Structural Integrity Rules

When it comes to hydronic piping systems, I would speculate that 99 per cent of radiant system designers are unaware that Canadian and U.S. concrete codes and standards address the placement and operation of embedded pipes. 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 systems 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 not a trivial statement and although more of a concern for the structural engineer, it does not grant the radiant designer impunity from understanding the relationship between pipes and concrete.

Construction

When it comes to “slab on grade” the 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 vapour/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). 

Concrete and Pipe Stress 

Concrete consists 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. They are also 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).

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 will 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. 

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, 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 — for all but special applications the 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 manufacturer that 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 Master Po would say to the young Grasshopper, do this and you will have alleviated the most common issues with embedded pipes in concrete.

^1. Bean, R., Pay Now or Pay Later: Modelling downward heat losses – a last opportunity before a lost opportunity, Heating Plumbing Air Conditioning Magazine, March 2011

^2. Bean, R., All Points Bulletin, Plan reviews and field inspections: Under slab insulation (redux), Heating Plumbing Air Conditioning Magazine, November/December 2011

^3. 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

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