I S S U E N O . 1 3

INSIDE

Project ProfileGrouted Stone Facade Kerf

Failures

Tech TipSpecifying

Exterior Grout

Lab NewsPortland Cement and Gypsum - An

ExpansiveCombination

A technical newsletter by Raths, Raths & Johnson, Inc. for the construction industry.

“Insight: to see into and understand; an item of knowledge gained by this power.”

TM

In this issue of RRJ Insight we discuss the effects of using the wrong grout in exteriorapplications such as railing pockets or stone cladding kerfs. The Project Profile articletalks about an investigation RRJ performed in order to determine the cause of stonespalling in an exterior facade. The Tech Tip article discusses how to avoid grout-relatedfailures, including connection failures, by following a few simple rules. And finally, theLab News article shows the expansive results of combining gypsum and portland cement-based grout materials.

Stone structures can develop connection problems if expansive grout is used in the kerfs.

INSIGHT

Project Profile

RRJ was recently called upon todetermine the cause of extensivestone kerf (slot in stone to accom-modate anchor) failures on a newbuilding in Washington, D.C. Theexterior facade consisted of largegranite panels up to 6 inches thickattached to the concrete buildingframe with stainless steel anchorsinserted into grouted plunge-cutkerfs in the edges of the stones(Figure 1). Semicircular spalls hadseparated from the stone panels atfailed locations. The failuresappeared to be clustered inlimited areas, with new spallsreported by the buildingmanager regularly.

From a distance the outwardappearance of the failed kerfsmimicked the conditions typicallyproduced by excessive anchorloads. Calculations evaluatinganchor loads and accompanyingstone stresses from dead loads,wind loads, and thermal expansionand contraction failed to explainthe locations and continuingappearance of the semicircularstone fractures. What then wascausing the problem?

Close examination of some of thefailed stones revealed expansivepatterns within the kerf grout(Figure 5). Laboratorypetrography analysis of the groutidentified clusters of ettringitewithin a matrix of portlandcement, gypsum, and silica sand.Ettringite is formed by anexpansive chemical reactionbetween portland cement and

gypsum under moist conditions(refer to this month’s Tech Tiparticle). The expansive conversionof portland cement and gypsumhad forced the failed kerfs apart,much like air inflating a tire,eventually causing the stone tofracture. Mystery solved! Groutexpansion from weather-relatedwet/dry cycling of the stone (andthe grout within the kerfs) hadcaused the kerf failures.

Review of the stone installer’sproject records revealed a newmystery; no purchases or deliveriesof either portland cement orgypsum were indicated. Instead, incompliance with the ProjectSpecifications, the installer had

purchased only pre-packaged groutmaterials from a majormanufacturer. Early and lateshipments were all of a cement-based product. Partly through theproject, however, a shipment of agypsum-based grout was receivedin lieu of the usual portlandcement-based material. Review ofthe project sequencing andcompletion records revealed theaffected areas corresponded withthe work performed while the twomaterials were concurrently onsite. Laboratory testing by RRJ(refer to this month’s Lab Newsarticle) confirmed thedestructive potential of mixingthe two products.

- Kurt R. Hoigard, P.E

Grouted Stone Facade Kerf Failures

Figure 1 - Plunge cut kerf anchor.

Over the past 15 years many articleshave been written discussing thenegative aspects of using gypsum-based grout products in moistureprone applications such as railingpockets or stone cladding kerfs.Gypsum grouts are made of gypsum(more commonly known as plaster ofParis) and silica sand, with traceamounts of other proprietarysubstances to reduce curingshrinkage. Advantages include rapidinitial set times (change from liquid tosolid), high strength, and low cost.Known problems with gypsumgrouts exposed to moisture includesoftening, wash-out, corrosion ofembedded aluminum and steelcomponents, and sulfate attack ofsurrounding concrete.

Grout manufacturers have developednew materials for exterior use inresponse to the growing concernssurrounding gypsum grouts. Thesenew products typically incorporateportland cement, silica sand, andsmall amounts of admixtures designedto accelerate initial set and reduce

shrinkage. Advantages include goodmoisture resistance, passive protec-tion of embedded steel components,and no sulfate attack of surroundingconcrete. Disadvantages includeslower initial set and higher cost.

The concurrent availability ofportland cement and gypsum-basedgrouts (with some manufacturersoffering both), combined with theslower set and higher cost associatedwith the portland cement-basedproducts, has led to the two producttypes being either inadvertently orpurposely mixed together at the jobsite (adding gypsum to portlandcement-based grouts speeds initial setand reduces cost). Subsequentmoisture exposure of the resultinghybrid typically causes destructiveexpansion from a chemical reactionresulting in the formation of ettringite(a by-product whose volume is greaterthan the original gypsum and portlandcement). Wetting and drying cyclesdriven by exterior exposurecompound the situation by formingmore ettringite with each cycle,

resulting in large volumetricexpansions of the original groutplacement that can split concreteand fail stone cladding kerfconnections (refer to Figures 2, 3 and5 of this issue for more information).

