FM 5-472 Chapter 4 - GlobalSecurity.org
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1. Place a suitable identifying label on the finished surface of the specimens Cover the entire specimens still in the mold with a double thickness of wet burlap Ensure that the specimens remain on site and are undisturbed for an initial curing period the first 16 to 48 hours after molding After this curing period move them to the testing laboratory and remove them from the molds for further curing The most satisfactory curing range for concrete is 68 to 86 F with 73 4 F being the most favorable temperature Moist cure the beams in saturated lime water totally submerged in a wet tank humidity room or keep them wet until they are tested FLEXURAL STRENGTH TEST ASTM C 78 94 EQUIPMENT STEPS 4 22 Concrete Perform this test to determine the flexural strength modulus of rupture of the test specimen to 10 psi Record the specimen identification modulus of rupture any defects noted and specimen s age Use the following items to perform this test in a laboratory environment e The flexural strength test apparatus e A concrete beam 6 x 6 x 21 inches e A measuring tape e A stopwatch e Pens e Pencils e Paper e Safety goggles e A proving ring with proving ring calibration and constant e Specimen identification A calculator Perform the following steps to determine the flexural strength Wear safety goggles throughout this test Step 1 Assemble the test apparatus and check for functional opera2. Add a second layer of concrete to the slump cone until two thirds of its volume is filled about 6 1 8 inches high Step 11 Rod the second layer in the same manner as the first with the rod just penetrating the underlying layer Step 12 Add the third and last layer of concrete overfilling if possible Step 13 Rod the third layer following the procedure in step 5 If the concrete height subsides below the top of the cone add additional concrete to keep it above the top of the mold Step 14 Strike off the excess concrete with a screeding and rolling motion of the tamping rod so the cone is completely filled Step 15 Removethe slump cone from the concrete a Place the hands on the handles and press downward b Step off the footholds c Raise the cone carefully and quickly in a vertical direction Raise the cone a distance of 12 inches within 5 to 7 seconds by a steady upward lift with no lateral or twisting motion d Place the cone directly beside the slumped concrete At this point about 2 1 2 minutes should have elapsed since the start of filling in step 2 Step 10 Measure and record the slump immediately see Figure 4 4 Figure 4 4 Measuring the slump of fresh concrete Concrete 4 19 FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I a Place the tamping rod along the top of the cone so it projects over the concrete b Measure the slump from the bottom of the rod to the top center of the concrete with a
3. Step 2 Prepare the compression tester see Figure 4 6 a Clean the tester s bearing plates loading surfaces b Check the tester for proper operation c Set the gauge to zero Step 3 Place the capped cylinder into the compression tester and center it on the bearing plates Secure the protective cage around the cylinder Step 4 Apply the test load at a rate of 20 to 50 psi per second not to exceed 50 psi per second 50 psi per second is about equivalent to a load gauge reading of 1 400 pounds per second Step 5 Read the gauge and record the load applied at the time of failure Step 6 Inspect the broken cylinder and record the following information e Identification number e Diameter e Cross sectional area in square inches e Maximum load applied in pounds e Compressive strength calculated to the nearest 10 psi e Type of break See Figure 4 7 e Defects in either specimen or caps e Age of specimen Step 7 Calculate and record the compressive strength of the cylinder using the following formula Compressive strength where P load at time of failure in pounds A nr C2 FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I Cone Cone and split Cone and shear Shear Columnar Figure 4 7 Types of fractures Concrete 4 31
4. II cement with an air entrained admixture is identified as Type IIA Water plays an important part in the concrete mix Its principal uses are to make the mix workable and to start the chemical reaction Any material in the water that retards or changes the reaction is detrimental A good rule of thumb is If it s good enough to drink it may be used for concrete The materials found in some types of water include organic compounds oil alkali and acid Each has an effect on the hydration process e Organic material and oil These compounds tend to coat the aggregate and cement particles and prevent the full chemical action and adherence The organic material may also react with the cement and create a weakened cementing action thus contributing to deterioration and structural failure of the concrete e Alkalies acids and sulfates Certain limiting amounts of these chemical impurities in the water tend to react adversely with the cement The result is inadequate cementing and weakened concrete Water must be substantially free of these chemicals for use in concrete mixing The salts in sea water are normally thought of as being corrosive However sea water is sometimes used for concrete mixing with satisfactory results A 10 to 20 percent loss in compressive strength can be expected when using the same amount of sea water as fresh water This can be compensated for somewhat by reducing the water cement ratio The aggregates common
5. Passing Indicated Sieve Nominal Sieve Size 4 312 3 212 2 112 1 3 4 1 2 3 8 No 4 Inches 31 201 1 2 100 200 ma o5 E 2 1 2 to 11 2 100 ch a to i 0105 wo Re Ee foes 2 to No 4 100 a a to 9 0to5 1 1 2 to 3 4 100 oe e 0to5 1 1 2 to No 4 100 a a to T to oig 1 to No 4 100 t00 A D 3 4 to No 4 100 90 ER 1 2 to No 4 100 ae z to a 3 8 to No 4 P T to 4 12 Concrete Table 4 4 Desirable gradation for fine aggregate in concrete Sieve Size Percent by Weight US Standard Passing 4 95 to 100 8 80 to 100 10 75 to 95 16 50 to 85 20 40 to 75 30 25 to 60 40 20 to 50 50 10 to 30 60 10 to 25 100 2 to 10 FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I FINENESS MODULUS Fineness modulus is an empirical factor that gives a relative measure of the proportional particlesize distribution of an aggregate s fine and coarse particles The fineness modulus does not represent any gradation of the material although the process is similar A 500 gram sample of sand is sieved through a series of sieves No 4 8 16 30 50 and 100 The weight retained on each sieve is converted into a cumulative weight and a cumulative percentage retained starting with the No 4 sieve The sum of the 6 percentages divided by 100 is the fineness modulus Another procedure for determining the fineness modulus is calculated using the cumulative percentag
6. concrete s quality and consistency throughout a project Take samples of concrete for test specimens at the mixer by repeatedly passing a receptacle through the entire discharged stream until sufficient concrete is collected into the pan In the case of ready mixed concrete take samples from the transporting vehicle while it s discharging the concrete see Figure 4 3 Figure 4 3 Sampling concrete from a truck mixer Concrete 4 17 FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I The contents of a paving mixer should be discharged into a pile and sample material taken by a shovel from at least five different portions of the pile The sample of concrete from which test specimens are made will be representative of the entire batch Obtain two or more samples by repeatedly passing a scoop or pail through the discharging stream of concrete from the middle portion of the batch to obtain the amount of material required by the test method Transport the samples to the testing site To counteract segregation mix the concrete with a shovel until the concrete is uniform in appearance Note the truck time and location of the placement of the concrete for future reference In the case of paving concrete samples may be taken from the batch immediately after depositing on the subgrade Take at least five samples from different portions of the pile and mix these samples thoroughly to form the test specimen SLUMP TEST ASTM C 143 90A EQU
