International Concrete Abstracts Portal

For the calculation of civil engineering structures, designers employ the mechanical aspect underestimating the physicochemical phenomena in connection with the hydration of cement paste. Although the mechanical approach is widely sufficient for classical structures, this is not the case for large structures like dams, where thermophysical phenomena play a leading part. After a short analysis of the degradation observed on a roller compacted concrete dam, showing the importance of the control of hydration effects on mass concrete, the authors present a thermomechanical model able to describe the main evolutions of concrete properties with aging. Application to the Riou dam shows the ability of the approach to simulate temperature, strains, and stresses and, as a consequence, the risk of damage for the structure. Cracks in the middle of the dam are properly represented. This approach permits determination of the position and number of construction joints and setting the schedule of construction as thickness of concrete layers or maximum delay between two layers.

Reinforcement corrosion due to chloride ingress is the most common cause of concrete deterioration in Norway. A wharf with dimensions of 270 x 25 m was built in 1965 to 1966 and required partial repairs in 1980, 1986, and 1989 to 1990. The repair work included some research and development. The conclusion of the 1989 inspection was that no corrosion activity was evident in the earlier repaired areas. Repair mortar with silica fume had somewhat lower chloride ingress and significantly higher electrical resistivity than mortar without silica fume. Latex addition to the repair mortar showed the same effect, as well as a reduced water content. The main conclusion is that materials and working procedures used for the 1980 repair have resulted in a satisfactory service life of at least 10 years.

Under the Strategic Highway Research Program (SHRP), Contract C-206, "Optimization of Highway Concrete Technology," constructibility and performance of concrete for early opening of highway repairs were evaluated. A variety of concretes mixed using different types of rapid strength cements and admixtures were used for full-depth repair (slab replacement) of concrete pavements and for bridge deck overlays in the states of Ohio, Kentucky, and Georgia. For pavement applications, eight mixtures with different strength-gain capacities allowing for a variety of traffic opening times ranging from 2 to 24 hr were evaluated. Latex-modified concrete with Type III cement and silica fume mixes were used for bridge deck overlays. Durability evaluation of these mixtures included freeze-thaw resistance, characterization of the air-void system and deicer scaling tests, and measurement of chloride permeability. Specimens for these tests were prepared in the field and were subject to standard field curing. Tests were also conducted on cores taken from pavements and overlays at opening time. Freeze-thaw tests on beams were conducted following a modified procedure of ASTM Method C 666B, using specimens wrapped in towels during the air freeze to reduce drying from the surface during the freeze cycle. Follow-up surveys were conducted to examine the performance of these concretes under the effects of environmental exposure and traffic loading. Test results showed that overlay mixes have excellent freeze-thaw resistance. Latex-modified concrete mixes showed moderate scaling using the deicer scaling test. Chloride permeability of cores taken from silica fume overlays were lower than those of latex-modified concrete overlays. Poor freeze-thaw performance of many of the pavement repair mixes indicates that many questions still remain regarding durability of concretes designed for early opening applications. Proper air content and adequate air-void systems are necessary, but not sufficient, conditions for obtaining the desired freeze-thaw durability. Microcracking in the concretes may account for some of the poor performance in freeze-thaw testing. The use of calcium chloride should be avoided, as it contributes to reduced freeze-thaw resistance.

A test method was developed to predict the risks of ASR expansion for actual field concrete compositions. Concrete prisms (7 x 7 x 28 cm) were cast, demolded, and measured for their initial length. They were then immersed in an alkaline solution at 150 C for 3 weeks in individual stainless steel containers and their lengths were monitored weekly. The concentration of the solution was adapted to match as closely as possible the composition of the interstitial pore solution by summing up the contribution of the binder constituents to the effective sodium and potassium contents, respectively. Despite this fact, the alkali balance before and after treatment of the prisms shows that the concrete is enriched in Na 2O and/or K 2O during the cure. The expansions reached at 150 C after 3 weeks were compared to those obtained at 100 percent relative humidity in air at 38 and 60 C after 12 and 4 months, respectively. Good correlations were obtained for the 67 different concrete mixes tested. Consequently, an expansion value of 0.11 percent was proposed as a provisional limit, which is rather conservative. Reaction products were studied by SEM, optical microscopy, and electron microprobe as gels, and semi-organized and crystallized compounds, and were shown to be present in cracks and pores as well as within the paste. The composition of the products formed at 150 C seems to be restricted to a narrower range of Ca, Si, Na, and K concentrations than what has been reported at lower temperatures.

Petrographic examination and the South African mortar bar test have been performed at SINTEF--Structures and Concrete during the last 2 to 3 years to evaluate the reactivity of Norwegian aggregates to be used in concrete structures. Paper presents the relationships between these two test methods. The purpose of the petrographic examination is to identify, quantify, and group different rock types in an aggregate. These groups are: reactive (with known reactive field performance), potentially reactive, and innocuous aggregates. In Norway, further testing by the mortar bar test is recommended when petrographic examination indicates 20 percent of reactive or potentially reactive rock types in the aggregates. The mortar bar expansion after 14 days of exposure is used for the evaluation of potential expansivity of the aggregates. One main conclusion from the investigation is that mortar bar expansion increases to an upper level with increasing content of reactive rocks in the aggregates. Beyond a "marginal" amount of reactive rocks in aggregates, the mortar bar expansion increases no further. A significant difference in mortar bar expansion between different reactive rock types has not been found. The established limit of 20 percent of reactive rocks in aggregates appears, in most cases, sufficient for classifying aggregates as innocuous; however, no verification of the limit has been made.