ADMIXTURE FOR CONTROLLINGBLEED IN CEMENT GROUTUSED IN POST-TENSIONING

Morris SchupackSchupack Associates and CompanyConsulting EngineersStamford, Connecticut

The bleed characteristics of neat cement grout are wellknown. With usual mixing procedures and groutadmixtures it can be expected that water separationwill occur due to sedimentation of the suspension ofcement and admixture in water. This is particularlyexaggerated with strand-type tendons.To control this characteristic and the general pumpingproblems of grout blockages with dewatering, a groutadmixture has been developed which can control thebleed as desired. The basic research in developingthis admixture used short strand tendons to determinethe different water retentive characteristic ofadmixtures. However, based on these tests, it was foundthat the vertical scale factor of pressure is important.Thus, a pressure filter test was also used to determinethe water retentive characteristic of various grouts.Based on this research, a grout admixture wasdeveloped with the property of high water retentivitysuch that for instance no water loss will occur in thegrout when the pressure is maintained at 10 atmospheresover 10 min. For shorter durations, water retentivityis fully maintained for even higher pressures.

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The sedimentation or bleeding charac-teristics of a neat portland cement groutare well known. With usual mixing pro-cedures and grout admixtures it can beexpected that water separation will oc-cur because of sedimentation of the sus-pension of cement and admixtures inwater (see Fig. 1).

It so happens that the water separa-tion is exaggerated with strand-typetendons which have a random degree ofvertical orientation. A filtering phenom-enon occurs with seven-wire strands orprobably any type of strand which haveouter wires tightly wrapped around oneor more center wires. Apparently, thesmall spaces between the outer wiresare large enough to permit passage ofwater but too small to permit most ce-ment particles to pass.

With water in the inner space andgrout surrounding it, a hydraulic driv-ing force is developed since groutweighs about 120 pcf (1920 kg/m3)whereas water weighs 62.4 pcf (1000kg/m3). This difference in head pushes

Morris Schupack

water through the outer wires and upthe interstices in the strand above thegrout as can be seen in the simple testof placing 3 ft (1 m) of strand in atransparent tube containing say 2 ft(0.7 m) of freshly mixed grout.

TOP

Jtarti ngLevel ofGrout

104

103

102

101

100

99

98

97

96

TOP OFI WATER

104

103

102

101

100

99

98

97

98

•. 95 .,

•950 2hr 4hr 6hr 8hr 0 2hr 4hr 6hr 8hr 0 2hr 4hr 6hr 8hr

NEAT GROUT 3-5/8 $ BARS 7—/2STRANDS

Fig. 1. Comparison of expansion and sedimentation. Note—Admixture: IntraplastRD-4 (water-reducing and expansion). Cement: Type II. 1.6-in. (4 cm) diameter

tubes, 14-ft (4 m) high.

PCI Journal/November-December 1974 29

Fig. 2. Bleed void at top of 20-ft t (6 m)fifty-four I/z-in, diameter strand verticaltest specimen amounting to 15 percent

of vertical height.

Fig. 3. Bleed test using 2-ft (0.6 m)high tube with 1.5 ft (0.45 m) of groutin which a 1/2-in. seven-wire strand is

placed.

Fig. 2 indicates that this bleed or wa-ter separation can accumulate to 15 per-cent of the vertical grout column. Invertically oriented tendons, used for in-stance in nuclear pressure vessels, rockanchors, and columns, voids such asshown in Fig. 2, are clearly unaccepta-ble. Furthermore, horizontal tendonswith vertical curvature would also ac-cumulate water voids at the high pointsof the tendon duct.

Recognizing the need for a solutionto the problem which is satisfactory forboth performance and efficient con-struction, an invesitgation was initiatedby the author. This was particularlyprompted by the need to grout 200-ft(60 m) vertical tendons for a nuclearcontainment structure in the near fu-ture.

