mac2T Test Facility

mac2T is a unique experimental facility for testing 100mm cubic concrete specimens under true multiaxial compression at any Lode angle (σ1≠σ2≠σ3) up to 400 MPa at temperatures up to 300oC.

Figure 1 Multiaxial compression testing in <i>mac<sup>2T</sup></i>

Load delivery and measurement

Figure 2 <i>mac<sup>2T</sup></i> (A) loading frames and (B) loading frame details

The loads are applied by using three independent loading frames, each equipped with a 4 MN servo-controlled hydraulic actuator and a 4MN low profile load cell (Figures 1 and 2A).

Heating and Temperature Measurement

The heat is generated by a set of six 240 W ceramic band (conduction) heaters installed around the heating platens, installed between the loading and cooling platens, all made of heat resistant steel (Durehete 1055).

The cooling platens are cylinders with a network of cross-drilled holes through which cold water is flushed throughout the tests. The temperatures are measured by an array of 24 thermocouples installed on the six loading platens (Figures 1 and 2B).

Deformation measurement

The deformations of the cube are measured, with an accuracy of 10-6 m, by a system of six laser interferometer devices, with reflector mirrors installed in tubes evacuated to 10-2 bar and in direct contact with the specimen (Figure 3A).

System for Data Acquisition and Control

In-house developed DA and control software allows complex multi-stage experiments involving load and temperature cycling, small stress probes, arbitrary (pre-defined) loading paths and experiments in which the execution of the test sequences depends on the response of the specimen (automatic peak detection) with the control parameters (switching between stress and strain control and changing of stress/strain rates, Figure 3B) programmed as functions of the material behaviour, such as changes in loading stiffness during the test.

Backup measurement systems and fail safe procedures enable long term load and temperature cycling tests to run unsupervised for weeks.

Figure 3 (A) system for strain measurement (B) diagram of control loops

Nominally uniform stress-strain field

Nominally uniform stress-strain field in the specimen is achieved by using:

  • self-positioning horizontal frames and actively controlled vertical frame to endure central loading on the six faces of the specimen (Figure 2A),
  • precision cut and ground specimens, and
  • 0.25 mm PTFE film to reduce friction on platen-specimen interface (Figure 3A).

Post-peak testing

Post-peak testing is enabled by a combination of:

  • rigid loading frames,
  • rigid platens,
  • accurate and reliable strain measurement system (that is not interrupted by damage of the specimens) and
  • a fast mixed-control servo-hydraulic system.