International Concrete Abstracts Portal

paper summarises results of studies carried out at the Building Research Establishment on the performance and longer term durability at 5 years of two concretes, which contained different normal portland cements, designed to give equal 28-day compressive strength by adjusting the cement contents and water/cement ratios. The two cements were chosen to provide examples of normal Portland cements with the widest difference in tricalcium silicate (C3S) content. It was necessary to obtain the low C3S cement from Israel to satisfy this requirement. The cements originating from the UK and Israel, had C3S contents of 67 and 33.5%, and tricalcium aluminate (C3A) contents of 8.4 and 12.3%, respectively. The concretes assessed were of similar mixture proportions, although an extra 25 kg of the low C3S cement and a lower water/cement ratio were required compared with the high C3S cement, to achieve equal 28-day strength concretes of 40 (-+ 2) MN/m2. Several types of concrete specimens were prepared using two curing regimes (wet and dry-curing), before carrying out a range of long-term tests. These included: compressive strength, seawater attack rating, carbonation, oxygen permeability, chloride ingress, and corrosion of rebar. This study showed that by designing concretes to give equal 28-day strengths, but using high-early strength cements, concrete performance should be quite satisfactory in most indoor and outdoor environments. However, concrete durability may be compromised, even with good curing, if the cement content is not sufficient or the w/c is too high for certain severe exposure conditions, such as in the marine tidal zone. In such cases the reduced cement content and higher w/c could result in discernible loss in long term strength development and reduced durability. These data are of direct relevance to the UK concrete industry practice and support the approach adopted in the current British Standards and Codes of Practice of specifying concrete in terms of minimum cement content and maximum W/C, as well as by minimum strength grade, rather than by 28-day strength attainment.

Results of an investigation of the deterioration of Portland cement products due to deleterious ettringite formation are described. The effects of thermal-drying on internal-sulfate attack in different concrete products is discussed. Comment on the relative significance of deterioration mechanisms is given. Delayed ettringite formation usually occurs in high-temperature cured products. It is usually associated with products exposed to conditions favorable to cracking (e.g. thermal-drying/re-wetting cycles).

In order to establish durable design of concrete structures, to estimate the service life of the bridges and to obtain data indicating whether the prestressed concrete bridges need repair, the durability of 267 prestressed concrete bridges in service for ten to thirty years in north- east district of Japan was investigated. Furthermore, results of investigation in north-east district were compared with those of Kyushu district. Characteristics of the deterioration of respective parts of prestressed concrete bridge are described from the viewpoints of materials and structures. The results establish the influence of moisture on efflorescence and cracking of superstructures and the influence of frost damage as in snow and cold regions. Furthermore, effective countermeasures for improving durability of concrete structures are discussed and proposed.

At the early stages of development, RCD concrete often tended to be inferior in strength and durability to any other conventional dam concrete because RCD concrete had the properties of stiff consistency and lean mixture. In this research, a laboratory test was conducted to determine its optimum sand percentage, sand ratio and optimum mixture proportions in order to improve durability of RCD concrete. In addition, a field test was conducted to determine the optimum lift height and optimum compacting method. Furthermore, a new construction method was developed to improve durability of RCD concrete that any existing RCD concrete can be strengthened by casting one of the conventional types of dam concrete on the upstream or downstream slope of the dam. Hence the proposed method in combination with the recommended mixture proportions and construction method has enabled one to rebuild many of the dams and to keep then in serviceable condition for a long time.

The degradation of samples of cement pastes, mortars and concretes in acidic solutions (pH 2-4) ha s b een investigated over relatively short periods, e.g. -72 hours at 25°C. The degradation rate of the cementitious material is taken as the rate of alkali leaching at constant pH; the latter is obtained directly from the amount of acid required, as function of time, to maintain the pH at a preset value. The data allow the calculation of apparent rate constants, the magnitude of which is expected to be related to the durability of the materials investigated in acidic environments. Paste degradation rates were obtained at two W/C (0.35 and 0.45); the role of several additives (silica fume, superplasticizers, air entraining agents) and the influence of solution composition (pH, ionic strength, other electrolytes) were investigated. Kinetic data were also obtained for the acid reaction rate of several mortar and concrete samples of varying mixture compositions and in the presence of S0,-2. Under the conditions of this study, the acid degradation rate appears largely controlled by H+ diffusion through the leached zone of the matrix. The diffusional rates values are moderately influenced by changes in mixture compositions, in reasonable agreement with expectations from greatly enhance the acid degradation rate, an effect zone and the related studies. Sulfate ions which can be understood from pH gradients in the leached activity coefficients of the H+ in the presence of sulfates.