Thomas Van Dam

Long-Term Effects of Magnesium Chloride and Other Concentrated Salt Solutions on Pavement and Structural Portland Cement Concrete: Phase I Results

Many state highway agencies use various chemicals to deice or anti-ice pavement and bridge surfaces. Most often these are aqueous solutions of various chlorides (e.g., magnesium chloride, sodium chloride, and calcium chloride) or other chemicals such as calcium magnesium acetate, urea, or others. Possible detrimental effects to concrete caused by these chemicals have not been fully examined and documented. Mortar specimens of three different water-to-cement ratios (0.4, 0.5, and 0.6) were immersed in concentrated solutions of various chemical deicers and held above freezing at a constant temperature of 40°F up to 84 days. Considerable expansion and cracking were noted in specimens immersed in the MgCl2 and CaCl2 solutions. Petrographic analysis and quantitative microanalysis were used to positively identify the presence of Mg(OH)2 (brucite) formation in the outer layers of the specimens. Furthermore, the results presented clear evidence of calcium oxychloride formation in the specimens analyzed. Further r...

Hardened Concrete Air Void Analysis with a Flatbed Scanner

Digital images collected from a polished concrete surface with a flatbed scanner are used to quantify air void characteristics. The surface is scanned a total of three times. Between the first and second scans, the surface is stained with phenolphthalein to color the cement paste pink. Between the second and third scans, the surface is painted black, and white powder is pressed into depressions left by air voids. The images collected from the three scans are aligned and input into a classification scheme to yield an output image. Each pixel in the output image is categorized as air void, cement paste, or aggregate. By digitally applying a grid of points and a series of lines to the output image, a modified point count is automatically performed according to ASTM C457, Standard Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete. A comparison is made between results obtained by an automatic analysis of the digital output image and results obtained by a manual analysis of the surface with an optical microscope.

PROPOSED PAVEMENT PERFORMANCE MODELS FOR LOCAL GOVERNMENT AGENCIES IN MICHIGAN

The state of Michigan has 191 432 km (118,950 mi) of roadway (paved and unpaved), including highways, roads, and streets. Local government agencies, which are responsible for 176 270 km (109,529 mi) of these roads and streets, commonly use a pavement management system (PMS) called RoadSoft to assist in managing their pavement network. A key element of any PMS is its ability to predict future pavement performance. A study is described in which various deterministic and probabilistic models were evaluated using data from two Michigan counties. It was found that the logistic growth model and the Markov model provided the best combination of predictive ability and potential for applicability in Michigan counties. A comparison between these models found that their predictive ability for four pavement segments with different deterioration rates was good, with the Markov model offering the added advantage of representing future performance as a probability distribution, not as a single condition state. Current plans are to implement the logistic growth model in RoadSoft by the end of 1999 and to add the Markov model as local organizations gather sufficient pavement condition data over the next 5 years. It is hoped that these two pavement deterioration models can be implemented in the RoadSoft PMS to improve pavement performance prediction.

Effect of Sample Preparation on Chemical Composition and Morphology of Alkali-Silica Reaction Products

The chemical composition and morphology of alkali–silica reaction (ASR) products is of great importance in studying the reaction mechanism and assessing the effectiveness of mitigation techniques. Epoxy-impregnated, polished thin sections were prepared from an in-service concrete pavement to examine ASR products that originate from chert particles in the fine aggregate. Both hydrous and anhydrous thin-section preparation techniques were used to investigate the influence of sample preparation on the results. Two distinct morphologies of ASR products were observed in the voids adjacent to the chert particles: a bladed crystalline type and a glassy amorphous type. The chemical compositions of the reaction products were determined with a scanning electron microscope via quantitative X-ray energy dispersive spectrometry. Both the chemical composition and morphology were influenced by sample preparation, with hydrous preparation resulting in leaching and degradation. It is recommended that, when conducting studies to assess the ASR mechanism and the effectiveness of mitigation techniques, anhydrous sample preparation be used.

Investigation of Portland Cement Concrete Exposed to Automated Deicing Solutions on Colorado’s Bridge Decks

The Colorado Department of Transportation (DOT) has identified potential performance problems in some portland cement concrete (PCC) bridge decks and approach slabs in the form of pattern surface cracking, spalling, and joint and crack deterioration; these problems are suspected to be materials-related distress (MRD). External factors such as deicing and anti-icing chemicals can initiate and increase the rate and magnitude of deterioration caused by MRD and thereby shorten the life of the structure. This study investigated whether highly concentrated deicer solutions that were applied through bridge deck deicing and anti-icing systems that used fixed automated spray technology disproportionately contributed to deterioration of PCC bridge decks and adjacent concrete approach slabs in Colorado and whether mitigation strategies employed by Colorado DOT addressed the problem. The investigation involved visual inspection techniques, materials sampling, and evaluation of sampled concrete by using petrographic methods. In bridge decks studied, the concrete evaluated seemed sufficiently resistant to damage from the intrusion of deicer chemicals. Where full-depth cracking was present, however, obvious signs of the movement of moisture and deicers through the deck were observed. Also, some initial signs of possible chemical attack from deicers were noted, and continued exposure to highly concentrated deicers may have contributed to long-term durability concerns. However, use of polymer-modified asphalt and fabric membranes in conjunction with a hot-mix asphalt overlay seemed effective in preventing the ingress of chlorides into the underlying concrete deck.