REDUCED SCALE GROUT

DEVELOPMENT TESTS

The principal effort of the investiga-tion was directed to ascertain if admix-tures could be used to thwart the bleedphenomenon in a strand tendon. It wasfound that just a 2-ft (0.6 m) high tube(Fig. 3) containing one strand wouldexhibit the bleeding phenomenon andcould be used to determine whether ornot a particular mix would decrease wa-ter separation. Extensive testing, inwhich over 20 different ingredients andcombinations were used, indicated thatthe use of relatively high quantities ofgelling agents might be a solution.

After selecting the most promisingthickener (gelling agent), tests weremade with 10-ft (3 m) high tubes (Fig.4) containing strands to see if the verti-cal scale factor had an important in-fluence on water separation. It wasfound that the scale factor does have aneffect, but that the basic direction ofincorporating a gelling agent appearedsound.

Based on the encouraging resultswith vertical 2- and 10-ft (0.6 and 3 m)

30

tests, two 40-ft (12 m) vertical testswere undertaken. Grouts with thicken-ers were used in the two 40-ft (12 m)high tests, one with an expansion agentand the other without.

The test results indicated that the useof an expansion agent as well as a thick-ener was desirable. Although the gasevolved from the expansion seems topromote bleeding in the presence ofstrands, most of this effect was re-strained by the presence of the thicken-er. Furthermore, the minimal expansioncompensated for any settlement, helpedexpel any bleed water that might haveevolved and facilitated complete pene-tration of the grout through all inter-stices in the anchorage components.

Results of the 40-ft (12 m) tests werepromising but the following problemsremain to be resolved:

1. Generally, inadequate mixingequipment was used in all of the abovetests. Since better mixing is likely to im-prove results, a test of various mixerswas conducted in a parallel effort.

2. Admixtures were specially pre-blended into the cement by the cementsuppliers. This dry blending had theadvantage of better dispersion of theadmixture throughout the cement massprior to incorporation of water; how-ever, production blending of the expan-sion agent is logistically difficult andmay be undesirable if done too far inadvance.

3. The influence of the vertical scalefactor was not fully known. A promisingperformance in a 40-ft (12 m) tendondid not assure performance in a 200-ft(60 m) tendon due to the larger pres-sure difference.

The latter two problems suggestedthat a means of model testing the bleedphenomenon of a tall tendon should bedeveloped and that a packaged com-pound suitable for field admixing wouldbe desirable.

Fig. 4. Bleed test using 10-ft (3 m)high tubes containing a % in. diameterstrand using various amounts and types

of gelling agents.

PRESSURE TESTS OFSTRAND TENDON MODEL

Pressure modelSince the strand essentially acts as a

filter in separating the cement particlesfrom the water under pressure, a con-cept of modeling the strand with a pres-sure vessel and a filter was developedand used to determine the effectivenessof various mixes.

The type of pressure vessel used wasa commercially available Gelman Pres-sure Filter Vessel (Fig. 5) which canoperate up to 200-psi (14 kg/cm2)pressure. Pilot tests with the apparatuswere performed to select the type offilter.

It was found that water separationobtained with a Gelman Type A fiber-glass filter could be reasonably correlat-ed to those obtained with earlier strandtests. This filter, which retains 99.7 per-cent of all particles greater than 0.3microns, was selected.

PCI Journal/November-December 1974 31

Fig. 5 Pressure filter, used as a strandfilter model, to evaluate water retentioncharacteristics of different cementgrouts. The concrete cylinder or plugshown on the right is actually a dehy-drated grout sample (with a conven-tional admixture) removed from pres-sure filter after applying about one-half

atmospheric pressure.

Testing procedureIn an effort to simulate the actual

field conditions of 200-ft (60 m) verti-cal tendons as closely as possible, thegrout was mixed for about 5 min witha shear mixer, placed in the pressurefilter and left to stand without agita-t^on for 10 min. Then pressure was ap-plied in 10-psi (0.7 kg/em 2) incre-ments at intervals of 3 min.

