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Mobile Concrete Laboratory Project Reports 0202Woodrow Wilson Bridge Foundation Concrete Thermal ModelingBackground: The replacement of the Woodrow Wilson Bridge represents a multibillion-dollar project that is being executed by the Maryland State Highway Agency and the Virginia Department of Transportation. A consortium of engineering and contracting firms (TKC) is building the structure. A significant portion of the overall cost of the new bridge is the work required for the pier foundations. The foundations are located both on land and in the Potomac River. Driven piles are providing the load carrying capacity of the foundation. Cast in place pile caps and pedestals complete the foundation structure. The cast in place pile caps and pedestals reflect the size of the bridge and thus can be considered mass concrete placements. Modeling of the combination of varying air temperature, river water temperature, concrete temperature, placement scheduling, and concrete insulation had been previously performed by CTL for TKC. The FHWA Maryland Division Office requested assistance from the FHWA Mobile Concrete Laboratory to perform independent verification utilizing a semi-adiabatic calorimeter, data collection, and modeling software. The semi-adiabatic calorimeter and software has the trade name of Quadrel^{TM}, a product of Digital Site Systems of Pittsburgh, Pa. The FHWA PCCP Lab at Turner Fairbank Highway Research Center became involved since the contact laboratory staff have experience with the Quadrel hardware and software. This report deals specifically with the results of modeling the pile cap mass concrete placements. Problem Statement: Predict the performance of pile cap concrete placements under varying air temperatures, river water temperatures, concrete temperatures, and placement times. Predict maximum concrete temperature, maximum temperature differential, and verify concrete strength development. Various cross sections and conditions are to be modeled. Note that modeling in Quadrel will not allow the inclusion of cooling pipes in the concrete mass as allowed with CTL's proprietary software. Procedure: Materials from the jobsite were secured and shipped to TFHRC. The FHWA Maryland Division Office supplied concrete mixture design information. The laboratory mix was batched and mixed and resultant plastic concrete data recorded. Compressive strength testing was done at 2, 7, 14, 28, and 56days. Concrete mixture data is found in Table #1. The design mixture for the pile caps along with the Quadrel batch is presented in the table Pile caps of particular interest were highlighted. Three types of conditions can be modeled in Quadrel. The conditions are; a symmetrical cross section with both faces with the same boundary conditions; an unsymmetrical cross section with each face with varying boundary conditions; and a foundation condition, with the concrete in contact with a base material and an exposed face. Cross section geometry of the pile caps and environmental conditions dictated what would be the most appropriate selection for modeling.
Compressive strength data is also found in Table #1. Two-day tests were required due to the slow rate of hydration. Quadrel Data: Figure #1 plots Temperature versus Time data along with calculated equivalent maturity hours from the Qdrum specimen Mix 20206-1. The rate of heat generation (the speed of the reaction) and the cumulative heat generated expressed in BTU/lb cementitious are plotted in Figure #2 as a function of Maturity Hours on a log scale Figure #1 Qdrum Temperature Data and Calculated Maturity Hours Mix 20206-1 Figure #2 Adiabatic Heat Signatures - Mix 20206-1 Modeling Concrete Placement with Quadrel: Two construction techniques are being used for the pile cap concrete placements. They are:
Modeling : Pile Caps- Insulated Forms in Contact with Ambient Air. Depths of these pile caps range from a minimum of 9 feet to a maximum depth of 16 feet. Widths of the pile caps range from 40 feet to 53 feet. All are cast on a concrete tremie slab. The critical dimension for heat loss from the placement will be the depth of the pile cap. Based on the above information, the best model selection from Quadrel for the insulated pile caps is the foundation model. Table #2 contains the range of conditions modeled for the insulated pile caps. The tremie slab concrete temperature is assumed to be 60 °F.
Modeling Uninsulated Forms (Deep Water Pile Caps) in Contact with the Potomac River The massive deep water pile cap placements are not insulated. There are potentially two paths of heat loss for the deep-water pile caps. With a length to depth ratio of 5.4 (87 foot length versus 16 foot depth) it would be expected that depth would govern the temperature distribution in the mass concrete placement. However, with the face of the uninsulated forms in contact with the water that has a thermal conductivity in the range of 7.0 Btu in/h ft^{2} versus 0.31 Btu in/h ft^{2} [1] for air, this condition also warrants investigation. For investigating the case that is governed by the concrete depth, the most appropriate model was the foundation model. For investigating the effects of water surrounding the forms, the most appropriate model was the symmetrical cross section.
