The majority of construction incommercial building today is

done by use of large building panelsof various types, connected by ahigh performance elastometric seal-ant material to form a waterproofseal. These panels consist of eitherprecast concrete, prestressed con-crete, or curtain wall which is pre-dominately glass and metal. Anycombination of these three typesalso is commonplace in modernconstruction.

It is vitally important that theconnecting link, the sealant, joiningall these panels be of top quality,capable of performing satisfactorilyfor a great many years. without thisperformance, water tightness soon islost, resulting in long, costly mod-ifications and repairs.

Many factors must be consideredin designing a commercial buildingwhich will give many years of trou-ble free service. These include jointdesign, selection of sealant, surfacepreparation of joints just prior tocaulking and, finally, selection ofthe proper backer rod (againstwhich sealant is placed). The rightdecisions in these important areas,plus proper installation of the prod-ucts, ensures desired results.

The backer rod evolves

Evolution of the backer rod from thefirst use of caulking to the present

Hidden, but essentialA technical reviewof backer rodsby J.F. Gibb

At left, the result of outgassingby a closed cell backer rod. Therod has been removed, and thepanels and sealant bead sawedin cross-section to showcavities caused by the bubbles.This is an unsatisfactory seal.Below, the sample at left seenfrom the side. A cured sealantbead applied above a closed cellbacker rod was ruptured whentrapped gases escaped from therod.

John F. Gibb invented and developedthe production equipment used to makeopen cell backer rods, and holds U.S.and foreign patents on this machinery.He is a member of the ASTM C-24 JointSealant Committee, and is GeneralManager/part owner of Backer Rod Mfg.& Supply Co., Denver.

40 THE CONSTRUCTION SPECIFIER/MARCH ’80

state of the art covers use of manydifferent types of materials. Theyrange from sand in horizontal joints,through wood, fiber board, jute,rope, and twisted paper. About 17years ago, a product evolved whichconsisted of an extruded plastic rod.

This new backer rod was a closedcell, flexible foam material. At first,it was made from polyethylene plas-tic. Now it consists of polyethyleneand polypropylene, both nearlyidentical in appearance and per-formance. This new rod is manufac-tured by plastic extrusion: A gas isinjected into the material, creatingplastic foam. The result might bedescribed best as being somewhatsimilar to a tube inflated underslight pressure as gas is trappedwithin its cellular structure. Thisrod proved to be far superior to whathad been used previously in thetrade, but was not without its prob-lems.

Problems — and solutionsOne of the more serious problemswas outgassing. When these closedcells and their outer skin somehowbecome ruptured, trapped gasesslowly escape to the atmosphere. If ahigh proportion of these cells be-come ruptured or deflated (so tospeak), much of the rigidity of therod is lost. The rod also loses muchof its ability to regain its originalround shape once it has been de-formed.

The problem of outgassing as-sociated with these new flexible foambacker rods resulted in sealantmanufacturers insisting on furtherresearch, in the attempt to find asuitable material that would elimi-nate this problem — without creat-

ing even more serious ones.For many years, in limited appli-

cations only, polyurethane foammaterials had been used success-fully. This promoted further de-velopment of this material, to de-termine its suitability for use inbacker rods in the caulking busi-ness.

Most early applications em-ployed a backing material cut intoshort strips (square or rectangular incross-section). The material exhi-bited a wide range of physical prop-erties, because much of it had beencut from scrap material used by thefurniture business. The type ofpolyurethane used in that industry isrequired to have a very high de-gree of resilency, and to retain thisproperty through severe usage. Thissame property is necessary in ex-pansion joint backer rod. Therefore,it proved to be an ideal material forbacker rod use.

About six years ago, a round,open cell, flexible polyurethanebacker rod was introduced. Theproduct was made from specialformulations of these flexiblefoams, producing a more uniformmaterial. (As noted, uniformity waslacking in previous urethane backerrod produced from scrap.)

Open vs. closed cell

There are some interseting (and im-portant) differences between openand closed cell backer rods.

Both open cell and closed cell areinert to all common and more pre-dominately used sealant materials onthe market today. Both have ex-cellent chemical resistance.