7. pycnometer 500 milliliter e A mold metal frustum half cone brass mold water absorption cone A metal flat head tamper Perform the following steps to determine the bulk specific gravity of fine aggregate in an SSD condition Step 1 Obtain a representative sample weighing about 1 000 grams Step 2 Dry the sample to a constant weight at 110 C Step 3 Cool the sample to a comfortable handling temperature Immerse it in water and allow it to soak for 24 4 hours Step 4 Decant the excess water carefully ensuring that no loss of fines occurs Spread the sample on a flat nonabsorbent surface and stir it to obtain uniform drying Continue drying the sample until it approaches a surface dry condition Step 5 Place the metal frustum water absorption cone half cone brass mold see Figure 4 2 with the large opening down on a smooth surface and fill it loosely with the aggregate Lightly tamp the surface raise the metal tamper about 5 millimeters and allow it to fall under its own weight of the aggregate 25 times with the metal tamper Figure 4 2 Water absorption cone and tamping rod Concrete 4 15 FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I Step 6 Remove the loose sand from around the base and lift the mold vertically The fine aggregate is at the SSD condition when it slightly slumps when you lift the mold If the material does not slump continue the drying accompanied by constant stirring Repeat the cone t
8. IPMENT STEPS 4 18 Concrete When the mixture appears to have reached the desired consistency perform a slump test This method of testing covers the procedure to be used in the laboratory and in the field for determining the consistency of concrete which is a characteristic of workability It is not an exact method but it gives sufficiently accurate results Use this test to measure the consistency of a concrete mix by measuring the vertical distance that the concrete settles to the nearest 1 4 inch NOTE This test is not applicable when there is a considerable amount of aggregate over 1 1 2 inches in the concrete Use the following items to perform this test in a field or simulated field environment e Aruler e A scoop e A trowel e A water source e A flat smooth surface e A slump cone with tamping rod e A pencil e Paper Perform the following steps to determine the slump Step 7 Moisten the inside of the slump cone and place it on a flat moist nonabsorbent rigid surface Hold it in place during filling by standing on the two foot pieces Step 8 Fill the slump cone to one third of its volume 2 5 8 inches high with plastic concrete FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I NOTE From steps 2 to 10 a total time of no more than 2 1 2 minutes should elapse Step 9 Rod the concrete by applying 25 evenly distributed strokes penetrating the full depth of the first layer in the slump cone Step 10
9. RETURN TO TOC Chapter 4 Concrete This chapter discusses the testing of both fresh and hardened concrete and the construction materials used to mix it These tests are performed to ensure that the concrete meets design requirements before it is poured Since there are many factors that contribute to the success of the finished product all test methods must adhere to ASTM standards Further information on concrete types components design and uses in military construction can be found in FM 5 428 SECTION I CHARACTERISTICS AND IDENTIFICATION Concrete is one of the most economical versatile and universally used construction materials It is one of the few building materials that can be produced by the user directly on the job to meet the specific requirements Concrete is an artificial stone which when first mixed forms a plastic or putty like mixture This mixture can then be placed into a form and allowed to harden or cure for a prescribed length of time When cured the finished concrete is a hard stonelike material It is used for pavements foundations dams and retaining walls bridges and buildings of all types DESCRIPTION AND COMPONENTS CEMENT Concrete is a mixture of portland cement fine and coarse aggregates entrapped or entrained air and water During mixing the cement air and water form a fluid paste that contributes to thorough mixing and effective placement of the concrete The cement and water when mixed c
10. able so the hydration can continue The curing process takes place over an extended period The most critical time is the first 7 days The extent and rate of curing depends on the e Temperature within the concrete e Presence of moisture The ideal temperature for concrete work is between 55 and 70 F Above this temperature rapid evaporation of moisture creates serious problems such as MOISTURE FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I e Increased water demand e Slump loss e Decreased setting time e Increased tendency for plastic shrinkage cracking The hydration process is delayed at lower temperatures Temperatures below 32 F completely stop the hydration process Since the chemical reaction gives off some heat proper methods must be used to keep the heat within the structure during times of low temperatures Cold weather construction may require heating the individual ingredients or the concrete and covering the emplaced concrete or providing a heated enclosure In hot weather extra care is required to prevent a high temperature rise and rapid drying of the fresh concrete Spraying the aggregate stockpiles with cool water helps lower the concrete temperature To keep the water as cool as possible reflective white or aluminum paint is applied to the water supply lines and storage tanks On massive construction projects such as dams and heavy retaining walls the mixing water is often kept cool by substituting ice
11. ained cements may also be available for use in some military situations The desired amount of air is generally from 4 0 to 7 5 percent of the total mix Perform this test to determine the percentage 0 5 percent of entrained air in a plastic fresh concrete sample Use the following items to perform this test in a field or simulated field environment e An air entrainment meter with 5 percent calibration cup and instructions e A trowel STEPS C2 FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I e A tamping rod 5 8 inch in diameter and 24 inches long with a rounded end e A sample of plastic fresh concrete e Water e Oil e Rags A pail A mixing pan from the concrete test set e A kitchen scoop e Paper e A pend e A rubber mallet There are many different air entrainment meters currently fielded and replacements of old equipment may not be the same For this reason it is recommended that the steps outlined in the manufacturer s user s manual be followed SECTION IV FLEXURAL STRENGTH TEST MODULUS OF RUPTURE TEST BEAMS The flexural strength of hardened concrete is measured by using a simple concrete beam and third point loading mechanism The flexural strength is determined by calculating measured breaks of the beam and is expressed as a modulus of rupture in psi Beam forms for casting test beams from fresh concrete are available in many sizes The most commonly used size is 6 x 6 x 21 inch
