2.3 Compressive Loading Rates & Controls

2.3 Compressive Loading Rates & Controls

Concrete is a viscoelastic, heterogeneous material. This means its mechanical behavior and measured compressive strength are highly dependent on the rate at which the compressive force is applied. If a cylinder is loaded too quickly, the concrete will exhibit an artificially high compressive strength. If it is loaded too slowly, creep deformation will occur, leading to an artificially low strength reading. To ensure standardized, repeatable, and accurate tests, ASTM C39 specifies precise loading rates and control procedures that must be maintained.

This section covers the viscoelastic physics of concrete loading, the loading rate specifications for hydraulic and screw-powered machines, calculations for converting stress rates to load rates, and critical control rules during the failure phase.

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The Viscoelastic Nature of Concrete Loading

When concrete is subjected to an axial compressive load, it deforms elastically at first, followed by micro-cracking and plastic deformation. The rate of this deformation directly impacts the cracking mechanism:

• Rapid Loading Effects: If load is applied very quickly, the micro-cracks do not have sufficient time to propagate along the path of least resistance (around the aggregate particles). Instead, the cracks are forced to go through the aggregate particles, which requires more energy. Consequently, the concrete appears stronger than it actually is.
• Slow Loading Effects: Conversely, loading concrete very slowly allows micro-cracks to grow slowly along the aggregate boundaries under sustained load (creep), causing failure at a lower ultimate load.
• The Standard Rate: To eliminate loading rate as a variable, ASTM C39 establishes a uniform stress rate that represents a standard compromise, matching the historical database of concrete testing.

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ASTM C39 Loading Rate Specifications

The loading rate depends on whether the compression machine is hydraulically operated or screw-driven.

1. Hydraulically Operated Machines

For machines that use hydraulic valves to control flow, the loading rate is specified as a constant rate of stress increase on the specimen:

• Stress Loading Rate: The load must be applied at a rate of movement corresponding to a stress rate of $35 \pm 7\text{ psi/s}$ [$0.25 \pm 0.05\text{ MPa/s}$].
• Timing of Rate Control: This designated rate must be maintained during at least the latter (second) half of the anticipated loading phase.
• First Half Relaxation: During the first half of the anticipated loading phase, a higher rate of loading is permitted to speed up the testing process. However, this initial load must be applied in a controlled manner, without shock or sudden surges.

2. Screw-Powered (Mechanical) Machines

For mechanical machines that control the movement rate of the crosshead directly:

• Head Travel Rate: The moving head must travel at a rate of approximately $0.05\text{ in./min}$ [$1.0\text{ mm/min}$] when the machine is running idle (not under load). This idle speed translates to a stress rate under load that is comparable to the hydraulic specification.

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Calculating Loading Rates in Force Units (lbf/s or kN/s)

While ASTM C39 specifies the loading rate in terms of stress (psi or MPa per second), most testing machines display the load in force units (pounds-force [lbf] or kilonewtons [kN]). The technician must calculate the corresponding force loading rate using the specimen's cross-sectional area: $\text{Force Loading Rate} = \text{Stress Rate} \times \text{Cross-Sectional Area}$

Calculation Example 1: Standard $6 \times 12\text{ inch}$ Cylinder

• Nominal Diameter: $6.0\text{ inches}$.
• Nominal Cross-Sectional Area ($A$): $A = \pi \times r^2 = \pi \times (3.0\text{ in.})^2 \approx 28.27\text{ in.}^2$
• Permissible Stress Rate Range: $28\text{ to }42\text{ psi/s}$ (which is $35 \pm 7\text{ psi/s}$).
• Calculated Force Loading Rate Range:
  • Minimum Force Rate: $28\text{ psi/s} \times 28.27\text{ in.}^2 \approx 792\text{ lbf/s}$
  • Target Force Rate: $35\text{ psi/s} \times 28.27\text{ in.}^2 \approx 990\text{ lbf/s}$
  • Maximum Force Rate: $42\text{ psi/s} \times 28.27\text{ in.}^2 \approx 1188\text{ lbf/s}$
• Result: For a $6\text{ in.}$ cylinder, the load should increase at a rate of $990 \pm 198\text{ lbf/s}$ during the latter half of the test.

Calculation Example 2: Standard $4 \times 8\text{ inch}$ Cylinder

• Nominal Diameter: $4.0\text{ inches}$.
• Nominal Cross-Sectional Area ($A$): $A = \pi \times (2.0\text{ in.})^2 \approx 12.57\text{ in.}^2$
• Calculated Force Loading Rate Range:
  • Minimum Force Rate: $28\text{ psi/s} \times 12.57\text{ in.}^2 \approx 352\text{ lbf/s}$
  • Target Force Rate: $35\text{ psi/s} \times 12.57\text{ in.}^2 \approx 440\text{ lbf/s}$
  • Maximum Force Rate: $42\text{ psi/s} \times 12.57\text{ in.}^2 \approx 528\text{ lbf/s}$
• Result: For a $4\text{ in.}$ cylinder, the load should increase at a rate of $440 \pm 88\text{ lbf/s}$ during the latter half of the test.

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Machine Controls During the Rapid Yielding Phase

As the cylinder nears its ultimate compressive strength, the concrete begins to yield rapidly. Internal micro-cracking accelerates, aggregate-paste bonds fail, and the specimen undergoes significant deformation.

The Rapid Yielding Phenomenon

During this yielding phase, the concrete becomes much more compressible. The display needle or digital display may slow down, stop, or even regress, even though the oil pump continues to pump oil into the piston at the same rate. This is because the volume of oil being pumped is being absorbed by the rapid compaction and cracking volume of the cylinder.

The No-Adjustment Rule

ASTM C39 enforces a critical procedural rule: Do NOT make adjustments to the control valve or controls of the testing machine when the specimen is yielding rapidly immediately before failure.

• Why Adjustments Are Banned: If a technician tries to turn the control valve open to force the loading needle to continue moving at the specified rate, they will pump an excessive volume of oil. This creates a massive dynamic overload when the concrete suddenly shears, leading to an explosive failure and a false low strength reading.
• Correct Procedure: The flow control valve must remain at the setting established during the latter half of the loading phase. Let the machine run at this constant rate of flow until the specimen fails.

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Determining Ultimate Failure and Recording

The technician must maintain the continuous application of load until the specimen displays a well-defined fracture pattern and the load indicator decreases steadily.

• Ultimate Capacity: The ultimate capacity is reached when the load indicator reverses direction or drops significantly (usually to less than 90% of the peak load) and a visible break pattern is established.
• Recording the Peak Load: Once failure is reached, the technician stops the test, reverses the piston, and records the maximum load (peak load) sustained by the specimen during the test.