Both, too, are classified as non-

staining. This means they containno materials that would bleed outslowly and seep into the sidewalls(substrate), later to reappear as dis-coloration on the surface (some dis-tance away). This is a particularlyimportant point, if marble or othermasonry materials are used.

The open cell rod is more flexiblethan its counterpart closed cell. Theopen cell must depend on its chem-ical and physical make up to attainrigidity, while closed cell rigidity isthe result of gases trapped in itsinterior (as explained earlier).

Rarely in commercial buildingconstruction do expansion jointwidths conform exactly to design.For instance, a one-half inch jointfrequently will vary from as little asone-quarter inch or less to as muchas three-quarter inch (or more). It isvirtually impossible to manufactureor to erect large building panelseconomically to such precise di-mensions that joints between suchpanels stay within close tolerances.A backer rod to be packed betweenthese panels must be flexibleenough for easy insertion.

Manufacturers of closed cellbacker rods recommend that theproduct not be compressed morethan 25 percent of its diameter. Con-sequently, the applicator mustchange sizes frequently, usingclosed cell in commonplace, vary-ing width joints, and open cell rod ifhe is to exceed the 25 percent max-imum compression limit.

Open cell backer rod must becompressed at least 25 percent; itdoes not hurt these rods to be com-pressed as much as 75 percent oftheir diameter. This allows theapplicator to use fewer sizes in vary-

THE CONSTRUCTION SPECIFIER/MARCH ’80 41

ing width joints. Many times, onesize will work the full length of thejoint (which might not be the casewhen using closed cell rod).

Three primary purposesA backer rod has three primary pur-poses. First, it forms a base againstwhich sealant is applied. (It there-fore determines the thickness of thesealant bead.) Also, it forces un-cured sealant out laterally undertooling pressure, resulting in 100percent contact of sealant tosidewall (substrate). These are thebond lines. Finally, backer rodsdictate the cross-sectional config-uration of the sealant bead.

Flexibility of the two backer rodmaterials plays an important part intooling the newly applied sealantbeads. A backer rod, although im-portant in an expansion joint, issecondary to the primary prod-uct—the sealant. The desired endresult is a satisfactory seal thatmeets or exceeds specifications.

The more rigid closed cell backer

rod is able to withstand a heavierpressure. The greater flexibility ofthe open cell rod requires thecaulker to use lighter toolingpressure, to minimize spring back.(It is not a difficult technique tomaster, for open cell backer rod hasbeen used successfully for 20 yearsor more.)

Tooling the hour glassTooling is done with a variety of in-struments. Most incorporate asemi-flexible blade, like a spatula. Itis the tooling operation that givesthe cross-sectional shape to thenewly applied sealant bead. Thisshape is referred to as an “hourglass” configuration.

The hour glass shape is impor-tant. It results in a maximum bond-ing surface on the sidewalls, and athinner section midway between.This means the adhesive bond to thesidewalls will be of sufficientstrength to eliminate failure causedby repeated elongations and con-tractions of the sealant material. Insimple terms, the strength of the

bond to the sidewalls forces ellasticmovement of the sealant. Elasticityof the sealant comes into play by al-lowing stretching and contractionat the thinnest point, avoiding ex-cessive stress at the bonds to thesidewall. Choosing the correctbacker rod obviously plays an im-portant part.

If a backer rod is placed incor-rectly (too deeply) or so it doesn’tforce an hour glass cross section,cohesive or adhesive failure be-comes a distinct possibility.

Proper cross sectionsFigures 1 and 2 show the dif-ferences between the proper crosssection of a newly applied sealantbead, and an incorrect one. Figure 2is incorrect because the ratio ofthickness of the sealant bead at thebond line to that at the midwaypoint is too small. Movement of thejoint sidewalls will put undue stresson the bond lines. The high cohe-sive strength of the overly thicksealant bead will make it very dif-ficult or impossible for the sealant toelongate. The bonds may fail even-tually because of this; they will notif they have the correct ratio de-picted in Figure 1.

This problem can be com-pounded if an incorrect back-up isused in cases where three-sidedadhesion occurs. In these cases, asealant cures while adhered to bothsidewalls, and to the backer rod.When the back-up rod is composedof a rigid, unyielding material,problems can arise.