12. an also be part of the temperature control during cold and hot weather concreting The increase of the concrete s compressive strength with age is shown by the curves in Figure 4 1 page4 8 Note the long time gain in strength that occurs when proper temperature and moisture conditions are maintained CONCRETE ADMIXTURES Chemical agents or admixtures are available for almost any purpose such as increasing workability durability and strength or compensating for inadequate curing conditions Concrete 4 7 FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I 150 125 10 75 50 25 Compressive strength percent Moist cured entire time In air after 7 days In air after 3 days In air entire time 180 Age in days ACCELERATORS RETARDERS Figure 4 1 Increase of compressive strength with curing age Sometimes it is desirable to accelerate the hydration process to obtain a high early strength and a high rate of heat production This combination is useful for cold weather concreting operations The addition of a chemical accelerating admixture generally calcium chloride to the concrete mixture produces the desired reactions The recommended maximum dosage for calcium chloride is 2 percent by weight of cement The ultimate strength of concrete will be slightly lower with the use of an accelerator Retarders are used when excessively high heat or too rapid setting of concrete will prevent full hydration o
13. awn from several increments When power equipment is not available take samples from at least three increments from the top third the midpoint and the bottom third of the pile Shove a board vertically into the pile just above the sampling point to prevent further segregation When sampling fine aggregate stockpiles remove the outer surface before taking the sample Take samples from near the top middle and bottom of the stockpile and recombine them to represent their particular stockpile Push a board into the stockpile just above the points of sampling to prevent the material above the sampling points from falling into the sample and causing size contamination Pit samples are sources of sand and gravel Sample them by channeling exposed faces or channeling in pits if exposures are not available Take care to ensure that the samples indude only materials that are below the overburden or strip zone GRADATION DETERMINATION Gradation of aggregate refers to the distribution of particles of aggregate among various sizes Aggregates having a smooth grading curve and neither 4 10 Concrete FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I a deficiency nor an excess of any one particle size usually produces mixtures with fewer voids between particles A too large proportion of coarse aggregate leaves voids that require more cement paste to fill This affects the economy of the mix Too much fine aggregate increases the amount of surface area t
14. b Oil the mold lightly c Assemble the mold Step 2 Make the cylinder a Fill the mold one third full with fresh concrete b Consolidate the concrete by applying 25 evenly distributed strokes over the mold s surface area with the tamping rod The tamping rod must totally penetrate the layer of concrete c Tap the side of the mold 8 to 10 times with the tamping rod d Add concrete to the mold so as to fill it two thirds full e Apply 25 evenly distributed strokes to the mold s surface area using the rounded end of the tamping rod which must pass entirely through the second layer of concrete and 1 inch into the preceding layer f Tap the side of the mold 8 to 10 times with the tamping rod g Add concrete to the mold to slightly overfill it h Repeat step 2e The tamping rod must pass entirely through the top layer and 1 inch into the preceding layer i Tap the side of the mold 8 to 10 times with the tamping rod j Trowel off the concrete so that it is flush with the top of the mold and smoothly finished Step 3 Label the mold The label should include as a minimum all of the following information e Thespecimen number e Thedate the cylinder was made e Theproject or placement that the concrete came from 4 26 Concrete FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I The system of labeling is optional The information should be recorded on a paper tag or gummed label and attached to the mold Step 4 Cover the
15. cal the mix as aggregate costs less than cement Larger pieces offer less surface area of the particles than an equivalent volume of small pieces Using the largest permissible maximum size of coarse aggregate permits a reduction in cement and water requirements One restriction usually assigned to coarse aggregate is its maximum size Large pieces can interlock and form arches or obstructions within a concrete form This restricts the area below to a void or at best fills the area below with the finer particles of sand and cement This is either a weakened area or a cement sand concentration that does not leave enough mortar to coat the rest of the aggregate The capacity of mixing equipment the spacing of reinforcement or the minimum width of forms limits the maximum aggregate size A listing of maximum sizes of coarse aggregate is indicated in Section II of this chapter PROPERTIES OF CONCRETE STRENGTH 4 4 Concrete To combine the ingredients correctly and to form the required concrete it is essential to know the required physical properties of both the plastic and the hardened concrete The hardened concrete must have the following properties e Strength e Durability e Watertightness e Workability e Consistency e Uniformity The quality and character of the hardened concrete is greatly influenced by the properties of the mix when it is plastic To attain optimum quality the plastic mix must be uniform consistent and w
16. cylinder with plastic or wet burlap to maintain moisture in the sample The covering should be tight around the cylinder but should not make contact with the fresh concrete Step 5 Allow the cylinder to cure undisturbed for 24 hours Step 6 Remove the covering and the mold from the cylinder after 24 8 hours Step 7 Transfer the label from the mold to the concrete cylinder The label itself may be transferred or the information may be recorded directly on the cylinder with a grease pencil Step 8 Cure the cylinder NUMBER OF SPECIMENS The number of specimens tested depends on the job specifications If no requirement is listed in the specifications a minimum of 2 will be molded for each test age for each 100 cubic yards or fraction thereof of each class of concrete placed in any one day A third specimen may be taken to assist in determining when forms may be removed The test specimens must remain on site and undisturbed for an initial curing period the first 16 to 48 hours after molding Normally the test ages are 7 and 28 days for compressive strength tests CURING AND STORING CYLINDERS After an initial curing period for 16 to 48 hours remove from the jobsite specimens that are intended for checking the strength of laboratory trial mixtures or to serve as the basis for acceptance or quality control of field concrete Take them to the testing laboratory and moist cure them at 73 4 F Store them in moist rooms in damp sa
17. dth and depth of the specimen in inches at the point of failure normally 6 x 6 inches Step 8 Determine the point of failure in the specimen and calculate the modulus of rupture If the specimen fails outside the middle third of the span length by more than 5 percent of the total span length then the specimen is Concrete 4 23 C2 FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I 4 24 Concrete considered unusable and should be discarded not more than 0 9 inches for an 18 inch span 18 x 0 05 0 9 a Use the following formula to calculate the modulus of rupture if the specimen fails within the middle third of the span length PXL where l bxd R modulus of rupture in psi P applied load in pounds L length of span in inches b width of specimen at failure point in inches d depth of specimen at failure point in inches b Use the following formula to calculate the modulus of rupture if the specimen fails outside the middle third of the length of span by not more than 5 percent of the span length R 3P xa b xd where R modulus of rupture in psi P applied load in pounds b width of specimen at failure point in inches d depth of specimen at failure point in inches a distance between the failure point and the nearest support measured along the centerline of the bottom of the specimen in inches Step 9 Record the following information about the test some information may be unavailable at the time of
