International Concrete Abstracts Portal

Deep beams are common elements in concrete structures such as bridges, water tanks, and parking garages, which are usually exposed to harsh environments. To mitigate corrosion-induced damage in these structures, steel reinforcement is replaced by fiber-reinforced polymers (FRPs). Several attempts have been made during the last decade to introduce empirical models to estimate the shear strength of FRP-reinforced concrete (RC) deep beams. In this study, the applicability of these models to predict the capacity of simply supported deep beams with and without web reinforcement was assessed. Test results of 54 FRP-RC, 24 steel-fiber-reinforced concrete (FRC), and 7 FRP-FRC deep beams were used to evaluate the available models. In addition, a proposed model to predict the shear strength of FRPFRC deep beams was introduced. The model was calibrated against experiments conducted previously by the authors on FRP-FRC deep beams under gravity load. The model could predict the ultimate capacity with a mean experimental-to-predicted value of 1.04 and a standard deviation of 0.14.

Highway bridges and parking structures, subject to coupled effects of mechanical loads and corrosion, often show early signs of distress such as concrete cracking and rebar corrosion leading to reduced structural performance and shortened service life. One solution to this problem is to use low-shrinkage low-permeability high-performance concrete (HPC) for bridge decks exposed to de-icing salts and severe loading conditions. A new HPC was formulated to achieve low shrinkage and low permeability, high early-strength, and 28-day compressive strength over 60 MPa (8,700 psi). Its mechanical performance and durability were tested both in the lab and field under severe test conditions, including restrained shrinkage, cycling loading, freezing and thawing cycles, and application of de-icing salts. Models were developed and calibrated to predict structural performance and service life of concrete bridge decks under severe exposure conditions. Prediction models indicate that bridge decks designed with low-shrinkage HPC can achieve a service life up to 100 years. Compared to normal concrete decks, short-t t-to-medium span bridge decks using low-shrinkage HPC could be built at a comparable initial construction cost, but at less than 35% of the life-cycle cost.

Un-bonded post-tensioned slabs were developed and principally flourished in the USA since the mid 1950’s. The continuous un-bonded one-way post-tensioned slabs became popular due to their predominant use in parking structures all over the country which is true even now. Two-way un-bonded post-tensioned slabs, mainly flat-plates and flat-slabs gained popularity since mid 1960’s. The use of a Banded System of placement of un-bonded post-tensioning tendons, introduced in the early 1970’s, made flat-plate and flat-slabs more competitive. Flat-plate/slab and shear-wall system became and remains very popular for mid-rise and hi-rise buildings. A brief discussion on the development of un-bonded post-tensioned slabs and its relevance in current design and construction is made in part-1 of this paper. Many of these slabs are now between 30 to 60 years old. These structures need routine repairing and retrofitting work. Existing methods are labor intensive and expensive. An alternative method could be repair work with composite materials. Use of composites (mainly CFRP) as a repair material for concrete structures is becoming very common. Most of the repair procedures are based on researches with reinforced concrete specimens and in some cases with pre-tensioned specimens. Research work using un-bonded post-tensioned specimens, especially two-way slabs is practically non-existent. A testing program was developed with the goal of finding the cracking and ultimate strength behavior of un-bonded post-tensioned slabs (before and after repair with CFRP) with different boundary conditions. A total of six slabs were tested. In the first phase two two-way simply supported un-bonded post-tensioned slabs were tested. In these tests CFRP repair configurations were varied. In one case CFRP was placed across the cracks in another case it was orthogonal (parallel to the edges). In the second phase, four more slabs were tested (three one-way slabs and one two-way slab). One one-way slab had two ends fixed, another one had one end fixed and one end simply supported and the third one had both ends simply supported. The CFRP placement configuration in these three slabs varied. CFRP was placed across the cracks in the supports and mid spans. The fourth slab was a two-way slab with simple support. In this case the CFRP repair configuration was similar to the first slab (but CFRP had 2 inches/5 cm overlaps). Sketches of different cracking patterns and CFRP configurations are shown inside. The repair and testing of slabs is discussed in part-2 of this paper.

Two-way flat slabs are extensively used in many structures, such as buildings, shopping centers, and parking garages, because their static efficiency allows to attain large span-depth ratios and to have more spaced columns. The reduction of the thickness, however, is limited by serviceability requirements (resulting from deflection criteria) and by the ultimate limit state of punching shear. This collapse mode has been widely studied in the past, with reference to ordinary conditions, but very limited attention has been devoted to the punching resistance of flat slabs in fire conditions, an issue which is of primary concern, especially in the case of parking garages. This paper deals with two key aspects of slab punching in fire conditions. The first is the sizable reduction of the punching resistance in a typical slab-column assembly, because of the thermally induced damage caused by the exposure to the fire, that is modeled by means of the temperature-time curve ISO-834 (that fits very well the points given by ASTM E119-08a. The second is the sizable load increase due to the redistribution of the internal forces ensuing from the fire that is modeled through a realistic fire scenario based on the available information coming from real car fires. Even if these two phenomena do not necessarily occur simultaneously in a real fire, they both testify to what extent punching shear can be critical for reinforced concrete slabs in fire.

Calcium nitrite corrosion inhibitor has been commercially available in the U.S. since 1978. In that period of time, over 200 parking structures, 100 marine structures, and more than 230,000 m 3 (300,000 yd 3) of precast/prestressed bridge girders have been constructed with concrete containing calcium nitrite. In this paper, several of the oldest structures, along with several test sites, were evaluated to determine the corrosion performance. The condition assessment included a visual evaluation of the structure, determination of chloride and nitrite contents in the concretes, and determination of the corrosion activity. The corrosion tests consisted of corrosion potential mapping and polarization resistance testing to determine the corrosion rates at the time of the evaluation. These assessment techniques are applicable to all steel reinforced concrete structures with or without some modifications. The assessment showed that all of the structures with calcium nitrite are performing well. In two cases, there is evidence that corrosion is in progress on adjacent structures that were not protected with calcium nitrite. The nitrite analyses document that calcium nitrite is stable in concrete and remains at the reinforcing bars. Diffusion of chloride is not increased in the concretes with calcium nitrite, and there is evidence of a reduction in chloride penetration in some cases.