International Concrete Abstracts Portal

The background to the development of two types of thin, fabric-reinforced, portland cement concrete sheets is described and range of properties given. Both normal weight and lightweight mortars (including cellular mortars) were used as a matrix. Glass or synthetic fiber continuous reinforcement in the form of fabric scrims and/or nonwoven three-dimensional fabric were used. The materials developed are potential substitutes for plywood, cement asbestos, and other types of sheet material that require the properties of weather resistance, incombustibility, nonbiodegradability, and economy. The test results also suggest that the matrix and reinforcement concepts developed will lead to applications in other reinforced concrete uses. The thin sheet materials lend themselves to easy manufacture in a comparatively simple plant.

A cementitious matrix capable of dispersing fibers using conventional mixing techniques was developed. The effects of reinforcing this matrix with different volume fractions (0 to 2 percent) of aramid fibers ranging in length from 1/8 to 1/2 in. (3 to 12.7 mm) on the composite material performance in the fresh and hardened states were assessed experimentally. The effects of matrix mix proportions on the fibrous material properties were also investigated. The test data generated in this study indicated that improvements in strength and toughness characteristics of cementitious materials can be achieved through aramid fiber reinforcement, with no need to use specialized manufacturing techniques.

Fibers are added to concrete to improve several of its properties. The ability of polypropylene fibers to modify different characteristics of concrete is controversial. This paper presents results on the influence of adding polypropylene fibers (0.1 to 0.2 percent by volume) on mortar permeability and plastic shrinkage. The influence of adding polypropylene fibers on the early stages of shrinkage is studied with 120 x 15 x 3 cm specimens. These were fabricated in mortar and then held in a chamber with controlled temperature and ventilation. The specimens have a special geometry to enable the shrinkage measurement in the plastic state, and the influence of this on mortar cracking. The variables studied were: water-cement ratio, sand-cement ratio, and fiber content. In addition, the ability of fiber concrete to absorb water and its permeability to CO2 were tested. Water absorption was measured in accordance with French standard NFB 10.502. Carbonation was studied by introducing fiber mortar specimens in a chamber saturated with CO2 and comparing the results with natural carbonation. Results show that the addition of fiber reduces plastic shrinkage when compared with the same type of mortar without fibers. Concerning water absorption, it is reduced when water-cement ratio is about 0.5; however, when the water-cement ratio is higher than 0.5, this behavior is reversed and the fiber mortar is more water absorbent. Accelerated and natural carbonation show that CO2 diffusion increases in mortar with the highest amount of fibers.

Traditionally, the first cracking strain of plain matrix is used as the material property in the fiber reinforced cement-based composites. It is used to indicate the tensile strength, and thus termination of the contribution of the matrix phase. In the presence of high volume fraction of fibers, formation of the first crack does not necessarily lead to the fracture instability; thus, matrix is able to carry increasing loads. The strength of the matrix is thus dependent on the type, volume fraction, bond, and strength of the fibers. Paper investigates the tensile stress-strain response of cement paste in the presence of glass fibers. A test procedure is described that can characterize the toughening effect of various fiber types on the matrix properties.

The production of a polypropylene-reinforced cement material marketed as an alternative to asbestos-cement is outlined. Typical tensile stress-strain curves of a number of alternative materials are compared with asbestos-cement. The load-deflection characteristics of corrugated sheets made from nonasbestos materials are also presented and discussed. The nonasbestos materials are generally much less brittle than asbestos-cement, although they have a lower first-cracking strength. The pseudo-ductile behavior exhibited, with multiple cracking before the ultimate load is reached, means that permissible loads in service must not be based solely on ultimate loads but on cracking and possible deflection criteria. Less well-defined stresses arising during installation and from restrained moisture movements, which may crack the nonasbestos materials, are likely to be critical for the effective performance of new sheeting materials.