18. e except that air entrainment is desired Entrained air improves workability and provides resistance to frost action freezing and thawing Type ll Cement used for general purposes especially when moderate sulfate resistance or moderate heat of hydration is desired It has a lower heat of hydration than the normal Type I generates heat at a slower rate and has improved resistance to sulfate attack Type II cement is used in locations where a high temperature rise in the concrete is objectionable as in structures of considerable mass such as large piers heavy abutments and heavy retaining walls Type IIA Air entraining cement used for the same purposes as Type II except that air entrainment is desired Type Ill Cement used when a high strength is needed quickly This may be due to a demand for early use or in cold weather construction to reduce the period of protection against low or freezing temper atures Type IIIA Air entraining cement used for the same purposes as Type III except that air entrainment is desired TypelV Cement used when a low heat of hydration is desired to keep the amount and rate of heat generated to a minimum Type IV cement develops strength at a slower rate than Type cement but helps prevent the development of high temperatures in the structure with the attendant danger of thermal cracking later when it cools Type V Cement used when high sulfate resistance is desired Sulfates react chemically w
19. e passing the usual means of expressing aggregate gradation The total number of sieves involved times 100 minus the sum of the cumulative percentage passing and divided by 100 gives the fineness modulus The fineness modulus values range from 2 20 for fine aggregate to 7 50 for coarse aggregate Typical values are 2 70 for fine aggregate 7 40 for coarse aggregate and 5 80 for 35 to 65 fine coarse combination Fineness modulus ranges for fine aggregate are shown in Table 4 5 Table 4 5 Fineness modulus ranges for fine aggregates Fineness Modulus Designation 2 3 to 2 6 Fine sand 2 6 to 2 9 Medium sand 2 9 to 3 1 Coarse sand TESTS FOR SPECIFIC GRAVITY ABSORPTION AND SURFACE MOISTURE Perform tests for specific gravity absorption and surface moisture on the aggregates before making the necessary calculations to design the concrete mixture For aggregates used in portland cement concrete measure to determine the bulk specific gravity of the aggregates in a saturated surface dry SSD condition This is the condition in which the pores in each aggregate particle are filled with water and no excess water is on the particle surface When used in concrete this moisture condition of an aggregate can be defined as neither absorbing water from nor contributing water to the concrete mixture Specific gravity is thus based on determining the total volume occupied by the aggregate particles including the pe
20. ed concrete presents a solid surface to prevent water penetration Superficial voids permit some water to enter below the concrete s surface but the water soon meets a dense solid mass that prevents further penetration As the W C ratio is increased the excess water forms more holes or voids that eventually interconnect to form channels into and throughout the concrete The end result is a more porous concrete that permits water to pass For watertightness 6 gallons of water or less per sack of cement will meet the requirement Concrete 4 5 FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I WORKABILITY CONSISTENCY UNIFORMITY Workability is the relative ease of difficulty of placing and consolidating concrete It is controlled primarily by the amount of each aggregate in proportion toa given quantity of cement paste As more aggregate is added toa given amount of paste the mixture becomes harsh and stiff The increased stiffness makes it more difficult to work the concrete into the forms and around the reinforcing bars The consistency needed depends on the conditions under which the concrete must be placed and finished Very dry and stiff mixtures may be placed in most situations where high frequency vibrations are used to assist in consolidating and compacting fresh concrete In other situations difficult placing conditions may require a more fluid concrete mixture to fill narrow forms and to flow around reinforcement Concrete is a fluid mi
21. ed in this manual refer to the metal caps due to their availability within the supply system If you must use sulfur caps ensure that sulfur vapors are not inhaled while heating the capping compound Ensure that there is adequate ventilation and that respiratory protection is used Used sulfur capping compound is a hazardous material and must be properly disposed of DETERMINING COMPRESSIVE STRENGTH OF A CYLINDRICAL SPECIMEN ASTM C 39 96 EQUIPMENT 4 28 Concrete Perform this test to determine the compressive strength of the concrete cylinder to within breakage and to determine anything unusual about the break Use the following items to perform this test in a well ventilated laboratory e A concrete cylinder 6 inches in diameter and 12 inches in height e A concrete capping set e Heat resistant gloves e Capping compound STEPS FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I e A ruler accurate to 0 01 inch e Calipers with at least a 6 inch opening e Paper e Pencils e Safety goggles and protective apron e A face shield e Oil e Rags e A calculator e A concrete compression tester with a 250 000 pound capacity A hammer ball peen or carpenter s Perform the following steps to determine the concrete cylinder s compressive strength Step 1 Prepare the concrete cylinder NOTE If rubber filled metal is used go to step 1j a Melt the capping compound in the melting pot Ensure that you melt enou
22. ended in water without being in contact with the aggregate or other cement partides This water eventually evaporates leaving holes or voids in the hardened concrete that cause additional losses in strength Minimum and maximum amounts of water are specified to assure an economical mix with no loss in strength This ranges from 4 to 8 gallons per sack of cement 94 pounds Durability is the concrete s ability to resist the elements of weathering and loading The primary elements affecting concrete are wind abrasion freezing and thawing wetting and drying and the chemical action of salts As the W C ratio is increased 4 gallons per sack more voids develop in the hardened concrete Therefore more surface area is available for the detrimental elements to attack resulting in a less durable structure Weak or easily crushed rock or other mineral particles that break down under applied loads introduce internal stresses that cause a breakdown of the concrete Rocks or mineral particles that are absorptive or susceptible to swelling when saturated will deteriorate when subjected to severe weather conditions Freezing moisture causes expansion stresses that can easily rupture absorptive rocks Rocks swollen from the sun s radiant heat and then subjected to shrinkage from sudden cooling by rain or temperature drop may break down from the severe weathering The concrete aggregate must withstand all these forces of nature A well mixed well proportion
23. er is molded so that the capping mixture will shrink before application Remove any free water or laitance a layer of fine particles on the surface from the end of the specimen Apply the paste to the top of the concrete and work it with a flat plate until it is smooth and level with the top of the mold Grind hardened concrete specimens to smooth the ends or cap them with a material having greater compressive strength than the concrete Prepared mixtures of sulfur and granular materials special high strength gypsum plasters and neat high early strength cement are satisfactory capping materials ordinary low strength plaster of paris compressible rubber or fibrous materials are not suitable for caps Apply the selected material ina plastic state and finish it to the desired plane surface by applying glass or metal plates and squeezing out excess material to provide a cap that is as thin as possible Apply sulfur caps in time to harden at least 2 hours before testing Plaster caps cannot be stored over 4 hours in the moist room Age neat cement caps 6 days or more in the moist room 2 days when Type II cement is used During capping protect moist cured specimens against drying by covering them with wet burlap There are numerous alternatives to sulfur caps listed in ASTM C 617 94 The metal cap with a rubber membrane is not an ASTM approved method however specific guidelines for their use are under review by the ASTM The test procedures us
