The Effect of Fly Ash on the Corrosion Behaviour of Galvanised Steel Rebarsin Concrete

The effect of fly ash on the corrosion behaviour of galvanised steel rebars in cracked concrete specimens exposed to wet-dry cycles in a chloride solution has been investigated. The obtained results show that the use of fly ash, replacing either cement or aggregate, always improves the corrosion behaviour of galvanised steel reinforcements. In particular, the addition of fly ash, even in the presence of concrete cracks, decreases the corrosion rate monitored in very porous concretes, as those with w/c = 0.80, to values comparable with those obtained in good quality concretes, as those with w/c = 0.45. Therefore, fly ash cancels the negative effect, at least from the corrosion point of view, of a great porosity of the cement matrix.

replacing either fine natural aggregate or cement; in the first case it leads to saving natural resources which are rapidly depleting; in the second case, the reduction of carbon dioxide emission, strictly related to Portland cement production, is obtained.
In this work, the effect of introducing fly ash in the concrete mixture at a dosage of 30% by cement weight, replacing either cement or fine aggregate,on the corrosion behaviour of galvanised steel reinforcements in cracked structures has been investigated. In particular, this work wants to verify if the simultaneous use of galvanised steel reinforcements and fly ash in concrete could assure not only the concurrence of the environmental and technical benefits derived from the use of both steel galvanisation and fly ash concrete, but it could further provide a useful synergic effect, as it could be really foreseen.

Specimens
Prismatic concrete specimens (28 cm 7 cm 7 cm) were manufactured with a nominal water/cement (w/c) equal to 0.45 or 0.80, with and without fly ash. This mineral admixture was added at a dosage of 30% by cement weight, replacing either cement or fine aggregate, thus changing the water to cementitious material (w/cm) ratio. The different concrete mixture proportions are reported in Table 1, where the compressive strength achieved after 1 month of air curing is also indicated. Each prismatic specimen was reinforced with a galvanised steel plate (7 cm 4 cm 0.1 cm) embedded with concrete cover of 4 cm. The zinc coating was 100 µm thick, obtained by molten zinc immersion, with an outer pure zinc layer about 20 µm thick. The galvanised reinforcements, just before their embedment in the fresh concrete, were submerged for 5 s in 15% NaOH solution to dissolve the ZnCO 3 layer eventually formed during atmospheric storage. The electric contacts among the reinforcing plates and the measuring equipment were arranged according to a previously reported methodology [12]. After 1 month of air curing the specimens were cracked by flexural stress so that a crack width of 1 mm was produced in a pre-formed notch area with the apex of the crack reaching the reinforcement. Then the specimens were exposed to weekly wet-dry cycles (2 days dry followed by 5 days wet) in a 10% NaCl solution.

Evaluation of the corrosion behaviour
The corrosion probability of the reinforced concrete specimens exposed to the aggressive environment was evaluated by free corrosion potential measurements with a saturated calomel electrode (SCE) as reference. The kinetic of the corrosion process was followed by polarisation measurements. The polarisation resistance was measured through the galvano-dynamic method, using an external graphite bar as counter-electrode, by calculating its average value between the anodic and cathodic ones. The corrosion rate was calculated through the Stearn and Geary law adopting the value of 26 mV/decade as B constant. The electrochemical values reported in the graphs are the averaged data obtained by measuring three specimens of each type during the immersion period.
In order to validate the electrochemical measurements, after 12 wet-dry cycles in the chloride solution, the concrete specimens were examined to evaluate the corrosion development by visual observation. After splitting the concrete specimens, the galvanised plates were removed and metallographic analyses were carried out on the cross section to evaluate the decrease in coating thickness due to the corrosive attack. The free chloride concentration on the galvanised steel plate was also measured at the end of the test by water extraction. Fig. 1 shows the free corrosion potential of the galvanised steel plates embedded in concrete specimens with w/c = 0.80 as a function of the number of wet-dry cycles. Just after the exposure to the chloride environment, all the galvanised steel plates are assumed to have activation values of about -1100 mV/SCE regardless of the type of the cement matrix, reflecting a general great corrosion risk. However, only the reference concrete without the pozzolanic additions remained at these activation values for all the test time, while, in the presence of fly ash, the free corrosion potentials moved towards more passive values after a few wet-dry cycles. At the same time, the corrosion rate ( Fig. 2) assumes initial values of about 30 µm/year, but while in the reference specimens it further increases with the test time, in the presence of fly ash the corrosion rate is kept constant at values significantly lower with respect to those detected in the reference matrix without the mineral admixture.

