L. ~ " ~ _ § § ‘ § 7,, ,, § § ,, ‘ f 3 AN EXPERIMENTAL STUDY OF THE EFFECT OF DRYING OUT CONCRETE IN EARLY AGES UPON LATER STRENGTH AND EFFECT OF SOAKING AT DIFFERENT AGES TO LEARN THE. ULTIMATE DAMAGE 0F DRYING Thesis for the Degree of B. S. (:L F Baker 1925, THESIS: WWW”- mm«~0".M AN EAfEnImfiNTAL STUDY OF THE.EEEECT OF DRYING OUT GONCnflTE IN EARLY AGES UPON LATJK STKAAGTH Add EFEEGT OF BOWL} AT Dfl‘h‘nhblN’R AGES TO LEARN THE ULTLuATE ONES OF BAKING. . A Thesis Submitted to the .Faculty of Michigan State College by G. F. Baker June~1928. Candidate for the Degree of Bachelor of Science. THESIS Introduction: It is a well known fact that the conditions under whi- ch concrete, probably one of the most important building materials known to the engineering profession today, is cu- red plays a very important part in the results which may be expected from its use as a building material. By experiment the ideal conditions have been found to be one day in damp sand and.about twenty eight days under water or at least in a very damp condition. The old theory of mixing a certain amount of concrete-with a Specified amount of fine and coarse aggregate in proportions that by previous experience had given accecptable results and adding water until by observ- ation the engineer thought that the proper consistency had been reached has been proven to be very poor aractice and is becoming more obsolete every day. It is now known that the strength of a concrete depends on the ratio of the amount of water to cement in the mixture and not, as was formerly supp- osed, simply on the amount of cement in the mixture. Poor curing conditions will however ruin the best concrete mixed and one of the big problems for the engineer is to furnish the proper amount of moisture to the concrete during the curing process. 93825 Object: The object of the test described in the following pages was to determine if possible the amount of damage that would be done to a concrete if it was completely dried out at some time during the curing process. When concrete is poured it begins to harden or set at once and its strength increases very papidly for a few days after which the action slows down but continues so that very nearly full strength will be Bea— 'ched in about twenty eight days. If however the concrete is dried out some time before full strength is reached the act- ion will be retarded and the curve of its compressive stren- gth will fall below the curve that would represent the stre- ngth at successive time intervals were the curing conditions ideal. It is supposed or I might say known that after retar- dation due to drying out the concrete will will slowly gain in strength but it will never attain the strength that could have been expected had it never been allowed to dry out. Proceedure: To make this test fifty six test cylinders were made as described later in three different batches with a two day in- terval between mixings. These cylinders were all dried to a constant weight at a constant temperature of one hundred deg- ,/rees E} with a trough of sodium chloride in the drier to col- lect the moisture. They were then divided up into fourteen groups of four cylinders each and at intervals of two days beginning two~days after the first bunch were made were set aside to cure immersed in water held at a practically const— ant temperature. One group being immersed in water two days after being dried out, another two days later and so on thus keeping each group dry two days longer than the preceeding group. At the end of twenty eight days these cylinders were all broken and the compressive force required for the first sign of failure noted. The average of the forces required for the different cylinders of each group was then computed and taken as the force required to break a cylinder from that group. The concrete was designed to withstand a compre- ssive force of three thousand pounds per square inch. At the same time as the cylinders for the test were ma- cie a set of ten