24. es Although equipment for obtaining sawed specimens may not be available the test may be performed on beams sawed from existing concrete structures for evaluation purposes FORMING THE BEAMS ASTM C 192 90A Assemble a standard 6 x 6 x 21 inch concrete beam mold and lightly oil the inside Fill the mold with two layers of concrete from the production batches each about 3 inches deep Consolidate each layer by rodding using one stroke per 2 square inches 63 per layer evenly distributed over the layer s surface Tap the sides lightly 10 to 15 times with a rubber mallet to close the voids left by rodding Lightly spade the concrete along the mold s sides with a trowel to help remove surface voids When rodding the second layer penetrate the first layer about 1 2 inch Strike off the top surface with a straightedge and finish it with a wood or magnesium float Concrete 4 21 FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I TAKING THE SPECIMENS Take test specimens at least once for each 100 cubic yards or fraction thereof for each class of concrete placed in any one day or as directed in the project specifications Make at least three specimens for each test age and mixture design being evaluated in the lab Additional specimens may be made for future testing Test ages are normally 14 and 28 days for flexural strength tests For testing field placed concrete a minimum of two specimens for each test age is required CURING THE BEAMS
25. ests at frequent intervals until the cone of fine artillery slumps slightly upon removal of the water absorption cone Step 7 Weigh 500 10 grams of the SSD sample and introduce it into a partially water filled 500 milliliter pycnometer Agitate the sample to remove all entrapped air bubbles Adjust the water temperature to 23 C 1 7 and fill the pycnometer to 90 percent of its calibrated capacity Roll invert and agitate the pycnometer 15 to 20 minutes to eliminate the air bubbles Fill the pycnometer to calibrated capacity weigh it and record the weight to the nearest 0 1 gram Step 8 Calculate the bulk specific gravity in an SSD condition as follows 5S B S C where B weght in grams of pycnometer filled with water to calibrated capacity S wedght in grams of SSD specimen C weght in grams of pycnometer filled with the sample and water to calibrated capacity COARSE AND FINE AGGREGATE ABSORPTION Equipment Steps 4 16 Concrete Absorption in aggregates is the aggregate s ability to steal moisture from the concrete mix design until its thirst or attraction is satisfied The following procedure is a continuation of the specificgravity determinations therefore the same equipment shall be used Perform the following steps to determine the percent absorption of coarse and fine aggregates Step 1 Weigh the coarse aggregate in water and the fine aggregate in the pycnometer Step 2 Remove the aggrega
26. f the cement Many materials retard the setting of concrete but the most common is hydroxylated carboxylic acid salts Sugar has also been used quite successfully AIR ENTRAINING AGENTS 4 8 Concrete The greatest improvement in watertightness and resistance to the disruptive action of freezing and thawing is obtained by incorporating 4 to 7 5 percent by volume of entrained air into the concrete Workability of fresh concrete is also FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I enhanced by entraining air Soaps oils acids wood resins alkali salts fine pozzolans and several proprietary compounds are available for use as air entraining admixtures with hydraulic cement These agents form very small uniformly spaced discrete air voids that relieve the buildup of damaging pressures from the expansion of freezing water into ice WATER REDUCERS PLASTICIZERS The concrete s workability is governed by the proportions of cement water and aggregate in a concrete mixture When a reduction of aggregate or an increase in cement is impractical the concrete s workability can be increased by adding a water reducing admixture or plasticizer Another primary characteristic is the strength gained from a decreased water demand Less water is required for the same workability which leads to a lower W C ratio and therefore higher strength Water requirements may be reduced as much as 10 percent for most water reducing admixtures Air entraining agent
27. for part of the mixing water The ice must be melted by the time the concrete is fully mixed and is ready to leave the mixer Large voids result from unmelted ice in the concrete Cement replacement materials such as pozzolans and diatomaceous earth pumicites or fly ash may be used to depress concrete temperature by reducing the heat of hydration in a structure However pozzolans vary widely and may have adverse effects on strength air content and durability if used in excessive amounts Concrete curing depends on a chemical reaction in the presence of water Moisture lost during the curing process by seepage or evaporation delays or prevents a complete hydration of the cement and ultimately prevents the development of optimum strength and watertightness Saturating the subgrade on which the concrete will be placed will delay if not prevent seepage from occurring Impervious membranes plastic or polyethylene sheets can also be used to prevent seepage through the subgrade Wood forms should be thoroughly wetted if they have not been otherwise treated with a moisture sealer One method of reducing evaporation is to cover the concrete with a material such as straw burlap plastic or a sprayed on chemical curing compound as soon after finishing as possible The preferred method of curing is by using continuous sprays and flowing or ponded water after the concrete has set initially so it does not damage the finish This water application c
28. gh compound to make several caps b Clean and lightly oil the baseplate of the capping apparatus c Set the baseplate into the capping apparatus stand d Pour asmall amount of the heated liquid capping compound into the baseplate e Position the cylinder at midheight against the backrest of the capping apparatus Slowly lower the cylinder into the baseplate while keeping the cylinder flush against the backrest If the cylinder is not kept flush with the backrest while capping the caps and the cylinder will not be perpendicular and a proper break will not occur f Remove the cylinder from the capping apparatus once the capping compound has solidified g Inspect the cap for uniformity and defects If you see any defects remove the cap and recap the cylinder then return to step la h Repeat steps 1a through 1g for the uncapped end of the cylinder i Determine and record the average diameter of the concrete cylinder The average diameter is the average of two diameters taken perpendicular to each other at midheight of the cylinder j Clean and examine the bearing surface of the steel cap if used for nicks gouges and warping Check the rubber inserts for tears rips cuts and gouges Replace them if they arein poor condition or if the maximum Concrete 4 29 C2 FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I 4 30 Concrete number of serviceable uses has been exceeded Place the steel cap firmly on the cylinder s ends
