303
other structures where tension stresses are invariably developed and
that would normally cause cracks to develop in plain concrete that
would propagate uncontrollably until failure occurs.
Concrete is fundamentally composed of a mixture of coarse and fine
aggregates, cement, and water. Synthetic cements are usually made
by grinding calcinated limestone and clay into a fine powder. On
mixing with water, an exothermal reaction occurs with the cement
that causes time-dependent hardening. Important variables include
its physical, mechanical (strength, stiffness, creep rates), and other
properties. Strengths increase with time and are highly dependent on the water-cement ratio of the mix (excessive water reduces
strength). Process factors include its ease of mixing and pouring
(largely related to the fluidity of the initial mix).
Given its importance in construction, a huge amount of research
and development has long gone into improving the quality and
properties of concrete. For example, there are many different types
of specially formulated cements available for specific purposes, such
as high early strength or low heat, and many additives are available to improve specific characteristics such as resistance to freezethaw action. The design of reinforced concrete elements has also
by now become quite developed. The fundamental driving characteristic of designing concrete subjected to forces is that concrete
is quite strong in compression but weak in tension. Plain concrete
cracks under very small tension stresses. As a consequence, reinforcing is placed within areas expected to experience cracking. Steel is
commonly used, but other materials are possible. Likewise, many
kinds of more sophisticated approaches involving pretensioning
or post-tensioning are in common use. Design procedures are well
developed but based on empirical understandings, given that stressdeformation relations are nonlinear.
The obvious routes for using nanomaterials to improve concrete
are generally either in process considerations (ease of mixing,
rate of setting, etc.) or in property enhancements. The first area
is very promising. The primary ingredients of modern cement are
lime, silica, alumina, and iron oxide. Raw mixtures are crushed,
ground, and fed into a high-temperature kiln, where the mixture
is chemically converted into a cement clinker. Small amounts of
other materials (e.g., gypsum) may also be added. This cement
mixture hardens when mixed with water in hydration reactions.
Calcium silicate hydrate (C-S-H) is formed in particular types of
organizational structures that act to bind the mixture together. The
speed and nature of these reactions is, in turn, influenced by the
Structural and Mechanical Environments
other structures where tension stresses are invariably developed and
that would normally cause cracks to develop in plain concrete that
would propagate uncontrollably until failure occurs.
Concrete is fundamentally composed of a mixture of coarse and fine
aggregates, cement, and water. Synthetic cements are usually made
by grinding calcinated limestone and clay into a fine powder. On
mixing with water, an exothermal reaction occurs with the cement
that causes time-dependent hardening. Important variables include
its physical, mechanical (strength, stiffness, creep rates), and other
properties. Strengths increase with time and are highly dependent on the water-cement ratio of the mix (excessive water reduces
strength). Process factors include its ease of mixing and pouring
(largely related to the fluidity of the initial mix).
Given its importance in construction, a huge amount of research
and development has long gone into improving the quality and
properties of concrete. For example, there are many different types
of specially formulated cements available for specific purposes, such
as high early strength or low heat, and many additives are available to improve specific characteristics such as resistance to freezethaw action. The design of reinforced concrete elements has also
by now become quite developed. The fundamental driving characteristic of designing concrete subjected to forces is that concrete
is quite strong in compression but weak in tension. Plain concrete
cracks under very small tension stresses. As a consequence, reinforcing is placed within areas expected to experience cracking. Steel is
commonly used, but other materials are possible. Likewise, many
kinds of more sophisticated approaches involving pretensioning
or post-tensioning are in common use. Design procedures are well
developed but based on empirical understandings, given that stressdeformation relations are nonlinear.
The obvious routes for using nanomaterials to improve concrete
are generally either in process considerations (ease of mixing,
rate of setting, etc.) or in property enhancements. The first area
is very promising. The primary ingredients of modern cement are
lime, silica, alumina, and iron oxide. Raw mixtures are crushed,
ground, and fed into a high-temperature kiln, where the mixture
is chemically converted into a cement clinker. Small amounts of
other materials (e.g., gypsum) may also be added. This cement
mixture hardens when mixed with water in hydration reactions.
Calcium silicate hydrate (C-S-H) is formed in particular types of
organizational structures that act to bind the mixture together. The
speed and nature of these reactions is, in turn, influenced by the
Structural and Mechanical Environments