Mitigation of Heat of Hydration with Large Diameter Drilled Shafts: Papers 2006-2012 and Projects

Lin, Lim. Large diameter, non-redundant drilled shaft foundations are becoming more popular in engineering and construction practice. When the drilled shaft diameters become large (generally 1.8-meter (6-foot) diameter or greater), mass concrete considerations need to be taken into account in the design and construction of these foundations. Mass concrete considerations due to the high heat of hydration are more acute in Hawaii because the local concrete mixes are very rich in cement content due to the angularity of the local crushed aggregates and the lack of cost-effective secondary cementitious materials (slag or structural quality fly ash are not readily available in Hawaii). Temperature measurements from several large diameter drilled shafts were taken in an effort to better understand the distribution of the temperatures in the constructed drilled shafts. In this paper, the temperatures at the center and edges of the drilled shafts and at various depths for several large-diameter drilled shafts are presented. The predicted temperatures from the finite element program compared well with the measured temperatures. Therefore, a discussion of the parameters used in the temperature predictions is presented.

Craeye, Geirnaert, De Schutter. High-performance concrete (HPC) with low w/b-ratio experiences a considerable chemical shrinkage and self-desiccation during its hydration process, leading to a rather high autogenous shrinkage deformation during hardening. In case the free deformation of the concrete is prevented, internal stresses are introduced, which can lead to premature cracks. These early-age cracks can severely affect the durability of a concrete structure. By adding super absorbing polymers (SAP) into the HPC as an internal curing agent, and by adding additional curing water to the concrete mixture, the chemical shrinkage and the self-desiccation during hydration of the concrete is counteracted and thus the autogenous shrinkage of the HPC can be significantly reduced. Unfortunately, this process of internal curing also has some disadvantageous effects on the mechanical properties. In search of an optimization of the internal curing process, an extensive experimental program was performed on HPC, using different degrees of internal curing, to assess the mechanical and thermal properties of the HPC, and to evaluate the effectiveness of the performed curing. The goal is to obtain a maximal autogenous shrinkage reduction and a minimal strength reduction. The resulting effect on the early-age cracking risk is simulated by means of finite element calculations. The simulations also include thermal stress development due to the heat of hydration. In case 70 kg/m3 of internal curing water is provided via the SAP, an optimal reduction of the cracking risk is noticed, mainly caused by the autogenous shrinkage reduction and the appearing expansive deformation peak directly after setting takes place.

Tia, Ferraro, et al. A finite element model for analysis of mass concrete was developed in this study. To validate the developed model, large concrete blocks made with four different mixes of concrete, typical of use in mass concrete applications in Florida, were made and monitored for their temperature and strain developments, and compared with the computed temperature and stress distributions from the finite element model. A parametric analysis was also conducted to determine the effects of various factors on the temperature distribution, induced stresses and the cracking risk. Investigation was also made on testing methods to measure the thermal and mechanical properties of mass concrete needed as input parameters for the finite element model. The findings from this study are as follows: (1) Results from the isothermal calorimetry test should be used for input for the heat generation function in the finite element modeling of concrete hydration; (2) Reliance on a limiting maximum temperature differential to control cracking in massive concrete applications should be supplemented with a suitable analysis to show that expected stresses will not exceed the strength of the concrete; (3) Adequate insulation should be used in conjunction with the usual formwork material to reduce the temperature differentials during the early age hydration of massive concrete; (4) A safety factor should be applied to the tensile strength values for concrete to guard against the initiation of micro-cracks; and (5) The current restrictions on maximum temperature imposed by state regulating bodies should take into consideration the type of cementitious materials that will be used in the concrete mix.

Brown. The great strength and ductility of drilled shaft foundations can be used to advantage on bridge projects in circumstances that favor their construction. However, drilled shaft construction methods contain risks of construction difficulties that must be addressed in the selection and design of drilled shafts. Some aspects of design and specification of drilled shaft foundations that can lead to constructability problems are described, and practical solutions that may satisfy the structural design requirements and lead to improved economy and reliability are offered. Many constructability problems relate to concrete specifications, congested reinforcement, splices, and construction of the column–shaft connection. The extent of the potential issues and the range of effective solutions often are dictated by the construction methods needed to accommodate the specific ground conditions at the site.

