Special Types of Concrete - Design and Control of Concrete Mixtures - page 315

Type and intensity of radiation usually determine the
requirements for density and water content of shielding
concrete. Effectiveness of a concrete shield against gamma
rays is approximately proportional to the density of the
concrete; the heavier the concrete, the more effective the
shield. On the other hand, an effective shield against neu-
tron radiation requires both heavy and light elements. The
hydrogen in water provides an effective light element in
concrete shields. Some aggregates contain crystallized
water, called fixed water, as part of their structure. For this
reason, heavyweight aggregates with high fixed-water
contents often are used if both gamma rays and neutron
radiation are to be attenuated. Boron glass (boron frit) is
also added to attenuate neutrons.

Additions

Boron additions such as colemanite, boron frits, and boro-
calcite are sometimes used to improve the neutron
shielding properties of concrete. However, they may ad-
versely affect setting and early strength of concrete; there-
fore, trial mixes should be made with the addition under
field conditions to determine suitability. Admixtures such
as pressure-hydrated lime can be used with coarse-sand
sizes to minimize any retarding effect.

Properties of High-Density Concrete

The properties of high-density concrete in both the freshly
mixed and hardened states can be tailored to meet job
conditions and shielding requirements by proper selection
of materials and mixture proportions.
Except for density, the physical properties of heavy-
weight concrete are similar to those of normal-weight con-
crete. Strength is a function of water-cement ratio; thus,
for any particular set of materials, strengths comparable to
those of normal-weight concretes can be achieved. Typical
densities of concretes made with some commonly used
high-density aggregates are shown in Table 18-3. Because
each radiation shield has special requirements, trial mix-
tures should be made with job materials and under job
conditions to determine suitable mixture proportions.

High-Density Aggregates

High-density aggregates such as barite, ferrophosphorus,
goethite, hematite, ilmenite, limonite, magnetite, and de-
greased steel punchings and shot are used to produce
high-density concrete. Where high fixed-water content is
desirable, serpentine (which is slightly heavier than
normal-weight aggregate) or bauxite can be used (see
ASTM C 637 and C 638).
Table 18-3 gives typical bulk density, relative density
(specific gravity), and percentage of fixed water for some of
these materials. The values are a compilation of data from a
wide variety of tests or projects reported in the literature.
Steel punchings and shot are used where concrete with a
density of more than 4800 kg/m 3 (300 pcf) is required.
In general, selection of an aggregate is determined by
physical properties, availability, and cost. Heavyweight
aggregates should be reasonably free of fine material, oil,
and foreign substances that may affect either the bond of
paste to aggregate particle or the hydration of cement. For
good workability, maximum density, and economy, aggre-
gates should be roughly cubical in shape and free of exces-
sive flat or elongated particles.

Proportioning, Mixing, and Placing

The procedures for selecting mix proportions for heavy-
weight concrete are the same as those for normal-weight
concrete. However, additional mixture information and
sample calculations are given in ACI 211.1 . Following are
the most common methods of mixing and placing high-
density concrete:
Conventional methods of mixing and placing often are
used, but care must be taken to avoid overloading the
mixer, especially with very heavy aggregates such as steel

Table 18-3. Physical Properties of Typical High-Density Aggregates and Concrete

Fixed-water,* Aggregate Aggregate bulk Concrete density,

Type of aggregate percent by weight relative density density, kg/m 3 (pcf) kg/m 3 (pcf)

Goethite 10-11 3.4-3.7 2080-2240 (130-140) 2880-3200 (180-200)

Limonite** 8-9 3.4-4.0 2080-2400 (130-150) 2880-3360 (180-210)

Barite 0 4.0-4.6 2320-2560 (145-160) 3360-3680 (210-230)

Ilmenite † 4.3-4.8 2560-2700 (160-170) 3520-3850 (220-240)

Hematite † 4.9-5.3 2880-3200 (180-200) 3850-4170 (240-260)

Magnetite † 4.2-5.2 2400-3040 (150-190) 3360-4170 (210-260)

Ferrophosphorus 0 5.8-6.8 3200-4160 (200-260) 4080-5290 (255-330)

Steel punchings or shot 0 6.2-7.8 3860-4650 (230-290) 4650-6090 (290-380)

* Water retained or chemically bound in aggregates.
** Test data not available.
† Aggregates may be combined with limonite to produce fixed-water contents varying from about 1 ⁄ 2 % to 5%.