NATIONAL BUREAU OF STANDARDS REPORT

6529

Flexural Tests of Prestressed Cellular Slabs

(Slabs F-3 and F-4)

by

Arthur F. Kirstein and Leopold F„ Skoda

Report to

Bureau of Yards and Docks

Department of the Navy

<qjbs>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1001-12-10411 September 8, 1959 6529

Flexural Tests of Prestressed Cellular Slabs

(Slabs F-3 and F-4)

by

Arthur F. Kirstein and Leopold F. Skoda

To

Bureau of Yards and Docks

Department of the Navy

IMPORTANT NOTICE

NATIONAL BUREAU OF ST

intended for use yvithin the

to additional evaluation and r listing of this Report, either the Office of the Director, Na

however, by the Government

to reproduce additional copie

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015.

rogress accounting documents

mally published it is subjected

reproduction, or open-literature

;ion is obtained in writing from Such permission is not needed,

prepared if that agency wishes

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

FLEXURAL TESTS OF PRESTRESSED CELLULAR SLABS

(Slabs F-3 and F-4)

Arthur F„ Kirstein and Leopold F„ Skoda

Two additonal specially designed specimens were constructed and subjected to pure bending following the completion of

the investigation in which twenty nine 5- by 5-ft prestressed

cellular slabs tested under concentrated loads (see NBS Report

6321). One slab was made of reinforced blocks and the other was made of unreinforced blocks. Both slabs contained epoxy resin

joints and were prestressed to 1000 psi in both directions. Deflections, strains, crack patterns, and maximum bending moments were recorded. Since the cellular portions of the slabs were

not subjected to shear, both slabs developed flexural failures

as the top flanges of the blocks failed in compression.

1. INTRODUCTION

In previous tests carried out in the investigation of properties of

prestressed cellular slabs, the specimens were 5- by 5-ft in size supported either on two or four edges and subjected to a concentrated load

at the center. The information derived from these tests served as the

basis for modification of the design of the cellular blocks and the

method of jointing which resulted in substantial increases in the shear

resistance of these slabs.

In the current phase of the study of these slabs, the object is to

determine the flexural behavior of prestressed assemblies. The test re- sults of two prestressed slabs subjected to pure bending are presented

herein as a continuation of an investigation presented in NBS Report 5825.

The results of this investigation are expected to serve as a basis for

further improvements in this type of construction.

2. DESCRIPTION OF TEST SPECIMENS

2.1 Cellular Blocks

The cellular concrete blocks used in this investigation were the

regular reinforced and unreinforced NBS blocks. They were hollow 6-in.

cubes having an opening 4.5- by 4.5-in. in cross section with a 1- by

2-in. elliptical access hole in each web to permit the passage of the

prestressing tendon. Both the reinforced and unreinforced block had the same principal dimensions which are shown in figure 1.
;
2 .

The blocks were made of a mix proportioned of one part Type III

cement and three parts of sand, by weight, with a water-cement ratio of

0.57. This mix had a 7-day compressive strength of approximately 6000

psi as determined by tests of 2-in. cubes. However, the actual units were moist-cured for a considerably longer period of time and then

placed in dry storage before they were assembled into slabs. Therefore,

the compressive strength of the concrete in the individual units might

have been somewhat greater.

The Young's modulus and Poisson's ratio of the concrete were determined in previous tests of the concrete units and other specimens. Axial

compression tests and sonic modulus tests indicated an average Young's

modulus of 4 x 10^ psi with a variation of ± 10 percent. Poisson's ratio

was found to be approximately 0.15.

2.2 Prestressing Steel

"Elastuff" steel prestressing tendons were used in the slabs. Tensile tests of this material indicated a stress-strain relationship that was essentially linear up to 95,000 psi, and exhibited a Young's modulus of approximately 30 x 10^ psi. The yield strength of the bar was

120,000 psi as determined by the 0.2 percent offset method and the tensile strength was 133,000 psi. Although "Elastuff" bars are made of the

cold-worked high carbon steel, they are fairly ductile and can be machined

easily.

2.3 Description 6f Prestressed Slabs

There were two slabs tested in this investigation. The nominal dimensions of the portion of Slab F-3 which was in pure bending were 2- ft

by 5-ft by 6-in. This portion of the slab contained 40 reinforced blocks.

