International Concrete Abstracts Portal

The use of high-strength lightweight concrete (HSLWC) in offshore oil and gas platforms is becoming more common. The constant wave action on these structures imposes continual fatigue loading on the concrete. Paper reviews previous research on both compressive and flexural fatigue behavior of HSLWC. The fatigue behavior of HSLWC is comparable or somewhat better than high-strength normal-density concrete (HSNDC) tested under the same conditions. The cyclic strain behavior of HSLWC is significantly different than for HSNDC and there is little change in strain behavior with increasing cycles of load until failure occurs. The fatigue life is reduced when the concrete is tested in submerged conditions. There is no significant difference between the S-N curves for reinforced and nonreinforced concrete. The mechanism that causes HSLWC to have comparable or better performance than HSNDC is attributed to the improved microstructure of the matrix-aggregate interface. This improvement reduces microcracking that typically leads to fatigue damage. The effect of crack blocking by sea salt depositions is discussed.

A 4-year study, conducted by a consortium of three universities, on the use of high-performance concrete in highway applications was recently completed. A major goal of this research project was to determine if high-performance concrete mixes could be successfully produced in the field. In addition, an evaluation was to be made of the long-term performance of this concrete under field service conditions. Field installations were constructed in five states for this purpose. Paper provides potential users of high-performance concrete with general recommendations and guidelines for production and placement.

In general, the structural member using high-strength concrete is accompanied by high brittleness, which may result in the unexpected dangerous failure. For economy and safety, high-strength concrete may be used for compressive members (vertical members) and low-strength concrete for flexural members (horizontal members). ACI 318-89 recommends that when the specified compressive strength of concrete in the column is greater than 1.4 times that specified for the floor system, the column concrete shall extend 600 mm into the slab from column face to avoid unexpected failure. The structural behavior of beam-column joints with two different compressive strengths of concrete for the beams and the columns has not been investigated adequately. ACI-ASCE Committee 352 recommends that for joints that are part of the primary system for resisting seismic lateral loads, the sum of nominal moment strengths of the column sections above and below the joint ( M c), calculated using the axial load, which gives the minimum column moment strength, should not be less than 1.4 times the sum of the nominal strengths of the beam sections at the joint ( M b). Thus, those recommended values should be examined before high-strength concrete can be used with confidence and convenience in structural members. The results showed that the ACI 318-89 extension distance of 600 mm is safe at least for members up to 300 mm in total depth, and the 2h (h is overall depth of the beam) extension distance was found to be safe also for members under flexural loading with a column-to-beam flexural strength ratio of 1.8.

The main topic focuses on a materials science approach to evaluating three major strengthening mechanisms of high-performance concretes: reduced porosity by low water-cement ratio, absence of macro-defects, and synthetic composition mechanism. The substantially improved cement matrix materials can be obtained by deliberately using one or more of the preceding mechanisms. The preliminary experiments were carried out by two computer coupled techniques, one utilizing an electromechanical linear variable differential transformer (LVDT), while the toughening experimental technique was based on determining the J-integral to obtain K 1 c and G 1 c in an indirect way suing small-size specimens. An acoustic emission system was also used. At different concrete maturity stages, the acoustic emission signal generated from the microstructure is transformed due to wave propagation and the transducer response. The data are analyzed numerically. The results obtained through this study are expected to contribute to the establishment of a new strengthening concept of high-performance concrete. The objective of this paper is to sketch a new approach to a group of strengthening phenomena that are as important from a theoretical viewpoint as they are useful for technology.

The Hibernia offshore concrete platform is being constructed in Newfoundland, Canada, and will be used in hydrocarbon production on the Grand Banks off the east coast of Canada. The 111-m tall concrete structure will contain approximately 165,000 m 3 of high-strength concrete. Construction of the concrete platform through 1993 consisted of a 108-m-diameter base slab that rested on a series of precast and cast-in-place concrete skirts. Specified 28-day compressive strengths (cylinder) for the skirts and base slab were 49 and 69 MPa, respectively. Actual average compressive strengths achieved were73.8 and 81.7 MPa, respectively. The remaining construction will be completed by 1996. The use of two different concrete production systems and their results are described.