LATERAL PRESSURE OF FLUED CONCRETE Thesis ior the Degree of M. S. MECHIGAN STATE COLLEGE Rfibmf (Swims Me Lravy 193%?! THESIS ' I“ :7 A‘ .4 I .-..\J"_' .. . .- -_. 7 .7 i , L...i,- m .. ; IIIIIIII IIIIIIIIII IIIIII I 1293 01067 1737 This is to certify that the thesis entitled LATEM PRESSURE 0F FLUID CONCRETE presented by ROBERT CHARLES MCLRAVY has been accepted towards fulfillment of the requirements for CIVIL ENGINEERING ii— degree in Major professor Date 0159 _ q \ T {:7 i‘ ’_ ‘ ”f7" \ A ffi—vf w {I I If“ TIL ‘ “ LATERAL PRESSURE OF FLUID CONCRETE BY Robert Charles MCLravy A THESIS Submitted to the School of Graduate Studies of Michigan State College of Agriculture and Applied Science in.partial fulfillment of the requirements for the degree of 8 MASTER OF SCIENCE Department of Civil Engineering 1950 ACKNOWLEDGEMENT The author wishes to acknowledge the many helpful comments and suggestions which he has received from members of the faculty and also his appreciation for the materials and assistance furnished by the Michigan State Highway Research Laboratory at East Lansing. I" I‘ 7. .. -~v'.\4 ' 3- ' 1‘; ")2 1‘ TABLE OF CONTENTS Page 1 Introduction 3 Proceedure ' 5 Sketch of Test Form 6 Computations for Test #1 ll Computations for Test #2 1h Graph Showing Results 15 Conclusion INTRODUCTION During the summer of l9h9, the occasion arose for the investigation and checking of the design of the formwork for a concrete bridge abutment. There was some doubt as to correct intervals for placing the walers and tie rods for holding the form together. In order to compute the pressure exerted by the fresh concrete against the form, some value for the unit lateral pressure of fresh concrete had to be selected. Upon consulting the text books, it was found, as was to be expected, different authors recommended various values for this pressure. These values ranged from as low as 120 psf to th psf, the full weight of the concrete. The majority of authors made no differ- entiation'between stiff or lean.mixtures of concrete, although they stated that this factor would influence the correct value to be used. If the concrete were to be vibrated into place, instead of hand spaded, most recommended the use of the full equivalent fluid pressure of concrete, generally accepted as th psf. The design of the formwork was then checked, using the th psf value. The results showed that the stresses in the tie rods would be almost at the failure point and no additional strength would be provided for impact, vibration and other factor of safety stresses. When the abutment was poured a close watch was kept upon the formwork. No failure or yielding of the formwork was observed either during the pour or the stripping of the forms afterward. As a result, the impression was formed that the lateral pressure of fluid concrete against formwork was not as great as generally believed. Thus, the idea of this thesis was born. PROCEDURE At the start of this thesis, it was thought that the Goldbeck Pressure Cells would be used. It was soon discovered, however, that they were not available through the Civil Engineering department and that the cost of them would be greater than this experiment justified. Consequently, another method had to be devised. After consulting with Dr. Harris, Professor Cade, and Dr. Pian, an arrangement was devised using electric strain gages. This arrangement was made in the form of a.wooden box approximately two feet square and five and one half feet high. The lower side of the box was cut out and.made into a sliding panel. This panel was held in place by two one-quarter inch circular steel tie rods acting upon h".x h" walers across the front of the panel and the back of the form. The box was made of three-quarter inch plywood fastened to 2".x h"'s and held together with built up h".x h"'s and one-quarter inch steel rods. It was so constructed, that the component parts could be stripped, cleaned and easily reassembled. The forms were erected in the concrete laboratory and the slid- ing panel shaped until