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.
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.Also see:
Designing Blast Hardened Structures For Military & Civilian Use - PDF, Page 53.
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DOD Protective Design Manuals Have Wide Application PDF