Thus, to obtain a pressure of 80 psi(5,6 kg/cm2), 24 min would elapse.This corresponds approximately to thepressure difference between 200-ft (60m) columns of grout and water and tothe tme that would be required to fill a

200-ft (60 m) high tendon composedof fifty-four 1 -in, diameter strands.

Having established a testing proce-dure, it was then possible to comparethe relative performance when param-eters such as brands of cement, differenttypes of cement, mixing time, mixingmethod, and conventional admixtureswere varied.

After extensive testing, results indi-cated that none of these variables hadany significant effect on water retention.It was surprising to find that vigorousextended mixing did not seem to im-prove water retention significantly.

Development of grouting admixtureusing strand tendon model

Numerous grouting admixtures andcombinations of admixtures were testedin various proportions in order to isolatethe most effective of the admixtures andto optimize the proportions. In a typicaltest, as noted, the pressure was in-creased in 10-psi (0.7 kg/cm2) incre-ments to 80 psi (5.6 kg /cm2).

As pressure was applied, the thresh-old pressure was noted at which wa-ter (if any) was forced through the fil-ter. Any water forced out of the groutwas collected and measured. The timeand pressure at which first water sep-aration occurred and the amount of wa-ter collected were used as an indicationof water retentive properties.

Of all the combinations, one contain-ing a gelling agent and water reducerswas found to be the most effective.

The tests performed with the pres-sure filter indicated that there was athreshold at which the water retentivityof different admixtures would breakdown. It was noted that using the classi-cal grout admixtures commerciallyavailable, a very rapid loss of water oc-curred and a solid cylinder of grout re-mained (see Fig. 5) before 10-psi (0.8kg/cm2) pressure was reached.

Virtually complete separation of thegrout and free water took place. Usingwater reducers or dispersents alone ac-

32

90

LL 80

200

0 70

F- v^60vo

150w

N 50

' 4C100 n

_ 3C

t- o.

50 2CoYw

1C

PRESSURE AT WHICH FIRST WATERLOSS OCCURS FOR DIFFERENTPERCENTS OF "GROUTING AID"

SYM. W/C

q000 0.41

0.450.47

0.50

___4- H

2.0UL 0

0 0.4 0.8 1.2 1.6

AMOUNT OF ADMIXTURE (% OF CEMENT WT.)

Fig. 6. Equivalent tendon height and pressure versus amount ofgrouting admixture.

tually increased the rate of water sep-aration.

Based on data from the pressure filtertests, Fig. 6 relates the proportion of theoptimized grouting aid, as a percentageof cement weight, required to maintainwater retention of the grout in the pres-sure filter for a given applied pressure.This is determined by observing thepressure at which the first water lossoccurs for different percentages of

grouting admixture.As can be seen, for a given proportion

of grouting aid there is a threshold pres-sure which causes water separation tooccur. This stresses the importance ofthe scale factor.

Note that without any grouting ad-mixture the water retention was lost onapplication of only a small pressure aswould be expected. Fig. 7 relates theproportion of grouting admixture to the

PERCENTAGE WATER LOSS OF INITIAL WATER

40VOLUME AT 80 PSI IN 24 MINUTES FORDIFFERENT PERCENTS OF "GROUTING AID"

30SYM. W/C

co

20 * 0.45N V 0.47-

10 o 0.50wF--

3

0 0.4 0.8 1.2 6 C

AMOUNT OF ADMIXTURE (% OF CEMENT WT.)

Fig. 7. Water loss versus amount of grouting admixture.

PCI Journal/November-December 1974 33

Fig. 8. Pressure vessel with strand usedas filter to check "pressure filter model."

Fig. 9. Pressure vessel with strandshowing grout and water loss afterbuilding pressure up to 90 psi (six atmo-

spheres) in about 30 min.

water loss at 80 psi (5.6 kg/cm 2) mea-sured as a percentage of the originalamount of water in the sample.

