The 7 Deadly Sins of Grout Testing

"We need to get real about ASTM C 109. First of all, it's a test method only, not a standard, and does not set criteria for acceptance or rejection. Secondly, it was developed for the portland cement industry to measure the compressive strength of portland cement."

When confronted with a new or different situation, we frequently try to deal with it based on a known point of reference which can lead us down the path of misunderstanding. Non-shrink grout all too often has to face this dilemma. So, it should be said loud and clear that non-shrink grout is not a concrete or a simple cement mortar and should not be tested as such. These materials are different with very different tasks to perform. While concrete testing has been greatly enhanced by the efforts of the American Concrete Institute (ACI) through its concrete technician certification program, there is not a similar program for non-shrink grout. All is not lost, however. Both ASTM and ACI have addressed non-shrink grout testing. The current guide ACI 351.1 (Grouting between Foundations and Bases for Support of Equipment and Machinery) deals with both the placement and testing of cementitious and epoxy grouts. Similarly, ASTM has produced a standard for cementitious non-shrink grouts that has gained worldwide acceptance in the form of ASTM C 1107, Standard Specification For Packaged Dry, Hydraulic-Cement Grout (Non-shrink). This specification details or lists acceptable test procedures and methods for the determination of physical properties of non-shrink cementitious grouts. It should be noted that ASTM C 1107 does not apply to epoxy grouts. At the present time epoxy grouts do not have a counterpart to ASTM C 1107. Likewise, very little space has been given to epoxy grouts in ACI 351.1.

The Deadly Sin of Grout Testing Misspecification
The primary testing of non-shrink grout in the field is for compressive strength, and as a deadly sin it's a doozy. The mistake often begins with writing of the job specifications. All too often the job specifications require the non-shrink grout to be tested in accordance with ASTM C 109, or even worse, to meet the "requirements" of ASTM C 109. We need to get real about ASTM C 109. First of all, it's a test method only, not a standard, and does not set criteria for acceptance or rejection. Secondly, it was developed for the portland cement industry to measure the compressive strength of portland cement. ASTM C 109 contains a formula for producing a standard mortar, which requires the cement, aggregate and water be mixed at a given ratio. The mortar that is produced has a very stiff consistency, whereas most non-shrink grouts have a fluid consistency. If a non-shrink grout is required to be tested in accordance with ASTM C 109 this can lead to our second deadly sin.

Standard brass mold of 3 2"X2" cubes
The Deadly Sin of Bad Procedure
If the non-shrink grout cubes are fabricated in accordance with ASTM C 109 the result will be a disaster. ASTM C 109 requires the test specimens to be 2" by 2" cubes, which is the proper type specimen for non-shrink grout testing. The consistency of the non-shrink grout becomes an issue during the molding of test specimens. ASTM C 109 requires each cube compartment be filled half full of grout and to be tamped 32 times, and the second and final layer of grout be placed in the cube mold and again, tamped 32 times. While this is a good procedure for a very stiff mortar, this method will drive the aggregate of a fluid consistency non-shrink grout to the bottom of the cube (segregation) and cause a loss of strength in the sample, not representative of the true strength properties of the grout. For this reason ASTM C 1107 (paragraph 11) gives a detailed procedure for fabricating and testing non-shrink grout cubes. This procedure modifies ASTM C 109 so that it can be used to fabricate cubes from fluid consistency non-shrink grout without causing segregation. To insure that grout cubes are properly formed, one should reference ASTM C 1107 for the proper fabrication and testing of non-shrink grout specimens and not ASTM C 109.

Dry pack grout
"Dry pack" is a very low water grout consistency that needs special attention when being either placed or formed into cubes for testing. It is essential that the dry pack grout test specimens be compacted to the same density as the in-place grout under the base plate. If the grout under test is compacted to a lesser density than that of the in-place grout, the grout under test will have a lower strength.

Standard 6" X 12" concrete cylinder mold is used for testing concrete, not non-shrink grout
The Deadly Sin of Bad Choices
A major problem uncounted when testing non-shrink grout in the field is the choice of molds in which the specimens are cast. ASTM C 1107 requires 2" by 2" cube molds to be in compliance with ASTM C 109. Using a cube mold that has faces that are rough or pitted or out of parallel with the opposite face will produce cubes with strengths that are much lower than the actual strength of the grout under test. Only brass or stainless steel molds should be used. Plastic cube molds cannot hold tolerance as well as brass or stainless steel and should not be used. Cube mold systems that use an insert in the cube molds should not be used for the same reason. It should be noted that ASTM C1107 requires the formed face of the cube under test to be in contact with the bearing blocks of the testing machine when being tested for compressive strength. This rules out capping the cube with sulfur and other capping materials or the use of neoprene pads.