The nominal dimensions of the similar portion of Slab F-4 were 5-ft by

5-ft by 6-in. This portion of Slab F-4 contained 100 unreinforced blocks.

The blocks in both slabs were arranged in a crisscross pattern so that the

axis through the open ends of one block was perpendicular to the axis of

each adjacent block. The holes in the webs of the blocks were arranged

to permit the longitudinal prestressing tendons to be located along the

mid-plane of the slab, and the transverse tendons were staggered above and

below the mid-plane. Thus, the resultant prestressing forces produced an axial compression of 1000 psi in two directions through the slabs.

Epoxy resin was used as jointing material in both slabs. This adhesive was made of a mix of 100 parts Epon 828, 8 parts diethy lenetr iamine,

and 5 parts Cab-o-sil, by weight. XJ

_1 / Epon 828 is an epoxy resin that cures at room temperature; diethylenetriamine is the curing agent for the epoxy resin, and Cab-o-sil is a finely divided silica filler.

3.

Both slabs were equipped with solid concrete end spans in the

longitudinal direction which resisted the shearing forces of the load

and served as anchorage blocks. With the use of these end spans and

the loading technique described later, it was possible to subject the

test section of the specimen to pure flexure.

2.4 Prestressing Procedure

The tensioning force was applied to the prestressing bars by

means of a hydraulic jacking rig that was equipped with a dynamometer

to determine the applied prestressing force. The calibration curve for

the dynamometer is shown in figure 2.

Approximately one-half of the prestress was applied to the slab in

small increments by tightening the anchorage nuts with a wrench. The

remaining prestressing force was applied by means of the hydraulic jacking rig. This final stage of the prestressing operation was accomplished

by using a suitable sequence of stressing the tendons so that no unduly

large differences in strain would be induced in the blocks.

3. TESTING PROCEDURE

3 . 1 Test Setup

A 600,000 lb capacity hydraulic testing machine was used to test

the slabs. Slab F-3 was simply supported by rockers that rested on the

testing machine platen, and the loads were applied through a knife edge

at one end of the test section and rollers at the other end. The loads were distributed across the ends of the concrete shear spans with stiffened I-beams. Figure 3 is a photograph of Slab F-3 and the test setup.

Slab F-4 and the test setup are shown in figure 4. This slab was simply

supported by I-beams that rested on the testing machine platen. In order

to give the slab more freedom to rotate at the supports 1- by 1-in.

aluminum bars were placed between the I-beam supports and the concrete

end spans. The loads were applied through 12- by 12- by 6- in. concrete

blocks which were arranged as shown (Fig. 4). All surfaces bearing on

the test specimens were firmly set in high strength plaster to ensure

intimate contact.

3.2 Instrumentation

The deflection measurements of Slab F-3 were made with 0.001-in.

micrometer dial gages attached to steel angles resting on the slab

directly over the joints between the test section and the end shear

spans. Thus, the datum plane was located on the top surface of the

specimen and at the ends of the test section. Figures 3 and 5 show

the symmetrical arrangement of these gages. The deflections of Slab F-4 were also measured with 0.001-in. micrometer dial gages which were attached to steel angles. These angles were supported on the top surface

4.

of the specimen directly over the supports. Thus, the datum plane for

Slab F-4 was located at the supports. This method of support for the

dial gages is shown in figure 4 and the symmetrical arrangement of

these gages is shown in figures 4 and 6.

The strains in the concrete on the top and bottom surfaces of the

slabs were measured with bonded wire strain gages of the AR- 1 and A-3

types. The locations of these gages are shown in figures 5 and 6.

Bonded wire strain gages of the A-3 type were placed at the center

of the two outer prestressing tendons of both slabs to measure the

strains in the steel due to the applied load.

3.3 Test Procedure

Slab F-3 was loaded as shown in figure 3. The load was applied in

increments of 1,000 lb, and gage readings were made for each increment

until the maximum load was reached. Figure 4 shows how Slab F-4 was

loaded. The load was applied in 2,000 lb increments up to 10,000 lb.

Then increments of 5,000 lb were applied until failure. Gage readings were made for each increment of load.

4. TEST DATA

4.1 Deflections and Load-Carrying Capacities of Slabs

Slab F-3 carried a maximum applied load of 16,350 lb and Slab F-4

carried a maximum of 35,000 lb which correspond, respectively, to total

bending moments, Mu, (including dead load) of 217.8 and 489.4 in-kips.