Closed cell backer rods, being anolefin-type plastic material, are wellknown for their anti-stick prop-erties. Usually, any inital adhesionto these back-ups break downquickly, and the sealant comes free.

This was thought to be the casewith open cell rods as well, sincethe weak open cell surface structurealso shears away from the sealantbead. In cases where it does not,however, the flexibility of the opencell rod allows it to move with thesealant bead (with absolutely no illeffects) for the life of the sealant.

This has been demonstrated both

42 THE CONSTRUCTION SPECIFIER/MARCH ’80

in laboratory tests of 10,000 cycles(excess of 25 years) and in over 20years of actual use in the field. Prob-lems do arise when a rigid back-up isused: the sealant bonds to ittenaciously. When the joint moves,the sealant doesn’t—and failurelikely will occur.

Closed cell rods tend to take a setwhen compressed for extendedperiods of time. This is not the casefor open cell rods. It is a basic re-quirement of these open cell foamsin the furniture and padding indus-try that they not take a set. On occa-sion, closed cell rods have beenknown to move back into a jointunder gunning pressure, if the rodfirst was placed in the joint too far.(This permits too thick a sealantbead, risking bond failure.)

Open cell rods have a high coeffi-cient of friction, and do not slideeasily over most surfaces. This re-sults in a good grip to sidewalls, anda resistance to movement undergunning presuure. Open cell rod canmove, however, if it is not com-

pressed the recommended 25 per-cent in joint packing.

Moisture AbsorptionOne of the myths that existed foryears about open cell backer rod wasits alleged moisture absorption.Many felt its open cell structuremade it perform as a sponge. Opencell foam will absorb some water,but under most situations it does notretain it for long. Twenty-four hourtotal submersion tests indicate 50percent absorption is about average.(This, of course, is not encounteredin the field, but whatever watermight be absorbed in actual practicestill is not retained long.) Closed cellrod, on the other hand, is virtu-ally non-absorbent; therefore, noproblem.

This open cell feature has provento be an important asset, because anescape route for gases always isreadily available. Should somemoisture become trapped betweenthe sidewalls and the backer rod, thearea usually is much slower to dry

out than is that part of the joint notcovered by the backer rod—and ex-posed to the air. (See Figure 3 andFigure 4.) The portion to be caulked,however, is dry enough to be sealed.

When this moisture problem oc-curs and the wetness begins toevaporate, vapor pressure is built upbetween the backer rod and thesidewalls. In the case of the open cellrod, when these vapors take thecourse of least resistance they gothrough the backer rod to escape. Asfor closed cell, being impervious,the only escape route is through thenewly applied sealant bead. Whenthis occurs, the sealant bead blis-ters. Channels can developthroughout it; at times, the bond linefails or is seriously weakened.

Another important plus for opencell rod which became obvious in theapplication of air cured sealants(silicones) was the fact that a cure isinitiated from both sides. Thisspeeds up the cure substantially(highly desirable with slow-curingsealants). This faster cure to a largeextent eliminates precure adhesive orcohesive failure problems.

Two-sided curing is impossiblewith closed cell rod material, there-fore air curing sealants remain in anuncured or semi-cured state for alonger period of time. (See Figure 5and Figure 6.)

OutgassingBubbling of newly applied or semi-cured sealant beads is referred to asoutgassing. The results are similarto those caused by trapped mois-ture. This is a common problemwhen using closed cell rod. Formany years, it was suspected thisbubbling in newly applied sealantbeads was caused by the sealant.However, more precise observationshowed it to be from closed cellbacker rod.

It was explained earlier thatclosed cell backer rod containstrapped gases within its surface skinand inside its closed cells. Also, itwas pointed out that if this skinand/or some of these cells becomeruptured, the gas begins to seep out.The impervious nature of the closedcell backer rod prevents escaping

THE CONSTRUCTION SPECIFIER/MARCH ’80 43

gases from going any place otherthan through the newly appliedsealant beads.

This seepage bubbles in the seal-ant, that, at times, can extend fromthe vent hole in the backer rodcompletely through the sealant bead,to its outer surface. This causes aleak in the seal. Most outgassing andbubbling isn’t quite this severe.Usually, these gas bubbles staywithin the sealant bead, though

they do detract from appearance,and weaken the sealant bead. (SeeFigure 7.)