29. hat must be coated with cement paste This may weaken the concrete and is uneconomical Good gradation results in e A dense mass of concrete with a minimum volume of voids e An economical mix e A strong structure Optimum strength water tightness and durability in the hardened concrete require careful control of aggregate gradation A gradation or sieve analysis indicates whether an aggregate s particle size distribution meets the project s requirements Dense aggregates can result in a concrete that is denser and stronger and more economical watertight and resistant See ASTM C 136 90 for analysis methods and Table 4 2 and Tables 4 3 and 4 4 page 4 12 for recommended size and gradation limits Table 4 2 Maximum recommended size of coarse aggregate Minimum Dimension Inches Structure 21 2to5 6 to 11 12 to 29 30 or More Reinforced walls beams and columns 1 2 to 3 4 3 4 to 1 1 2 1 1 2 to 3 1 1 2 to 3 Unreinforced walls 3 4 1 1 2 3 6 Slabs heavily reinforced 3 4 to 1 11 2 1 1 2 to 3 1 1 2 to 3 Slabs lightly reinforced 3 4 to 1 1 2 1 1 2 to 3 3 3 to 6 bars NOTE Maximum size not to exceed 1 5 of minimum dimension of a wall or similar structure 1 3 of slab thickness for horizontal slab or 3 4 of minimum clear spacing between reinforcing APPARATUS TEST PROCEDURES AND CALCULATIONS The apparatus test procedures and calculations required to determine the grada
30. ith the cement compounds causing undesirable expansion of the mixture The sulfates may be present in the water used to mix the concrete or may be created by sulfurous gases from nearby industrial areas The principal source of sulfate attack however occurs on foundations and other concrete in contact with the earth in certain regions and is caused by a reaction between the groundwater containing dissolved reactive minerals or acid and the hardened cement Type V cement is low in calcium aluminate and is highly resistant to sulfate attack Concrete made with air entrained cement is resistant to severe frost action and to salts used for ice and snow removal In general air entrainment may be controlled to a much greater extent by using admixtures with normal cements during mixing This combination results in concrete with tiny distributed and separated air bubbles up to millions per cubic foot The WATER Ordinary Water Sea Water AGGREGATES FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I entrained air bubbles improve the workability of fresh concrete These bubbles reduce the capillary and water channel structure within water which prevents the buildup of damaging water Air entrained concrete has greatly increased durability in outdoor locations exposed to freezing weather Each of the first three Types I Il and III are available as air entrained To signify this characteristic a letter A is added after the type For example a Type
31. le time to cool to 50 C immerse it in water and allow it to soak at room temperature for 24 hours Step 4 Remove the sample from the water and roll it in a large absorbent cloth until all visible films of water are removed The surfaces of the particles will still appear to be slightly damp The larger fragments may be wiped individually The aggregate sample is now in an SSD condition Weigh the samplein air in its SSD condition Record this and subsequent weights to the nearest 0 5 gram on DD Form 1208 Step 5 Place the weighed SSD sample immediately in the wire basket container Determine its weight in water at 23 C 1 7 Shake the basket or container while it is immersed to remove any entrapped air This weight is the immersed weight or weight in water Step 6 Calculate the bulk specific gravity in an SSD condition as follows B C where B weght in grams of SSD samplein air C weight in grams of SSD samplein water FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I SPECIFIC GRAVITY OF FINE AGGREGATE ASTM C 128 93 Equipment Steps This test method covers the specific gravity and absorption of fine aggregate The specific gravity of fine aggregate may be expressed as bulk specific gravity bulk specific gravity SSD or apparent specific gravity For this test method fine aggregate is defined as material smaller than the No 4 sieve and larger than the No 200 sieve Use the following items to perform this test e A
32. ly used for concrete are natural deposits of sand and gravel where available or crushed stone Crushed aggregate may cost more to produce however this may be the only way to obtain substantial quantities of large sized stone Artificial aggregates such as a blast furnace slag or specially burned shales and clays are used Aggregates are divided into the following types e Fine aggregate e Coarse aggregate When properly proportioned and mixed with cement these two groups will yield an almost voidless stone that is strong and durable Aggregate should be equal to or better in strength and durability than the hardened cement paste if it is to withstand the design loads and effects of severe weather Concrete 4 3 FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I Fine Aggregates Fine aggregates are the material that will pass a No 4 sieve and will be predominantly retained on a No 200 sieve To increase workability and for economy as reflected by using less cement the fine aggregates should have a rounded shape Their purpose is to fill the voids between coarse aggregate particles and to modify the concrete s workability This workability characteristic is discussed more in the description of finished concrete Coarse Aggregates Coarse aggregates are the material that will be retained on a No 4 sieve In determining the maximum size of coarse aggregate other factors must also be considered The coarser the aggregate used the more economi
33. nches long capped with a suitable material to provide smooth bearing surfaces on each end Load is applied to the end surfaces through metal platens on the testing machine cylinder breaker causing compressive stress in the longitudinal direction of the cylinder Make the cylinders as near as possible to the place where they will be stored for the first 16 to 48 hours Sufficient concrete about 1 cubic foot for the desired number of cylinders must be available in the trial mixture or field sample Material from the air content test must not be reused since this may be contaminated with excess water Use appropriate sampling procedures for procuring your sample as stated in Section III Use the following items and information to perform this test in a field or simulated field environment e A tamping rod 5 8 inch in diameter and 24 inches long with a rounded end e A sample of fresh concrete e A trowel e Oil e Rags e A disassembled cylinder mold e A sheet of plastic or burlap e A kitchen scoop e A pan 24 inches wide x 24 inches long x 3 inches deep e A grease pencil Concrete 4 25 FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I e Waterproof paper tags e Gummed labels e Anink pen e Paper e Water e The test specimen number e Theorigin of a concrete sample STEPS Perform the following steps to produce and label a concrete cylinder for testing Step 1 Prepare the mold a Clean and dry the mold
34. nd or sawdust or in limewater to maintain free water on all surfaces of the specimen at all times Occasionally test specimens are made in the field to determine when forms may be removed Form these in addition to the specimens formed for strength determination Give these specimens as much as possible the same protection from the elements on all surfaces as is given to the portions of the structure that they represent Store them in or on the structure as near as possible to the point of use Test them in the moist condition resulting from the specified curing treatment Specimens intended for testing to determine when a structure may be put into use are removed from the molds at the same time the forms are removed from the structure When shipping specimens from the field to the laboratory for testing pack them in a sturdy wooden box or other suitable container surrounded by wet sawdust or wet sand Provide protection from freezing during storage or shipment Moist curing is continued when the specimens are received in the laboratory Concrete 4 27 FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I CAPPING CYLINDERS Plane the ends of compression test specimens within 0 002 inch and within 0 5 degree of being perpendicular to the axis of the cylinder Cap with neat cement specimens formed in strong metal molds having accurately flat baseplates 2 to 4 hours after molding Make a stiff paste of portland cement and water at the time the cylind