During the tests, signs of distress were noted in both slabs at moments

considerably below the maximum. At about 0.65 applied/ultimate moment,

(141.6 in-kips), audible cracking took place in Slab F-3 which is indicated by the change of slope of the deflection curve in figure 7. Audible cracking in Slab F-4 occurred at a moment of 351.7 in-kips, but this

is not indicated by the corresponding curve in figure 7 because the deflections exceeded the range of the dial gages.

The deflections along transveerse sections on Slab F-4 are shown

in figure 8. The deflections presented in this figure are total deflections measured with respect to a datum plane through the supports.

4.2 Concrete and Steel Strains

The steel strains are plotted against the ratio of applied moment

to ultimate moment for convenience in presenting the data from Slabs F-3

and F-4 on the same set of coordinate axes in figure 9. The open circles

represent data from Slab F-3, and the solid circles represent data from

Slab F-4.

5.

The average concrete strains on the tensile and compressive faces

of Slabs F-3 and F-4 at midspan are shown in figure 10. Again the open

circle represent data from Slab F-3, and solid circle represent that of

Slab F-4.

The longitudinal strains in cpncrete along transverse sections of

Slab F-4 are presented in figure 11. A complete tabulation of the con- crete strains measured in Slab F-4 Is given in Table 1.

4.3 Mode of Failure

Both slabs failed in compression as did Slabs F-l and F-2 (NBS

Report 5825). The crack patterns in Slab F-3 are shown in figure 12.

The shaded and missing portions of the blocks indicate the location of

the compression failure. When Slab F-4 was dismantled after testing,

most of the block fell apart. Therefore, no crack patterns could be

presented for this slab. However, a photograph of the compression

failure of Slab F-4 is presented in figure 13.

5. DISCUSSION

When considering a slab subjected to concentrated loads, such as Slab F-4, it is expected that the slab will bend in the transverse direction as well as the longitudinal direction. Furthermore, it is ex- pected that local depressions will be formed around the loaded area. The deflections presented in figure 8 and the strains presented in

figure 11 do not appear to bear this out. However, when the rigidity

of the solid concrete end spans is considered, these findings are not

unexpected.

Audible cracking, having occurred at 24,200 lb, can explain the

erratic behavior of this slab at higher loads. The photograph of the

compression failure in the area of sections 3-3 and c-c will also ex- plain these erratic deflections and strains. Figures 9 and 10 show

marked changes in the slopes of the curves representing steel and con- crete strains, respectively, after the cracking occurred. As pointed

out before, figure 7 also indicates this change of slope in the deflection curve for Slab F-3.

The crack patterns of Slabs F-l and F-2 (presented in NBS Report

5825) were more widespread than the crack patterns of Slab F-3.

Limited crack formation in Slab F-3 is due to the use of welded wire

fabric as web reinforcement. Since the unreinforced blocks of Slab

F-4 fell apart when the slab was dismantled, no crack patterns could

be presented.

6 .

6. SUMMARY

The preceding work can be summarized as follows:

1. Both slabs exhibited abrupt compression failures of the compres

sive flanges of the concrete blocks.

2. Since the solid concrete end spans of Slab F-4 were very rigid

compared to the test span, no measurable transverse bending due to the

concentrated loading technique was observed.

3. Both slabs showed signs of distress in the form of cracking

prior to the final compression failure. The formation of cracks in the

slabs caused large increases in steel strains, concrete strains, and de

flections at approximately two thirds of the ultimate moment for Slab

F-3 and three fourths of the ultimate moment for Slab F-4.

