International Concrete Abstracts Portal

This paper deals with the effect of different curing methods on the strength and durability of high performance concrete exposed to medium temperature. Strength was measured in terms of compressive strength while durability was indicated in terms of initial surface absorption of surface layer concrete or covercrete. High performance concretes were prepared with the water-binder ratio of 0.35. Cylindrical specimens were cast for the test of compressive strength and they were cured under three types of curing conditions such as standard (2O°C) and moderately elevated curing temperatures (35°C & 5O°C). Initially the three groups of specimens were cured for 3, 7 and 14 days respectively at 20°C. Later, the curing continued at 35°C and 50°C up to 9 1 days. The aim was to determine the most efficient curing method, period and temperature to get higher compressive strength. Test results indicated that the performance of water curing as well as wrapped curing was consistently better. Specimens at the age of 7 and 14 days of initial water curing provided good results. Silica fume (SF) concrete produced the highest compressive strength at the age of 91 days under these curing conditions. This finding suggests that high performance concrete should be cured by water and the minimum curing period ought to be at least 7 days. Test results also showed that higher compressive strength develops in the temperature range of 20°C to 35°C. Besides, cubical specimens were also prepared, cured and tested at the age of 28 days to determine the initial surface absorption. Test results revealed that water-cured specimens of silica fume concrete had the lowest initial surface absorption. Hence, the performance of concrete containing silica fume was consistently better with water curing.

The paper discusses the methods used to ascertain the cause of damage to berth No. 4 of the Lae city port in Papua New Guinea and outlines the procedure followed to repair and rehabilitate it. The berth comprising of a concrete deck and supported on steel tube piles was built in 1970. By mid 1990’s, it began showing signs of extensive visible deterioration with widespread corrosion of steel piles and spalling of concrete. To assess the extent of damage, investigation was carried out in two stages with global visual survey in the initial phase and diagnostic testing at selected points in the second stage. The investigation revealed that the mechanism responsible for corrosion was chloride attack. Based on the investigation, remedial measures were recommended for the repair of the berth. The repair and rehabilitation of the berth has been successfully completed and tests done after restoration, estimate the life of the berth to have increased by 15 years.

Today, in Russia, carbide - silicon and aluminate - silicate packing masses are generally used for lining blast - furnace chutes. They contain re-fractory clay, coal-tar pitch and resins as binders which emit carcinogenic sub-stance dangerous for a human organism. Thirty compositions of chute concrete masses excluding any carcinogenic substance were studied and tested on a chute by the Siberian State University of Industry in conjunction with the Kuznetsk Metallurgical Combine company. The best results were obtained with the following composition: 75 % fused electrocorundum, 20 % refractory clay, 5 % high-alumina cement and 7.3 % water (above 100 %). Thermal resistance in heat changes was above 25 cycles at 800 ‘C, apparent density was 2.54 to 2.75 g/cm3, compressive strength was 76.6 and 79.2 MPa at 110 ‘C and 1450 ‘C, respectively, slag resistance was 0.1 to 0.2 mm at 1450 ‘C, firing shrinkage was 0.2 % with no corrosion observed. The composition developed increased the service life by 10 times compared with the composition generally applied and does not emit any carcinogenic matters. However, in view of the economic crisis and high cost of the electrocorundum, its application is limited. Therefore, we have developed compositions with a high - alumina product (HAP), the waste from the Yurga abrasive works, as a replacement for the electrocorundum. They are as follows: (i) 35 % HAP, 20 % fireclay powder, 15% refractory clay, 30 % waste from the production of silicon carbide; (ii) 48 % HAP, 20 % fireclay powder, 15 % refractory clay, 32 % waste of silicon car-bide with a particle size distribution of 3 to 0 mm. These compositions exhibit < 50 % reduction in strengths (from 80 to 40 MPa) at 1450 ‘C with other indices (slag resistance, iron resistance, apparent density and shrinkage) being the same as for compositions containing pure fused electrocorundum. Their cost is simi-lar to that of the concrete masses generally used but the service life is 4 times longer which was proved by testing in a central chute of a blast furnace.

Concrete structures in aggressive environments are prone to a reduction in their design service-life due to one or more of the following causes: reinforcement corrosion, sulfate attack, chemical attack etc. In addition to utilizing a good quality concrete, protective measures, such as use of coated bars, and/or application of surface coatings, are now commonly utilized by the construction industry worldwide. Cement-based coatings are now widely used as they are considered to be safe from the point of view of preserving the alkaline nature of concrete pore solution. This paper presents results of an investigation conducted to evaluate the performance of cement-based surface coatings in environments commonly occurring in the field. The performance of selected cement-based surface coatings was evaluated by ascertaining their adhesion to the concrete substrate, crack bridging ability, chloride diffusion, moisture resistance, water permeability, carbonation diffusion, and chemical resistance. Best adhesion was exhibited by the epoxy-modified cementitious coating. Similarly, the chloride permeability was also the lowest in this type of coating. The crack bridging ability of the polymer modified cementitious coating was better than other cement-based coatings investigated in this study. These results indicate that epoxy modified cement-based coating provides adequate protection to concrete.

The moisture condition of the substrate surface prior to repair influences the plastic shrinkage and resultant physical properties of the repair material after repair. Absorption of moisture by the substrate may cause not only plastic shrinkage of the repair material adjacent to the interface, but also expansion at the surface to be repaired due to uptake of moisture and may lead to the premature debonding of the repair material. In this investigation, substrate prisms of four different thicknesses (30, 48, 66 and 1OOmm) under three different moisture condition (oven dry, natural dry and saturated surface dry) were employed to investigate the absorption and expansion of the substrate when it is immersed in water or comes into contact with the freshly cast repair material. Based on the test results obtained, a suitable pre-wetting time before repair work is begun is proposed.