International Concrete Abstracts Portal

The authors have proposed a partially pre-cooling system for massive structures, such as gravity concrete dams. It is discussed in this paper how the effectiveness of the proposed method is discussed using the finite element analysis. In the ordinary pre-cooling system, pre-cooled concrete is placed in the entire region (width and depth) of a massive structure. In the proposed system , pre-cooled concrete is placed only in the surface layer. In order to evaluate the effectiveness of this system, a thermal stress analysis was conducted by the finite element method. The key parameters were the dimensions of the cooling system and cooling temperatures. The results show that the proposed system is rather effective than the conventional cooling system in terms of the thermal stress condition of massive concrete structures. In addition, the cost benefit is adequately expected.

In the past decades, cement-based materials have been increasingly used for the construction of radioactive-waste barriers. The design of durable structures for this specific application requires a precise knowledge of the evolution of the material engineering properties over a 1000-year period. Among the wide range of degradation phenomena to which the structure can be exposed upon its service life, the leaching of calcium appears to be the most critical. Given the intricate nature of the leaching process, a reliable prediction of the long term behavior of the concrete barriers can only be made through numerical modeling. In order to generate more quantitative information on the subject, a new accelerated leaching test has been developed. The operating principle of the test is quite simple. The leaching process is accelerated by applying an external electric field through the material. This paper details the different electrochemical phenomena involved during a LIFT experiment. It also presents several experimental data obtained for various neat paste samples. In this series of tests, the evolution of the hydrated cement paste microstructure was studied using various experimental techniques such as X-ray diffraction, DTA/TG analyses and scanning electronic microscopy. The advantages and limitations of this new procedure are discussed.

High fluidity concrete has been used to meet requirements for the marine construction thanks to its superior durability and ease of placing. High-fluidity concrete is, however, so viscous and has less bleeding to have the cold joint that may harm the uniformity of the structure. We have executed series of experiments to study the effect of interval and method of making joints on the strength of placing-joint of 5 types of high-strength and high-fluidity concrete and high-fluidity lightweight concrete for the marine construction . The strength of the placing-joint has shown no substantial degradation compared to those without placing-joint by rodding the joint within 120 minutes after the first placing under an ambient temperature of 20 C, while specimens without rodding, cured under the standard water bath, have shown 2/3 of the strength of those without placing-joint at an interval of placement less than 60 minutes.

In this paper, the authors present results of an experimental program carried out to study the pull-out behavior of a new type of high yield strength steel deformed reinforcing bars (fY = 830 MPa) embedded in high strength concrete (f, = 100 MPa). Influence of embedded length, concrete strength and yield steel strength, on bond strength has been investigated by means of pull-out tests. The final aim of this research program is to define design and specification requirements for this new material. Variations of the average ultimate bond strength with embedded length, for concretes with compressive strength from 40 MPa to 100 MI%, produce horizontal curves, which indicates that the mean ultimate bond stress is an appropriate bond strength indicator. Ultimate bond strength increases proportionally with both the increase of tensile strength and compressive strength of concrete. No significant effect of the yield steel strength of reinforcement on the value of the mean ultimate bond stress has been observed. In the case of a pull-out failure, the relationships between the ultimate mean bond stress and the main characteristics of the bond between steel and concrete (embedded length, mechanical strengths of concrete and yield steel strength) are similar to the ones of standard deformed reinforcement (fY = 500 MPa).

The development and evaluation of chemical admixtures requires experimental verification of many admixture samples in the laboratory prior to testing the most promising formulations in concrete. Since quantities of new admixture formulations are usually limited, it is necessary to carry out small-scale tests on fresh cement pastes and mortars. The large number of test methods used by individual researchers makes it difficult to compare results from different laboratories. In an attempt to solve this comparability problem, an advanced methodology for testing of water-reducing admixtures has been developed and is discussed in this paper. This methodology is based on studying the properties of fresh cement-water systems, namely pastes and mortars, with or without the addition of a chemical admixture. It includes and modifies some traditional tests such as the Vicat test for cement pastes and the flow test for cement mortars plus some more recently developed tests such as the mini-slump test. It also includes some new test methods such as the torque test and the mini-flow test. The applicability of the new set of test methods was examined during the evaluation of lignosulphonates as water-reducing admixtures for concrete. The determination of rheological properties, air-entrainment and set-retardation was conducted on cement pastes and mortars. The values describing the plasticity/fluidity, air content, and time of setting were obtained by different methods and under different conditions. The results were then compared and their interpretation is discussed. The methodology presented here is suitable for preliminary evaluation of concrete admixtures. Incorporation. of these test methods into national and international standards is suggested.