Slow Dynamics in Concrete: Effects of Temperature, Strength Variation, and Microcracking Damage
Synopsis
This study conditioned concrete prisms of four compressive strengths (f′c = 32–56 MPa) and two alkali-silica reaction (ASR) damaged specimens by compressive loading, monitored the subsequent velocity recovery with coda wave interferometry (CWI), and applied a self-referencing temperature correction to remove drift from ambient fluctuations of about ±0.2°C; for intact specimens the recovery rate mv and velocity drop magnitude |c| increased with strength (|c| from 3.08×10⁻⁴ to 6.02×10⁻⁴, mv from 6.77×10⁻⁵/s to 13.76×10⁻⁵/s) and recovery time shortened from 9.9 h to 6.
Fig. 1 provides an example of monitoring the recovery of a concrete specimen. When a
· Page 6Interpretation
A self-referencing temperature correction procedure was proposed and validated, using heating and cooling thermo-acoustic coefficients (TACs) from thermal modulation tests to subtract the thermal contribution to velocity change and isolate the purely mechanical slow dynamics recovery. Prior slow dynamics studies generally relied on stable thermal environments or reference specimens; this work transfers the authors' earlier correction method for acoustoelastic tests on metals to concrete, requires only the specimen under test, and separates heating and cooling TACs to match concrete's hysteretic thermal response. Thermal modulation tests were run in an environmental chamber at 0.5°C/h with surface and internal temperature measurements in agreement; after correction, the coefficient of variation of mv across four repeated tests on one specimen dropped from 16.1% to 4.3% and that of |c| from 5.0% to 3.6%.
In intact concrete, the recovery rate mv and velocity drop magnitude |c| increased with compressive strength, and stronger specimens recovered faster, with recovery time shortening from 9.9 h (32 MPa) to 6.6 h (56 MPa). Most prior slow dynamics work on concrete focused on damage characterization rather than strength; this study uses companion mixes made from the same materials and differing only in mix proportions (water-cement ratio 0.61 to 0.30) to relate slow dynamics parameters directly to design strength. Three prisms per mix were tested for four mixes, with strengths measured on cylinders per ASTM C39 at 32.2, 40.4, 48.8, and 56.3 MPa; recovery parameter CVs ranged from 5.5% to 14.3%, exceeding the batch strength CVs of 1.4% to 4.0%.
ASR-damaged specimens under the same conditioning stress showed greater softening and faster recovery rates, yet far longer recovery times than intact specimens, and different damage mechanisms produced different recovery times. The work shows that in damaged specimens increased softening is not accompanied by a proportional increase in recovery rate, breaking the trend seen in intact specimens where velocity drop and recovery rate changed together; it also notes that the RFA specimen, despite lower expansion (0.0230% vs. 0.0721%), had a recovery time more than three times longer than the RCA specimen. The two ASR specimens (RFA, RCA) had |c| of 11.85×10⁻⁴ and 15.84×10⁻⁴, mv of 19.06×10⁻⁵/s and 28.04×10⁻⁵/s, and tr of 458 h and 124 h; damage morphology draws on the authors' earlier SEM and resonant frequency results.
Two additional intact specimens cast about five years earlier, with strengths of 34.1 and 35.0 MPa, retained intact-concrete recovery behavior but their specific parameters fell outside the strength prediction intervals of the primary groups. This suggests recovery parameters are sensitive to age- and moisture-dependent microstructural changes, so comparisons across specimen sets should account for these factors rather than extrapolating from 28-day strength alone. The two specimens had tr of 2.4 h and 3.1 h, shorter than all four primary groups, but their recovery behavior aligned more closely with the 40–49 MPa groups; companion specimens stored in the same conditions had a moisture content of 2.5% by mass.
Perspective
The results apply to concrete prisms under controlled laboratory conditions with 6 kN compressive conditioning applied by a testing machine, and the temperature correction targets the gradual, limited-range temperature changes encountered here (about 21.5–23.5°C); for intact specimens, recovery parameters vary in the same direction as design strength (water-cement ratio 0.30–0.61), supporting strength comparison within a set of companion materials; for damaged specimens, recovery time is sensitive to the spatial distribution and connectivity of microcracks, which may help distinguish damage mechanisms.
Recovery parameter CVs exceed batch strength CVs, and the ASR and additional specimens are few, so the generality of the strength–recovery parameter relationship still needs more specimens; the relative roles of age, moisture content, pore structure, and aggregate grain size versus strength have not been separated; the influence of alkali-silica gel and gel-filled cracks on slow dynamics requires further investigation; the applicability of the temperature correction to non-monotonic or rapid temperature changes remains to be tested; this reading covered the full text, but figures and tables appear as text, so specific curve details should be checked against the original figures.