T he control of cracking is a problem that con-
c e rns tho who design an
d build with concr
e t e.
But there hasn’t been any standard
test that e valuates quantitatively what the effect is on drying shrinkage cracking in the field when one mater-ial or condition is changed. Now a test is being deve l-
oped that has been introduced for consideration by the
A m e rican Society for Testing and Ma t e ri a l s. The test
ts up adver conditions that incorporate the major
caus of cracking. This maximizes the cracking
漫语potential of the test specimen. The first numerical re-
orientaldailysults of the test can become available within a matter
of hours and complete results are normally ava i l a b l e
within 24 hours.
Un f o rt u n a t e l y, many people equate the cracking po-
tential of concrete with its drying shrinkage chara c t e ri s-
t i c s. Drying shrinkage is usually measured according to
ASTM C 157, “St a n d a rd T est Method for Length Change
of Ha rdened Cement Mo rtar and Co n c re t e.” Using this
test to control drying shrinkage cracking, howe ve r, has
had questionable success and may mislead tho who
t ry to u it for this purpo.
A propod new drying shrinkage test was descri b e d
deem什么意思in “Propod Field Test for Drying Sh rinkage of Co n-
c re t e,” CONCRETE CO N S T RUCTION, July 1984, page
663. That article listed the following as some of the many
factors that contribute to drying shrinkage cra c k i n g :
• materials and design
• placing and finishing techniques
• tempera t u re and humidity
• curing pra c t i c e s
• ground conditions (for slabs on gro u n d )
The article described a test for measuring dry i n g
s h rinkage bad on 4x4-foot test specimens 8 inches thick. The test described here, howe ve r, is not a mea-s u re of drying shrinkage but rather of cracking potential.It makes u of smaller, 2x3-foot specimens only 3/4inch thick (Fi g u re 1).Maximizing cracking potential The cracking potential is determined by compari n g the cracking of two test panels expod simultaneously to a t of ve re conditions designed to cau cra c k i n g .One panel is a control panel; the other is a similar panel e xcept that a single material is altered to study its effect.The cond panel could also be made identical in mate-rials but then be subjected to different tempera t u re or d rying conditions.For the control panel tho influences on dry i n g s h rinkage cracking are chon that are thought to m
axi-m i ze the amount. In one recent version of the test the f o l l owing conditions we re applied to the control panel for this purpo:Thin specimen with large expod are a . The concre t e test specimen is only 3/4 inch thick, but the expod top s u rface is 2x3 feet to accelerate the rate of eva p o ra t i o n and shri n k a g e.No coar aggre g a t e .Becau the specimen is only 3⁄4inch thick, no coar aggregate is ud in the panel. Ce-A propod test to determine the cracking potential due to drying shrinkage of concrete This method permits evaluation of
many factors that may be studied
B Y P AUL P . K RAAI
P ROFESSOR -L ECTURER
S AN J OSE S TATE U NIVERSITY
Figure 1. Test specimens after 21⁄2hours of testing. A control panel (left) is designed to have maximum cracking potential. A test panel (right) differs from the control panel in one respect that is to be checked for its effect on changing the cracking potential. In this particular instance the test panel contains polypropylene fibers while the control panel does not.四级网考成绩查询
m e n t - t o - a g g regate pro p o rtions are normally 1:4 by
weight but the can be changed to suit specific test re-
q u i re m e n t s.
High cement factor . Cement content is 705 pounds per
cubic yard .
High water-cement ra t i o. Wa t e r-cement ratio is 0.70.
In the mix ud for the test this produced a flow of 100
to 110 percent when tested by applicable provisions of
ASTM C 109, “St a n d a rd Test for Co m p re s s i ve St re n g t h
of Hyd raulic Cement Mo rt a r s.”
Movement of free water only in upw a rd dire c t i o n .T h e bottom of the form is cove red with polye
thylene film to p re vent absorption of water at the bottom. Bleed water m oves up tow a rd the top surf a c e.Abnce of re s t raint by bottom surface. The polye t h y l-ene film also rves to pre vent the bottom surface fro m re s t raining the volume change of the concre t e.Perimeter re s t ra i n t .A strip of 1/2xl-inch-mesh hard-w a re cloth is bent to an L-shape and installed just inside the perimeter of the form as shown in Fi g u re 2. This re-s t rains movement caud by drying shrinkage which is the direct cau of cra c k i n g .