35. ombine chemically to bind the aggregate particles together This combining process called hydration results in a rapid development of strength in the first few hours after mixing followed by less rapid gains in strength during the following weeks Cement is a substance that hardens with time and holds or entraps objects or particles in a definite relation to each other For concrete portland cement usually is used Portland cement is a substance that when mixed with water hardens and binds objects or particles together to form concrete This process begins immediately and continues as long as moisture and temperature conditions are favorable As hydration continues concrete becomes harder and stronger Most of the hydration takes place during the first 30 days Hydration can continue well over 50 years but at a much slower rate References to cement in this manual mean portland cement The ASTM specifies eight common Concrete 4 1 FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I AIR ENTRAINED CEMENT 4 2 Concrete types of portland cement ASTM C 150 97 These are adequate for most purposes The various types of portland cement are known as hydraulic cements because they are capable of hardening and developing strength in the presence of water These cements include Typel Cement used for general construction when special properties for any other type are not required TypelA Air entraining cement used for the same purposes as Typ
36. orkable This permits placing the concrete without developing segregation honeycombing or other defects in filling the forms or in producing the desired smooth hard and resilient surface Strength is the concrete s ability to resist a load in compression bending or shear see Sections IV and V of this chapter The desired design strength is DURABILITY WATERTIGHTNESS FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I obtained by proportioning the mixture with correctly graded aggregates an adequate amount of cement to coat the surface area of the particles and the proper amount of mixing water The most important influencing factor on strength is the ratio of water to cement W C ratio For plastic and workable mixes lower values of the W C ratio give higher strengths Two and one half gallons of water is the minimum amount necessary to hydrate a sack of cement adequately This minimal amount of water is not sufficient to economically provide the needed plasticity and workability for freshly mixed concrete Additional water must be added to the mixture to improve workability but must be minimized to obtain the desired strength with an economical cement content Additional water thins the paste content and therefore coats more particles This increases the yield from each sack of cement and produces a more economical mix Excessive amounts of water too high a W C ratio weakens the paste by allowing the cement particles to hydrate while susp
37. rmeable pore space Absorption and surface moisture determinations are necessary to calculate the amount of mixing water used in a concrete mixture SPECIFIC GRAVITY AND ABSORPTION OF COARSE AGGREGATE ASTM C 127 88 This test method covers the specific gravity and absorption of coarse aggregate The specific gravity may be expressed as bulk specific gravity bulk specific gravity SSD or apparent specific gravity Concrete 4 13 FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I Equipment Steps 4 14 Concrete Use the following items to perform tests for bulk specific gravity SSD percent absorption and surface moisture e A balance sensitive to 0 5 gram capable of suspending the sample container in water from the center of the weighing platform or pan of the weighing device e A wire sample basket or a bucket with a 4 to 7 liter capacity for 1 1 2 inch or smaller aggregate and a larger basket or bucket for larger aggregate sizes e A water tank large enough to hold the basket e A pycnometer 2 to 3 cubic feet e A heat source oven or hot plate e A metal sample container e A metal spatula e An absorbent towel Perform the following steps to determine the bulk specific gravity of coarse aggregate in an SSD condition Step 1 Wash a representative sample over the No 4 sieve to obtain a sample size according to Table 4 1 page 4 10 Step 2 Dry the sample to a constant weight at 110 C 5 Step 3 Allow the samp
38. ruler c Record the slump to the nearest 1 4 inch SUPPLEMENTARY TEST PROCEDURE After completing the slump measurement gently tap the side of the specimen with the tamping rod The behavior of the concrete under this treatment is a valuable indication of the cohesiveness workability and placeability of the mix A well proportioned workable mixture will gradually slump to lower elevations and retain its original identity A poor mix will crumble segregate and fall apart Slump is usually indicated in the project specifications as a range such as 2 to 4 inches or as a maximum value not to be exceeded When it is not specified an approximate value can be selected from the list in Table 4 6 Table 4 6 Recommended slumps for various types of construction g Slump in Inches Types of Construction z a Maximum Minimum Reinforced foundation walls and footings 3 1 Plain footings caissons and substructure walls 3 1 Beams and reinforced walls 4 1 Pavements and slabs 4 1 Mass concrete 2 1 May be increased 1 inch for consolidation by hand methods such as rodding and spading AIR CONTENT TEST ASTM C 231 97 EQUIPMENT 4 20 Concrete Add an air entraining admixture to the concrete mix so that enough air will be entrained to improve the mixture s workability durability watertightness and freeze thaw resistance but not enough to substantially reduce the strength Air entr
39. s are also considered as plasticizers because the void system reacts as a lubricant in concrete SECTION II AGGREGATE TESTING Aggregate used in mixing concrete is a mixture of fine and coarse material usually sand with either natural gravel or crushed rock It serves as an inert filler to provide the bulk material required Well graded aggregates contain particles of all sizes from the largest permitted by the dimension of the member to be formed to sand fines The smaller particles fill the spaces between the larger particles thus providing a dense material that requires a minimum of cement paste for binder The aggregate materials must be clean and hard resist weathering and have no unfavorable reaction with the cement An aggregate must provide maximum strength and durability in a concrete mixture Fineness coarseness and aggregate gradation are factors considered when deriving the correct concrete mix for a specific construction purpose Specific gravity absorption and moisture also affect the aggregate s ability to bind well with cement and water in a concrete mix The components of the final mix cement water and aggregate must bond adequately for structural strength and must resist weather and loads Correct aggregate selection also reduces the project s cost An engineering analysis determines the aggregate best suited for a particular purpose Testing allows the best selection For the aggregate tests to be worthwhile