Mullins, Winters, Johnson. Drilled shafts are large diameter cast in place concrete foundation elements that until recently were not viewed with the same scrutiny as other massive concrete elements when considering mass concrete aspects. This study addressed three aspects of temperature generation in drilled shafts: 1. construction of drilled shaft with a full length centroidal void to minimize internal heat, 2. thermal integrity measurements of drilled shafts, and 3. field measurements of mass concrete elements taken to corroborate model findings with the intent of refining the definition of detrimental mass concrete conditions.

Bentz. The goal of long lasting concrete for critical infrastructure applications can only be achieved when early-age cracking is avoided. This includes nuclear facilities, including waste processing, containment and storage facilities and power plant facilities. Consequently, this topic is crucial to the mission of the Cementitious Barriers Partnership (CBP). Since most concrete is cast in place, field conditions, including environmental and workmanship parameters, can significantly influence early-age cracking tendencies. Beyond this, two inherent contributions to early-age cracking are thermal and autogenous deformations. In this chapter, these latter two contributions are reviewed from the three perspectives of basic mechanisms, relevant material properties, and successful mitigation strategies for portland cement-based concrete. Cementitious waste forms have unique chemistry and will need to be considered on a case by case basis. For thermal deformations, key considerations are hydration rates and the thermophysical properties of the cement paste or concrete. The heat of hydration of the binder sets the limit on the ultimate possible temperature rise of the concrete. Equally important to this ultimate heat of hydration is the hydration rate that governs when and how fast this heat is produced within a cement paste or concrete element. Thermophysical properties of relevance include heat capacity, thermal conductivity, and coefficient of thermal expansion. Methods for measuring these properties are discussed and representative data presented.
Autogenous deformations are driven by the volumetric chemical shrinkage that accompanies the reactions of cementitious binders. Under non-saturated conditions, this chemical shrinkage leads to self-desiccation and the creation of internal stresses and strains. Autogenous shrinkage is generally increased in lower waterto-cementitious materials ratio (w/cm) systems and in systems that contain fine supplementary cementitious materials such as silica fume and slag. Measurement of internal relative humidity provides a convenient method for onsite monitoring of the self-desiccation process. A wide variety of mitigation strategies have been successfully employed to mitigate thermal and autogenous contributions to early-age cracking. Modifications to the mixture proportions such as an increase in w/cm ratio, the utilization of a coarser cement, or a partial replacement of cement with a coarse limestone powder can effectively reduce both the maximum temperature rise and the autogenous shrinkage experienced by a concrete mixture. Two other well-developed mitigation strategies, specifically for reducing autogenous shrinkage, are the utilization of shrinkage-reducing admixtures and the application of internal curing, using pre-wetted lightweight aggregates for example. Both of these have progressed from laboratory evaluation to field applications in recent years and their ability to reduce plastic shrinkage cracking (as well as early-age cracking after set) has been recently documented.

Yao, Bittner. In construction of drilled shafts under water, placing concrete in the shafts is technically demanding and involves complex construction logistics. Past construction experience has demonstrated that high quality concrete can be placed in drilled shafts under water with a proper concrete mix and proper placement techniques. However, a significant number of failures have occurred which have resulted in excessive cost overruns and delays. These problems may have occurred because proper underwater concrete construction techniques have not been widely disseminated within the industry. This is a technical area where competent design and sound construction planning can achieve a significant reduction in both risk and cost. This paper will discuss some key technical issues in the concrete mix design, concrete production and placement for the drilled shaft construction. The paper also describes two lesson-learned case histories from drilled shaft construction projects.

Mullins, Kranc. Drilled shaft foundations have historically been plagued with construction related defects (i.e., soil inclusions or poor concrete cover) that most often go undetected due to the blind construction techniques necessary. These defects can affect the quality of the bond interface between the shaft concrete and the insitu geomaterial as well as the durability of the reinforcement. This project investigated the merits of a new easy-to-use method of assuring shaft integrity via infrared thermal detection methods that can assess the presence or absence of intact concrete both outside and inside the shaft reinforcement cage. The objectives included (1) field temperature measurements of mass concrete structures with specific interest in drilled shafts and (2) numerical modeling to verify the anticipated temperature response within a drilled shaft or mass concrete structure. Both small scale and full scale shafts were constructed and monitored during the curing/hydration phase. The temperature measurements obtained were used to verify the presence of anomalies while also obtaining temperature dissipation information to the surrounding materials. These case studies were used to calibrate a 3-D thermal modeling software developed expressly for mass concrete conditions with emphasis on drilled shaft integrity evaluation. Further evaluation of these sites along with case studies from larger diameter shafts showed that mass concrete conditions could be experienced by shafts even when small in diameter. Using the new thermal modeling software, the physical dimension of the test shafts as well as the location of known anomalies were inputted and signal matched against the measured temperature traces. The good correlation obtained showed that forward modeling shafts could be used to predict the size and location of anomalies. Inverse modeling algorithms were also developed with encouraging results. Finally, prospective construction techniques using voided shafts were conceived, modeled, and evaluated for feasibility. Initial results indicate that benefits could be derived from both mass concrete temperature control and reduction in concrete volume/usage.