cylinders were made for the purpose of con- structing an ideal curve for the strength of the concrete. Tfliese cylinders were cured twenty four hours in damp sand 31nd then twenty seven days immersed.in water at a constant temperature. That is one of the cylinders was cured the full twventy eight days. The other nine were broken successively at intervals of three days and the compressive strength noted. From these compressive forces an ideal curve for the concrete was constructed showing what strength at different time int- ervals might be expected and what ultimate strength might be looked for. Forms used for the cylinders were one quart ice cream boxes having an end area of nine and six one hundredths square inches. When the cylinders were broken one end was covered with a cap made of plaster of paris and fashioned perfectly smooth thus a uniform bearing surface was procured. The results obtained were as given in the following tables and graphs. Michigan State College Concrete Laboratory Sand C. A BULKING OF AGGREGATE : Wt. of damp sample .......................................... ‘ .................................. Wt. of dried sample .......................................... i ................................. Wt. of water, damp sample .................................... I ........ i Per cent of moisture. . . . . . . . . . . . . . . . . . . . .. ...................17.7 I‘m. Wt. per cu. ft., damp loose .................................... 1 ,,,,,,,, ..‘.--- Wt. dry aggr. in 1 cu. ft. damp loose (x) ........................ i ....................... l ............ I Wt. Waterin 1 cu. ft. damp loose .............................. _ _, ' Wt. per cu. ft., dry rodded (y) ................................. ! BULKING FACTOR (y/x) .................................. _. . , . , I __.__. l- I I _ in All W _ m SIEVE ANALYSlS For Fineness Modulus SAND COARSE AG TIREGATE SIEVE Wt. on % on Total % Wt. on % on Total % Sieve Sieve Coarser Sieve Sieve Coarser 13/2" 0 ....... 0 2.51_____ l 1 2 ..5___ %’ 0 0 O O .0 %" 0 H __. g _ o __ __-__6_1§j.._____ _ 44 .5 # 4 608 0068 965 92.7 # 8 54 ’2 _§ _-__1 53: ~ 197 7 g, __ ,,___ __ 97 .4 #14 129-? 191915 _______11_.9__________ 9s.2 #28 179.5 __ _39.965 _.1o_.o . 98.7 # 48 409-75 71.94 7.2 (2-9-...o_ __ #100 202 . o _9&._].4__ ______5lo__l ___ __ __ __-99L,4l__._ Pan 2 l . 0 xxx 4 , 0 xxx TOTAL 1099-2 7379 .08 5 2319.9. l- -14 1 . 5 Fineness Modulus ........................... 3 o 39 . ............................ 7 42 Maximum Size .............................. 5/8 ........................... j/l-l Max. Size of Mixed Aggr. (based on 94», E’} Mix.) 6% ,,!' , ,.., Michigan State College Concrete Laboratory DESIGN OF CONCRETE MIXTURES Design Data Job ........................ ‘ > Date June. .1928- ....... Source of Materials .................................... Sand (f. A. Wt.ofdampsample........................... ............... _. Wt; of dried sample .......................................... ...-.83 {5.502. ..-. . , W98 # Wt. of water in damp sample .................................. __ __. Percent of moisture ......................... -. ................ - ........................................................ Wt. of one cu. ft. damp loose .................................. . .. . ..................................... Wt. of one cu. ft. dry rodded .................................. l.1.2-_..ii_-_.l_.202-_-____ ........ 708.1}: 80Z ...... Wt. dry material in 1 cu. ft. damp loose ......................... ..................................... Wt. water in 1 cu. ft. damp loose. . . . . Q ......................................................................... Bulking factor ............................................... ____________________________________________ Fineness modulus ............................................ 3....39. ._ . ...... 7-42 ........... Maximumsize ............ 3/8 ............ _ 1.4 ................. Desired 28 day strength-.---.3.C.OO..-----_# sq. in. Exposure .. . ._ _. . Slump 6-7 Real Mix--1033 .............. Gals. water per sack Cement.