In order to avoid grout-relatedfailures, including the connectionfailures that befell the granite facadediscussed in this month’s ProjectProfile article, RRJ recommendsadhering to the following simplerules when specifying grout forexterior applications:

• Never use gypsum-based grouts inexterior applications or interiorapplications subject to wetting,because softening, wash-out, metalcorrosion, and concrete attack mayoccur.

• Avoid preparing your own “custom”grout. Instead, stick with a provencommercially available product.

• If you must prepare your own grout,never add gypsum to a portlandcement base. Ettringite expansioncan fracture concrete, granite, andother building stones.

• When using commercially availablegrout products avoid mixingdifferent products, even from thesame manufacturer. The hybridformed may have poorerperformance than either of theparent materials and could result inexpansive cracking from theformation of ettringite.

- Kurt R. Hoigard, P.E.

Tech TipSpecifying Exterior Grout

Figure 2 - Expansive grout causes surrounding stone to crack therebyweakening the connection resulting in an unsafe condition.

Lab NewsPortland Cement and Gypsum — An Expansive Combination

The granite kerf failures discussedin this month’s Project Profileprompted RRJ to initiate alaboratory testing program to assessthe destructive potential of mixingportland cement and gypsum-basedgrout products available from amajor manufacturer. This workwas undertaken to confirm RRJ’ssuspicion that mixing of the twoproducts had caused the observedgranite kerf failures via theformation of ettringite (an expan-sive by-product of a chemicalreaction between portland cementand gypsum).

RRJ’s test program included thepreparation of 2 inch grout testcubes (figure 4) and grout-filledkerfs (figure 6) in two types ofgranite and two types of limestone.Six test grouts were prepared withthe ratios of the portland cement andgypsum-based products shown inTable 1. Grout A was the gypsum-based product and Grout B was theportland cement-based product.Laboratory analyses of Grouts Aand B provided information on thebase constituents for each used indetermining the reported portland/gypsum/sand/other ratios aspercents by weight.

After curing according to themanufacturer’s recommendationsthe test specimens were subjectedto repeated cycles of wetting anddrying. Each wet/dry cycle included24 hours of water submersionfollowed by 24 hours in an oven at125 degrees F. The cube specimenswere checked for unrestrained

Figure 3 - Grout cube test plot of percent expansion versus wet/dry cycles.

Table 1 - Test Grout Contents

Grout Cube Expansion

Wet/Dry Cycles

Perc

ent E

xpan

sion

Mix Number Grout A (lbs.) Grout B (lbs.) Portland/Gypsum/Sand/Other Gypsum Portland (percents)

0 5 44.1 / 0.0 / 55.2 / 0.7

1 4 35.7 / 13.6 / 50.2 / 0.5

2 3 27.3 / 27.2 / 45.1 / 0.4

3 2 18.8 / 40.8 / 40.1 / 0.3

4 1 10.4 / 54.4 / 35 / 0.2

5 0 2.0 / 68.0 / 30.0 / 0.0

1

2

3

4

5

6

expansion by visual examination for cracking andperiodic measurement of the cube specimendimensions. The kerf specimens wereperiodically examined for cracking of the stone.The results were astounding. As shown in Figure3, Mix 1 (no gypsum) experienced no expansionor cracking. Mixes 2, 3, and 4 all rapidlydisintegrated due to expansive fracture. Mix 5initially cracked and expanded, but thenstabilized. Mix 6 experienced minor initialcracking, and then shrank in size as surfacematerial dissolved during repeated wetting. Ofthe stone kerf specimens, stone fracture wasobserved on all Mix 2 samples, regardless ofstone type, after only 15 wet/dry cycles (Figure6). After 80 cycles no other kerf failures wereobserved.

The results of these tests confirmed RRJ’ssuspicion that mixing of the two productscould result in expansive fracture of the groutand failure of grout-filled stone kerfs, andformed the basis for the recommendationsprovided in this month’s Tech Tip article.

- Kurt R. Hoigard, P.E.

Figure 5 - Close up of kerf grout pocket. Expansion of grout intojoint and outward caused stone to fail.

Figure 4 - Test grout cube samples after 80wet/dry cycles.

Figure 6 - Stone specimen with expansive grout in the kerfsimulates field performance in the laboratory.

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RRJ is available to help you with any questions you may have regarding our servicesor a specific article in our newsletter.

For additional information call:

Ms. Barbara J. Smith, S.E., P.E.at 1-800-826-6822

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