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# Civil Engineering/Crack Control in Water Tanks

Question
Sir,
I have compared ACI methods to calculate crack widths. The philosophy has been changed much from ACI 318-95 to 2005. Now according to ACI 350-06 we need to limit fs only against fsmax given in code. My question is if we have temperature loads in water tank that is above ground, do we need to check crack widths on axial loads due to temperature? if yes then how can i calculate that? by ACI 224.2R? or I just need to calculate flexural crack widths only?

Hi Waseem,

I am not well versed with the ACI codes but have designed water tanks to BS 8007, from which the quoted extract below may assist. You do need to cater for crack widths due to temperature, not just due to flexure.

2.6 Causes and control of cracking

Direct or flexural tension in the concrete arising
from applied external service loads, from
temperature gradients due to solar radiation, or
from the containment of liquids at temperatures
above ambient, may cause cracking in the concrete.
The limitation of cracking from applied loading is
dealt with in 2.2.3.3 and in the appropriate design
sections. Crack widths arising from flexure and
direct tension in mature concrete may be calculated
as indicated in appendix B.

2.6.2 Temperature and moisture effects
2.6.2.1 Origins. Changes in the temperature of the
concrete and reinforcement and in the moisture
content of the concrete cause dimensional changes
which, if resisted internally or externally, may crack
the concrete. The distribution and width of such
cracks can be controlled by reinforcement, together
with the provision of movement joints. In this
clause, i.e 2.6.2, temperature and moisture changes
and methods for their control in relation to the
particular problems of liquid-retaining structures
are considered; it supplements information given
in BS 8110-2:1985.
Heat is evolved as cement hydrates, and the
temperature will rise for a day or more after casting
and then fall towards ambient. Cracking usually
occurs at this time while the concrete is still weak.
Subsequent lower ambient temperatures and loss of
moisture when the concrete is mature will open
these cracks, although the loss of moisture at the
surface under external drying conditions is usually
low. A structure built in the summer but not filled or
an external structure standing empty will usually
be subjected to greater drops in temperature than
the same structure filled. Structures constantly full
and protected from climatic effects (e.g. by earth
cover, shading or reflective treatment) will have a
temperature near that of the liquid stored.
The designer should allow for both the greatest drop
in temperature below the peak temperature arising
from the heat of hydration and the maximum drying
that can be expected, bearing in mind the effects of
delays in construction and of conditions that may
occur when structures are emptied for maintenance
or repair.

Civil Engineering

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•1984: International Conference on the Art and Practice of Structural Design, London •1994: 3rd Int. Kerensky Conference in Structural Engineering, Singapore •2008: International Conference on High-Rise Towers, Abu Dhabi •2013: IEK International Conference, Kisumu, Kenya

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BSc, 1st Class Hons, in Building Engineering, University of Bath, UK MSc in Concrete Structures and Technology, University of London. Diploma of Imperial College, UK.

Awards and Honors
•Science Congress Special Award (for 2-seater Hovercraft - 1968) •Institution of Civil Engineers Award for outstanding performance at Bath University (1975) •Concrete Society Postgraduate study Bursary Award (1976) •Consular Representative for British High Commission, Nairobi. (1995 to 1998) •Examiner for Institution of Civil Engineers Professional Interviews, Nairobi. (1997 to 1998) •Branch Representative in Vienna for PI assessment for Inst. of Struct. Engrs. (1999 to 2010)