CHARACTERIZATION OF THE RHEOLOGICAL AND SWELLING PROPERTIES OF SYNTHETIC ALKALI SILICATE GELS IN ORDER TO PREDICT THEIR BEHAVIOR IN ASR DAMAGED CONCRETE
Open Access
- Author:
- Gholizadeh Vayghan, Asghar
- Graduate Program:
- Civil Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 28, 2017
- Committee Members:
- Farshad Rajabipour, Dissertation Advisor/Co-Advisor
Farshad Rajabipour, Committee Chair/Co-Chair
William D. Burgos, Committee Member
Carlo G. Pantano, Committee Member
James L. Rosenberger, Outside Member - Keywords:
- Alkali-silica reaction
Alkali-silicate gels
Swelling properties
Rheological properties - Abstract:
- Alkali–silica reaction (ASR) is a major concrete durability concern that is responsible for the deterioration of concrete infrastructure in the world. The resultant of the reaction between the cement alkali hydroxides and the metastable silicates in the aggregates is a hygroscopic and expansive alkali–silicate gel (referred to as ASR gel in this document). The swelling behavior of ASR gels determines the extent of damage to concrete structures and, as such, mitigation of ASR relies on understanding these gels and finding ways to prevent them either from formation, or from swelling after formation. This dissertation focuses on the synthesis and characterization of ASR gels with wide ranges of compositions similar to what has been reported for the filed ASR gels in the literature. The experimental work consisted of three phases as follow. Phase I: Investigation of rheology, chemistry and physics of ASR gels produced through sol–method. Inspired from the existing literature, two sol–gel methods have been developed for the synthesis of ASR gels. The rheological (primarily gelation time, yield stress, and equilibrium stress), chemical (pore solution pH, pore solution composition, osmotic pressure, solid phase composition, stoichiometry of gelation reactions) and physical (evaporable water, solid content, etc.) properties of synthetic ASR gels have been extensively investigated in this phase. Ca/Si, Na/Si and K/Si, and water content were considered as the main chemical composition variables. In order to investigate the suppressing effects of lithium on the swelling properties of ASR gels, the gels were added with lithium in a part of the experimental program. The results strongly suggested that Ca/Si has a positive effect on the yield stress of the gels and their rate of gelation. Na/Si was found to have a decreasing effect on the yield stress and gelation rate (especially at low Ca/Si levels). K/Si and Li/Si had second–order (i.e., polynomial) effects on the yield stress of the gels, causing a significant drop in this parameter followed by some increase as they approached their upper values. Na/Si and K/Si were both found contribute to the osmotic potential of the ASR gels, while increase in Ca/Si generally led to a drop in this parameter. The presence of all components (Ca, Na, and K) were found to contribute to the pH of the gels’ pore solution, and Ca/Si and Na/Si showed a synergistic effect on this parameter. Lithium, on the other hand, was found to be able to drop the OH- concentration of the pore solution by a factor of five in the case of high–sodium gels, which could partially explain its ASR mitigating effect. Phase II: Investigation of the free and restrained swelling behavior, hydrophilic potential and viscoelastic properties of ASR gels produced through the “paste method”. 20 gel compositions were selected (using the central composite design method) with Ca/Si, Na/Si and K/Si molar ratios varying in the ranges (0.05–0.5), (0.1–1.0) and (0.0–0.3), respectively. The gels were produced by batching appropriate amounts of certain precursors containing different chemical components. After curing, the gels were tested for the abovementioned parameters using some innovative test methods as explained in the relevant chapters. The results suggest that increasing the alkali content (Na/Si and K/Si) in ASR gels resulted in an increase in the gels’ free swelling and water absorption, and a reduction in the equilibrium relative humidity (ERH). However, no significant effect was found for Ca/Si with respect to the ERH. Ca/Si was found to have a multi–episode effect on the swelling and water absorption properties of the gels. An increase in Ca/Si up to 0.18 led to a considerable reduction in the swelling strain, followed by a slight increasing effect as it approached 0.4. Further increase in Ca/Si resulted in complete elimination of swelling strain. While Na/Si and K/Si could constantly increase the free swelling strain, their excessive presence was found to have a softening effect on the gels’ structure, leading to a drop in their swelling pressure. Finally, all gels were found to show viscoelastic behavior that could be best explained via Burger’s model. The elastic and viscous components have been measured for each gel and related to their composition using regression. Phase III: An Extended Chemical Index Model to Predict the Fly Ash Dosage Necessary for Mitigating Alkali-Silica Reaction in Concrete. In order to have an applied and ready–to–implement contribution to the realm of alkali–silica reaction, a predictive statistical model was developed that determines the optimum fly ash dosage for ASR mitigation depending on the acceptable risk of ASR and structure’s importance. The model uses the oxide compositions of portland cement and fly ash, as well as the reactivity of the aggregates. Seventy-six experimental data points (published in the last two decades across the U.S. and Canada) on CPT expansion results for plain portland cement and fly ash-blended concrete mixtures were used to develop and evaluate the model. The model was found to be capable of predicting CPT expansion of concrete incorporating both class F and class C fly ash and reactive aggregates within ASTM C1293 precision criteria. It allows for the use of a wider range of fly ashes compared to what is currently being prescribed by the standards for use in concrete susceptible to ASR.