freeze and thaw in concrete. D-cracking. building codes.seismicity

1. Freeze and Thaw And Seismicity
Presented to: Sir Yasir Sarfaraz
Presented By : Kamran Ahmed
Roll Number : 127
Institute of Geology UAJK

2. Freeze and Thaw
Definition:
 Freeze-thaw weathering is a process of erosion that happens in cold areas
where ice forms.
 A crack in a rock can fill with water which then freezes as the temperature
drops. As the ice expands, it pushes the crack apart, making it larger.
 When the temperature rises again, the ice melts, and the water fills the
newer parts of the crack.
 The water freezes again as the temperature falls, and the expansion of the ice
causes further expansion to the crack. This process continues until the rock
breaks.

3.  When water freezes, it expands about 9 percent.
 As the water in moist concrete freezes it produces pressure in the pores of
the concrete.
 If the pressure developed exceeds the tensile strength of the concrete, the
cavity will dilate and rupture.
 The accumulative effect of successive freeze-thaw cycles and disruption of
paste and aggregate can eventually cause expansion and cracking, scaling,
and crumbling of the concrete.
Freeze-Thaw Resistance

4. Prevention of Concrete Scaling
Scaling is defined as a general loss of surface mortar or mortar
surrounding the coarse aggregate particles on a concrete surface. This
problem is typically caused by the expansion of water due to freezing
and thawing cycles and the use of deicing chemicals; however properly
specified, produced, finished, and cured quality concrete need not
suffer this type of deterioration. There is a distinct chain of
responsibility for the production of scale resistant concrete.
Since scaling damage to pavements of all types is caused by physical
salt attack, the use of high strength (4,000 psi or more), low
permeability, air entrained concrete is crucial to good durability in
these applications.

6. D-Cracking
Cracking of concrete pavements caused by the freeze-thaw
deterioration of the aggregate within concrete is called D-cracking.
D-cracks are closely spaced crack formations parallel to transverse
and longitudinal joints that later multiple outward from the joints
toward the center of the pavement panel.
D-cracking is a function of the core properties of certain types of
aggregate particles and the environment in which the pavement is
placed.

7. This problem can be reduced either by selecting aggregates that
perform better in freeze-thaw cycles or, where marginal aggregates
must be used, by reducing the maximum particle size. Also,
installation of effective drainage systems for carrying free water out
from under the pavement may be helpful.

8. Air entrainment
The severity of freeze-thaw exposure varies with different areas. Local
weather records can help determine the severity of exposure. The
resistance of concrete to freezing and thawing in a moist condition is
significantly improved by the use of intentionally entrained air. The tiny
entrained air voids act as empty chambers in the paste for the freezing
and migrating water to enter, thus relieving the pressure in the pores
and preventing damage to the concrete. Concrete with a low
permeability (that is, a low water-cement ratio and adequate curing) is
better able to resist freeze-thaw cycles. In rare cases, air-void clustering
can occur, leading to a loss of compressive strength.

9. Freezing temperatures
Concrete gains very little strength at low temperatures. Accordingly,
freshly placed concrete must be protected against freezing until the
degree of saturation of the concrete has been sufficiently reduced by
cement hydration. The time at which this reduction is accomplished
corresponds roughly to the time required for the concrete to attain a
compressive strength of 500 psi. Concrete to be exposed to deicers
should attain a strength of 4,000 psi prior to repeated cycles of freezing
and thawing.

10. Optimizing the Use of Fly Ash in Concrete
Cold weather and winter conditions can be challenging when concrete contains
fly ash. Especially when used at higher levels, fly ash concrete typically has
extended setting times and slow strength gain, leading to low early-age strengths
and delays in rate of construction. In addition, concretes containing fly ash are
often reported to be more susceptible to surface scaling when exposed to deicing
chemicals than portland cement concrete. It is therefore important to know how
to adjust the amount of fly ash to minimize the drawbacks, while maximizing the
benefits.

11. Seismicity
Seismicity, the worldwide or local distribution of earthquakes in space,
time, and magnitude. More specifically, it refers to the measure of the
frequency of earthquakes in a region—for example, the number of
earthquakes of magnitude between 5 and 6 per 100 square km (39
square miles).

12. Earthquakes generate waves that may be slow and long, or short and abrupt.
The length of a full cycle in seconds is the Period of the wave and is the inverse
of the Frequency. All objects, including buildings, have a natural or fundamental
period at which they vibrate if jolted by a shock. The natural period is a primary
consideration for seismic design, although other aspects of the building design
may also contribute to a lesser degree to the mitigation measures. If the period
of the shock wave and the natural period of the building coincide, then the
building will "resonate" and its vibration will increase or "amplify" several
times.

13. The aforementioned seismic measures are used to calculate forces that
earthquakes impose on buildings. Ground shaking (pushing back and forth,
sideways, up and down) generates internal forces within buildings called
the Inertial Force (FInertial), which in turn causes most seismic damage.
FInertial = Mass (M) X Acceleration (A).
The greater the mass (weight of the building), the greater the internal inertial
forces generated. Lightweight construction with less mass is typically an
advantage in seismic design. Greater mass generates greater lateral forces,
thereby increasing the possibility of columns being displaced, out of plumb,
and/or buckling under vertical load (P delta Effect).

14. SELECT/DESIGN APPROPRIATE STRUCTURAL
SYSTEMS
Seismic design objectives can greatly influence the selection of the most
appropriate structural system and related building systems for the project.
Some construction type options, and corresponding seismic properties, are:
Wood or timber frame (good energy absorption, light weight, framing
connections are critical).
Reinforced masonry walls (good energy absorption if walls and floors are well
integrated; proportion of spandrels and piers are critical to avoid cracking)
Reinforced concrete walls (good energy absorption if walls and floors well
integrated; proportion of spandrels and piers are critical to avoid cracking)
Steel frame with masonry fill-in walls (good energy absorption if bay sizes are
small and building plan is uniform)

15. Steel frame, braced (extensive bracing, detailing, and proportions are
important)
Steel frame, moment-resisting (good energy absorption, connections are
critical)
Steel frame, eccentrically braced (excellent energy absorption, connections
are critical)
Pre-cast concrete frame (poor performer without special energy absorbing
connections)
Structural and architectural detailing and construction quality control is very
important to ensure ductility and natural damping and to keep damages to a
limited and repairable range. The prospect of structural and nonstructural
damage is not likely to be eliminated without the prudent use of energy-
dissipating devices. The cost of adding energy-dissipating devices is in the
range of 1–2% of the total structural cost. This is not a large number,
particularly when related to the life-cycle cost of the building. Within a 30–50
year life cycle the cost is negligible.

16. RELEVANT CODES AND STANDARDS
Many building codes and governmental standards exist pertaining to
design and construction for seismic hazard mitigation.Building code
requirements are primarily prescriptive and define seismic zones and
minimum safety factors to "design to." Codes pertaining to seismic
requirements may be local, state, or regional building codes or
amendments and should be researched thoroughly by the design
professional.
Many governmental agencies at the federal level have seismic
standards, criteria, and program specialists who are involved in major
building programs and can give further guidance on special
requirements.

17. Federal Emergency Management Agency (FEMA)
International Code Council (ICC)
National Earthquake Hazards Reduction Program (NEHRP)