Load Frames: Workhorses of the Laboratory

Load Frames: Workhorses of the Laboratory

A construction materials testing laboratory is utilized for performing many standardized test methods to measure the physical properties of various materials. Specimens of soil, asphalt, concrete, and other composite materials must have their resistance to load, deformation, compressive strength, and shear strength characterized to ensure adequate performance over time as structural elements or stand-alone features. Protocol details are unique to each test, but similarities in the application and direction of loading forces, specimen sizes, and sample form factors have allowed a design concept to evolve.

Load frames today are basic loading devices able to perform many separate tests in geotechnical, soil mechanics, and asphalt laboratories using specialized fixtures and components. That’s great news for lab budgets. Can you imagine if a separate, dedicated testing machine was required to run each of a dozen tests?

Load frames are required to run these common tests for geotechnical and asphalt applications:

  • Unconfined compressive strength of soils
  • Unconfined compressive strength of soil cement
    • Unconsolidated/Undrained (UU)
    • Consolidated/Undrained (CU)
    • Consolidated/Drained (CD)
  • Marshall Stability and Flow of Asphalt
  • Asphalt Semi-Circular Bend Test
  • Indirect Tensile Strength of Asphalt (Lottman Test)
  • Tack Bond (Interlayer Shear Strength of Asphalt)

Increased Versatility Offers Increased Functionality & Testing Capabilities

Starting with a quality load frame with the right mix of capacity, platen speed, and performance tolerances offers an immediate improvement in operating efficiency and helps your bottom line just by optimizing versatility. Suddenly you have the flexibility to perform a wide range of test procedures on different materials, variations on individual procedures, and even custom applications. Adding accessories to perform particular tests would only take place as the need arises, avoiding unnecessary expenditures. Individual components are available singly or in packaged sets and can be upgraded or replaced with no need to replace an entire system.

Popular load frames for construction materials testing from different manufacturers share similar basic features. They are compact enough to fit on bench tops for easy operation and convenient locating in the lab. Drive systems and controls are located in the base and two heavy upright rods carry a sturdy, adjustable crosspiece to load against. Typical loading systems consist of an electric motor connected to a gearbox that drives a mechanical jack screw. Drive system components are carefully matched to ensure loads are generated at the desired strain rate and controlled to specified tolerances. Manually operated load frames are not common, but still available. The cost of producing a high-quality manual loader are prohibitive when compared to the precision control offered by a mechanical/electrical system.

What to Consider When Selecting a Load Frame

  • Total capacity of the frame should be well beyond the highest loads likely to be tested. Any load frame can be loaded to its maximum rated capacity during testing, if required. However, loading to or near that maximum time after time will eventually degrade components and materials, resulting in frequent repairs and a shortened service life. We recommend selecting a frame with total capacity about 20% higher than typical maximum loads for your application. In addition to load capacity, the entire system must be stiff enough to minimize deflection, which can cause inconsistent test results, premature wear, and damage to the machine.
  • Strain rate (platen speed) must meet the requirements and tolerances for desired testing applications. Frames with low strain rates and close tolerances for speed often require DC or stepper motors with a higher degree of speed accuracy and subsequently higher pricing. Modern drive systems apply platen speeds and forces smoothly across the loading spectrum.
  • Daylight opening is the maximum space between the two vertical rods and from the top of the lower platen to the bottom of the horizontal crosshead. This dimension must be large enough to accommodate the specimen, fixtures, test cells, platens, and accessories when testing is underway.
  • Bottom platen supports the specimen including whatever test cells and fixtures are called for in the test method. It needs to be large enough to fully support samples and fixtures, and thick enough to carry maximum loads and resist deflection. Corrosion-resistant metal or plated steel is a good choice of material since there is often free moisture from the specimens or test cells. Scribed concentric circles make it easy to center fixture during setup.
  • Horizontal crosshead bears all the forces applied by the drive system to the test specimen. Strength, rigidity, and ease of adjustment are important features, and it is often used as an anchoring or reference point for various loading and deflection instruments.
  • Vertical height adjustment accommodates a wide range of sizes and types of specimens, fixtures, and cells. Sturdy, coarse-threaded vertical rods with top and bottom adjustment nuts for the crosshead offer a simple system for quick and easy height adjustment.
  • Controls should be simple, logically positioned, and easy to read and understand to reduce operator error and cut training time.
  • Components that integrate quickly and easily with the main load frame simplify setup and change over. Well-designed components can often be easily installed on different brands of load frames.

