eXpert 1000 - ADMET

Summary of Test

ASTM A370 is performed by steel manufacturers and commercial and government laboratories serving the construction industry to measure the strength of rebar. Values of ultimate tensile strength, yield and elongation are required for the most common sizes.

Why is it done?

Steel manufacturers are required to test the products they sell to ensure that the specifiction cited is met. Significant liabilities can result from providing noncompliant product. Tensile testing to failure is the most common method employed. These often include the use of extensometers for the measurement of Yield.

In the United States, the size designations of these mild steel bars used to reinforce concrete are set by ASTM International. Rebar is generally supplied in 20- and 60-foot lengths.

Most bars are “deformed,” that is, a pattern is rolled onto them which helps the concrete get a grip on the bar. The exact patterns are not specified, but the spacing, number and height of the bumps are. ASTM A615 defines these markings.

Typical deformed reinforcing bar (courtesy of Concrete Reinforcing Steel Institute)

Three grades are defined:

Inch-pound Grade	Metric Grade	Minimum Yield Strength
Inch-pound Grade	Metric Grade	Pounds per Square Inch	Megapascals
Grade 40	Grade 280	40,000	280
Grade 60	Grade 420	60,000	420
Grade 75	Grade 520	75,000	520

The size designations up through size 8 are the number of eighths of an inch in the diameter of a plain round bar having the same weight per foot as the deformed bar. So, for example, a number 5 bar would have the same mass per foot as a plain bar 5/8 inch in diameter.

Users of rebar are required to test the rebar they receive using independent sources prior to acceptance. There have been issues with substitution of grades and materials not meeting the intended specification. Testing helps to minimize the possible installation of out of spec rebar.

Why is it important?

The strength of rebar is a fundamental requirement in the design and construction of buildings, dams, bridges, roads and airports. The failure of a single rebar may result in significant loss of strength in the structure under construction with the possibility of catastrophic failure.

Rebar installation (courtesy HDR Engineering)

What ADMET equipment is needed?

Many steel manufacturers and independent testing laboratories own tensile testing machines of considerable force capacity. These older machines constitute a considerable investment so automating an existing system via upgrade/retrofit is a prudent decision. The upgrade/retrofit is performed at the customer’s site including training.

If new equipment is desired, ADMET ExPress servohydraulic testing machine of capacities 20,000 to 300,000 LBF depending upon rebar size and specification are available.

Rebar fixtures including holders and long gauge length extensometer are among the tools ADMET offers for the testing of rebar.

Changes in the force rate being applied to the rebar can affect the yield strength, tensile strength and elongation values which are highly strain rate sensitive. In general the yield strength and tensile strength will increase with increasing strain rate. Elongation values generally decrease as the strain rate increases.

Rebar testing using manually operated machines places a burden on the technician performing the test. The rate of testing is deliberately slow and thus the length of time to test can be long. All the while the operator is expected to be monitoring and adjusting the controls of the machine to ensure there is consistent force application at the specified rate. This is often not the case and inconsistent force application from test to test is the inadvertent result. Undesirable scatter in the test data can occur and sometimes good product gets rejected while at other times out of spec product can be deemed acceptable. Automatic control minimizes this scatter and provides inherent documentation of the testing force application.

Rebar No.	Diameter (in)	Area(in^2)	40ksilbf neededto test yield	60ksilbf neededto test yield	75ksilbf neededto test yield
3	0.375	0.11	4400	6600	8250
4	0.500	0.20	8000	12000	15000
5	0.625	0.31	12400	18600	23250
6	0.750	0.44	17600	22000	27500
7	0.875	0.60	24000	36000	45000
8	1.000	0.79	31600	47400	59250
9	1.128	1.00	40000	60000	75000
10	1.270	1.27	50800	76200	95250
11	1.410	1.56	62400	93600	117000
14	1.693	2.25	90000	135000	168750
18	2.257	4.00	160000	240000	300000

Table 1. Rebar size versus yield strength in pounds force for 3 grades

Note: Not all sizes are available in all strengths. Ultimate tensile strength is typically between 1.33 (larger sizes) to 1.75 (smaller sizes) the yield strength.

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What do you do when your outdated hydraulic materials testing machine breaks or your customer requires a test that cannot be performed with your existing system? Do you replace the testing system with a new one or do you upgrade/retrofit with new software and controls?

There are two hydraulic universal testing machine designs. The first uses a single sided ram to apply force and requires gravity for the piston to return home. This type of hydraulic testing machine is the most common and is used for static tests only. A schematic of a ram based hydraulic tester is shown in Figure 1. The second type of hydraulic machine uses a dual acting piston. The dual acting piston design provides control in both the up and down direction and is commonly used for fatigue testing applications.