To confirm that the pressure filterwas a reasonable model, a test was de-signed by Burns and Roe, designers of agrouted nuclear containment structurefor General Public Utilities.

The test consisted of a pressure ves-sel into which a strand was sealed in,with one end penetrating but sealed onthe outside as shown in Fig. 8. Groutleakage occurred through the strandand probably some water as shown inFig. 9.

The results of several tests using thisdevice confirmed that the pressure filterwas a practical method of determiningthe water retention characteristics ofgrout. These tests, carried out simul-taneously with pressure filter tests, useda grout mixed in a full-sized shear mix-er, rather than small laboratory mixerswhich were used for the developmentalfilter tests.

FULL-SCALE TESTSOF ADMIXTURE

Since the resulting grout is basicallythixotropic, various full-scale tests wereperformed to determine the best pro-cedure in mixing, pumpability, andgeneral water retention under field con-ditions. Some of these tests were:

MixingFor greatest ease in mixing, preblend-

ing of the grouting admixture is theoret-ically most desirable. This, however,creates a logistic problem in requiringthe preblending or field blending of thematerials.

It was found that for quantities ofgrouting admixture less than about 0.8percent by weight of the cement, mostpropellor-type mixers would work well.For quantities over about 1 percent byweight of cement, shear mixers or highenergy propellor mixers were prefer-able.

34

The nature of the grouting aid is such, that when added to the surface of the grout, surface gelling occurs. This has to be broken up by the mixer.

It was found that the best way of adding the grouting admixture was to place it through a perforated long tube into the vortex of previously mixed ce- ment and water.

Furthermore, it was found that mix- ing time is increased somewhat for this grouting admixture. Since for most ap- plications only about 0.6 percent of grouting admixture by weight of ce- ment would be used, most usual mixing procedures would suffice, except that the grouting admixture cannot be plac-d directly into the water. I t has to be placed in the already mixed grout.

The preferable mixing sequence based on experience to date is to use a shear-type mixer using the loading se- quence of water, cement, mix until well dispersed, and then incorporate the grouting admixture gradually while mixing.

Pumpability Grout was pumped through a 30-ft

(9 m) length of 2Y4-in. flexible metal conduit with constricted end fittings in a qualitative test of pumpability. This conduit was not surrounded by concrete and was the usual flexible conduit used for enclosing post-tensioning tendons. Pumping was stopped and resumed with no difficulty.

The ease with which grout can be pumped depends on the water-cement ratio, how well it is mixed, the propor- tion of grouting aid, and to some ex- tent, the fineness of the cement. Using cement with a Blaine fineness in the or- der of 3800, grout as thick as made with 0.41 water-cement ratio and 1.9 per- cent grouting admixture have been pumped.

In one particular test, grouts with 0.48 water-cement ratio and 1.6 per- cent grouting admixture, required the same power to be pumped as conven-

PCI Journal/November-December 1974

tional water-cement grout incorporating a water reducer. This seems to indicate that even though the grout has thixo- tropic properties, in most cases it can be pumped as readily as conventional grout.

Water retention The pumping test made above, and

another test made with 70-ft (22 m) length of 21%-in. (6 cm) diameter flexi- ble metal hose, indicated absolutely no leakage of water or grout through the joints in the flexible metal hose even pumping against a 30-ft (9 m) head. The dramatic difference could be seen when water alone was pumped through the same hose; water leaked profusely.

Penetrability

Several tests were made using nhe- teen ?$-in. diameter strands strapped together in a 4-in. (10 cm) coyduit. These were later cut and complete fill- ing of the spaces between the strands was achieved (Fig. 10).

To further prove the suitability of the thixotropic grout on curvature, Burns and Roe had a test run Ily Stres- steel Corp. in which a 54-strand te l - sioned tendon on a 20-ft (6 m) radius and 30 ft (9 m) long was grouted. The excellent results of a section through the grouted tendon is shown in Fig. 11.

