1、Standard Method of Test for Estimating the Cracking Tendency of Concrete AASHTO Designation: T 334-08 (2016)1 Release: Group 1 (April 2016) American Association of State Highway and Transportation Officials 444 North Capitol Street N.W., Suite 249 Washington, D.C. 20001 TS-3c T 334-1 AASHTO Standard
2、 Method of Test for Estimating the Cracking Tendency of Concrete AASHTO Designation: T 334-08 (2016)1Release: Group 1 (April 2016) 1. SCOPE 1.1. This method covers the determination of the cracking tendency of restrained concrete specimens. The procedure determines the effects of variations in the p
3、roperties of concrete as related to the time-to-cracking of concrete when restrained. The procedure is comparative and not intended to determine the time of initial cracking of concrete cast in a specific type of structure. 1.2. This standard may involve hazardous materials, operations, and equipmen
4、t. This standard does not purport to address all of the safety concerns associated with its use. It is the responsibility of the user of this procedure to establish appropriate safety and health practices and to determine the applicability of regulatory limitations prior to use. 2. REFERENCED DOCUME
5、NTS 2.1. AASHTO Standards: M 210M/M 210, Use of Apparatus for the Determination of Length Change of Hardened Cement Paste, Mortar, and Concrete R 39, Making and Curing Concrete Test Specimens in the Laboratory T 160, Length Change of Hardened Hydraulic Cement Mortar and Concrete 2.2. ASTM Standards:
6、 A53/A53M, Standard Specification for Pipe, Steel, Black and Hot-Dipped, Zinc-Coated, Welded and Seamless A501/A501M, Standard Specification for Hot-Formed Welded and Seamless Carbon Steel Structural Tubing 3. SIGNIFICANCE AND USE 3.1. This test can determine the effects of concrete variations on cr
7、acking tendency. These variations might include aggregate source, aggregate gradation, aggregate-paste bond, cement type, cement content, water content, mineral admixtures, silica fume admixtures, fiber reinforcement, or chemical admixtures. 3.2. Actual cracking in service depends on many variables
8、including bridge type, degree of restraint, hydration effects, construction and curing methods, and environmental factors. The method is useful for determining the relative likelihood of early concrete cracking and for aiding in the selection of concrete mixtures that are less likely to crack. The t
9、est method may also be modified to evaluate other factors that may affect cracking such as curing time, curing method, evaporation rate, or temperatures. 2016 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.T
10、S-3c T 334-2 AASHTO 3.3. The test method measures the strain in a steel ring as a surrounding concrete ring shrinks. The time-to-cracking of the concrete ring is measured as the time when an abrupt drop in strain is seen in the steel ring. Simple visual monitoring of the time-to-first-cracking may a
11、lso be useful, but it is less accurate and more time-consuming than monitoring strains, and large errors may result with visual monitoring. The test can also evaluate environmental and construction factors by modifying the test environment or curing procedures. 3.4. Other concrete tests that may be
12、related to cracking tendency are unrestrained free-shrinkage according to T 160, compressive strength, tensile strength, elastic modulus, Poissons ratio, and creep. 4. APPARATUS 4.1. Steel RingThe standard steel ring shall have a wall thickness of 12.7 0.4 mm (1/2 1/64in.), an outside diameter of 30
13、5 mm (12 in.), and a height of 152 mm (6 in.). As shown in Figure 1, the inner and outer faces shall be machined smooth, round and true, and polished. Note 1Theoretical elastic analysis indicates that decreasing the steel thickness increases stresses in the steel significantly but only slightly affe
14、cts concrete stresses. Bond strain gauges at four equidistant mid-height locations on the interior of the steel ring. Note 2Structural steel pipe conforming to ASTM A501/A501M or A53/A53M 12-in. extra-strong pipe with an outside diameter of 324 mm (123/4 in.) and wall thickness of 13 mm (1/2in.) may
15、 be substituted if its inner and outer faces are machined to a smooth, polished surface. 4.2. Data AcquisitionThe data acquisition unit shall be compatible with the strain instrumentation and automatically record each strain gauge independently. Note 3Often when cracking occurs, only one or two gaug
16、es indicate significant strain relief. 4.3. FormsThe forms shall be nonabsorbent. Fabricate the base forms of resin-coated or polyethylene-coated plywood to minimize friction restraint of the concrete. Thin 3-mm (1/8-in.) polyethylene sheeting works well as the outside radius form. 4.4. CuringWet cu
17、re the top surface, using prewetted burlap covered with plastic. 4.5. Curing and Test RoomAfter wet curing, store the samples in a controlled-environment room with a constant air temperature of 21 1.7C (73.4 3F) and a relative humidity of 50 4 percent. Note and record the evaporation rate near the r
18、ing surfaces as described in T 160. 5. SPECIMEN FABRICATION 5.1. Secure the steel ring to the base with a central hold-down device during casting. 5.2. Coat the steel ring surface in contact with the concrete with a release agent such as paraffin wax dissolved in solvent or other suitable form-relea
