Structural Concrete - Hassoun

70 Chapter 2 Properties of Reinforced Concrete
(280 kg/cm 2 ), a value of 145 lb/ft 3 (2320 kg/m 3 ) can be used, whereas for higher strength
concretes, w can be assumed to be equal to 150 lb/ft 3 (2400 kg/m 3 ).
2. Unit weight of plain concrete of maximum aggregate size of 4 to 6 in. (100 to 150 mm) varies
between 150 and 160 lb/ft 3 (2400 to 2560 kg/m 3 ). An average value of 155 lb/ft 3 may be used.
3. Unit weight of reinforced concrete, using about 0.7 to 1.5% of steel in the concrete section,
may be taken as 150 lb/ft 3 (2400 kg/m 3 ). For higher percentages of steel, the unit weight, w,
can be assumed to be 155 lb/ft 3 (2500 kg/m 3 ).
4. Unit weight of lightweight concrete used for fireproofing, masonry, or insulation purposes
varies between 20 and 90 lb/ft 3 (320 and 1440 kg/m 3 ). Concrete of upper values of 90 pcf or
greater may be used for load-bearing concrete members.
The unit weight of heavy concrete varies between 200 and 270 lb/ft 3 (3200 and 4300 kg/m 3 ).
Heavy concrete made with natural barite aggregate of 1 1 in. maximum size (38 mm) weighs about
2
225 lb/ft 3 (3600 kg/m 3 ). Iron ore sand and steel-punchings aggregate produce a unit weight of
270 lb/ft 3 (4320 kg/m 3 ) [20].
2.15 FIRE RESISTANCE
Fire resistance of a material is its ability to resist fire for a certain time without serious loss of
strength, distortion, or collapse [21]. In the case of concrete, fire resistance depends on the thickness,
type of construction, type and size of aggregates, and cement content. It is important to
consider the effect of fire on tall buildings more than on low or single-story buildings because
occupants need more time to escape.
Reinforced concrete is a much better fire-resistant material than steel. Steelwork heats rapidly,
and its strength drops appreciably in a short time. Concrete itself has low thermal conductivity. The
effect of temperatures below 250 ∘ C is small on concrete, but definite loss is expected at higher
temperatures.
2.16 HIGH-PERFORMANCE CONCRETE
High-performance concrete may be assumed to imply that the concrete exhibits combined properties
of strength, toughness, energy absorption, durability, stiffness, and a relatively higher ductility
than normal concrete. This improvement in concrete quality may be achieved by using a
new generation of additives and superplasticizers, which improves the workability of concrete
and, consequently, its strength. Also, the use of active microfillers such as silica fume, fly ash,
and polymer improves the strength, porosity, and durability of concrete. The addition of different
types of fiber to the concrete mix enhances many of its properties, including ductility, strength,
and toughness.
Because it is difficult to set a limit to measure high-performance concrete, one approach is
to define a lower bound limit based on the shape of its stress–strain response in tension [22]. If
the stress–strain relationship curve shows a quasi-strain-hardening behavior—or, in other words,
a postcracking strength larger than the cracking strength with an elastic-plastic behavior—then
high performance is achieved [22]. In this behavior, multicracking stage is reached with high
energy-absorption capacity. Substantial progress has been made recently in understanding the
behavior and practical application of high-performance concrete.