International Concrete Abstracts Portal

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.

Nine bridges under the control of the Roads and Traffic Authority (RTA), in New South Wales (NSW), were inspected in two different regions of the State, and seven with various degrees of cracking and/or repair were selected for examination, particularly with respect to alkali-aggregate reaction (AAR). Twenty-eight cores were drilled from four bridges in the north-east coastal area and 14 cores from three bridges in the same area but 250 km inland. Detailed petrographic examination and scanning electron microscopy of the cores has shown that of the three inland bridges, definite signs of AAR are present in two bridges but the third bridge has a less clear diagnosis, although the aggregate is the same. For the bridges in the coastal area the glassy basalt may have reacted, although typical visual signs of AAR products were not evident. Clay alteration products in the basalt used for two of the bridges may have caused some dimensional instability. For one of the bridges, signs of alkali-carbonate reaction (ACR) were seen in some cores, but the contribution of this reaction to cracking of the bridge is not clear. This is the first report of ACR in Australia. The expansive behavior of cores and alkali levels are also discussed. subjected to elevated temperature, humidity,

Cores from five concrete pavement sites in eastern South Dakota, U.S.A., have been examined for microstructural and microchemical evidence of alkali-carbonate reactivity (ACR) using optical microscopy, scanning electron microscopy, and energy-dispersive X-ray microanalysis (EDX). Dolomitic aggregate used at each of the five sites shows evidence of reaction with the cement paste resulting in dedolomitization products and textures. In altered aggregate, dolomite (CaMg(CO3)2) is replaced by an irregular, fine-grained (l-10 um) intergrowth of crystals that have either Ca or Mg as the only peaks in their EDX spectra. These reaction products are interpreted to be calcite (CaC03) and brucite (Mg(OH)2), respectively, as would be expected from dedolomitization. Dedolomitization is most prevalent in fine aggregate, and is generally concentrated just inside the aggregate rim or along cracks penetrating into the particle. An unreacted rim of dolomite, 5-10 um thick, is present, however, even in the most altered particles. Unreacted dolomite and dedolomitized zones have nearly indistinguishable Ca/Mg, indicating that Ca and Mg are conserved in the reaction, and Si02+A1203 is typically < 1 % by mass, so it is unlikely that clay mineral impurities play a role in the deterioration of these aggregates.