|Standards Enable New Cement and Concrete Testing Technology
These are interesting times to be a member of ASTM International Committees C01 on Cement and C09 on Concrete and Concrete Aggregates. Modern technology and automation has finally entered the cement and concrete testing laboratory. Old standards are being updated to accommodate new technology and new standards are evolving based on developmental work using new pieces of laboratory equipment.
Such devices as automatic paste and mortar mixers, automatic time- of-setting machines, automatic fineness measuring systems, pre-programmed jolting and vibration tables for fabrication of flexural and compressive strength specimens, and tamping machines for fabrication of strength specimens are already on the market. Some of these machines have been widely accepted by specifying agencies while others are still in various stages of consideration. For the construction industry, the specification and use of such new technologies is highly dependent on the availability of standards and Committees C01 and C09 are working to meet this challenge.
For the past 80 years, the physical testing of cement and concrete has depended on conscientious and skilled testers performing work that is labor-intensive, time-consuming, and sometimes tedious. The volume and quality of work produced has been limited by the speed and capability of the tester. In conducting tests and training, the staff at the Skokie, Ill., construction materials testing company, CTLGroup, has often wondered why some of these tests were not automated. There seemed an obvious need to be filled.
Advances in automation over the last 10 years are now permitting the typical cement laboratory to go high-tech. The use of microprocessors, computer-controlled systems, robotics and optics has permitted increased precision and accuracy in testing as well as greater laboratory efficiency. The use of automation can improve reproducibility and reduce variability. Other benefits include increased efficiency, improved workflow and reduced turnaround time and training costs.
While cement and concrete test methods may vary throughout the world, the cement and concrete properties being evaluated are the same. The use of automation to test the properties of cement and concrete has been facilitated by changes to our test methods. The last decade has seen the inclusion of alternative methods of testing that have offered the industry numerous options for automated testing.
Automated Vicat Testing
ASTM C 114, Test Methods for Chemical Analysis of Hydraulic Cement, was the first standard to change, using technology that allows for rapid methods to obtain test results. When C 114 was first approved, cement chemistry was performed by wet or bench methods. Proper analysis could take days to complete and required highly skilled, specially trained chemists. With the advent of modern analytical equipment (atomic absorption and X-ray fluorescence) the standard needed to change to permit the use of such technology.
Therefore, about 20 years ago, a task group of Subcommittee C01.23 on Compositional Analysis developed a qualification procedure that became part of the C 114 standard. This allowed for the use of alternative methods of analysis. With this change, tests that previously took days to complete could now be performed within one workday.
In the late 1990s, ASTM subcommittees on the physical testing of cement began to investigate the possibility of alternative methods. A number of subcommittee members were aware of an automatic Vicat time-of-set machine that had come on the market. Cement manufacturers in Europe had been using this technology for a number of years, but requirements such as temperature and humidity were not as stringent in European standards as in ASTM standards. This presented an issue for the manufacturers of the automatic apparatus.
After five years of subcommittee work and equipment modifications by manufacturers, ASTM C 191, Test Method for Time of Setting of Hydraulic Cement by Vicat Needle, was revised to allow alternative methods for determining setting time. The standard includes a qualification procedure as well as targeted acceptable ranges for the performance of the automated tests. In support of Subcommittee C01.30 CTLGroup’s laboratory provided all the developmental data for the evaluation of the new apparatus. Hundreds of tests were performed to develop the exact protocol necessary for the qualification procedure. The approach used in ASTM C 191 followed that of C 114 by using the alternative-methods approach to implement new technology.
Once ASTM C 191 was approved, CTLGroup purchased a unit for its cement and mortar laboratory. The implementation of this unit dramatically increased productivity. Using the manual method, only 20 to 25 samples could be tested in a normal work week, one entire day being devoted to this activity. It required two physical testers and one lab assistant to measure the samples. The physical testers would cast time-of-set specimens all morning and then the lab assistant would take time-of-setting measurements, sometimes well into the night. This was a weekly ritual at CTLGroup’s cement laboratory for over 30 years.