Ex p o s u re to wind. Immediately after casting, the two
panels are expod to a wind of 10 to 12 miles per hour.
No curing.No curing is ud at any time during the pe-
riod of exposure to wind (which can be 4 or 5 hours).
Performing the test
Mix pro p o rtions are established for the control mix
and a batch is made of sufficient size to make one pan-
el. Another mix is made to study one material or condi-
tion not ud for the control. For example, this cond
mix might contain an admixture, an aggregate thought什么洗发水去头屑
to either increa or reduce cracking, or fibers or other
m a t e rial that might affect cracking potential. This mix is
ud for the companion panel.
Special mixing pro c e d u res can be ud. In a norm a l
c o n c rete mix the coar aggregate woul
d b
e coated with
cement paste. Since the size of the cra c k i n g - p o t e n t i a l
test specimen does not permit the u of coar aggre-
upgradeable
g a t e, coar aggregate can be first introduced into the
mix and then re m oved with its cement paste coating.
This is accomplished by mixing cement and water and
adding coar aggregate (Fi g u re 3). The coated coar
a g g regate is then screened out (Fi g u re 4) and the slurry
re t u rned to the mixer. Sand and any other ingre d i e n t s
such as fibers are then added and mixed. Mixing is con-
tinued for 5 minutes after all fibers, if ud, have become
c o a t e
d .
Figure 2. Partial ction of restraining form for test panel.
The mortar specimen cast in the form will be 3⁄4inch by 2
feet by 3 feet. It remains within the form throughout the
test and the anchored hardware cloth provides the needed restraint.
As soon as a concrete mix has been placed in the form it is screeded and troweled with a 40-inch length of 1-inch angle iron that has been polished on one side to p roduce a smooth surf a c e. In the current version of the test both panels are t at the same time in front of fans ( Fi g u re 1) that prod
uce the drying effect. To direct the wind onto the plastic concrete surf a c e s, the fans are tilt-ed by means of a 3⁄4-inch piece of molding placed under their back edges. Although drying conditions are need-ed, ambient conditions including air speed need not be exact so long as they are the same for the two panels be-ing compared. Ex p o s u re to this induced wind should continue for 5 hours. Initial cracking begins to occur at the age of about 1 hour. In Fi g u re 1 the test was begun at 11:00 A.M., and so the picture shows the condition of this t of panels at an age of 21⁄2h o u r s. Most of the cra c k-ing occurs within 4 hours.
Measuring and calculating
Panel evaluation starts at 24 hours. At that time cra c k s a re likely to be slightly wider and possibly more numer-ous than after 4 hours. Panels re e valuated at 3 and 6months have shown no significant further change in c racking pattern s.
Crack lengths and ave rage crack widths are measure d and re c o rded for each panel. Crack widths fall into four c a t e g o ri e s, each of which is assigned a weighted va l u e :Crack width Weighted va l u e _________________________ _________________L a rge (about 3 millimeters)3识别英文
Medium (about 2 millimeters)2
Small (about 1 millimeter)1
六级考试时间2014Hairline (about 0.5 millimeter)0 . 5
For each crack the weighted value is multiplied by the
c rack length in millimeters to give the weighte
d av
e ra g e va l u e. The sum o
f the weighted ave rage values of all c racks in a panel is called the “total weighted ave ra
g e va l u e” for the panel. This re p rents the cracking poten-t i a l .For example, at the end of the t
est of the panels show n in Fi g u re 1, calculations we re made as follow s :L e f t .The control had 1090 millimeters of larg e, 1120millimeters of medium, 760 millimeters of small, and 150 millimeters of hairline cra c k s. He n c e, (1090x3) +(1120x2) + (760x1) + (150x0.5) = 6350, whic
h is the total weighted ave rage va l u e.Right. The other specimen, containing polypro py l e n e f
i b e r s, had 250 millimeters of small and 230 millimeters of hairline cra c k s. He n c e, (250xl) + (230x0.5) = 370, the total weighted ave rage va l u e.Ex p resd as a perc e n t a g e, the total weighted ave ra g e value in this example is 5.8 percent of the control, or 94.2p e rcent reduction from the contro l .Some examples of test data In an early t of tests, ven panels we re all compare d with a single control panel. The ven panels re p re s e n t mixes made with ven kinds of polypro pylene fiber.Fibers we re added at the rate of 0.7 percent by weight of cement in mixes containing 705 pounds of cement per cubic yard. The cement was plastic cement, a modified p o rtland cement ud in Ca l i f o rnia for exterior port l a n d cement plaster (stucco) and some other applications.The cracking potentials are given in Table 1. The data indicate that there can be considerable difference in the e f f e c t i veness of va rious kinds of polypro pylene fiber in reducing cracking. Table 2 gives the compre s s i ve and tensile strengths for the mixes ud in the control panel
and three other panels.
