface with optical microscope Optical microscopy revealed that the concrete added nitrite and the reference had a carbonated surface (yellow-brownish), while the concrete added nitrate had no carbonate precipitate as seen from Fig. 4. When the concrete thin sections are photographed in ultra violet light as in Fig. 5, microcracks and voids are more visible. There is a tendency that the matrix in the concrete with nitrite has a few more cracks than the others. However, this is even more visible in the scanning electron images.

Figure 3 – The total nitrate content vs. distance from surface for concrete wall element 1 (added nitrite with 10% nitrate) and wall element 2 (added nitrate).

Figure 4 - Images of concrete surface with optical microscope indicating carbonation depth.

Figure 5 - Images of concrete surface with optical microscopy in UV light highlighting cracks and voids (yellow).

Scanning electron microscope (SEM) investigations Back-scattered electron (BSE) images of the microstructure of the three different concrete walls studied by SEM are shown in Fig. 6 and 7 near the surface and for the inner part, respectively.

The binder of both the reference and the concrete with nitrite seems much more altered than the concrete mix with nitrate as seen from Fig. 6. The concrete with nitrite had some carbonate precipitated at the surface, but more notable a network of cracks. The wall element with nitrite also had some spalling of the surface layer (concrete skin) for some reason, while the others were intact. The cracking of the binder of concrete with nitrite is more visible at higher magnification in Fig. 8.

The inner part of the concrete shown in Fig. 7 does not reveal any big difference. There are very few unreacted grains of cement, so the assumption of 90% hydration for calculating nitrate content seems justified.

The crystalline "band" shown in Fig. 8a was analyzed with SEM/EDS and determined to be Thaumasite according to the chemical composition of the band. The composition of the band was 22.7 Ca, 6.2 Si, 16.2 S and 2.1 Al in atom percent. Thaumasite has formula Ca3Si(OH)6(CO3)(SO4)·12H2O, resulting in atomic ratios Ca/Si = 3 and Ca/S = 3. However, thaumasite is isostructural with ettringite of formula Ca6Al2(SO4)3(OH)12·26H2O with atomic ratios Ca/Al = 3, Ca/S = 2 and they may therefore be intimately mixed together. In a detailed study of the reference concrete after 10 years [6], the presence of thaumasite in air voids was found. However, from the analysis there seems to be much more sulphate relative to calcium than explained by any of these two phases, and some intermixed gypsum cannot be ruled out.

Figure 6 - BSE images near the surface of the different mixes (note that the surface of the Nitrate sample is at the bottom of the image, while for the others it is on the top).

An area within an air void near the surface of the concrete with nitrite shown in Fig. 8b is analysed by wavelength dispersive analyses of X-rays (WDS) in a number of points marked in the photo with corresponding element distribution listed in Table 6 (oxygen just calculated assuming oxides). The chemical composition only revealing Ca for the bright rim around the area (point 7 and 8) indicates that this is calcium carbonate. The bright crystals marked 9 and 10 are potassium feldspar as the elements present are K, Al and Si. The points 1 and 2 in the middle of this area corresponds well with thaumasite with atomic ratios Ca/S ≈ 3 and Ca/(Si+Al) ≈ 3 accepting some coexistence of thaumasite with isostructural ettringite. The analysis point 3, 4, 5 and 6 in the periphery of the area also have a composition close to that of thaumasite, so it appears that the whole grey area more or less is thaumasite with a precipitated layer of calcite around it.

Figure 7 - BSE images from the interior of the concrete walls

During the electron microscopy session, an element mapping was made for selected areas of the first 6 mm from the surface for all 3 wall elements. The elements selected for mapping was calcium (Ca), aluminum (Al), silicon (Si), sulphur (S), sodium (Na), magnesium (Mg), iron (Fe) and chlorine (Cl), all related to a back scattered electron (BSE) image so concentration of certain elements can be related to microstructural details. The element mappings of wall elements, 1 (nitrite), 2 (nitrate) and 3 (reference) are reproduced in Figs. 9, 10 and 11, respectively. The color code for the concentration of the elements can be found to the right in all these figures.