Gilson Recommends:

Gilson Load Frames are high-quality instruments designed and manufactured in cooperation with Karol-Warner to meet the widest range of requirements for ASTM and AASHTO testing. Choose from five different models built for geotechnical laboratory testing. Two additional Marshall Stability Load Frames are purpose-built and quickly adapt for Marshall stability and related asphalt tests. These models have capacities of 10,000lbf (44.5kN) or 10,000lbf (44.5kN) to 20,000lbf (89kN), and feature fixed platen speed.

Testing Components and Fixtures

Each test procedure has its own requirements for measuring load forces and deflection, deformation, or penetration values. Gilson offers a selection of individual Load and Displacement Measurement products and sets to configure load frames for a wide variety of laboratory tests. The individual components are available separately to suit user preferences and allow for custom applications.

Technicians of a certain age may recall manually turning a loading crank at just the right speed and visually reading a stopwatch, load ring gauge, and deformation dial gauge, all while simultaneously recording the values with a pencil. Talk about multitasking.

Fortunately, technology has allowed us to leave those difficult operations behind. A power-driven load frame outfitted with quality component sets automates or eliminates many of those manual processes, thus optimizing efficiency and reducing operator error. Components to measure load forces and sample deformation or deflection are available separately or matched up in complete sets. Components from different manufacturers are often compatible, but selection of sets ensures easy installation and integration. Whether guided by budget constraints or aiming for maximum performance, you can tailor your loading system for current and future needs with some basic planning.

Additionally, you can optimize the features and convenience of electronic measurement systems with software applications that automatically collect, display, analyze, and report data in real time. Software is a big step in ease of operation, efficiency, and reduction of operator error.

Analog and digital measuring instruments each have their advantages and there are some points to consider before making your selection:

Analog Load Frame Components
Rugged and cost-effectiveCan require reference charts to interpret readings
Simple to install and maintainRequires experience and concentration to use
Instrument accuracy meets test method requirementsData must be recorded, calculated, and reported manually
Digital Load Frame Components
Higher degree of accuracy and repeatabilityHigher initial expense
Automated data collection, calculation, and reportingInitial setup can take more time
Reduces training time and operator errorRequires computer integration for optimum results

Among Gilson's Analog Load and Displacement Measurement products are:

Analog Triaxial Component Set Load Ring
  • Load Rings are crafted from aircraft grade anodized aluminum and include a dial gauge and calibration curve for accurate force measurements.
  • Mechanical Dial Indicators feature low-friction analog mechanisms and large, easy-to-read dials with clear graduations. Housings rotate 90° for convenient setup.

Digital Load and Displacement Measurement products include:

Load Displacement Readout Kits
  • Load Cells (S-Type) are machined from stainless steel or nickel alloy and measure force in tension or compression to ±1.0% of full scale.
  • Linear Variable Displacement Transducers (LVDTs) measure deformation, deflection, penetration, or other dimensional changes of specimens during testing.
  • Digital Readouts interpret and display digital signals from load cells, LVDTs, and other electronic instrumentation. They also export values to testing software or spreadsheet applications on computers.

One of Gilson’s latest offerings is Data Acquisition Software available for Marshall Stability and various soil testing applications, such as CBR and LBR, Triaxial, Unconfined Compressive Strength, and Soil Cement.

  • Gilson software records loads and displacement, displays real-time stress-strain data, and calculates and reports results for a number of soils and asphalt laboratory tests. Unique applications for California bearing ratio, triaxial compression, soil unconfined compressive strength, soil cement, consolidation, direct/residual shear, and Marshall stability are designed to integrate with digital components and component sets for Gilson Load frames. All programs follow ASTM/AASHTO requirements for collection and calculation of results.

If you'd like to receive additional information on geotechnical applications and testing with load frames, please contact our knowledgeable technical support staff for assistance.