Snyder, Mark B.

“D”-cracking and other forms of aggregate-related freeze-thaw damage have often been associated with concrete pavements in Minnesota. The best approach for preventing these types of distress is to avoid using aggregate sources that are known to be susceptible to freeze-thaw damage in concrete applications. The most widely accepted methods of evaluating aggregate freeze-thaw durability involve the preparation and freeze-thaw testing of concrete beams that contain the aggregate in question. These tests are generally time-consuming, sometimes requiring months to complete, and often require the use of expensive equipment and/or highly skilled operators. Furthermore, the variable nature of many aggregate sources necessitates frequent testing to ensure the adequate freeze-thaw resistance of material being produced at any given point in time. A more rapid test of aggregate freeze-thaw durability was developed under the Strategic Highway Research Program in 1994. This test, called the Washington Hydraulic Fracture test (WHFT), was relatively inexpensive and allowed a single laboratory technician to assess the freeze-thaw durability of several samples of aggregate in as few as seven working days. Broader evaluations of the WHFT revealed several deficiencies, however.

This report presents freeze-thaw durability results of an investigation regarding the application of high performance concrete (HPC) to prestressed bridge girders. This study included a total of 30 concrete mixes and more than 130 specimens, with the following variables: aggregate type, round river gravel, partially-crushed gravel, granite, high-absorption limestone, and low-absorption limestone; cementitious material composition, Type III portland cement only, 20 percent fly ash, 7.5 percent silica fume, and combination of 20 percent fly ash with 7.5 percent silica fume replacement by weight of cement; and curing condition heat-cured or seven-day moist-cured. No air-entraining agents were used in the study's initial phase to simulate the production of precast/prestressed bridge girders. Results indicate that it is possible to produce portland cement concrete with high strength and freeze thaw durability without the use of air-entraining agents. Overall, the moist-cured concrete specimens exhibited better freeze-thaw durability than the heat-cured concrete specimens. The reference concrete mixes--containing only portland cement performed better than the concrete containing pozzolan material of fly ash or silica fume. The low-absorption limestone aggregate concrete mixes exhibited the best freeze-thaw durability performance--in some cases, enduring more than 1,500 freeze-thaw cycles without failing. The study found that the moisture content of the coarse aggregate at the time of mixing had a significant impact on the concrete's freeze-thaw durability.

This report reviews eleven field and laboratory studies that have been performed to address concerns about the use of recycled concrete aggregate in pavement foundations. Performance concerns have centered on the possible impairment of drainage systems by deposits of calcium carbonate precipitate and other fines derived from the recycled concrete base materials. Environmental concerns have focused on the relatively high pH of the effluent produced by drainage systems that remove water from untreated recycled concrete aggregate foundation layers. The studies considered in this report demonstrate that all recycled concrete aggregates are capable of producing various amounts of precipitate, with the precipitate potential being directly related to the amount of freshly exposed cement mortar surface. It appears that selective grading and blending with virgin aggregates are techniques that should significantly reduce precipitate potential. One study suggests that washing recycled concrete products will reduce accumulations of crusher dust and other fines in and around the pavement drains. Others indicate that the use of filter fabrics with sufficiently high initial permittivity will allow the accumulation of precipitate and other fines without significantly impairing drainage function. This report discusses study results related to environmental concerns and provides recommendations for revisions to current specifications.

Cracking of the concrete decks on newly constructed bridges in Minnesota has become a significant concern. Since 2005 MnDOT has been collecting bridge deck construction and early age cracking information on a "Bridge Deck Placement Data Form." The information collected has been entered into a database, along with early age crack surveys, concrete mix design information and concrete testing information. There currently is information on over 120 bridges stored in the database. Crack surveys were performed on 20 of the bridges contained in the database. A statistical analysis of the data, including the updated crack surveys, was performed to determine if there were any relationships between variables collected on the forms and crack frequency, type, or time of development. The analysis showed that, in general, the data collected was not sufficiently consistent to draw significant conclusions. A relationship for temperature restraint cracking for bridges with integral abutments was developed for lineal feet of cracking as a function of bridge deck age, water/cementitious material ratio, and total cementitious content. Recommendations were made for modifications to current construction practices and improving the uniformity of the data collected on the "Bridge Deck Placement Data Form" in the future, so that additional analysis could be performed with more consistent data.

Study goals included: 1) identify mechanisms causing premature failure in Minnesota concrete pavements; 2) evaluate the accuracy of existing tests of aggregate freeze-thaw durability using Minnesota aggregate sources and pavement performance records; 3) develop a new methodology for quickly and reliably assessing aggregate freeze-thaw durability; and 4) evaluate techniques for mitigating D-cracking. Research results indicate that the poor durability performance of some Minnesota PCC pavement sections can often be attributed to aggregate freeze-thaw damage. However, secondary mineralization, embedded shale deposits, poor mix design and alkali-aggregate reactions were also identified as problems. Petrographic examination can help to differentiate between these failure mechanisms. A reliable and universal method for quickly identifying D-cracking aggregate particles was not identified. A test protocol was developed for improved aggregate durability evaluation. It includes several tests which are selected for use based on aggregate geological origin and composition and the results of previous tests. Further validation of the proposed test protocol is recommended. Several techniques appear to be effective in improving the freeze-thaw durability of concrete prepared using marginally durable aggregate: mix design modifications, reductions in aggregate top size, and the blending of durable and nondurable aggregates. Some chemical treatments showed promise, but may not be economical.