Figure 3. Mixing coar aggregate into the slurry. A coar aggregate can be initially mixed in for its effect on the mix proportions but the aggregate and its paste coating are then removed before completing the mixing and making of specimens.
Figure 4. Coar aggregate coated with paste after being screened from the slurry. The slurry is then mixed with sand and any other materials still to be added.
In another t of early tests (T able 3), four panels we re also compared with a single control panel. The mixes contained 705 pounds of Type II portland cement per cubic yard plus 2 percent flake calcium chloride by weight of cement or 0.7 percent polypro pylene fiber by weight of cement, or both. Data for Panels 2 and 3 show that under the test conditions calcium chloride caud a significant reduction in cracking potential. This part i c-ular polypro pylene fiber controlled cracking completely, whether calcium chloride was ud with it or not. It might be noted that unre s t rained shrinkage tests (not c racking tests) by Ronald Zollo (Re f e rence 1) showed a l a rge reduction in total shrinkage when collated fibri l l a t-ed polypro pylene fibers we re ud in a mix made at a w a t e r-cement ratio of 0.65. His measurements we re made on 11-inch gage lengths on the top surfaces of 1 1⁄2 x3x14-inch test specimens.
The test described in this article is also being ud to e valuate the effects of different aggregate types on s h ri n k a g e.
Some general comments on the procedure
Adam Neville has written (Re f e rence 2) that inve s t i g a-tions have shown that even before any load is applied to c o n c rete there are cracks between the coar aggre g a t e and the cement paste. “The ove rall stress under which they deve l o p,” he says, “is nsitive to the water- c e m e n t ratio of the paste.” The cracks develop slowly but at 70 to 90 percent of the ultimate strength they open thro u g h the mort a r, according to Ne v i l l e.
Since most drying shrinkage cracks start as micro c-racks in the mortar fraction of the concre t e, the cra c k -
e valuation test described here has been bad on this
f raction. In a cross ction of concrete sawed thro u
g
h d rying shrinkage cra c k s, the cracks can frequently be en detouring around coar aggregate particles ra t h e r than going through them. This is becau micro c ra c k s h a ve already begun to form adjacent to the aggre g a t e p a rt
i c l e s.
英语语法新思维The cracking that develops in this test begins while the c o n c rete is still in the plastic condition, and re a d e r s might object that it is really plastic shrinkage cra c k i n g , not regular drying shrinkage cracking. Ne ve rt h e l e s s, I b e l i e ve that it is directly related to regular drying shri n k-age cracking and that it provides a valid measure of the potential cracking caud by drying shri n k a g e. Another cau of cracking of re s t rained concrete is volume change due to changes in tempera t u re. This test can be adapted to study the effects of tempera t u re. References
1. Zollo, Ronald F., “Collated Fibrillated Polypropylene Fibers in FRC,” Fiber Reinforced Concrete: International Symposium, SP-81, 1984, pages 397-409, American Con-crete Institute, Detroit, Michigan 48219.
2. Neville, Adam, Hardened Concrete, ACI Monograph Number 6, page 4, American Concrete Institute, Detroit, Michigan 48219. (Out of print. Order from University Micro-films International, Attent
ion: Books on Demand, Box 1467,
Ann Arbor, Michigan 48106.)
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