International Concrete Abstracts Portal

This paper describes the development and applications of several lithium-based chemical admixtures for set control of cementitious-based construction and building materials. Comparative evaluations show the effect of these admixtures on the set time of (1) calcium aluminate cement; (2) portland cement; and (3) blended systems of calcium aluminate cement and portland cement. General information is provided to address the principles of material selection, dosage rate and application of cementitious systems.

The paper examines the rheological and mechanical behavior of super-plasticized microcement grouts to be used for the consolidation of porous systems (e.g. sand deposits, masonry walls of historical buildings). Five microcements were used: they differed in chemical composition of the clinker, type of mineral addition (natural pozzolan or silica fume), and particle size distribution. Three different super-plasticizing chemical admixtures - acrylic, naphtalene, and melamine based - were used, all with an active polymer content of 0.4% by mass of cement. The following properties of the grouts were investigated compressive strength, stability (bleeding test). fluidity (Marsh cone), The acrylic polymer proved to be more effective than the naphtalene and melamine based products in reducing the mixing water to produce a given initial fluidity. Furthermore, the grouts produced with this super-plasticizer displayed a lower fluidity loss with time. The performance of a specific mixture was also strongly dependent on the fineness of the binder and on the type of mineral addition. The compressive strength of the hardened grouts was primarily affected -especially at latter ages - by the water-binder ratio, independently of the type of super-plasticizer. None of the mixtures considered showed any significant bleeding. A limited number of injection tests in columns of dense sand were also performed. For a given water binder ratio, the groutability of a mixture was greatly improved using the acrylic based super-plasticizer in place of the melamine- and naphtalene-based products.

New world record of altitude transportation of concrete (532 m) reached on the 2nd June 1994 in Riva de1 Garda Hydroelectric Power Plant. The requirement involved the construction of a new piezometric well and intake conduit and the replacing of the existing surface penstock with a new one built underground to reduce environmental impact. Two parallel tunnels were excavated: one for Ledro - Garda lakes location, the other for transporting penstock’s metal elements. The self supporting metal pipe (0 2, 30 mm, thickness 23 mm) has been blocked for all its length (790 m) by filling of the ring space between the excavation wall and the piping with 5.000 m2 of concrete. In order to reduce environmental impact it was decided to pump concrete from the l owe r end from an excavated chamber nearby the Powerhouse. The record level: 532 m (vertical) and 790 m (total length) widely exceeded the former one (432 m) obtained in 1985 in Spanish ESTANGENTO SALLENTE Power Plant.

Concrete tiles were produced according to a new technology based on a vacuum treatment combined with a vibro-compacting placement of superplasticized mixture. Three concrete mixtures (all vibro-compacted according to this technique) were manufactured with different water-cement ratio (0.50 or 0.32) and in the absence or in the presence of the vacuum treatment. Measurements of flexural strength, dimensional stability, SEM, image analysis and particle size distribution of anhydrous cement in concrete tiles were carried out. The vacuum treatment, which removed entrapped air voids from the fresh mixture during the vibro-compacting placement, increased flexural strength but did not change the dimensional stability. The reduction in the W/C from 0.50 to 0.32 significantly increased flexural strength. It also improved the dimensional stability in terms of a lower curling effect caused by different humidity exposures of the two opposite faces of the .concrete tile. The combination of vacuum treatment with low W/C produces a macro-defect free and low micro-porosity cement matrix which is responsible for the high strength and low curling effect of these concrete tiles.

Concrete is one of the most widely used construction materials in the world. In applications where appearance, durability, and permeability are of concern, reduction of cracking is a key need. A common cause of cracking is restrained drying shrinkage. Until very recently, drying shrinkage cracking has been controlled by reducing joint spacing, increasing reinforcement, and using expansive cements and admixtures. In this paper a new shrinkage-reducing admixture (SRA) is discussed. SRAs provide a convenient means of reducing drying shrinkage as they are added during batching and mix easily into the concrete. The SRA material discussed in this paper improved workability and finishing as an added benefit. The effects of mixture proportioning, curing conditions, and SRA content on unrestrained and restrained drying shrinkage were determined. The results show that long-term drying shrinkage reductions of 50% can be achieved, and that there is a significant improvement in restrained shrinkage performance. Even though the lowest absolute values for drying shrinkage occur with proper curing, there is still a substantial reduction in drying shrinkage for specimens cured for short times. It is shown that shrinkage reduction is directly related to the SRA addition rate as a percentage of the mixing water. Furthermore, data on large-scale field experiments show that substantial reduction in cracking is obtained for concretes treated with SRA.