Effective load transfer across Portland cement concrete pavement joints significantly decreases pavement deterioration. Dowel bars placed transversely across a joint or crack provide a mechanism for effective load transfer to take place. Dowel bars are used in new construction as well as retrofitted into existing pavements for restoration of load transfer. Areas of concern with using dowel bars include high costs, due to the labor-intensive procedure of retrofitting, and corrosion associated with standard mild steel epoxy-coated dowels. This research addresses these problems by evaluating four dowel bar details tested in an accelerated manner. Retrofit testing was performed using mild-steel epoxy coated dowels and fiber reinforced polymer (FRP) dowels. The details tested provide comparisons among dowel bar materials, depth of placement, number of dowels used, and dowel diameter. Verification testing of previously tested details is also presented.

An area of concern common to portland cement concrete (PCC) pavements is load transfer across joints and cracks. The current design standard for load transfer in new jointed PCC pavements and the rehabilitation of old PCC pavement is to place steel dowel bars at mid-depth of the pavement across the joint or crack (1). The main issues with the use of retrofit and/or new dowels are the high expense associated with the retrofitting operation and the corrosion that has been associated with the use of steel dowels. Three new and experimental dowel bar retrofit designs, that address the issues of high retrofit cost and corrosion susceptibility, were tested in an accelerated manner in order to determine the potential viability of their use for the restoration of load transfer in PCC pavements. Innovations in the three designs included the use of fiber reinforced polymer dowels, grouted stainless steel pipe dowels, and a change in the geometric configuration of the design. An evaluation of test results and recommendations, regarding the use of the designs for the restoration of load transfer in PCC pavements, are presented.

The Washington Hydraulic Fracture test was developed under the Strategic Highway Research Program to address the need for a rapid, inexpensive test for concrete aggregate freeze-thaw durability. The original test and analysis procedures were not sufficiently reliable and accurate to merit widespread adoption and implementation. Several follow-up research efforts have been performed and each has resulted in improvements to the test. This report describes the results of recent research efforts to improve the test. The "hydraulic fracture index" has been replaced by a model that predicts freeze-thaw test dilation as a function of the distribution of particle mass retained on various sieves after testing. This model was developed using data obtained from freeze-thaw and hydraulic fracture testing of 18 quarried carbonate and gravel aggregate sources, and the resulting correlation is exceptional (r-squared = 0.98). In addition, a large test chamber was developed to allow testing of aggregate samples five times larger than the original small chamber, thereby allowing aggregate durability characterization with a single test run. It is believed that the hydraulic fracture test is now ready for more broad-based validation testing and eventual widespread acceptance and implementation as an accurate screening tool for concrete aggregate freeze-thaw durability.

This research project assesses the nature and extent of premature deterioration of segmental concrete block retaining walls (SCBRWs) along roadways in the Minneapolis-St. Paul area. Researchers conducted a two-stage condition survey on 104 SCBRWs. The first stage, a general distress survey, focused on determining the type, severity, and extent of distresses present. The second stage, a peak winter survey, assessed the extent of snow/ice cover and exposure to winter sun. According to research results, only 7% of the SCBRWs surveyed were in poor or very poor condition. But researchers observed many distress types in 50% or more of the walls surveyed, including freeze-thaw damage, scaling, manufacturing flaws, and efflorescence. Freeze-thaw damage and scaling were most highly associated with decreases in overall wall condition. Efflorescence and freeze-thaw damage were found to be at least partly dependent upon SCBRW age and block manufacturer. Durability problems appear to be directly related to the lack of durability of the block units, which suggests problems with the use of inadequate mix designs, non-durable aggregate, and/or inadequate curing procedures. The report includes recommendations to address possible deficiencies in manufacturing processes and quality.

A laboratory-based linear loading pavement test stand, the Minnesota Accelerated Loading Facility (Minne-ALF) simulates the passage of heavy traffic loads moving at speeds up to 65 kph (40 mph) over small, full-scale pavement test slabs. Hydraulic actuators control a rocker beam, which simulates loads. Researchers simulated the passage of 40-kN (9-kip) single-wheel loads at a rate of 172,000 per day, although wheel loads up to 100 kN (22 kip) can be simulated at varying speeds. Full-axle simulations are possible with frame modifications. Concrete slabs were cast and dowels were installed in slots across cracks/joints. Test variables included joint face texture, repair backfill material, and dowel material and length. Test outputs included measurements of load transfer efficiency and differential deflection across the joint/crack. The effect of joint/crack face texture was great when the joint/crack remained tight. Load carrying performance was improved using Speed Crete 2028 in place of 3U18 concrete backfill with similar joint and dowel bars. Load transfer was unaffected by the use of stainless steel-clad dowel bars in lieu of epoxy-coated dowel bars. Researchers recommend additional testing to examine the effects of dowel length and dowel materials.