Quadrel Modeling Details:
Modeling Results Results- Pile Caps- Insulated Forms in Contact with Ambient Air. An example output from Quadrel is found in Figure #3. This is for the midpoint range of allowable concrete placement temperature 70°F and ambient air temperature of 60°F. Temperature curves for various depths into the concrete mass and the two surfaces are plotted Figure #3 - Pile Caps- Insulated Forms in Contact with Ambient Air Model - Air temperature=60°F and Concrete Temperature at Placement= 70° Depth =16 ft Comments on Results Pile Caps- Insulated Forms in Contact with Ambient Air Model:
Maximum Temperature Differentials: The influence of concrete placement temperature and maximum temperature differential are plotted in Figure #4. The maximum temperature differentials are plotted over the whole range of anticipated concrete placement temperatures and pile cap depths. The plot illustrates that the major factor for the maximum temperature differential through the pile cap cross section is the initial concrete placement temperature, with the change in pile cap placement depth from 16 feet to 9 feet having a minor effect. Figure #4 - Maximum Concrete Temperature Differential for Insulated Forms in Contact with Ambient Air Peak Concrete Temperature Figure #5 is a plot of the maximum concrete temperatures for the varying thickness of the insulated pile caps. Ambient air temperature is not a significant factor. The concrete placement temperature is the main factor that affects maximum concrete temperature. Figure #5 - Maximum Concrete Temperature for Insulated Forms in Contact with Ambient Air Time of Peak and Maximum δ Temperature: The near adiabatic conditions that exist in the pile cap placements influence the time when peak concrete temperatures and maximum temperature differentials occur. The influence of concrete placement temperature and pile cap thickness is found in Figure #6. There is at minimum a doubling of the time to reach peak concrete temperatures and maximum temperature differential when the concrete placement temperature is reduced from 90°F to 50 °F. Figure #6 Time of Peak and Maximum δ Temperature Events for Insulated Pile Caps Form in Contact With Ambient Air Results - Deep Water Pile Caps- Uninsulated Forms in Contact with the Potomac River An example of the simulation results for this model is found in Figure #7. Figure #9 plots the simulation results for a concrete placement temperature of 70°F and a water temperature of 70°F. The data on the simulation temperature versus time plot have been labeled for easier identification. The simulation was run for a period of two months (1320 hours). Figure #7- Pile Caps- Uninsulated Forms in Contact with the Potomac River - Investigation of Steel Form- Symmetrical Wall Model -Water Temperature=70°F and Concrete Temperature at Placement= 70° Comments on Results Deep Water Pile Caps- Uninsulated Forms in Contact with the Potomac River.
Maximum Temperature Differentials Figure #8 plots the maximum temperature differential for the uninsulated pile caps. As discussed previously, the maximum temperature differential comes at later ages in the pile cap since the center of the cross section remains at adiabatic conditions while the form face is cooling to the river water temperature. Maximum temperature differentials are almost twice that of the insulated piles caps exposed to air discussed previously, Figure #8 Deep Water Pile Caps- Maximum Temperature Differential (δT) for Uninsulated Forms in Contact With Potomac River Water Results - Deep Water Pile Caps-Insulated Form Model in Contact With Ambient Air: Away from the interface of river water and the uninsulated form of the deep water pile cap the most appropriate model is the insulated form model at a fixed depth of 16 feet. Heat generated by hydration will escape either through the concrete tremie slab or through the blanket insulation at the surface. Results were discussed previously. Superposition of the Two Models - Deep Water Pile Caps The major difference between the two models is the eventual reduction of the temperature at the center of the concrete mass over time for the insulated foundation model that is in contact with ambient air. The actual maximum temperature differential for the deep-water pile cap placement is somewhere between the two model values. That is, the maximum temperature differential is not as severe as the uninsulated wall model in contact with the Potomac River or as moderate as the insulated forms in contact with ambient air. This is because the temperature at the center of the concrete mass is controlled by heat loss to the top and bottom (due to height being the least dimension), while temperatures near the edges are controlled by heat loss to the river (due to the greater thermal conductivity of water). By measuring temperatures at discrete intervals an estimated temperature differential can be determined by superposition. The minimum temperature of the two models is compared and the lowest value selected. The differential at time interval t is then determined by subtracting the peak temperature of the insulated form model -minimum temperature. Results are plotted in Figures 9, 10 and 11. Figure #9 - Estimated Maximum Temperature Differential- 50°F Concrete Placement - Deep Water Pile Cap Figure #10- Estimated Maximum Temperature Differential- 70°F Concrete Placement- Deep Water Pile Cap Figure #11- Estimated Maximum Temperature Differential- 90°F Concrete Placement - Deep Water Pile Cap Examination of Figures #9 through #11 indicates that temperature differentials exceed the limit of 35 °F for most scenarios. In fact, the temperature differentials exceed 35°F for extended periods of time. Again, the lower the initial concrete placement temperature, the lower the temperature differential in the deep-water pile cap placement. When the concrete placement temperature is lowered to 50°F and exposed to warm river water with a temperature of 70°F, the differential is no more than 30°F. This is best illustrated with Figure #12 . The most effective way to reduce the concrete temperature differential would be to place the deep-water pile caps when the river temperature is elevated provided the concrete placement temperatures can be lowered to the range of 60°F. Achieving a placement temperature of 60°F should be feasible with a combination of cooling methods for concrete materials and the mixing process. Figure #12 - Deep Water Pile Cap Concrete Placement Temperature Differentials at - River Water 70°F Conclusions: The following conclusions can be made about the pile cap concrete placements and modeling with Quadrel based on the materials received, mixed, and tested: General:
Insulated Pile Caps Exposed to Ambient Air
Deep Water Uninsulated Pile Caps Exposed to River Water
Dobrowksi, Joseph A,[1]"Concrete Construction Handbook- 4^{th} Edition", pp 20.3-20.4, McGraw-Hill Book Company, New York, 1998 |
More InformationContactGary Crawford |
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Updated: 04/07/2011 |