International Concrete Abstracts Portal

This study focused on the investigation of durability characteristics of very early strength concretes incorporating polypropylene fibers and application feasibility of these into the highway pavements. A series of laboratory tests were performed to assess the strength development and scaling resistance. The test of surface scaling resistance was performed according to ASTM C 672 exposed to frost in the presence of de-icing chemicals. The experimental variables included cement types, fiber contents and concrete mixtures. The compressive strength of very early strength concrete at 3 hours was measured as 22 MPa, which corresponded to the strength at 7 days or 28 days of ordinary portland cement concrete. This may enable the repaired concrete pavement to be opened to traffic 3 hours after concrete placement. There was little effect of polypropylene fiber reinforcement on compressive strength. The fiber reinforced concretes exhibited noticeably higher flexural strength than the reference concrete at all ages. The plain concrete slabs were given a visual rating of 3 or 4, and according to ASTM Standard C 672. This corresponds to moderate to severe scaling. The fibrous concrete performed better than the plain concrete slab which was given a visual rating of 0 or 1. This corresponds to no scaling or very light scaling. As the amount of fiber reinforcement increased the surface scaling resistance increased. The scaling resistance of very early strength concrete was comparable to that of ordinary concrete from the visual rating. Thus, the developed very early strength concrete may be satisfactory for use in repair works for pavements.

This paper presents results of an investigation into the tension stiffening effects of steel fiber reinforced concrete members in direct tension. Tension stiffening effects and losses of strain energy were analyzed from the load-defl .ection curves with the main experimental variables such as concrete strength, steel fiber content, and concrete cover depth. Tension stiffening effect of RC members increases linearly until first crack, decreases inversely with number of cracks, and then decrease rapidly when splitting cracks occur. The higher the content of steel fiber the higher tension the stiffening due to its bridging effect after cracking inside of member. Therefore, it is necessary to consider the tension stiffening effects with a nonlinear analysis. From the comparison between the results of experiment and existing models, it is found that the existing models could not incorporate the effects of concrete strength, cover depth and fiber reinforcement. Thus, it is required to develope a new model which could include these factors. This study proposes a new model which is defined by tension stiffening factor by considering concrete strength. and the ratio of cover depth with al and a2, respectively. The tension stiffening model for steel fiber reinforced concrete is, also, proposed as a shape of u-i-bilinear, considering concrete cover. depth. The analysis using the proposed model shows good agreement with that of experiment on direct tension and flexural members at all concrete strength levels.

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.