Keith Kesner

CYCLIC RESPONSE OF HIGHLY DUCTILE FIBER-REINFORCED CEMENT-BASED COMPOSITES

The response of highly ductile fiber-reinforced cement-based composites (DFRCC) to uniaxial monotonic and cyclic loading is examined. The DFRCC studied herein is a portland cement-based mortar matrix with a low volume fraction (~2%) of high modulus polymeric fibers. Various cyclic testing schemes were employed to examine the effect of cyclic loading on the materials compressive and tensile stress-strain envelope. The response of different tensile specimen geometries was also examined. A distinctive feature of DFRCC is its pseudo-strain hardening response in tension. Test results identified a unique unloading and reloading response for DFRCC, which is a consequence of fibers debonding and pulling out of the matrix. Reversed cyclic loading was seen to affect the tensile response of the material if the uniaxial compressive strength during loading was exceeded, and not to affect the tensile response if compressive strength was not exceeded. Evaluation of tensile specimen geometries revealed significant variations in strain capacity between the different geometries.

Detection and Quantification of Distributed Damage in Concrete Using Transient Stress Waves

The feasibility of using transient stress waves to detect and quantify cracking caused by distributed damage mechanisms, specifically alkali-silica reactivity (ASR) or delayed ettringite formation (DEF) was evaluated. Numerical studies were used to determine how distributed damage in concrete plates affects propagation of impact-generated stress waves. It was found that distributed damage produces significant and quantifiable changes in the waveforms and spectra obtained from impact-echo results. To verify results obtained in the numerical simulations and develop correlation between changes in impact-echo signals and the actual amount of damage, impact-echo tests were performed on a specially prepared plate specimen made from concrete modified to produce distributed cracking over time. The amount of damage in the plate was determined periodically using images obtained from samples removed from the plate and subjected to neutron radiography. A correlation between amount of damage in the samples and changes in impact-echo signals was established and verified in a field study in which impact-echo tests were conducted on and core samples taken from concrete box beams experiencing cracking caused by distributed damage mechanisms.

Tendon Corrosion in Precast Segmental Bridges

Tendon corrosion of grouted internal tendons in precast segmental bridges is reviewed, with an emphasis on the role of grouting in the corrosion protection of posttensioning. An analysis and compilation of the available information on the subject was conducted as part of an NCHRP research study (NCHRP Project 20-7, Task 92) with emphasis on those structures located in the United States and the United Kingdom. It was determined that the corrosion problems with precast segmental and posttensioned bridges in the United Kingdom principally were due to poor design choices and poor quality construction, not a significant intrinsic susceptibility to corrosion. The study found no evidence of corrosion or other durability problems with precast segmental bridges in the United States, although it must be recognized that this conclusion is based on reported information primarily gathered by visual inspection. Similar to the experience in the United Kingdom, the few reported problems with grouted posttensioned structures in the United States were due to poor quality construction. Additionally, laboratory research indicates that properly designed and constructed precast segmental and posttensioned structures are corrosion resistant. To ensure good future performance of precast segmental bridges, high-quality design details and proper construction practices must be continued, especially when using internal tendons. It is important to adopt more stringent grouting requirements and inspection procedures in the United States, similar to what has been done in the United Kingdom, to ensure that the tendons are completely encapsulated in a high-quality grout.

Use of the Impact-Echo Method to Evaluate Damage due to Distributed Cracking in Concrete Plate Members: Theory and Field Trials

The research evaluated the feasibility of using transient stress waves to detect and quantify cracking caused by distributed damage mechanisms, specifically alkali-silica reactivity and delayed ettringite formation. It was found that distributed damage produced significant and quantifiable changes in the waveforms and spectra obtained from impact-echo results. Numerical (finite element) studies were used to determine how distributed damage in concrete plates affected propagation of impact generated stress waves. To quantify the damage, the rate of stress wave attenuation can be assessed and compared with values predicted by theory. To verify the results obtained in the numerical simulations and to develop correlation between changes in impact-echo signals and the actual amount of damage, impact-echo tests were performed on a specially prepared plate specimen made from concrete modified to produce distributed cracking over time. The actual amount of damage in the plate was determined periodically by using neutron radiography. A correlation between the amount of damage in the samples and changes in impact-echo signals was established. The correlation was verified in field and laboratory studies on concrete sections with distributed damage. The amount of cracking predicted from the analysis of impact-echo tests agreed with the actual amount of cracking obtained from neutron radiography of the core samples. This research demonstrates that the impact-echo method can be used to detect and quantify the amount of distributed damage in concrete plates. However, the mechanism responsible for the damage cannot be ascertained from either impact-echo test results or neutron radiographs.

When Bad Things Happen to Good [Buildings]

The title of this paper borrows from the 1981 book by Harold Kushner entitled “When Bad Things Happen to Good People”. In his book, Kushner attempts to explain why a universe created by a deity who is of a good and loving nature still holds so much pain and suffering for good people. In the context of this paper, the title is meant to be an epigraph that suggests that although a building may be meeting its intended structural purpose, bad things, at least as they are perceived, can happen during design, construction and service of the building that bring its safety into question. One of the main circumstances that can bring into question the integrity of an unbonded post-tensioned building is corrosion of the strands and anchorage components. This paper will highlight the unnecessary demise of a modern high-rise post-tensioned structure due to corrosion, and contrast that outcome to several existing unbonded post-tensioned buildings that experienced corrosion and were successfully repaired and continue to function.