International Concrete Abstracts Portal

Leaching degration of cement composites is a phenomenon involving the removal of various solid constituents and may result in loss of strength. The leaching of calcium is not a major problem for most concrete structures, as the degradation rate is very slow. However, it is important for structures such as marine structures, dams, and radioactive-waste repositories that are in an environment that direct contact with water for a long term. In order to evaluate the leaching behavior, investigation of the fundamental degradation mechanism must be undertaken. Described in this paper is the degradation mechanism of cement composite derived from accelerated testing. Investigations were conducted using three different accelerated test methods, I.e. the immersion test method, the dissolution test method and the permeation cell test method. The main results of the study were as follows: 1) 30-50% of calcium in cement composites was leached out in several moths in the accelerated tests (the immersing test method and the permeation test method); 2) in all tests, calcium leaching from Ca(OH)2 and C-S-H was observed, and the change in the porosity and physical properties corresponding to the leaching degree was found; and 3) a possibility to predict the long-term degradation behavior of structures by using numerical analysis was demonstrated.

The state-of-the-art on thaumasite formation is discussed, stressing aspects of its formation in mortars an in portland-cement concretes, and also the effect its formation has on their durability. Methods for material synthesis, mentioned in the bibliography including a detailed report of the characterization of thaumasite through XRD, IR, DTA/TG, NMR, electron microscopy and microanalysis, are described.

Durability of reinforced concrete structures (RCS) seems to be poor when compared with those of ancient un-reinforced structures. When ordinary durability (service life of 40-50 years) is needed, the poor behavior of RCS stems from human negligence in adopting the well consolidated and available experiential knowledge. However, for long-term durability requirements (service life of 100 years and more) the inherent vulnerability of the steel-concrete system must be taken into account. The inherent vulnerability of RCS substantially depend on the following "weak points" of concrete: (I) Low tensile strength (ii) High modulus of elasticity (iii) Microcracking caused by restrained thermal and drying shrinkage or service loading. This paper critically examines some possible future scenarios to achieve long-term-durability in RCS, including: a) Improvement in the corrosion behavior of the metallic reinforcement through the use of corrosion inhibitors, protection of the reinforcement with a coating, chance in the composition of reinforcing bars, or cathodic protection. b) Use of non metallic reinforcement. c) Increase in the tensile strength and/or ductility of concrete mixtures based on rubber-like polymer additions. d) Surface coatings for concrete protection.

Since 1990 the Anode-Ladder-System has been used world-wide to monitor the corrosion risk of new concrete structures. This sensor-system is an embedded macrocell system indicating the depth of the critical chloride content initiating corrosion of the reinforcement. Subsequently the time-to-corrosion of the reinforcement can be determined continuously, enabling the owners of buildings to initiate preventive protection measures before damage like cracks and spalling occur. However, this system can only be installed before placement of the concrete. Therefore a new sensor system has been developed to monitor the corrosion risk for the reinforcement also in existing or repaired structures. This new Expansion-Ring-System consists of 6 anode rings separated by sealing rings and a cathode bar, which are installed in small holes drilled into the concrete and connected by a special expansion mechanism. Actually it is in the stage of final testing in laboratories and pilot projects.

The development of a curing time estimator is described: it is based upon an existing model of the microstructure and porosity gradients in the cover concrete that correlates well with relevant hydration and pore structure measurements. The same model also yields capillary porosities that correlate with measurements of compressive strength and water absorption rate. The objective is to provide a single method to estimate curing times for CEM I and CEM II (portland and portland/fly ash) concretes in a wide range of climatic conditions and achieve a consistent, well-defined measure of cover concrete performance. New hydration and pore structure measurements are briefly reviewed in relation to the existing model of microstructure and porosity in cover concrete. Recent developments regarding European standards for curing and concrete durability are considered. A criterion of capillary porosity in the matrix of cover concrete is used to unify the durability-related effects of curing period, cement type, water/binder and climatic conditions. The initial input to the estimator is the cement type to be employed. The nest input is a maximum water/binder, as necessary to ensure durability under the expected exposure conditions; this automatically sets a target capillary porosity in the cover concrete, based upon recent curing period recommendations from European standards committees. Subsequent inputs define the climatic conditions in terms of exposure capillary porosities in the cover concrete for a wide range of curing periods so that a period can be chosen without exceeding the target porosity. Capillary porosities for reduced water/binders, 95and 90% of the input value, are also tabulated to illustrate the reductions in curing period that are possible with these higher concrete qualities. Examples are given to illustrate the effect of each of the eight inputs; water/binder, exposure relative humidity and cement type are the most influential. It is evident that in many cases control of cover concrete performance via curing options is limited relative to the control offered via small changes in the concrete mix proportions of alternative cements.