Evaluation of Potential Long-Term Durability of Joints Cut With Early-Entry Saws on Rigid Pavements

Early-entry sawing is an attractive operation to expedite the construction of jointed concrete pavements; however, there are some concerns that early-entry sawing may compromise the pavements long-term performance. The Illinois Department of Transportation initiated this study as an initial effort to investigate the durability of joints sawed by using early-entry sawing. Joint performance as a function of saw-cut depth and timing was also considered. The study was integrated into an active construction project along Illinois Route 59 in Plainfield. During construction, paving and sawing operations were observed and documented; of particular interest were the sawing operations, during which signs of surface scarring, joint raveling, and slab edge breakouts were recorded and the extent of sawing-related damage was subjectively assessed. In addition to general observations, climatic conditions were monitored, as was pavement temperature from time of paving onward. Ambient climate conditions, portland cement concrete mixture, and slab temperature data were used to perform a HIPERPAV analysis to assess the potential for early-age cracking. Compressive strength cylinders were also cast and tested at 3, 7, and 28 days. In addition, cores were retrieved from joints throughout the test section and a battery of durability tests—including petrographic analysis, freeze–thaw testing, and resistance to salt scaling—was conducted. Overall observations from the field construction and findings from the laboratory testing indicate that the use of early-entry sawing is viable and that joint durability is not compromised.

Field Study of Air Content Stability in the Slipform Paving Process

This study evaluated the impacts of construction on the air content and air void system structure of portland cement concrete pavements. The primary intent was to quantify the air content of fresh concrete before and after it had gone through the slipform paver. The air void system parameters of hardened concrete were then assessed with cast cylinders and extracted core specimens. The results of the air content testing on fresh concrete and the concrete cylinder specimens cast in the field suggested that some loss of air (approximately 1%) occurred as the concrete passed through the paver. Laboratory testing performed on cores extracted from the pavement did not provide conclusive evidence that entrained air was lost during the slipform paving process. In fact, many extracted cores had measured air content values that were much higher than those measured in the fresh concrete and even higher than the specification requirement. If excessive, such values could result in increased permeability and low-strength-related issues. The results suggested that the air content testing on fresh concrete did not capture the true air content of the concrete as it was placed with a slipform paver. The fresh concrete air content in general was lower than was the air content measured in the cores.

Blast Furnace Slag as Sustainable Material in Concrete Pavements

Slag materials are byproducts of metallurgical processes that include metal production from ore and refinement of impure metals. Air-cooled blast furnace slag (ACBFS) has been used for different pavement-related applications. Appropriate use of ACBFS can significantly enhance sustainability, rather than disposing of it, by effectively contributing to all aspects of the “triple bottom line.” Proper use of ACBFS can also result in economic, environmental, and social benefits as long as the performance of the concrete structure is not compromised through such use because any short-term economic and environmental gain would be rapidly eclipsed by the economic, environmental, and social costs of poor pavement performance. Thus, it is essential that engineers and contractors who use ACBFS aggregates in concrete understand its unique properties to make sure that the expected performance of the pavement over its design life is achieved. The chemical composition of ACBFS may affect its performance and has to be considered when ACBFS is used as a coarse aggregate. Physical properties of ACBFS, such as texture, absorption, and specific gravity, also have to be considered when ACBFS is used in concrete. ACBFS also affects fresh and hardened properties of concrete. Specific design, construction, and quality control considerations have to be taken into account when ACBFS is used. In this paper, sustainability aspects of using ACBFS as a coarse aggregate in concrete pavements and considerations for its use in this application are presented. A best practices guideline recently published by FHWA presents the same discussions in more detail.

EVALUATION OF PREMATURE DETERIORATION OF CONCRETE BRIDGE BARRIERS BY PETROGRAPHIC EXAMINATION

Many of the current-generation concrete bridge barriers used in Michigan have deteriorated at a rate greater than expected. For an assessment of the possible causes of this premature distress with the objective of alleviating future occurrences, sixteen core specimens from eight sites constructed between 1983 and 2001 were evaluated. The evaluation included visual inspection, stereo and petrographic optical microscopy, and scanning electron microscopy. The major observations revealed through this analysis were that poor consolidation, marginal air-void systems, alkali-silica reaction associated with chert and siltstone in the fine aggregate, corrosion of reinforcing steel, and possibly frost susceptibility of fine siltstone aggregates were responsible for the deterioration. These results provide the basis for improving the quality of such barriers in future construction.

Deterioration in Concrete Pavements Constructed with Slag Coarse Aggregate

In Michigan, sections of an Interstate-type pavement are suffering extensive cracking and joint deterioration after 10 years of service, having been constructed in 1992. An adjacent section constructed in 1993 with comparable design features and materials remains in good condition, with little visual sign of distress. A study was conducted to determine, if possible, the cause of the observed distress in the highway built in 1992. In all, cores from nine different projects were evaluated, all of which were made with iron blast-furnace-slag coarse aggregate and natural fine aggregate containing chert constituents. The analyses conducted included stereo and petrographic microscopy and chemical extractions to determine levels of exchangeable and soluble potassium and sodium, as well as sulfates. The findings indicate that, in distressed pavement sections, the chert constituents in the fine aggregate are deleteriously alkali–silica reactive (ASR), whereas these same constituents are not deleterious in the sections rated as fair. Further, the distressed sections all had sulfate levels significantly higher than predicted by the mixture design. It is hypothesized that, in addition to the ASR in the fine aggregate, dissolution of the calcium sulfide dendrites in the slag coarse aggregate is providing excess internal sulfates, resulting in in-filling of the air-void system with ettringite and potentially sulfate attack. The exact nature of the deterioration mechanisms is not fully understood, but it seems clear that some type of interaction exists between the ASR and excess sulfates.