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For a deeper discussion on this topic see
our ASHRAE Denver Slides on
Embedded Pipes in Concrete. |
Concrete Code and Standards: Regulation of Embedded Piping
Systems
Copyright ©
2013 Robert Bean, R.E.T., P.L.(Eng.) All world rights reserved. Originally
published in September 2013 issue of
HPAC
Magazine Canada.
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When it comes to hydronic piping systems I
would speculate 99% of radiant system designers are unaware that
Canadian and U.S. concrete codes and standards address the
placement and operation of embedded pipes, and certainly when it
comes to structural concerns, hydronic codes play second fiddle
to the piping pressure and temperature test and operating
requirements of the concrete regulations.
When working with embedded pipe system in
Canada, radiant designers should familiarize themselves with CSA
A23.1, A23.2, and A23.3, respectively Concrete Materials and
Methods of Construction, Test Methods and Standard Practices for
Concrete and Design of Concrete Structures. For those working in
the U.S., the governing document is ACI 318 Building Code
Requirements for Structural Concrete.
These documents address embedded pipes and
are for the most part consistent in addressing test pressures;
temperatures, tube spacing density, and allowable pipe diameter
and tube placement/depth (see Figure 1 and Table 1).
As noted in
CSA A23.3, section 6.2, “Embedded pipes…shall be located so as
to have negligible impact on the strength of the construction or
their effects on member strength shall be considered in the
design.” This is a non-trivial statement and although more of a
concern for the structural engineer it does not grant the
radiant designer immunity from understanding the relationship
between pipes and concrete. |
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Figure 1 Tube placement, spacing and diameter are
all regulated by concrete codes and standards. Fortunately these
restrictions rarely have a negative impact on the design of
radiant systems.
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Construction
When it comes to “slab on grade” radiant
designer should be aware of the different control joint types,
purpose and layout (Figure 2);
insulation types and
characteristics including compressive strengths 1,2;
and vapor/gas barrier and placement (Figure 3). For slabs above
grade designers should be aware of consequences of tube
placement (Figure 4) within the slab itself or in bonded or
unbonded toppings (Figure 5).
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Figure 2 Joints for slab on grade construction
are inserted to control cracking and if necessary to connect
slab sections. Pipes passing through or under these joints
should be sleeved as per pipe manufacturer’s instructions.
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Figure 3 Vapour/soil gas barriers should be
placed under the slab on top of the insulation.
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Figure 4 FEA simulation
effects of tube depth on
cooling surface temperatures. Note the surface temperature
efficacy (consistency) and back losses/gains as a result of
placement. The mid position is optimal and meets concrete design
and construction regulations.
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Figure 5 Typical tube placement in a hollow core
structural slab with or without insulation and/or toppings.
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Concrete and Pipe Stress
Concrete consist of cement, aggregate (sand
and gravel) and water. In simple terms adding water causes the
cement to harden around the aggregates
(Figure 6).
For most
slabs, stress in concrete will be due to shrinkage experienced
during a typical 28 days curing cycle and will with a few
exceptions exceed the stress caused by increases in
slab
temperature due to heating with embedded pipes. Thus control
joints in slabs are not there for the exclusive benefit of the
radiant slab but there to regulate cracking as a result of the
curing process, changes in slab depths, intersections or changes
in direction and changes to slab grading (slopes) (Figure 7). |
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Figure 6 Microscopic image of PEX-a pipe embedded
in concrete. Make note of the protective air barrier layers in
the right hand side image. Stress due to encased pipe being
heated and cooled is absorbed internally at the molecular level.
Image courtesy of Uponor, all rights reserved, used with
permission.
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Figure 7 Summary of control joint placement due
to length and thickness, change of direction, change in slab
thickness and grade (slope).
