Progress in Concrete

Technology203332 Concrete Technology

19 February 2004

High-Strength Concrete Definition

For mixtures made with normal-weight aggregates, high-strength concretes are considered to be those which have compressive strengths in excess of 6000 psi (40 MPa).

Properties

Workability-In their earlier stages of development, the high strength concrete mixtures containing large amounts of fines (high cement content, plus pozzolan) a low water/cement ratio, and normal water-reducing admixtures had a tendency to be stiff and sticky. The advent of superplasticizers has changed this. It is now possible to obtain high consistency with adequate cohesiveness for placement of concrete by pumping, without causing segregation, even at lower than 0.3 water/cement ratios.

Strength What is also noteworthy about strength is the

capacity of high-strength concrete mixtures to develop strength at a rapid rate without steam curing.

Durability Due to the high cement content thermal cracking

can be a durability problem in high-strength concrete structure.

It seems that superplasticized, low water/cement ratio, high-strength concretes containing a high cement content and a good-quality pozzolan should have a great potential where permeability of durability, not strength, is main consideration.

Fiber-Reinforced Concrete Definition and Significance

Concrete containing a hydraulic cement, water, fine or fine and coarse aggregate, and discontinuous discrete fibers is called fiber-reinforced concrete. It may also contain pozzolans and other admixtures commonly used with conventional concrete. Fibers of various shapes and sizes produced from steel, plastic, glass and natural materials are being used; however, for most structural and nonstructural purposes, steel fiber is the most commonly used of all the fibers.

Toughness Mechanism Examination of fractured specimens of fiber-

reinforced concrete shows that failure takes place primarily due to fiber pull-out or debonding. Thus unlike plain concrete, fiber-reinforced concrete specimen does not break immediately after initiation of the first crack. This has the effect of increasing the work of fracture, which is referred to as toughness and is represented by the area under the load-deflection curve. Explaining the toughening mechanism in fiber-reinforced composites.

Toughness and impact resistance Related to flexural toughness are the impact

and fatigue resistance of concrete, which are also increased considerably.

In regard to strength, it has been shown that the addition of fiber to conventionally reinforced beams increased the fatigue life and decreased the crack width under fatigue loading.