Johnson, Mullins, Winter. As the concrete in drilled shafts cures, extremely high internal temperatures can be generated (even when relatively small in diameter) giving rise to mass concrete conditions. When the concrete temperature exceeds safe limits, the concrete may not cure correctly and can ultimately degrade via delayed ettringite formation. Minimizing the peak temperature (and the associated defects) can be undertaken by casting the shafts without concrete in the core thereby removing a large amount of the energy producing material in a region that is least likely to benefit the structural capacity and that is less able to dissipate the associated core temperatures due to the presence of the more peripheral concrete. This paper presents the field temperature results of shafts cast under normal conditions and the surprisingly alarming findings. A numerical model was subsequently calibrated and used to show the tremendous potential for voiding shafts. Finally, construction issues that are likely to be encountered are summarized with possible solutions.

Brown, Schindler. Drilled shaft foundations on major bridge projects often require the use of underwater placement of large volumes of concrete through densely placed rebar, with the cage often made even more congested with the placement of additional tubes for post-construction integrity testing. Such conditions represent the most difficult circumstances for constructors in terms of concrete placement to achieve a shaft which is free of defects or anomalies in the integrity testing. The concept of "high performance" for drilled shaft concrete is that the mixture should address the most critical performance requirements related to workability. This paper describes the important considerations in concrete mixture design for drilled shaft construction including the use of admixtures and mixture components associated with self-consolidating concrete (SCC). Some case histories of difficulties associated with concrete are also described.

Elias (thesis). This report describes the use of two models, ABAQUS and Schmidt, for predicting the peak temperature in the center of cast-in-place concrete piling. Five concrete piles with varying diameters and comprised of different concrete mixes were used. The temperature profiles predicted using the finite element model ABAQUS were compared to the temperatures predicted by the step-by-step method ACI 207 - Schmidt model. The results indicated that the ABAQUS model correlated better to experimental results and yielded better predictions of the temperature profiles than the Schmidt model. Using the results from the ABAQUS Model, a specification was recommended that would result in acceptable concrete for large cast-in-drilled hole (CIDH) concrete piles..

Liwu, Min. Thermal cracks that usually occur in mass concrete are closely related to the thermal behavior of cement matrix, such as heat liberation, temperature rise, and thermal shrinkage. Cement pastes added with large-volume mineral admixtures that are usually used for thermal controlling were cast into well-sealed plastic cylinders and covered by heat insulation materials to simulate the pseudo-adiabatic condition of mass concrete. The deformation and temperature rise of cement specimens under the heat insulation condition were examined at early hydration age. Results show that with addition of fly ash, coal gangue, and blast furnace slag, the heat liberation and peak temperature of cement paste decrease, while its total shrinkage increases. There is no shrinkage but expansion of the pastes during the temperature rise process, which may be ascribed to the complete compensation of the shrinkage by thermal dilation of the pastes. The thermal dilation coefficient (TDC) of cement paste changes drastically with the hydration duration, and it is also related to the addition of mineral admixtures.

Jalinoosa, Haramyb. Continuous temperature monitoring of two drilled shaft foundations was performed for a period of seven days using geophysical temperature logging method. Such temperature monitoring studies are important for mitigating thermal cracking and durability
problems of concrete. From this study, it appears that during primary curing (about first seven days), the curing strength of the concrete in a drilled shaft is a complex function of time, position in shaft’s profile (center is warmer than the sides), the physical properties
of the surrounding formations, and the depth of the groundwater table. Specifically, two parameters from the soil/rock profile that is noteworthy: thermal conductivity and permeability. Thermal conductivity controls relative changes in temperature and
permeability controls small relative changes in the moisture content. These parameters in turn control curing age of concrete as it relates to the relative strength.