The most striking difference is that the ingress of sulphates as measured from the sulphur mapping (reasonable to assume that all sulphur is in the form of sulphate in this case) is much higher for the element added calcium nitrite. The estimated values for wall element 1 (nitrite), 2 (nitrate) and 3 (reference) from Figs. 9, 10 and 11, are 4.0, 1.3 and 1.0 mm, respectively, for similar concentration.

Figure 8 - Concrete with added nitrite showing formation of thaumasite according to SEM/EDS.(a) showing mainly thaumasite crystals in a crack to the left. (b) showing a domain in a pore with WDS analysis in different numbered points referring to Table 6.

Table 6 – Element analyses (atom%) by WDS in different points marked in the right photo of Fig. 8

No. Na O S Fe Mg K Al Ca Si Cl 1 0.12 71.25 5.05 0.16 0.00 0.05 0.57 17.13 4.69 0.98 2 0.18 68.58 5.65 0.42 0.04 0.07 0.85 17.87 4.81 1.53 3 0.09 69.29 3.87 0.12 0.00 0.04 0.25 18.99 5.30 2.05 4 0.11 70.08 4.08 0.15 0.09 0.07 0.33 19.01 4.47 1.62 5 0.11 70.77 3.57 0.15 0.02 0.05 0.32 18.47 4.72 1.82 6 0.17 70.08 4.29 0.12 0.00 0.08 0.64 18.07 4.89 1.68 7 0.12 73.08 0.60 0.02 0.01 0.02 0.10 25.30 0.63 0.11 8 0.11 70.04 0.23 0.02 0.00 0.05 0.40 27.34 1.71 0.09 9 0.18 63.75 0.01 0.02 0.00 7.90 7.16 0.04 20.93 0.01

10 0.39 63.81 0.00 0.02 0.00 7.74 7.22 0.02 20.81 0.00

In Fig. 9, it can be seen from the sulphur mapping that the sample from the wall element with nitrite has a rather large sand grain just below the surface blocking for further increase, but that the sulphates apparently is concentrated up in the interfacial zone between this sand grain and the binder indicated by the white color for the highest concentration.

The sample is epoxy impregnated before cut and plane polished, and there is a chlorine residue in the epoxy from epichlorohydrin, that is why all voids appearing as black in the upper left image in Figs. 9-11 has a relative higher concentration in the chlorine mapping than the binder.

Otherwise, there is not much else to deduct from the element mapping at this low magnification. There is a few white "hot spots" of chlorine, but they are too small to see the crystals in the image, most likely coming from small crystals of Friedel's or Kuzel's salts.

Figure 9 – element mapping near the surface of wall element 1 with nitrite.

Fig. 10 – Element mapping near the surface of wall element 2 with nitrate added

Fig. 11 – Element mapping near the surface of wall element 3 without admixtures

CONCLUSION

From the condition survey of 12 year old concrete wall elements added either calcium nitrite or calcium nitrate as corrosion inhibitors and compared to a reference without admixtures, the following conclusions can be drawn:

The compressive strength of the concrete with nitrite was higher than concrete with nitrate, which again was higher than the reference without admixture. On the other hand, the compressive strength of both concrete with nitrite and reference concrete seemed to have a significant drop in strength from 1 year to 12 years, while the strength of concrete with nitrate was rather constant in the same period.

In spite of this, from the chloride ingress profiles, the calculated apparent diffusion coefficient was about double for the concrete with nitrite compared to the two others. The binder near the surface was substantially cracked in the concrete with nitrite, and the sulphate ingress was higher than for the two other concretes. Part of the concrete "skin" had actually spalled off for the wall element with nitrite, while it was intact for the 2 other wall elements. In some cracks and voids of the concrete element with nitrite, evidence was found for the formation of thaumasite that could be a reason for the spalling.

There was also a substantial nitrate content in the concrete with added nitrite, indicating that some nitrite could have oxidized to nitrate over time, even though the commercial calcium nitrite added had about 10% nitrate relative to nitrite.

REFERENCES

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