As stated earlier, the non-shrink grout industry has adopted the 2" by 2" cube as its standard test specimen for compressive strength testing, whereas the concrete industry adopted the 6" by 12" cylinder as its standard. Cylinder molds are inexpensive and widely available at construction sites. Whereas, cube molds are very expensive, costing $300 to $400 per set, and many times are simply not available. The temptation to use cylinder molds should be resisted. The reason is that when non-shrink grout is tested as a 6" by 12" cylinder it can test significantly lower in strength (as much as 20% lower) than when tested as a 2" by 2" cube. This lower strength is the result of the geometry of the test specimen (the ratio of length over diameter) and not the material being tested.

The Deadly Sin of Poor Field Protection
Once the grout cubes are molded in the field they must remain undisturbed at that location for at least 18 hours. It is an absolute must that the molded specimens be protected from extremes in temperature, vibration and shielded from direct sunlight. ASTM C31/C31M, Making and Curing Concrete Test Specimens in the Field, paragraph 9.2.2 Initial Curing, gives very good guidelines for protecting freshly molded cementitious specimens in the field. Three key points made are: 1) store the samples in a temperature range of 60 to 80°F, 2) do not store in direct sunlight and 3) prevent moisture lost. Even though the cube mold has a cover plate, it is a good practice to cover the specimen with wet burlap or rags. This will help reduce moisture loss and control the temperature of the test specimens.

Often overlooked, but very important, is vibration. All sources of vibration can cause segregation of aggregate and cement paste causing low-test strengths. It can also cause low strengths by disturbing the hardening process of the grout. Test specimens should not be molded and stored in area where vibration can be physically felt.

The Deadly Sin of Transport
As Rodney Dangerfield might say, "grout specimens get no respect on the way to the laboratory," and in a number of cases he is right. After initial field curing of at least 18 hours, grout specimens should be taken directly to the laboratory. They are not intellectuality enhanced by a daylong tour of the scenic job sites in the area. Grout cubes at this tender age are undergoing rapid strength gain and any damage can and will cause the cubes to test low. Test specimens should be treated as if they were eggs.

The Deadly Sin of Improper Curing
There are many ways to cure concrete and mortars and grouts. When it's comes to non-shrink grouts, ASTM C 1107 has some very specific procedures to be followed. These procedures can be found in paragraph 11.5.3. "For 1 day compressive strength specimen, strip molds at 24+/- ½ hour after molding. Strip remaining molds at 72 +/- 1 hour after molding and place specimens in moist cabinet or moist room protected from dripping water. Three cubes shall be tested at each age: 1, 3, 28 days." ASTM C 1107 does not allow the specimens to be immersed in lime-saturated water.

Compressive strength machine set up for testing 2"X2" non-shrink grout cubes
The Deadly Sin of Inappropriate Testing Equipment
Practically all the laboratories testing non-shrink grout are concrete laboratories and, of course, testing concrete is the major part of their business. Their testing machines may be set up to test 6" by 12" concrete cylinders. Unfortunately, many testers do not know that the 2" by 2" grout cubes must be tested on a machine using a smaller upper bearing block. If the cubes are tested on a machine setup for 6" by 12" concrete cylinders, the cube face may be unevenly loaded, causing a low and inaccurate strength to be reported.

In conclusion, of all the most quoted deadly sins my favorite is pride, or rather the lack of pride in our work. I am convinced that if more pride was taken in our testing programs we could avoid the seven sins just discussed. One might say that if we give the little guy (the test cube) a break, he will give you a good break (test). While this may be a bad pun, it is good advice.

Note: ASTM C 1107 can be found in ASTM volume 04.02 Concrete and Aggregates and ASTM C 109 can be found in ASTM volume 04.01 Cement; Lime; Gypsum. These standards can be downloaded from the ASTM web site at www.astm.org for a small fee.


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© 2003 L&M Construction Chemicals, Inc. | ConcreteNews Winter 2002/2003.

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