it slid smoothly. Six inch electric strain gages were applied to the two tie rods holding the panel in place. (These gages and the potentiometer for reading them were supplied through the courtesy of the Michigan State Highway Research Laboratony.) HarkS'were established at distances of two, three, four and five feet above the bottom of the form. The forms were then oiled with mineral oil to prevent bonding between the concrete and the wood. Concrete was then.placed in the forms to the desired height and the deformation of the tie rods recorded by means of the electric strain gages. The concrete was vibrated before each reading using an electric powered, flexible cable type vibrators. When the forms were filled to the five foot mark, a platform was placed on the concrete and a surcharge added. This was an attempt to determine the elapsed time required to produce enough set in the concrete so that any additional increment of concrete would not produce a correSponding increase in lateral pressure at the bottom of the form. At the end of the experiment the electric strain gages and the tie rods were placed in a tension machine and calibrated. Knowing the area of the movable panel and the force exerted upon the tie rods, the lateral pressure was computed for each depth of concrete. This information was plotted in the form of graphs for comparison between the stiff and the lean mixtures of concrete on the theoretical fluid pressure of each mix. -x— The completion of this thesis was greatly facilitated by the help of Mr. Larry Childs of the Michigan State Highway Research Laboratory. SKETCH OF TEST FORM ' ’ " l-6 3e. Ins/dc e .I}? h o 6 ”5/9.: #1.: 5 rm), 6090 "' 801'!) 51%: Data Sheet Trial #1 Time Depth AStrain Gap (micro in. Surcharge (ft.) West Gage East Gage (lbs.) 11:02 AM 0 595R8 moss 0 11:15 2.0 680R8 BhORS 11:20 3 .0 75538 910RS 11:25 h.o 780R8 93535 11:31 5.0 800R8 950R5 Time of Set 12:0h PM 5.0 79OR8 920R5 O 12 :07 5.0 790R8 960125 115 12 =32 . 5 .0 79SR8 960R5 1.15 12 :15 S .0 800128 968125 210 1:00 5.0 800R8 965125 305 Strain Gages: Length - 6.0" Concrete: Slump - 1/2" to B/h" Aggregate - 6A Cement - Air Entraining 28 day strength - To :58°F 1;: 62° F Penninsula 2,085 psi Computations Test #1 Head='l.5' West Gage East Gage 680 8&0 52.: mo. difference 85 80 micro-inches 100 x 85 /7h.33::11h.5 lbs 100 x 80 /75.2;:19§,g 1bs 220.7 lbs Panel area 18.82 x 11. 87: 1.550 sq. ft. "IIE_' Lateral pressure 220.7 / 1.550 : lh2.5 psf. Heaizz.5' 755 910 £22 129 160 150 100 x 160 / 7b.33 : 215 lbs. 3 100 x 150 / 75.2 : 199.8 lbs. 215 + 199.8 2 h1h.8 lbs. hlh.8 / 1.55 : 267 psf. Head : 3. .5' 780 935 5.25 mg 185 175 185 x 100 / 7u.33 : 2h9 lbs. 100 x 175 / 75.2 = 233 lbs. 2h? 4- 233 = h82 h82 / 1.55 2 311 psf. Head 2‘4.51 'West G389 800 £25 205 100 x 205 / 7h.33 : 276 lbs. 3 276 253 529 / 1.55 East Gage 950 ggg 190 100 x 190 / 75.2 = 253 lbs. 529 lbs. 3&2 psf. weight per cubic foot volume of container weight of container full weight sample #1 #2 #3 0.1962 cu. ft. 0.50 lbs. 29.19 lbs 28.87 .2192 BI 86.69 28.896 - 0.5 : 28.396 lbs. 28.396 / 0.1962 : 1hh.§ lbs. per cu. ft. Compressive strength 10 day average 28 day 2,018 pS i o 2085 psi. Strain Gage Calibration for Trial #1 East Gage No Load Gage Reading Increment (pounds) (micro inches) 1 1,000 729R6 2 2,000 1h91R6 762 3 2,000 8h6R7 b 3,000 1587R7 7&1 1503/20 - 75.15 micro inches per 100 pounds ‘flest Gage 8.2.3....911 mm _1 100 530 2 200 605 75 3 300 682 75 u too 756 7b 5 500 832 76 6 600 905 73 7 700 976 73 hh6/6 ' 7h.33 micro inches per 100 pounds Data Sheet Trial #2 Time Depth(ft) Strain Gage (micro in.) Surcharge ' west Cage East Gage (lbs. 10:05 AM 0 530R8 68OR8 10:15 2.0 65OR8 755R8 10:20 3.0 730R8 86OR8 10 : 21: 11.0 785118 920118 10:30 5.0 860R8 lOOOR8 I____ Time of Set 11:20 5 .0 850118 1005118 0 11:30 5.0 835118 995118 0 11 :35 5 .0 8110118 1000118 205 11th 5.0 8110118 1000118 205 12:00 PM 5.0 8h0R8 1000R8 205 Strain Gages: length.