Now it only takes one person to perform this test. With the automated time-of-set, testing can be performed any day of the week, any time of the day. The unit is computer-controlled and stores all test data, allowing test data to be evaluated and analyzed as needed. The unit is easy to use and virtually eliminates operator variability.
Because the automatic equipment performs an allowed alternative method within ASTM C 191, it requires calibration against the manual method. Once qualified per the requirements of the standard, the unit can be used immediately. At CTLGroup, initial set values obtained manually vs. the automatic method compared favorably and required no correction factor. With a correction factor applied, final set values readily meet the qualification criteria of the standard.
Automated Cube Consolidation
Mortar cube fabrication is probably the most operator-dependent of all tests conducted in the cement/ mortar laboratory. Reproducibility between physical testers and between laboratories can vary greatly. In 2005, ASTM C 109, Test Method for Compressive Strength of Hydraulic Cement Mortars (Using 2-in. or 50-mm Cube Specimens), was changed to allow alternative methods for fabrication of cube specimens.
The standard method requires the consolidation of cube specimens by hand tamping. Consolidation requires 32 tamps in 10 seconds in rounds of eight and that the cubes be filled in two equal layers. The technique a physical tester employs (how the tamper is held, wrist-action, pressure used when tamping) impacts the measured strength of the cube. Training laboratory staff to fabricate cubes can be a long and costly process. Currently, there are two alternative methods for the consolidation of 50-mm cubes an automatic tamping machine and an automatic vibration table.
Subcommittee C01.27 on Strength developed a qualification procedure for alternative methods for fabrication of C 109 mortar cubes. The qualification procedure is included in an annex to the standard that provides tools for the user to statistically analyze the data developed using the new method as compared with baseline control samples cast the traditional way. The standard requires the use of recent proficiency samples from the ASTM Cement and Concrete Reference Laboratory, for which results have been published. The C01.27 task group worked closely with CCRL so they would support this method. The implementation of this alternative method is leading to improved laboratory efficiency and is minimizing variability between operators.
New Technology Developments
Currently, subcommittees for fineness (C01.25), workability (C01.22) and chemical admixtures (C09.23) are considering changes to allow for automation in their areas of testing. Additionally, a new standard is being developed for the calorimetry testing of cement and cement/admixture combinations.
Some of the new technologies now standardized for the industry include:
• Machines to automatically compact 50-mm cube specimens;
• Automatic determination of initial and final set of cement pastes;
• Microprocessor controlled laboratory mixers;
• Automated measurements of the fineness of powdered materials;
• Devices that measure and monitor hydration reaction and heat generation of cementitious mixes (calorimetry);
• Wireless technology for determining the initial and final set of concrete mixtures;
• Concrete air-void analyzer for concrete air void systems in fresh concrete;
• Image analysis for analyzing air void systems in hardened concrete; and
• Fully automatic aggregate gradation units.
Others in the industry are also monitoring developments in technology and how its application fits into the laboratory. The Federal Highway Administration’s Mobile Concrete Laboratory has led the way in presenting new laboratory test methods to local and state agencies and to independent laboratories serving the construction industry. The fully equipped laboratory can perform physical tests on concrete as well as nondestructive techniques for evaluating in-place concrete.
In December 2005, the American Association of State Highway and Transportation Officials developed a steering group for technology deployment. The group, referred to as the Technology Implementation Group, will be evaluating innovative technologies as well as ways of delivering these technologies to state and local transportation agencies and the industry in general.
The future likely will bring more automation to the physical testing of cement and concrete, which in turn should allow us to improve the quality of laboratory testing. The commitment of ASTM subcommittee members, the development of new equipment by manufacturers and the support of ASTM International is helping to facilitate the standardization of new technologies. The recently developed ASTM Interlaboratory Study Program, as well as the Cement and Concrete Reference Laboratory proficiency sample programs, are providing necessary tools to help us bring these important changes to our industry. //