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As it relates to pipe stresses, concrete
and PEX pipe have very different
co-efficients of expansion3;
as such when embedded pipe, restrained by the concrete, is
heated and cooled expansion/contraction stress must show up
within the pipe since it is not possible for relief through pipe
elongation as would be the case with uncased pipe. Due to the
inward distribution of this stress over the internal structure
and surface area of the pipe, any distortion would be at a
microscopic level (see Table 2 and Figure 6). I won’t pretend to
be a “plastics engineer” but the stress is real and pipe should
be engineered to accommodate this long term function. Beyond
that I’ll leave the academic debate, as to which method of PEX
has better characteristics at handling the long term results of
internal “stress relief”, to the various polyethylene chemists
and pipe engineers. |
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Temperature vs. strength of concrete
Concrete design documentation is explicit
in stating excessive concrete temperatures during the curing
cycle can destroy compressive strengths (Figure 8). Under no
circumstances should fluid in radiant slabs be operated at those
design conditions exceeding the concrete codes and standards
until the concrete has obtained its design strength. Failing to
adhere to this requirement can destroy the structural integrity
of the slab.
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Figure 8 Relationship of concrete strength to
curing temperature. Image adapted from Design and Control of
Concrete Mixtures, Portland Cement Association, 2003.
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Final thoughts…
When it comes specifically to structural
concrete never assume the plumbing codes and standards trump
concrete codes and standards. Requirements for structural
integrity will always supersede piping pressures and temperature
test protocols and operating conditions.
Tube depth and spacing matters; John
Siegenthaler and I have discussed the effects on back losses,
operating temperatures and thus plant efficiency and surface
temperature efficacy. Keep the tubes within the recommended
depth of the concrete codes and standards, and for all but the
special applications tubes are best located in the upper portion
of the slab. Embedded pipes are permanent and affect system
efficiency – use only the highest quality product available and
use lots of it to guarantee the lowest temperatures in heating
and highest temperatures in cooling.
For slab on grade construction ensure those
responsible for the placement of the slab have considered the
proper layout and construction of control joints and safeguard
any pipes passing under or through the joint following
manufacturers procedures. When selecting rigid insulation make
certain the compressive strength is suitable for the
application. I know of one prime manufacturer which offers
loading analysis for special applications such as heavy
equipment rolling across slabs in industrial buildings.
Vapour and soil gas barriers should be
placed under the slab and on top of the insulation. This
mitigates damage to the barrier and prevents slab moisture from
accumulating within the insulation. For slabs placed on soils
bearing on high water tables use low water absorption and low
vapour permeance insulations such as Type 4 extruded polystyrene
board stock.
As Caine would say to the young
grasshopper, do this and you will have alleviated the most
common issues with embedded pipes in concrete.
References:
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Bean, R., Pay Now or Pay
Later: Modelling downward heat losses – a last opportunity
before a lost opportunity, HPAC Canada, March 2011
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Bean, R., All Points
Bulletin, Plan reviews and field inspections: Under slab
insulation (redux), HPAC Canada, November/December 2011
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PEX co-efficient of
expansion is appx. 1" (25mm) every 100' (30.5m) of tubing
for every 10°F (5.6°C) of temperature change. For
co-efficient of expansion of concrete using various
aggregates see, Design and Control of Concrete Mixtures,
Portland Cement Association, 2003
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Bean, R., The
Fundamentals of Radiant Cooling System Design and
Construction, Radiant Slabs On-Site Fabricated Heat
Exchangers, ASHRAE Denver 2013, Seminar 38
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Related reading:
Do I need an
engineer? A Guide to HVAC/Indoor Climate Design Service
Providers
Where
will your indoor climate system score?
How to "ball park" your budget for
indoor climate control.
Indoor environments: Self assessment
Built to code: What does it mean for consumer thermal comfort?
The Total Comfort System -
The "Un-minimum" System
Thermal
Comfort: A 40 grit perspective for consumers
Thermal
Comfort: A Condition of Mind
Do-It-Yourself HVAC - Should you do it?
The Cost of HVAC Systems - Are You Paying Too Much for Downgrades?
Radiant Installations - The Good, Bad
and Ugly
Thermal Comfort Surveys - Post Occupancy, Part I
Thermal Comfort Surveys - Post Occupancy, Part II
HVAC does not equal IEQ
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