International Concrete Abstracts Portal

After damage due to reinforcement corrosion in carbonated concrete, repair principle W - limiting the water content of concrete - as specified in the Draft Recommandation for Repair Strategies for Concrete Structures Damaged by Reinforcement Corrosion (l), can be applied in the repair of concrete compo-nents. The repair principle involves sealing the concrete surface with a suitable surface protection system after exchanging and reprofiling the damaged concrete areas. As a result the water content in the concrete shall be reduced to such an extent as to bring the corrosion process of the reinforcement to a standstill. The effectiveness of surface coatings as corrosion protection systems has been investigated on carbonated concrete specimens using the simultaneous measurement of corrosion rates and electrolytic concrete resistivities.

Throughout the world, sizable portions of national budgets must be appropriated for the repair and rehabilitation of concrete structures which have suffered damage due to corrosion of reinforcing steel. This paper presents a critical review of the current state-of-the-art on the mechanisms responsible for deterioration of concrete and reinforcing steel. The protective effect of the passive film and the depassivation of reinforcing steel by carbonation and chloride ingress are discussed. In addition to the passive film, some researchers believe that a mineral scale may contribute to the protection of embedded steel against corrosion. The following ambiguities in the current understanding of the deterioration of concrete due to corrosion of the reinforcing steel are discussed: mechanisms of passivation of steel in the concrete environment; stoichiometry of the passive film; applicability of Fick’s second law for the prediction of chloride diffusion in concrete; mechanisms of depassivation of steel by chloride ions; threshold Cl-/OH ratio; the composition and mechanism of protection provided by mineral scales; mechanisms of expansion of the steel corrosion product in concrete; and the detrimental effects of aggressive ions on concrete properties.

Maintenance costs for concrete structures that are exposed to de-icing salts and marine environments represent large expenditures for owners as a results of corrosion of steel in concrete. To solve this problem, the US Federal Highway Administration sponsored a comprehensive program todevelop a new alloy to be used as a sacrificial anode for reinforced and prestressed concrete structures. Based on intensive studies of existing metals and alloys, a new alloy was developed to overcome the observed deficiencies in the existing materials. The new aluminum-zinc alloy can be thermally sprayed on concrete surfaces to protect steel from corrosion in chloride contaminated concrete. This paper presents the results of both laboratory testing and field installation of this new alloy.

Laboratory and field studies were carried out to quantify the effect of AAR damage on other deterioration mechanisms in concrete. Laboratory concretes were cast with various mixture proportions selected to provide a range of AAR expansions after 3 months curing at 60°C. These specimens were then tested in compression and evaluated by an electrical test to predict chloride . permeability (ASTM C 1202), and oxygen permeability. and freezing and thawing tests. Expansions of 0.06% led to significant reductions in strength and stiffness, but no change in permeabilitv. Indeed, the perrneability characteristics were onlv adverselv affected in concretes that expanded bv 0.1 o%. The abilitv of concrete to tolerate low expansion (0.04 to 0.06%) without increases in permeability is probably a function of the discontinuous crack distribution. However, such cracks still result in a softening of the mechanical response. Cores extracted from ten hvdraulic structures exhibiting a wide range of distress due to AAR, were also tested in the laboratory. Quantitative petrographic damage ratings revealed damage ratings in the range 30 to 680. Concrete with minor levels of AAR gave damage ratings slightlv less than 100 and performed similarlyto undamaged concrete. Significant increases in permeability and electrical charge passed were observed when the damage rating exceeded 200. Concrete with damage ratings above 300 were characterized by extensive internal fracturing with crack widths frequently greater than 0.5 mm. These concretes had Coulomb ratings > 10,000 and og en perrneability coefficients > lO-15 m2 Cores -taken from fly ash concrete structures showed excellent performance (< 1, 0 00C and < IO-” m2) despite relatively high water-cementitious material ratios (:> 0.55).

The paper presents the results of a study on the durability of alkaline portland cement concrete made with potentially alkali reactive aggregate. An alkaline portland cement, into which alkaline component was introduced by 3 methods, namely: - in a form of easily dissolvable active compounds of alkali metals creating an alkaline reaction in dissolving; -in a form of insoluble alkali-containing compounds that are themselves feldspar rocks containing 10-20 % by mass of alkali metal oxides (feldspar, nepheline, mariupolite), and - combined method (feldspar rock was ground with 25-50 % by mass clinker and a resultant blend was mixed with a solution of active alkaline compound), and andesite (alkali-reactive aggregate) and granite (reference) were used. The assessment of durability of the concrete was done by measuring variations in strength, volume, microhardness in a contact zone cement paste- aggregate and phase transformations. It was established, that corrosion of alkali-reactive aggregate while hardening of concrete is associated with structure forming processes in a contact zone that are directed at formation of alkaline and alkaline-alkali-earth aluminosilicate hydrate compounds. The formation of the latter initiates the binding of the alkaleis that were introduced into the cement and released during destruction of feldspar rocks and corrosion of aggregate. These processes take place at the highest rate when an alkaline component is introduced by the combined method. In this case, an active alkaline component introduced together with water initiates the destruction processes, and feldspar rocks are the crystallization centers of hydrate formations. These processes result in the formation of a dense shell around aggregate grains, this shell having width of 50 um and microhardness of up to 5200 MPa. A value of volumetric changes, when andesite aggregate is used, remains at a level of that of granite aggregate, and concrete strength increases with time testifying to its long-term durability.