Hydraulic Machine Anatomy

Figure 1 — Schematic of a single acting ram based hydraulic universal testing machine

Comparing the purchase of a new machine or an upgrade to an existing hydraulic testing machine, it is almost always less expensive to upgrade the existing machine regardless of force capacity. New ram based static 20,000 lb and 400,000 lb hydraulic testing machines cost between $35,000 and $200,000, respectively. In general, retrofits cost between $500 for the integration of a simple digital indicator to $30,000 for a new servo-hydraulic power unit with a PC/Windows-based materials testing system. As a rule, the amount of money you save retrofitting a hydraulic tester increases as the capacity of the machine increases. So, a retrofit would save well over $100,000 on a large machine and thousands of dollars on a smaller machine.

In addition to the direct machine costs, there are other considerations that come into play, such as your investments in grips, fixtures, and test jigs. The larger the investment, the greater the replacement expense. If all of your testing systems are similar, then it is best from a fixturing, training and maintenance standpoint to retrofit the testing machines. Finally, there is the very pressing situation of when your existing machine has failed. A retrofit can be installed much more quickly than it would take to purchase a new machine.

Hydraulic testing machines are generally used to test metals and metal components at higher forces. Their ability to perform millions of static tests and fatigue cycles with little mechanical wear makes the hydraulic testing machine well suited for these testing applications. Because of their durability, the hydraulic testing machine is an ideal candidate for retrofitting.

Types of Hydraulic Retrofit Packages

A number of upgrade packages are available for retrofitting hydraulic testing machines. In general, the upgrade packages, once installed and calibrated, will extend the ASTM E4 certified force measuring range of the machine. A description of possible upgrade package types for manually operated hydraulic testing machines follows:

Basic Upgrade: The simplest upgrade keeps the existing manual control valves and replaces the dial gage or existing digital indicator with a new single channel digital indicator to display load and report ultimate tensile strength. Some of the more sophisticated single channel indicators are also capable of calculating yield by the halt of the pointer method and downloading results and load vs time data to a computer for reporting and plotting.

GaugeBuster 2

Figure 2 — ADMET Gauge Buster 2 Digital Indicator. Displays live load, peak load, load rate and ultimate tensile strength. Results and XY data can be uploaded to computer to generate reports and plots

Three Channel Upgrade: The next step up when keeping the existing manual control valves is to incorporate a two- or three-channel digital indicator that is capable of measuring force, displacement, and strain. A testing machine equipped with a two or three channel indicator is capable of generating a stress-strain curve and calculating offset yield, Young’s modulus, ultimate tensile strength, and percent elongation according to ASTM E8 Standard Test Method for Tension Testing of Metallic Materials. Results and stress-strain curves can be uploaded to a computer for further review and analysis. New or existing extensometers can be connected to the indicator for measuring strain.

eP2

Figure 3 — ADMET’s eP2 Digital Controller can be installed on both manually operated and servo controlled testing machines. The eP2 features a load and position input plus an optional strain channel. Results and XY data can be uploaded to a computer running the GaugeSafe Data Exchange Program to perform analysis according to ASTM E8 and generate stress vs. strain curves and test reports

PC/Windows Based Upgrade: Installing a PC/Windows based materials testing system is the most sophisticated upgrade package for a manually controlled hydraulic testing machine. It provides all of the capability of the three channel digital indicator but adds calculations for plastic strain ratio (ASTM E517), K and n values (ASTM E646), plus many more. PC based materials testing systems also provide more data handling and reporting capabilities than a standalone digital indicator.

Manual Retrofit

Figure 4 — A 400,000 lb capacity manually operated Tinius Olsen Testing Machine retrofitted with ADMET’s MTESTQuattro® Materials Testing System. MTESTQuattro features load, position, axial strain, transverse strain and auxillary input channels

The next level of retrofits convert manually operated machines to automatic closed-loop servo hydraulic testing systems that are microprocessor controlled. This is accomplished in one of two ways.

Basic Servo Retrofit: The least expensive servo retrofit option is to keep the existing hydraulic power unit, bypass the existing manual control valves and install a new servovalve manifold. Either a multi-channel digital indicator or a PC/Windows based materials testing system can be used to control the machine. The standalone digital indicator is more suitable for running similar tests repeatedly, whereas the Windows based system provides more control capability for performing a variety of tests.

Full Servo Retrofit: The more expensive servo retrofit is to replace the existing hydraulic console with a new servo hydraulic power unit (see Figure 5). Again, either the multi-channel digital indicator or Windows PC-based system can be used to perform the tests.

New Servo Retrofit

Figure 5 — A 60,000 lb capacity Baldwin Universal Testing Machine retrofitted with a new servo-hydraulic console equipped with ADMET’s MTESTQuattro® Materials Testing System. The servo based system ensures that tests are performed automatically according to ASTM specifications

In addition to upgrading existing manually operated testing machines to servo control, it is also possible and quite common to upgrade the controller and software on an existing servo equipped static and fatigue rated hydraulic testing machine. This an economical solution because the new controller and software only has to be attached to the existing load frame and hydraulic power unit.

Regardless of whether you retrofit or replace, it is important that you keep your testing capabilities current. Up-to-date equipment will reduce test times, eliminate data entry errors, and speed materials and product development. Employing testing systems that automatically perform the tests according to specification, automatically calculate results and seamlessly communicate with computers and programs running on your corporate network are paramount to operating a successful testing laboratory.

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