Compressive concrete

Variation of strain over the compression region up to failure for the beams was found to be almost perfectly linear. Failure occurred when the ultimate compressive strain in the PC reached a value of at least 0.005. As the beams failed, the compressive concrete piece separated as a V-shape, a phenomenon already observed before with other steel-reinforced PC systems. 

Considering that x = 3.5-5 cm (Eq. [6.6]), the thickness of the epoxy-rubber coating 8 = 12-16 cm. This means that the sum of the thickness of the polymer coating layer and the height of the compressed concrete zone is 17-20 cm, which is approximately equal to the total height of the beam (20 cm). 

I Cast iron (tension) Cast iron (compression) Concrete (compression) 120-290 69-480 340-1,400 10-70 1... 

Torsional capacity of FRP-confined concrete member Torsional capacity of the compressed concrete element FRP contribution to the torsional capacity... 

Generally, the deformation capacity in the plastic range of the members is limited by the failure of compressed concrete. FRP confinement on these members (mainly columns) leads to an increase in the ultimate deformation of compressed concrete, conveying a greater dnctifity to members. 

A compressive concrete strength may be examined as an example of such inconsistencies. According to the rules that have been established since the beginning of the twentieth century, the compressive strength was tested on specimens of various shapes and dimensions, always subjected to axial compression and calculated assuming that stress was distributed uniformly over the cross-section and that its value corresponded to the failure of the specimen. The main discrepancies between such procedures and reality are ... 

Figure 8.2 Diagrams obtained after measurement with a photoelastic coating on a lateral face of a compressed concrete element, after Dantu (1958) ...

Figure 8.5 Schematic stress-strain diagram for a compressed concrete element. By 8s apparent strain is denoted under stress due to microcracks.

Similar behaviour occurs when trying to locate voids in concrete cast behind steel plates, e g. the steel liner in nuclear containment walls. Our own experience has shown that in the case of a steel liner (encast at depth 250 mm) the reflected compression waves are dominant regardless of the condition of the concrete behind the plate. 

A crack in concrete with an air gap thickness of as little as 0.025 mm will hinder significant transmission of seismic compression waves [1]. 

The methods that are based on the reflection of compression waves will generally not give information about the concrete which lies deeper than the most shallow large planar defect (crack or void ). 

The transmission times can be used to determine the depth to the defects. As yet the use of this kind of testing in concrete is based on the rektilinear propagation of compression waves from the surface. No directional transducers for use on concrete are known to exist. 

A problem obviously exists in trying to characterise anomalies in concrete due to the limitations of the individual techniques. Even a simple problem such as measurement of concrete thickness can result in misleading data if complementary measurements are not made In Fig. 7 and 8 the results of Impact Echo and SASW on concrete slabs are shown. The lE-result indicates a reflecting boundary at a depth corresponding to a frequency of transient stress wave reflection of 5.2 KHz. This is equivalent to a depth of 530 mm for a compression wave speed (Cp) of 3000 m/s, or 706 mm if Cp = 4000 m/s. Does the reflection come from a crack, void or back-side of a wall, and what is the true Cp ... 

As with SMC, appHcations are limited to high volume because of the capital investment in equipment and tooling. Thermoset compression molders require additional heating and material Handling equipment to adapt their process to thermoplastic sheet fabrication. AppHcations include automotive bumper beams, load floors, radiator supports, battery trays, and package shelves. Chair sheUs, military containers, material Handling pallets, trays, and concrete foaming pans are also produced. 

In contrast to other polymers the resistance to water permeation is low due to the hydrolysis of the poly(vinyl acetate) (163,164). Ethylene copolymers have been developed which have improved water resistance and waterproofness. The polymer can be used in the latex form or in a spray-dried form which can be preblended in with the cement (qv) in the proper proportion. The compressive and tensile strength of concrete is improved by addition of PVAc emulsions to the water before mixing. A polymer-soHds-to-total-soHds ratio of ca 10 90 is best. The emulsions also aid adhesion between new and old concrete when patching or resurfacing. 

Fig. 20.8. The stress-strain curve for cement or concrete in compression. Cracking starts at about half the ultimate strength.

There are less exotic ways of increasing the strength of cement and concrete. One is to impregnate it with a polymer, which fills the pores and increases the fracture toughness a little. Another is by fibre reinforcement (Chapter 25). Steel-reinforced concrete is a sort of fibre-reinforced composite the reinforcement carries tensile loads and, if prestressed, keeps the concrete in compression. Cement can be reinforced with fine steel wire, or with glass fibres. But these refinements, though simple, greatly increase the cost and mean that they are only viable in special applications. Plain Portland cement is probably the world s cheapest and most successful material. 

The sound speed c, m s , is the velocity of propagation of the pressure variations. This depends on the physical properties of the medium and increases with the density of the medium. In air, for example, it is. 344 m s, while in water, 1410 m s and in concrete, 3000 m s . The elapsed time between successive compressions is called the period time T. 

It is important to note material such as those plastics or wood that are weak in either tension or compression will also be basically weak in shear. For example, concrete is weak in shear because of its lack of strength in tension. Reinforced bars in the concrete are incorporated to prevent diagonal tension cracking and strengthen concrete beams. Similar action occurs with RPs using fiber filament structures. 

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