DISCUSSION OF TEST RESULTS
The better corrosion behaviour monitored in the presence of fly ash, regardless of the substitution type, cannot be attributed to the lower chloride concentration at the galvanised steel surface due to densification and chloride adsorption of the cement matrix with the pozzolanic additions. In fact, in this case, cracks represent preferential paths for chloride penetration through the concrete cover and, at the end of the test, the chloride concentration on the surface of the reinforcements is almost the same regardless of the presence of fly ash ( Table 2) and, however, greatly overcoming the concentration threshold (1.2% by cement weight) generally reported as the critical value able to induce the corrosion process in galvanised reinforcements [6].Therefore, in the presence of concrete cracks, the better corrosion behaviour observed in the presence of the mineral admixture could be attributed foremost to the lower alkalinity of the cement matrix due to the pozzolanic reaction, that seems to effectively improve the corrosion resistance of the galvanised reinforcement, as it was already suggested in the literature, even in the presence of concrete cracks [11]- [13]. On the other hand, a good quality concrete matrix with a w/c ratio as low as 0.45 seems to mask the beneficial effect of the pozzolanic additions. The corrosion risk described by the free corrosion potential measurements (Fig. 3) is the same regardless of the cement matrix while the corrosion rates (Fig. 4)    The post exposure evaluation confirmed the electrochemical measurements. As a matter of fact, in the reference high quality concrete (w/c = 0.45 without FA), the corrosive attack really appeared much localized at the crack apex and also very deep, even showing well recognizable iron corrosion products (Fig. 5a). Moreover, far from the crack apex, Fe-Zn alloy appeared on the plate surface meaning that total consumption of the pure zinc layer due to the corrosive attack occurred, as later confirmed by metallographic analysis (Fig. 5b).
On the other hand, the galvanised steel plates extracted from high volume fly ash concrete showed a surface coating made of white zinc corrosion products (Fig. 6a), later identified as calcium hydroxyzincate by X-ray diffraction. Calcium hydroxyzincate is a well passivating zinc corrosion product whose formation seems to be effectively favoured by low alkalinity of the concrete pore solution [13] as achieved when high volume fly ash is added because of its pozzolanic activity. Once formed, calcium hydroxyzincate protects the underlying pure zinc layer from further corrosion as metallographic analysis well put into evidence (Fig. 6b).

CONCLUSIONS
The use of fly ash, as partial replacement of either cement or aggregate, always improves the corrosion resistance of galvanised steel reinforcements in cracked concrete specimens exposed to wet-dry cycles in a chloride solution. In particular, the pozzolanic addition of fly ash, even in the presence of concrete cracks, decreases the corrosion rate monitored in very porous concretes, as those manufactured with w/c = 0.80, to values comparable with those obtained in good quality concretes, as those manufactured with w/c = 0.45. In other words, fly ash cancels the detrimental effect, at least from the corrosion point of view, of a great porosity of the cement matrix.
As a matter of fact, the use of galvanised steel reinforcement in fly ash concrete, not only assures the concurrence of the technical and environmental benefits derived from the use of both steel galvanisation and fly ash addition to concrete, but it can further provide a useful synergic effect because the pozzolanic activity favours the formation of a dense protective passive layer on galvanised reinforcement, which remains stable even in the presence of concrete cracks. The actual research is about the study and the development of innovative and environmentally friendly materials for building applications prepared also by re-cycling industrial by-products. Furthermore, her research work is focused on the field of geo-polymeric materials for rehabilitation and restoration of ancient and modern buildings.