USCOMM-NBS-DC

Table 1. Concrete Strain in Slab F-4

Load, lb

Gage 2,000 4,000 6,000 8,000 10,000 15,000 20,000 25,000 30,000

101 2 5 9 14 19 27 36 47 99

201 -3 -7 -8 -12 -15 -27 -37 -44 -27

102 -10 -19 -27 -39 -49 -71 -100 -147 -215

202 5 10 20 30 38 57 69 68 97

103 -24 -46 -64 -86 -106 -153 -209 -282 -441

203 22 44 64 79 106 148 19 6 217 236

104 2 2 2 2 7 7 11 23 40

204 0 - 3 - 3 - 6 - 10 - 20 - 46 - 93 - 78

105 - 6 -15 -22 -35 - 45 - 70 - 99 -153 -224

205 6 13 20 31 41 61 78 56 61

106 -19 -38 -60 -80 -100 -146 -205 -285 -430

206 21 41 63 82 104 160 218 249 258

107 5 7 9 r*9 It 17 20 27 49

207 -'•3 - 9 -10 -10 - 14 - 21 - 29 - 31 - 37

108 -10 -22 -30 -43 - 53 - 83 -119 -155 -263

208 9 13 24 36 42 61 83 99 93

109 -21 -45 -67 -90 -113 -172 -236 -301 -543

209 22 41 64 86 108 158 212 253 258

110 - 2 - 2 - 3 - 4 - 4 - 10 - 10 - 9 - 17

210 1 - 4 - 3 - 1 - 4 - 6 - 14 - 24 - 36

111 -10 -20 -29 -47 - 57 -'93 -132 -180 -294

211 9 15 23 40 5.2 . 77 104 125 122

112 -24 -45 -67 -90 -114 -171 -232 -294 -504

212 22 41 63 89 112 165 223 266 269

113 - 3 - 6 - 7 -11 - 15 - 22 - 31 - 39 - 46

213 4 4 8 14 16 22 24 21 19

114 -10 -24 -35 -54 - 72 -112 -158 -202 -323

214 12 19 31 50 66 99 134 137 153

115 -23 -50 -74 -96 -125 -191 -253 -310 -513

215 24 42 66 91 114 169 227 226 243

Table 1. Concrete Strain in Slab F-4 (Continued)

Load, lb

Gage 2,000 4,000 6,000 8,000 10,000 15,000 20,000 25,000 30,000

116 -16 -38 -53 -72 - 93 -141 -196 -270 -377

216 23 37 58 85 106 158 204 213 263

117 -18 -39 -55 -75 - 98 -140 -201 -273 -428

217 14 28 46 66 81 116 139 140 152

118 -29 -44 -62 -84 -106 -164 -223 -286 -533

218 22 41 65 91 113 173 235 274 265

119 -19 -41 -58 -79 -100 -151 -211 -281 -512

219 21 38 60 83 105 161 223 270 269

120 -24 -48 -70 -98 -118 -179 -246 -315 -587

220 20 45 64 91 110 170 236 288 329

121 -19 -44 -65 -86 -106 -165 -226 -292 -506

221 21 41 61 88 107 163 221 264 260

122 -21 -42 -69 -91 -115 -176 -238 -300 -480

222 27 54 73 100 127 187 250 279 309

123 -21 -45 -65 -89 -111 -165 -221 -282 -500

223 28 44 66 88 112 166 226 265 279

124 -21 -44 -67 -90 -111 -169 -228 -289 -524

224 22 40 62 84 108 159 220 261 286

125 -21 -43 -64 -88 -110 -167 -221 -281 -916

225 20 40 61 89 110 165 227 275 256

126 -21 -47 -70 -91 -119 -179 -247 -321 -1059

226 28 49 71 99 124 184 251 300 216

127 -18 -40 -60 -74 - 98 -147 -203 -262 -682

227 20 45 60 87 112 165 227 262 196

128 -21 -41 -62 -82 -107 -162 -227 -290 -741

228 20 41 63 91 117 173 240 290 158

129 -22 -45 1 vO 00 -92 -116 -180 -244 -303 -953

229 20 39 59 81 102 153 209 250 123

Avg.

Top

-21 -43 -65 -85 -110 -167 -228 -291 -870

Avg.

Bottom

22 43 63 89 113 168 231 275 190

Compressive strain indicated by minus sign (-).

FIG. I -NOMINAL DIMENSIONS OF NBS

REINFORCED BLOCKS.
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No. 100 SERIES STRAIN GAGES TOP SURFACE

No. 200 SERIES STRAIN GAGES BOTTOM SURFACE

GAGES 0-1 THRU O' 20 DEFLECTION

FIGURE 6 INSTRUMENTATION DIAGRAM OF SLAB F-4

TOTAL

APPLIED

MOMENT/

ULTIMATE

MOMENT

FIGURE 7 CENTERLINE DEFLECTIONS OF SLABS F~3

AND F-4 IN CONSTANT MOMENT SPAN.

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FIGURE 12 CRACK PATTERNS SLAB F-3

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U.S. DEPARTMENT OF COMMERCE

Frciloiiik II. Mui llt r. SrtrHary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director THE NATIONAL BUREAU OF STANDARDS

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