-.5.734-...Allowable Fin. Mod. Mix-ml Aggr- 5,_25_ , Percent. Sand (r) ............ 56 .............. Percent C. A. (l-r). 1+4 Wt. 1 cu. ft. separated aggregates (dry rodded).. .--.l..l 0.9.1 7’; . . Wt. 1 cu. ft. mixed aggregates (dry roddcd)-.--..-...-.-... . ._ .. ._ . ._ ... ........ Shrinkage factor.-. Nominal Mix... _- _- _. .._ , Field Mix.-- I . 1.76-:- .;.1 38 _ ....-.-_-._..Bnlkc(l Field Mix ....... Water carried by aggregates in a l sack batch .................................................................. . ............. , Absorbed water in aggregates in a 1 sack batch ......... 1% ...... ..................... . ...... Batch Data—Cement ........................ lbs“ W :fiitm _ , ..igilsn Smd llis. . (‘. A. . lbs- Crew ()porzit or. Results: For ideal curve. Cylinder. 2:?ain. giggggifiii: lbs. stggggggsiivfibs. per sq. in. No.1 9.06 8400 934-7 No.2 9.06 18920 2094.7 No.3 9.06 28120 3137.5 N0.4 9.06 36230 4032.} No.5 9.06 35280 3920.0 No.6 9.06 43470 4874.4 No.7 9 .06 40000 4444 .4 No.8 9.06 35000 3999. 9 No.9 9.06 38960 4323.8 No.10 9.06 38670 4260.0 Results: For fifty six test cylinders. Group Area sq. in. Compressive Average Comprestive strength lbs. strength lbs. No.1 per sq. in. a. 9.06 28560.0 b. 9.06 30000.0 0. 9.06. 28250.0 d. 9.06 25430.0 28060 3110.0 No.2 a. 9006 30450'0 b. 9.06 29050.0 0. 9.06 27140.0 d. 9.06 31530.0 29540 3250.0 No.3} a. 9.06 27150.0 b. 9.06 29670.0 00 9006 27360'0 d. 9.06 27700.0 27970 3980.0 No.4 a. 9.06 23670.0 be 9006 2660000 0. 9.06 23920.0 d. 9.06 25270.0 24890 2750.0 No.5 8. 9006 2043000 b. 9.06 17830.0 0. 9.06 23340.0 No.6 a. 9.06 24930.0 b. 9.06 26200.0 0. 9.06 26220.0 d. 9.06 27700.0 26262 2890.0 No.7 a. 9.06 23880.0 b. 9.06 24280.0 ((13. 9006 2475050 No.8 a. 9'06 2269000 b- 9-06 19730.0 3’ 9006 1775000 . 9.06 21810.0 21410 2360.0 Results cont'd.: Group Area sq. in. Compressive Average Compressive strength lbs. strength lbs. per sq. in. No.9 8.. 9.06 2396000 b. 9.06 20000.0 00 9‘00‘6 2366000 d. 9.06 27330.0 23737 2610-0 No.t0 a. 9006 23780.0 b. 9.06 22815.0 0. 9.06 21580u0 No.11 a. 9.06 23070.0 b. 9.06 18420.0 Co 94-06 T584000 d. 9.06 18590.0 20027 2210.0 No. 12 a. 9.06 19310.0 0. 9.06 19270.0 d. 9.06 19900.0 19493‘ 2150.0 lfio.13 a. 9.06‘ .15720.0 0. 9.06 19430.0 0. 9.06 13710.0 14913 1645.0 DID-14 b- 9.06» 18040.0 00 9‘06 17540 0-0 ' d. 9.06 17500.0 18077’ 1990.0 ...xiR I mfilk xxx 08\ Donn coco ‘er 3'3. I I], ./h&/ IA‘FCC K09. 9%“ Q 0°. 50. .- 4‘3 DA..1 / IL- Kiri- Explanation of tabulated results: 1. For ideal curve. The compressive force required to break each cylinder was noted and tabulated in a separate column. This force was then divided by the end area of each cylinder and the compressive force in pounds per square inch was tabulated in another col- umn. 2a For fifty six test cylinders. These cylinders were divided into fourteen groups, the grouping being governed by the number of days that the cylind- ers were kept dry. These groups were composed of four cylind- ers each designated by a, b, c, &d. The compressive force req- uired to break each cylinder was tabulated and the average of the forces for the four cylinders of each group taken as the compressive strength of the group.,This average was then div— ided by the end area of each cylinder to give the compressive strength in pounds per square inch. Explanation of graphs. 1. For ideal curve. The time of curing was plotted against the compressive strength in pounds per square inch. A dotted straight line (graph was then drawn through the various points. A smooth curve ‘was then drawn which.struck an average for the various break- ing points. The formula given by "Duff A. Abrams" Professor in charge of laboratory at Lewis Institute, Chicago for the curve of ideal strength of a concrete id‘s-13000 in which a is the ccompressive strength in pounds per sq;Z:;_inch, and x is the inater cement ratio. In this test x was found to be equal to Explanation of graphs cont'd. .78 and the constant