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VHSC Very-High-Strength-Concretes For Use In Blast And Penetration-Resistant Structures PDF

Dr. J. Donald Cargile, Impact and Explosion Effects Branch;
Ed F. O'Neil and Billy D. Neeley, Concrete and Materials Division;

Geotechnical and Structures Laboratory, US Army Corps of Engineers, Engineer Research and Development Center, Vicksburg, MS

You may be thinking that there is nothing we can tell you about concrete that won't cure insomnia, but you'd be wrong. How does advanced concrete 4 to 5 times stronger than standard concrete sound? The folks at ERDC are working to drastically improve this ubiquitous material, both in its general compressive strength and its resistance to fragmentation in impact events. Donald Cargile and his colleagues present the experimental data and demonstrate that concrete has a lot of development potential left in it.

VHSC Very High Strength Concretes - PDF, page 61.

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Introduction

Most fixed protective structures employ concrete in some way. The US Army Engineer Research and Development Center (ERDC) is conducting research to provide force protection in everything from foxholes to fixed facilities and against threats ranging from small arms to advanced conventional, and even terrorist weapons. Concrete is a highly economical material, it can be cast into many shapes, and can be formulated for varying degrees of strength and durability. It is primarily used for its compressive strength, as concrete is much stronger in compression than it is in tension. With the proper use of tensile reinforcement, concrete can be used in many tensile-loaded applications, such as flexural members, eccentrically loaded compression members, and direct tension members. Because of the wide use and availability of concrete, it is useful to elaborate on its fundamentals. Additionally, a better understanding of the complex creation of concrete variants will assist engineers and architects in choosing the best materials that address aesthetic, engineering, and protective considerations.

Advantages of Higher Strength Concrete and its Application to Structural Protection

Limitations of Conventional Concrete

VHSC Principles ( Very High Strength Concrete )

Tensile Properties

The tensile strengths of VHSCs can be higher than those of conventional concretes. As mentioned previously, tensile strength of VHSC may nominally be only 10 MPa, while it's compressive strength is on the order of 180 MPa. The addition of steel fibers increases the first-crack load, increases the ultimate load-bearing capacity, and dramatically increases the flex- ural toughness.

Very-high-strength concretes exhibit near-linear stress-strain characteristics up to failure when fabricated without the addi- tion of fibers. Their fracture energy, defined as the area beneath the load-deflection curve, is somewhat less than 140 J/m2.The addition of fibers to the matrix improves the behavior of the concrete in the post-first-crack region of the load-to-failure cycle. In VHSC, various percentages and types of steel fibers have been used but the best overall results (incorporating cost considerations) have been obtained with hooked-ended, steel fibers 30 mm in length and 0.5 mm in diameter. The large number of small fibers which cross the path of potential cracks, coupled with the good bond between fiber and matrix, provide high resistance to fiber pullout during ten- sile-cracking, and greatly increase the toughness of the materi- al. Figure 1 shows the load-deflection curve of a typical VHSC beam. By comparison, a load-deflection curve for a conven- tional concrete and a conventional fiber-reinforced concrete are added. Comparison of the areas under the curves gives a rela- tive relationship for the increase in toughness afforded by the very-high-strength concrete. The greatest effect is in the area of the curve beyond the first-crack load, where the sample's load- deflection behavior transitions from linear to non-linear. Up until this load, the tensile-carrying-capacity of the concrete has been responsible for the shape of the curve. In the unreinforced concrete, the magnitude of the first-crack load is about one- tenth that of the VHSC and the load and deflection of the post-first-crack portion of the curve is very small. Likewise, even with conventional fiber-reinforced concrete the first-crack strength is low er than VHSC and the post-first-crack portionof the curve is also smaller. Toughness is a measure of the amount of energy that must be expended to open cracks in the matrix under tensile loading. An example of toughness would be the resistance to a projectile passing through a material. This toughness is important in the performance of protective structures. The amount of energy required to penetrate the VHSC concrete will be greater than that required to penetrate conventional concrete. This means that some projectiles will be less effective at penetrating the structure, and perhaps will even be stopped by the VHSC. If the projectile completely passes through the VHSC, the exit velocity will be lower than that through the same mass of con- ventional concrete. Also, the amount of material fragmented from the back of a protective-structure member as the projec- tile passes through (also called spall) will be reduced by the steel fibers in the VHSC matrix.

Continue reading: AMPTIAC Advanced Materials & Processes Technology Information Analysis Center - Special Issue Quarterly PDF, 68 Pages.