Test of 54-strand 30-ft (9 m) high vertical tendon

To determine that a large group of strands in a 30-ft (9 m) high tendon does not have undetermined perfor- mance which would negate the results of the filter tests, Burns and Roe tested a 54-strand tendon at a 30-ft (9 m) height. Using 1.5 percent grouting ad- mixture by weight of cement, the test indicated freedom from water separa- tion.

The above test is a prelude to a 180- A (54 m) test which will be run for the same project for General Public Utili- ties Service Corporation.

Fig. 10. Section cut through nineteen %-in. diameter tendon using grouting

admixture.

Fig. 11 . Grout penetration test section through fifty-four %-in. diumeter strand tendon on a 420-ft. (6 m) radius, stressed to 80 percent of ultimate

strength.

PROPERTIES OF GROUT CONTAINING ADMIXTURE

In general, there are three important properties of grout containing a water retentive admixture, namely, setting time, strength, and porosity.

Setting time The setting time of grout is a func-

tion of the characteristics of the cement, proportion of grouting admixture, tem- perature, and to a lesser extent, water- cement ratio. The values shown in Ta- ble 1 were obtained for grouts pre- pared with a 0.47 water-cement ratio and a normal setting cement kept at 70 F (20 C):

Strength The cube compressive strengths at 28

days were in the 3700 to 5000-psi (260 to 350 kg/cm2) range for different ce- ments, 1.3 percent admixture, and 0.45 water-cement ratio.

Porosity The porosity could be described

qualitatively as low and comparable to other grouts used. -

The prime purpose of this develop- ment was to achieve the desired water retention of a cement ~ a s t e . Besides ac- complishing this, several other interest- ing characteristics of this type of grout admixture were found. These are:

1. Once the grout is mixed, it is dif- ficult to intermix more water without supplying adequatc encrgy. This makes cleaning equipment more deliberate but has the significa~~t advantage of displac- ing water with a minimum of dilution. This property can be controlled by the quantity of grouting admixture used.

2. With quantities generally in ex- cess of 0.6 percent by weight of ce- ment, the material is apparently thixo- tropic. The thixotrophy can be con- trolled to the extent one desires by quantities of grouting admixture. In fact, the apparent viscosity can be such

that the grout can be handled in acaulking gun. It should be pointed outthat high quantities of grouting aid re-duce strength and increase retardation.

3. Based on the work done to date,the grouting aid does not appear to besensitive to the type or brand of cement.

4. The grouting aid has been used upto a grout temperature of 95 F (35 C)without difficulties. This temperature,however, is not recommended.

PRACTICAL APPLICATIONS

At the present time, the patentedgrouting admixture is available throughSika Chemical Corporation in theUnited States only. Later on, arrange-ments will be made for distribution ofthe grouting admixture in other coun-tries.

The material has been selected by theTennessee Valley Authority for grout-ing rock anchors.

It is being considered by Burns andRoe for General Public Utilities SeviceCorp. for the complete grouting ofpost-tensioned tendons for a nuclearcontainment vessel subject to the com-pletion of the 180-ft (54 m) verticaltendon demonstration test.

One interesting application of the ad-mixture has been the grouting of an in-terconnected system of ten 130-ft (40m) thirteen 1/z-in, diameter strandtendons. These tendons are 70 ft (21m) above the grout pump with about2000 ft (600 m) of interconnected hoseand conduit with valves at the end ofeach tendon.

The grouting procedure is showndiagrammatically in Fig. 12. The rea-son for this approach, as reported byWalt Driscoll of Stresstek Corp., sup-plier of the tendons was:

"The proximity of traffic, pedestriansand architectural concrete prohibited theusual dirty method of pumping groutthrough and splashing it out a vent onthe ground until it reaches its proper

Table 1. Setting time (in hr) of groutfor various percentages of admixture.