b which in Abrams curve was 7.0 was found to be equal to 7.26 For fifty six test cylinders. In this curve the time that the cylinders were kept dry was plotted against the compressive strength of each group. In deter- mining the compressive strength for each group it was seen that the compressive strength of some of the separate cylinders var- ied a great deal from what should be eXpected. These cylinders were there fore disregarded in the computation of the average strength of a group. Thus in group 5 b was thrown out, also 0 in group 8, c in 11, b in 12, and c in 13. On the graph a straight- line curve was first drawn through the various points, the a smooth curve was drawn which struck an average for the various strengths starting at the strength determined by curve N0. 1 for the ideal strength at the end of twenty eight days. 1. ‘II-wdiilrl 1, ll .11. ..... . .. I '8' i!!\. Ollltlmllv'i 1|! . i . 1 1111 ..III Conclusions: Although.this concrete for the test was designed for a compressive strength of three thousand pounds at the end of twenty eight days, an examination of the results and the graph N0. 1 for ideal strength will show that the actual strength at the end of twenty eight days was about four thousand pounds per square inch. This was undoubtedly due to two different factors. First, since the time that the curve showing the proper water cement ratio was determined the more refined methods of prod- uction of cement have brought about a cement that is vastly superior to the grade used for the determination of this curve and from this alone an increase over the designed strength of five or six hundred pounds is not uncommon. Second,it is an established fact that the amount of water in the concrete is a governing factor as far as the strength is concerned. In this test dried aggregates were used and one per cent of water added for absorption, however the mixing pans were not moistened be- for the mixing was done and a certain amount of moisture would be used up in wetting the pans. This would lessen the amount of water in the concrete and thus increase the strength. It will be seen thag the strength of the concrete after being dried dropped a very appreciable amount by an examination of the results and the enclosed graphs. Since the scope of this work is so limited a specific statement of the decrease in str- ength cannot be made with any very great degree of certainty. An examination of graph No. 2 showa an initial drop in compre- ssive strength of very nearly 37% though and this result may iiiliiullbll. failing. 1 1 be accecpted as approximately the correct figure. At the end of twenty eight days a drop of about 53% was found to exist. This is equally as correct as the preceeding drop. These results show that it is extremely important for the contractor to cure his concrete in the presence of external moisture and not give it a chance to become dried out at any time during the curing process. This is especially true of highway work where during the summer if no precautions are taken such as covering with soil and spri- nkling the concrete is exposed to the direct rays of the sun as well as the high summer temperature and may thus be-materially damaged.as far as its compressive strength is concerned, and compressive strength and wearing qualities go hand in hand as required qualities of highway concrete. This is also true of concrete used in columns and other structural members and is of equal importance to the structural contractor. Although this test is inadequate and fails to reach any absolutely definite conclusion as to the exact amount of damage that will be done by drying, it proves conclusively that it will be very much to any contractor's or engineer's advantage to take special care to see that his concrete is cured under as nearly ideal cond- itions as possible, that is,in as damp a condition as may be consistent with local conditions and at the same time soon- omical. °6 1411171 ‘ WWII 12111111111 1311 3 H” H, «1‘1 1‘1 ‘1