8/9/2019 2005Win TechTip Brennan

1/218 Shotcrete • Winter 2005

esting Shotcrete for Bond

by Edward Brennan uc s wr tten a out t e requ rementsfor materials (sand, aggregates, cementan a m xtures , equ pment, p acement

methods (wet versus dry), and personnel withrespect to t e s otcrete process. But, ow o wecheck the quality of the nished product in placea ter nsta at on?

When testing shotcrete in place, which perfor-mance cr ter a matter t e most? Genera y, w tmost shotcrete applications, the density, strength incompress on, goo res stance to weat er ng reez ng-and-thawing durability) and bond to substratemater a s matter t e most. Preconstruct on test ngof the shotcrete materials used can yield predeter-m ne va ues or t e rst t ree. Per ormance

parameters in job-site testing , however, are requiredto assess t e qua ty o on .

Laboratory testing of prepackaged materials toe use at a spec e water-cement rat o can

produce resu lt s that closely correlate wi thtest resu ts on actua s otcrete n t e e . T eselaboratory test results will depend on the w/c beingcons stent w t t at ac eve n t e e anthe weather conditions at the job site not being

rast ca y erent. O v ous y, t e est test ngmethod for the shotcrete is to conduct tests onsamp es cut or core rom actua s ot pane s orthe in-place shotcrete) rather than specimens castn t e a oratory. W at mean ng u resu ts o we

actually receive from shotcrete test panels? Compress ve strengt ASTM C 42 ; Density absorption and permeable voids

ASTM C 642 ; Freezing-and-thawing durability (ASTM C 666); F exura strengt ASTM C 78 ; an Modulus of elasticity (ASTM C 465).

In t e nsta at on o an -app e concreterepair materials, the bond test frequentlyre erence or many years as een t e s antshear test, better dened as ASTM C 882 or ASTMC 1042. For ot o t ese test met o s, an s oconcrete are cast with a 60-degree angle and a base4 n. 100 mm n ameter. T e an s are t ennormally cured for 28 days. The surface of the

an s prepare to ac tate goo on etween

the 28-day-old blank and the new material. Therepair material is placed in the mold on the blank.At the desired age ( 1, 7, 14, and 28 days), the 4 x8 in. diameter (100 mm x 200 m) cylinder receivesa compressive load and the resulting failuredetermines the bond strength. With respect to bothASTM C 882 and C 1042, both tests’ original

function was to identify the effectiveness of a bonding agent applied to the surface. Because theuse of bonding agents is not recommendedfor shotcrete applications, and casting the repairshotcrete against the concrete blank does not

provide the same quality of bond as shooting, thistest method is not recommended for shotcrete.

In recognition of the limitations of the slant sheartest method as it relates to the bonding of newshotcrete material to a substrate, the InternationalConcrete Repair Institute (ICRI) developed its own testfor measuring the quality of bond of new concrete

to existing concrete surfaces. The resulting ICRITechnical Guideline No. 03739 provides a directtensile test for in-place testing for the purpose ofevaluating bond in concrete structures. The differ-ences between this test and the previously referenceds ant s ear test are o v ous. ICRI as eterm ne t attensile loading can provide a meaningful reectiono t e a ty o a concrete mater a to on to asubstrate. ICRI Technical Guideline No. 03739s s m ar to t e tens e on strengt test s own

in ACI 506.4R, Fig. 3.2, and the just-publishedASTM C 1583-04, “Test Met o or Tens e Strengtof Concrete Surfaces and the Bond Strength orTens e Strengt o Concrete Repa r an Over ayMethods by Direct Tension (Pull-off Method).”

W en app y ng concrete s otcrete to ex st ngconcrete, the intent is to create a composite system,t at s, a com nat on o mu t p e mater a s t atresult in a monolith. Effective and lasting bond

etween t e mater a s s requ re to proper ycreate the single composite system.

Creat ng proper on requ res: Proper surface preparation including creatingan a equate sur ace roug ness pro e re er toICRI Technical Guideline No. 03732) and propermo sture precon t on ng o t e su strate;

8/9/2019 2005Win TechTip Brennan

2/2hotcrete Winter 2005 19

Edward Brennan managesUS Concrete Pro ucts in

Baltimore, MD. He is a Past Presi ent o t e AmericanShotcrete Association (ASA),

as een an ASA Director or3 years, and currently serveson t e Pu ications Committee.

He also serves as a local board member for the Internationa Concrete Repair Institute ICRI . Brennan has been in the industry for more than

15 years.

• Good compaction between the new materialan t e ex st ng su strate;

• Similarity/compatibility of materials;• Contro e cur ng o t e new mater a ; an• Controlled job site conditions, particularly

w t respect to s otcrete su strate an am enttemperature conditions.Proper a erence to t e prev ous requ rements

creates a composite of the new material and existings u strate. In-p ace on tes t ng can t encommence at appropriate ages.

T e rect tens e test met o requ res cor ngof the composite concrete sample by using a core

t to r t roug t e new y app e s otcrete anhe substrate concrete. Once the core has been cut,

a r g stee s s attac e to t e top o t e coreusing a high-strength adhesive (epoxy).

T e test ng ev ce app es a tens e oa per- pendicular to the bond line. The testing deviceapp es t e oa at a constant rate unt a ure. T efailure load is reported and divided by the crosssect ona a rea to eterm ne t e tens e onstrength at failure in lb/in. (MPa).

T s rect tens on test met o measures t e onof shotcrete to concrete and other substrates in acre e way. It s mportant to note t at t e resu tsof the slant shear test will generally be much higher,as state n n. MPa , t an t e resu ts o ta nefrom the direct tension test. Thus, performance

equ rements etween t e two test met o s wdiffer substantially and should not be confused.Common y spec e rect tens e on strengt

alues for shotcrete to properly prepared concretesu strates ten to range rom 100 to 150 n n.(0.69 to 1.00 MPa).

In summary, t e rect tens e on strengt testethod provides the best means of evaluating the

on o n-p ace s otcrete. For eta e n ormat onon ICRI Technical Guideline No. 03739, go to

ww.icri.org .

Calculating the tensile bond strength:

U.S. un ts: Metr c un ts:

Tota pu -o orce = 1500 6672 N

Core ameter = 3.5 n. 88.5 m

Core area = D 4 = 3.14( 3.5) /4 = 9.61 in.

88.9 4 = 6204 mm

Tensile bond strength =1530 9.61 = 159 ps

6672 N/6204 mm