19、se agent. Do not use form-release agents on the outside radius form. 5.3. Make and cure the test ring specimens following the applicable requirements of M 210M/M 210 and R 39. 5.4. Cast at least two concrete rings, using the apparatus shown in Figures 1a and 1b, for each batch. Rod the concrete into
20、 the molds in three equal lifts, using a 16-mm (5/8-in.) diameter round-nosed rod. Rod the concrete equidistantly 75 times per layer, ensuring that the rod slightly penetrates into the previous layer. Spade the inside and outside surfaces of the mold after each lift to eliminate large voids along th
21、e form faces. Lightly tap the base of the mold with a rubber mallet to close any 2016 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-3c T 334-3 AASHTO holes left by rodding and to release any large air bu
22、bbles; do not tap on the exterior radius of the molds or on the steel ring. Note 4While not recommended, if external or internal vibration is used, vibrate the concrete following R 39 and record the vibration frequency and time. Notes: 1. Not to scale. 2. Assume a tolerance of 5 mm (0.2 in.) through
23、out. Figure 1aCracking-Tendency Test Apparatus 2016 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-3c T 334-4 AASHTO Figure 1bCracking-Tendency Test Apparatus (LeftEmpty; RightFull of Concrete) 5.5. After
24、 consolidation, strike off and wood-float the concrete surface. Clean any excess concrete from the top and sides of the forms to achieve a level surface. Finish with minimum manipulation necessary to achieve a finished, flat, and even surface. Immediately transfer the specimens to the cure room. 5.6
25、. Loosen the tie-down holding the steel ring to the base form. Connect and begin monitoring the strain instrumentation immediately after placing the specimens in the cure room, cleaning the strain gauge, and connecting wires with an emery cloth before attachment. After the concrete hardens sufficien
26、tly to resist indentation of the burlap, cover the specimens with wet burlap followed by plastic. Leave undisturbed for 24 h. Keep the burlap wet until the forms are removed. 5.7. Remove the forms from the concrete rings at an age of 24 1 h. Gently slide the ring or lift and tap the base to break th
27、e specimen free from the base form. Check that no debris is caught between the concrete and the base form. Keep the ring in contact with the base during testing or seal the bottom to prevent drying. Lightly dress the top outside edge of the concrete to remove sharp edges. Seal the top surface by run
28、ning a bead of silicon caulk on the inside and outside edge of the top of the concrete ring and pressing a rubber mat or plastic into the caulk. Because some silicone caulks will corrode the steel ring, protect the top edge of the steel ring with a coat of varnish or do not allow the caulk to contac
29、t the steel. 6. TESTING 6.1. Record ambient temperatures. 6.2. Temperature compensation for the steel ring gauge readings shall be performed by monitoring a strain gauge mounted on an unstressed piece of steel. 6.3. Monitor the strains in the rings as soon after casting as practical, recording strai
30、ns every 30 min. Measure each strain gauge separately. Every 2 to 3 days, review the strain measurements, and visually inspect the ring for cracking. Note 5A strain decrease of more than 30 microstrain in one or more gauges usually indicates cracking. 6.4. After cracking, note the cracking pattern a
31、nd crack widths on the exterior radial face. Monitor the specimens for two additional weeks after cracking, measuring crack widths so the strain decrease and crack pattern can be characterized. 2016 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplic
32、ation is a violation of applicable law.TS-3c T 334-5 AASHTO 6.5. Measure crack width at three equidistant locations 33, 76, and 114 mm (1.3, 3.0, and 4.5 in.) across the height of the ring and calculate the average width. 7. CALCULATION 7.1. Time-to-cracking is the age when strains measured by one o
33、r more of the strain gauges mounted on the steel ring suddenly decrease. Average the results from each specimen cast from the batch, and report the age at cracking to one tenth of a day. If compressive strain increases in the steel ring are followed by gradual decreases and the concrete rings do not
34、 crack, report the results as “no cracking” and record the age when the test was terminated. 7.2. Plot the free-shrinkage strain of the unrestrained samples and determine the unrestrained shrinkage at the average time-to-cracking measured in accordance with T 160 (optional). 8. REPORT 8.1. Report th
35、e following information as pertinent to the variables studied: 8.1.1. Properties of the Concrete MixtureBatch materials and proportions, air content, consistency, and unit weight of fresh concrete; 8.1.2. Variations in ring dimensions, forming, casting, or curing; 8.1.3. Steel ring thickness and out
36、side diameter; 8.1.4. Casting and curing temperatures; 8.1.5. Temperature, relative humidity, and evaporation rate of the test room; 8.1.6. Time-to-cracking in days for each specimen, and the average to one tenth of a day; 8.1.7. Average strain of steel ring at cracking; 8.1.8. Plots of steel ring s
37、train versus time; and 8.1.9. Average unrestrained free-shrinkage at the average time-to-cracking. 8.2. OptionalPattern of the cracking and the measured crack widths on the exterior face. 1Formerly AASHTO Provisional Standard PP 34. First published as a full standard in 2008. 2016 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.