40. tes and dry to a constant weight at a temperature of 110 C 5 Step 3 Weigh and record the oven dried samples Step 4 Calculate the percent of absorption using the following formula S A P a x 100 where P absorption of the aggregate in percent S weight of SSD specimen in grams A waght of SSD samplein the oven dried state in grams FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I The percent absorption represents the moisture content oven dried basis of the aggregate when it is in an SSD condition SURFACE MOISTURE Surface moisture is the excess moisture remaining after the absorption requirement of the aggregate has been met This excess moisture determines how much water is added to the concrete mix to meet the required W C ratio for the proper strength requirements Perform this test just before mixing the concrete as designed This allows for adjusting the water coarse and fine aggregate weights to retain design integrity Surface moisture is the water present in both the fine and coarse aggregates exceeding that which corresponds to an SSD condition This water will become part of the mixing water when the aggregate is used in making concrete The amount of mixing water used must be corrected to allow for its presence See ASTM C 566 89 and ASTM C 70 79 SECTION III FRESH CONCRETE TESTS After concrete is first mixed a slump test and an air content test are performed and used as a control measure to determine the
41. the samples for testing must be representative of the aggregates to be used Take aggregate samples as close as possible to the finished product to give the best representative sample of the aggregate Take a sufficient size and number of samples from the processing plant s discharge point to represent the material in the stockpile The sample should consist of at least four times as much material as is needed for the tests and should be reduced to the desired size through splitting and or quartering the sample Minimum sample sizes can be found in Table 4 1 page 4 10 Concrete 4 9 FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I Table 4 1 Minimum sample sizes Nominal Maximum Size Minimum Weight of Test Sample mm in kg Ib 12 5 or less 1 2 or less 2 4 4 19 0 3 4 3 6 6 25 0 1 4 8 8 37 5 11 2 5 11 0 50 0 2 8 18 0 63 0 2 1 2 12 26 0 75 0 3 18 40 0 90 0 3 1 2 25 55 0 100 0 4 40 88 0 112 0 41 2 50 110 0 125 0 5 75 165 0 150 0 6 125 276 0 STOCKPILE SAMPLING ASTM D 75 87 It is difficult to ensure that unbiased samples are obtained from stockpiles This is due to the segregation that often occurs when material is stockpiled with coarser particles rolling to the outside base of the pile For coarse or mixed coarse and fine aggregates every effort should be made to enlist the services of power equipment to develop a separate small sampling pile composed of materials dr
42. the test e Specimen s identification number e Average width to the nearest 0 05 inch e Average depth to the nearest 0 05 inch e Span length in inches e Maximum applied load in pounds e Modulus of rupture to the nearest 10 psi e Curing history how the specimen was cured and apparent moisture content of the specimen at the time of the test e Any defects noted in the specimen e The age of the specimen FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I SECTION V COMPRESSIVE STRENGTH TEST The compressive strength of hardened concrete as measured by compression tests on standard forms of cylindrical specimens is used in the design of structures Compressive strength tests are made on concrete trial mixtures to evaluate the performance of available materials and to establish mixture proportions that give the required strength Strength tests are used also to control the quality of concrete being manufactured in the field Compressive strength is defined as the average of the strengths of all cylinders of the same age made from a sample taken from a single batch of concrete At least two cylinders are required to constitute a test Therefore a minimum of four specimens are required if tests are to be made at 7 and 28 days The test results will be the average of the strengths of the two specimens tested at 28 days CASTING A CONCRETE CYLINDER EQUIPMENT The standard test specimen is a cylinder 6 inches in diameter by 12 i
43. tion see Figure 4 5 C2 FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I Apply load here Steel rod Loadi gading Specimen i Steel rod Steel ball 1 T ae f i e a Ti 173 v3 173 z Length of span Figure 4 5 Apparatus for flexural strength test Step 2 Measure the length of span and record the measurement on a piece of paper The length of span is determined by measuring the distance from center to center of the two loading points or supports on the base of the apparatus see Figure 4 5 The normal length of specimen is 21 inches and the normal length of span is 18 inches Step 3 Place the specimen in the tester and bring the loading surface into contact with the test specimen see F igure 4 5 Step 4 Zero the gauge Some apparatus are equipped with a hydraulic pump and corresponding gauge while others are equipped with a loading jack and proving ring Step 5 Apply a load at a continuous rate that constantly increases the extreme fiber stress between 125 and 175 psi per minute This is an approximate load of 1 500 to 2 100 pounds per minute Step 6 Obtain the total load in pounds at the time of specimen failure and record the weight on the paper provided On machines equipped with hydraulics take the reading directly from the gauge For machines equipped with a proving ring this reading is the product of the dial gauge reading and the proving ring constant Step 7 Determine and record the wi
44. tion of aggregate for portland cement concrete are the same as explained for sieve analysis except that the No 4 sieve is taken as the dividing line between fine and course aggregates The minimum sample size required in the sieve analysis of fine aggregate is 500 grams The result of this test is a gradation curve for the aggregate concerned MATERIAL FINER THAN 075 MILLIMETERS No 200 SIEVE The extremely fine mineral material clay silt dust or loam occurring in most aggregates requires relatively large increases in the amounts of mixing water Fines tend to work to the surface of concrete and cause cracking upon drying due to shrinkage If the fines adhere to the larger aggregate particles they also tend to interfere with the bond between the aggregate particles and cement water paste Specifications limit the amount of such material to a small percentage ASTM C 117 95 gives the standard test method for fine Concrete 4 11 FM 5 472 NAVFAC MO 330 AFJMAN 32 1221 I materials The apparatus test procedures and calculations to determine this percentage are described in the test for impurities Fine material not to exceed 3 to 5 percent of the total aggregate weight is generally not harmful to concrete For some purposes a small amount of such fines may improve the workability Table 4 3 Desirable gradation for coarse aggregate in concrete Percent
45. xture containing particles of different size shape and mass Heavier particles have a tendency to settle out through the mixture faster than lighter particles Often the result is a segregated mixture of a very poor quality When concrete is properly proportioned and mixed and carefully handled segregation is held to a minimum The mixture must have the proper proportion of cement sand mortar to prevent the larger coarse aggregate particles from separating from the batch during mixing transporting and placing When cement is allowed to drop free fall over a considerable distance it can cause segregation of the mixture To minimize segregation for drops in excess of 3 to 5 feet bottom dump buckets should be used to place concrete as close to the final location as possible See TM 5 742 for construction procedures Uniformity refers to a single batch of concrete and to all batches for an entire project The same amount of each ingredient should be mixed into each batch or a nonuniform structure will result Design would not be met in all sections of the structure and possible failure of these sections could result Proper supervision in mixing and handling of the concrete ensures uniformity CONCRETE CURING TEMPERATURE 4 6 Concrete Concrete does not develop its full strength until the chemical process of curing hydration is complete Cement must have sufficient water to continue its hydration Curing is the means of keeping water avail
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