Moisture Sensitivity of Hot Mix Asphalt (HMA) Mixtures in Nebraska - Phase II
As a consequential effort to the previous NDOR research project (P-564) on moisture damage, this report presents outcomes from this project incorporated with the previous project. Performance changes and fundamental material characteristics associated with moisture damage due to various anti-stripping additives in asphalt mixtures are studied through various experimental approaches and a numerical simulation. Three additives (i.e., one reference additive, hydrated lime, and two alternative additives: fly ash and cement) are investigated by adding them into two types of mixes (SP2 for low-traffic-volume roadways and SP5 for high-traffic- volume roadways) where two different asphalt binders (PG 64-22 for the SP2 mix and PG 70-28 for the SP5) are used. Two asphalt concrete mixture scale performance tests, the AASHTO T-283 and the APA under water, and two local-scale mixture constituent tests, the boiling water test (ASTM D 3625) and the pull-off test, are conducted to characterize the effects of binder-specific anti-stripping additives on the binder-aggregate bonding potential in mixtures. The pull-off tensile strength tests are then numerically modeled through the finite element technique incorporated with the cohesive zone modeling approach to seek more fundamental scientific insights into the effect of each anti-stripping additive on the overall moisture damage resistance. Results from laboratory tests and numerical simulations indicate that the SP5 mixtures, where high-quality aggregates and polymer-modified binder are used, are fairly self-resistant to moisture damage without treating any anti-stripping additive and do not show any visible sensitivity among additives, whereas the effects of additives and their sensitivity are significant in the SP2 mixes that use the unmodified binder PG 64-22 and low-quality aggregates. With the limited amount of test data, hydrated lime seems to perform slightly better than other additives, particularly with longer moisture-conditioning time. Fly ash contributes to reducing moisture damage by improving binder-aggregate interfacial properties, which are validated from the integrated experimental-computational evaluation.
Integrated Experimental–Numerical Approach for Estimating Material-Specific Moisture Damage Characteristics of Binder–Aggregate Interface
An integrated experimental–numerical approach is presented for estimating the material-specific moisture damage characteristics of binder–aggregate interfaces. A pull-off test is conducted with a pneumatic adhesion tensile testing instrument to determine the bond strength between asphalt film and the aggregate surface to which various types of antistripping additives are applied with various levels of moisture conditioning. A sequentially coupled moisture diffusion–mechanical analysis is implemented with a finite element technique to model the bond strength and progressive interfacial degradation caused by moisture diffusion, followed by mechanical pulling pressure. To simulate the adhesive fracture at the binder–aggregate interface, a cohesive zone model is incorporated into the approach. The integrated approach results in a degradation function that characterizes interfacial damage caused by moisture uptake with two model parameters representing the remaining bond strength and the degradation trend. With the two model parameters, moisture damage mechanisms and damage resistance potential, both of which are material specific, can be estimated.
Tests of composite beams with web holes
Tests of composite beams comprising a concrete slab supported on a steel deck and a steel wide-flange shape containing large web openings are described. Special attention is directed to the amount of shear connection between slab and steel section in the region of a web hole, and on the effect of construction loads acting on the steel section prior to composite action being effective.It is shown that limited shear connection at the hole will significantly affect the strength when loading produces a high shear-to-moment ratio, and a theory is presented which conservatively takes this into account. The effect of construction loads on unshored construction is shown to be small when these do not exceed 60% of the non-composite beam resistance at the hole.