- 6.0" Ratio - 2.13 Concrete: 28 day strength Slump - 2 1/2" to 3" Aggregate - 6A Cement - Air Entraining T,=65:I T,=58 F Penninsula - 3350 psi 10 Computations Test #2 Head : 1.5' West Gage East Gage 650 : 755 1.3.9 622 difference 120 75 micro-inches 100 x 120 / 7h.33 : 161.5 lbs. 3 100 x 75 / 75.6 : 99.2 lbs. 161.5 + 99.2 = 260.7 lbs. Lateral pressure 260.7 / 1.55 : 172 psf Head 2 2.5' 730 860 5.32 .6112 200 180 100 x 200 / 7h.33 : 269 lbs. 3 100 x 180 / 75.6 : 238 lbs. 269 + 238 : 507 lbs. Lateral pressure 507 / 1.55 ==327 PSfo Head -_- 3.5f 785 920 .519. 613.2 255 280 100 x 255 / 7h.33 : 3th lbs. 3 100.x 210 / 75.6 : 318 lbs. 3hh + 318 = 662 lbs. Lateral pressure 662// 1.55 :.h27 psf. 12 ‘ Head : 4.5' West Gage East Gage 860 1000 5.22 629. 330 320 100 x 330 / 71.33 : 8&3 lbs. ; 100 x 320 / 75.6 : 123 lbs. 113 +~uz3 = 866 lbs. Lateral pressure 866 / 1.55 2.560 psf. 'Weight per cubic foot volume of container 0.1962 cu. ft. weight " " 0.50 lb. full weight sample #1 30.25 lbs. #2 30.19 #3 12.22 31M 30.25 - 0.5 : 29.72 lbs. 29.72 / 0.1962 : 150.6 lbs. per cu. ft. Compressive strength 1h day average 2,598 psi. 28 day " 3,350 psi. Strain Gage Calibration for Trial #2 East Gage N0 (sgfiigs) (338:0Riiiigi) Increment 1 200 885 2 300 9115 60 omit 3 too 1020 75 h 500 1096 76 5 600 1170 7h 6 700 1215 75 7 800 1323 78 378/5 = 75 .6 micro inches per 100 pounds We 3 t Gage N° (£133.) (Siiiofiijgfié‘g) Increment 1 100 530 2 200 605 75 3 300 680 75 h too 756 76 5 500 832 76 6 600 905 7 3 7 700 976 73 [1116/6 = 711.33 micro inches per 100 pounds 15 CONCLUSION The following formulae for lateral pressure may be developed using the results of Test #2, which was performed with a 3,000 psi. concrete having a slump of 2 1/2" to 3", a common mix in construction work. Since the graph of this test resulted in a straight line not passing through the origin, the equation has the fem ofAI +51 I K, or in this particular case 89X - 0.7Y :10, where Y equals the lateral pressure in pounds per square foot, when X equals the depth of fluid concrete measured from the surface. This equation can be arranged into the two different forms as shown below. 1. Total lateral pressure per square foot. where Y : lateral pressure per square foot Y=89X -10 at a depth of X feet of fluid 0.7 concrete. 2. Lateral pressure per square foot per foot of depth of fluid concrete. where Y : the lateral pressure per square 1’ 2‘— 89X -I0 foot per foot of depth X of fluid 0.7T concrete. 16 Substituting values for X and Y in equation (2) gives the following results. X = 1.0' Y = 1l3.0 psf X : 2.0' Y 2'. 110.0 psf X z: 3.0' Y : 112.0 psf X =- h.0' Y : 12.3.8 psf I x : 5.0' Y : 124.3 psf ! As X + no the value of Y approaches a limit , thus: lim 89x -10 = 1m 22. = 1.2-7.3 psf X-u- —0_.'7'X— X-*°°0.7 Consequently, the results of these tests indicate that for forms whose dimensions compare with those of the test forms and where the rate of filling is not over five feet per thirty minutes at a minimum temperature of 60° F., the formwork should be designed to resist a lateral pressure of 125 psf per foot of depth of fluid concrete. This value of 12.5 is believed to be preferable to the actual computed value of 1253psf for a five foot depth because it will include all the above stated depths and the designer has only one value to remember instead of several. It was originally thought that as a collary of this thesis the determination of time of set of concrete in forms could be measured. The data gathered from this part of the work, however, was not conclusive. A different apparatus should be used for that deter- mination. It can be seen from the data sheets that as the time interval increased after the last increment of concrete had been added, the strain gage readings started to decline although a surcharge was 17 placed upon the fresh concrete. This was caused probably by the shrinkage of the concrete as it began to set. This does, however, indicate that as concrete takes its initial set, more concrete can be poured into the forms without an increase in the lateral pressure at the bottom of the form. HICHIGAN STATE UNIV. LIBRQRIES 31293010671737