Backer rod ruptureRupture of closed cell backer rod canoccur in several ways. Roughmasonry surfaces often (and easily)cut open the surface skin and under-lying cells, allowing enclosed gasesto escape. Rupture also can becaused by equipment used to pack

the backer rod into the joints. Asmentioned earlier, closed cell rodshould not be compressed over 25percent of its diameter. Variation injoint width is so predominant thatover-compression occurs often; thistoo can cause rupture of the skin andunderlying cells.

Over-compression in packing thejoint requires heavier pressure inuse of the packing tool, which in-troduces more chance of rupture ofthe skin and underlying cells, at thepoint of contact of the tool. Themore over-compression, the morepoints of contact, therefore the morechance of rupture. Should the jointbe in a rough masonry wall, theproblem is compounded. The roughsurfaces also will tear open thebacker rod when it is subjected tosuch over-compression.

This problem can be com-pounded further if heat is involved,and rapid expansion of the buildingpanels occurs. The more rapid clo-sure of the joints (and expansion ofthe gases) due to heat forces gasesinto the uncured sealant faster, onlyadding to the troubles.

Another cause of outgassing withclosed cell rod is sharp bends.Again, the surface skin and underly-ing cells can be ruptured. It is betterto break closed cell backer rod atsharp turns than to bend it. Thebreak provides an avenue of escapefor these gases, away from the seal-ant bead.

One proposed partial solution tothis outgassing problem was to packthe joint well in advance of (days be-fore) the caulking operation. How-ever, this would increase labor costssubstantially. The closed cell rodwould take a permanant set, par-tially releasing its grip to thesidewalls. This would risk move-ment during caulking (what’s more,it was found closed cell rod stillgassed, even after a year in a joint).

Outgassing is one problem thatdoes not exist with open cell rod. Thelack of an outer skin (or closedcells) totally eliminates any possi-bility of this occurring. In fact, pre-vention of outgassing was the pri-mary reason open cell backer rodwas developed.

44 THE CONSTRUCTION SPECIFIER/MARCH ’80

Other consequences of heatClosed cell backer rod is a thermo-plastic olefin material. This charac-teristic limits its usage if highertemperatures are encountered. (Themaximum allowable temperaturerecommended by manufacturers is160°F.) Open cell backer rod is athermosetting plastic material withmuch higher heat resistance. It hasbeen used successfully for limitedperiods under hot-applied sealantmaterial (up to 500°F), with no ill ef-fects.

The effects of elevated tempera-tures on successful application of asatisfactory sealant bead are of vitalimportance. New construction inhot climates reportedly will en-counter temperatures of nearly300°F for some materials. A backerrod material installed under suchconditions must be able to with-stand these temperatures withouthaving a detrimental effect onapplied sealants. The possibility ofbacker rod outgassing or shrinkingbrought about by contact with hotsurfaces should be considered care-fully.

Both closed cell and open cell ma-terials have good cold temperaturecharacteristics under normal caulk-ing environments. However, in ex-tremely cold temperatures the opencell has a definite edge, remainingflexible at minus 60°F.

A minor role?

The backer rod in an expansion jointappears to play somewhat of aminor role. After the sealant has at-tained a full cure, the rod serves nofurther purpose other than, perhaps,offering some insulating propertiesin the expansion joints.

Although the role backer rodsplay may seem minor, it is impor-tant that architects, specificationwriters, caulking contractors andbuilding owners ensure that seals inexpansion joints perform well, anddo not become the prematurelyweak link in the structure.

It is in making possible that im-portant seal that the backer rod per-forms its hidden, but essentialtask.

THE CONSTRUCTION SPECIFIER/MARCH ’80 45

From top to bottom: (1) Atypical rough masonry jointwhich will be sealed with acaulking material, after abacker rod has been packedbetween the panels. (2) Anopen cell backer rod beingpacked into an expansionjoint prior to caulking. (3) Aproperly caulked joint inrough masonry, using anopen cell backer rod (notethe absence of bubbles in thecured sealant bed).