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Fundamentals of indoor environmental quality / thermal comfort and air quality solutions using radiant based HVAC

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.

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.Bookmark and Share
 

 

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).
 

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. Bookmark and Share
 

Figure 3 Vapour/soil gas barriers should be placed under the slab on top of the insulation. Bookmark and Share
 

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. Bookmark and Share
 

Figure 5 Typical tube placement in a hollow core structural slab with or without insulation and/or toppings. Bookmark and Share
 

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).

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. Bookmark and Share
 

Figure 7 Summary of control joint placement due to length and thickness, change of direction, change in slab thickness and grade (slope). Bookmark and Share
 

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.

 

 

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.
 

Figure 8 Relationship of concrete strength to curing temperature. Image adapted from Design and Control of Concrete Mixtures, Portland Cement Association, 2003. Bookmark and Share
 

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:

  1. Bean, R., Pay Now or Pay Later: Modelling downward heat losses – a last opportunity before a lost opportunity, HPAC Canada, March 2011

  2. Bean, R., All Points Bulletin, Plan reviews and field inspections: Under slab insulation (redux), HPAC Canada, November/December 2011

  3. PEX co-efficient of expansion is appx. 1" (25mm) every 100' (30.5m) of tubing for every 10F (5.6C) of temperature change. For co-efficient of expansion of concrete using various aggregates see, Design and Control of Concrete Mixtures,  Portland Cement Association, 2003

  4. Bean, R., The Fundamentals of Radiant Cooling System Design and Construction, Radiant Slabs On-Site Fabricated Heat Exchangers, ASHRAE Denver 2013, Seminar 38


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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