Time of initialProportion of setting from initial

admixture by weight water-cement ratioof cement, percent contact, hr

0 71/20.8 (for usual

tendon) 111.1 (up to 100 ft

vertical rise) 121.3 (up to 150 ft

vertical rise) 131/21.6 (up to 200 ft

plus vertical rise) 141/2

consistency. It therefore became neces-sary, in order to run a clean groutingoperation, to interconnect a series oftendons in a series of beams with grout-ing hose and then conduct the spillageout away from the structure to a safelocation at ground level."

"The grout mixing operations were setup at a central location on the groundlevel, once again to avoid making a mess.This resulted in pumping distance of ap-proximately 2000 ft (610 m) when lengthsof grout hose 3/4 in. (2 cm) and tendonsheathing 23/4 in. (7 cm) with thirteenrig-in, diameter strands were added. Withthe 80 ft (21 m) static head on the topfloor, grouting pressures at the pumpran continuously at 250 to 300 psi (17 to20 atmospheres) with the ambient temper-ature 85 F (30 C).°"

"After the tendons and beams were in-terconnected, clear water was flushedthrough the system as much to check forleakages as to determine whether theremight be obstructions. With 2000 ft oflines full of grout you can't afford to stopto replace a grout cap gasket or exchangea leaky hose connection. Upon completionof water pressurizing, the water was leftin the system and the grout allowed todrive it out. A copius quantity of grout wasexpelled, once the system was filled, toinsure there was no dilution. In fact, verylittle mixing took place. Good grout fol-lowed clean water out of the exit hose ina rather abrupt fashion. The grout re-mained homogeneous and fluid under highpressure, it didn't bleed and left all thegrout caps full."

PCI Journal/November-December 1974 37

2-SPAN CONTINUOUS BEAM

61 FT. (19 M.) 61 FT. (19 M.)

UWASTE ATGROUND LEVEL

10 CONNECTEDTENDONS

TOTAL GROUTINGLENGTH 2000 FT.(600 M.)

ELEV. 7 FT (21 M.)

ELEV. 0

Fig. 12. Typical interconnecting grouting system for 10 two-span continuous ten-dons. The total grouting path length is approximately 2000 ft (600 m).

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CONCLUSION

The use of the water retentive grout-ing admixture seems to resolve theproblem of water separation from ce-ment grout.

Particularly for tendons with any ver-tical rise, the proper use of the grout-ing admixture will greatly minimize oreliminate the bleed phenomenon.

For vertical strand type tendons, two-stage grouting can be avoided.

which were performed at StressteelCorp.

The author, as consultant to GeneralPublic Utilities observed and helpedguide the above work and performedadditional small scale and model testsfrom which the water retentive grout-ing admixture was developed.

REFERENCES

1. Schupack, Morris, "Grouting Testson Large Post-Tensioning Tendonsfor Secondary Nuclear ContainmentStructures," PCI JOURNAL, Vol.16, No. 2, March-April 1971, pp.85-97.Wern, Allen H.; Schupack, Morris,and Larson, Walter; "PrestressingSystem for H. B. Robinson NuclearPlant," Journal of the Power Divi-

sion, ASCE, March 1971."Tendon Grouting Means," UnitedStates Patent No. 3,762,937, Octo-ber 2, 1973.

ACKNOWLEDGMENT

Most of the tests for the strand ten-dons were sponsored by General Public 2.Utilities Service Corp. for its two sub-sidiaries, Jersey Central Power andLight Company and Metropolitan Edi-son Company.

Burns and Roe, architect-engineers, 3.planned the large tendon tests and theverification test of the pressure filter

Based on a paper presented at theFIP/PCI Congress, New York City, May 30, 1974.Discussion of this paper is invited.Please forward your discussion to PCIHeadquarters by April 1, 1975.

PCI Journal/November-December 1974 39