Fire behavior of alkali-activated material (AAM) systems with applications in the retrofitting of masonry walls and the construction of new AAM-based reinforced concrete elements
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Date
2023-10-20
Authors
Arce Campos, Carlos Andres
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Abstract
As climate change effects become more severe the importance of taking actions to reduce it becomes clearer. The construction sector is a big player in the fight against climate change sue to the high CO2 contributions of around 1.28 billion a year [1]. The production of Portland cement alone contributes to 5 % - 8 % of total global emissions [2]. Alkali-activated materials represent an alternative to the use of Portland cement due to the lower carbon emissions attributed to their productions and the value of reutilization of by-products that otherwise would end up in landfills. This research is focused on identifying and characterizing the mechanical and fire behavior of an alkali-activated material mix design for civil engineering applications. This thesis shows the reader the journey to develop an alkali-activated mix starting at the paste level, passing through the mortar design phase and arriving at the concrete design phase. The mix designs were later tested for structural applications and the main conclusions reported.
The first Chapter introduces alkali-activated materials, their role as alternative binders is described and their benefits highlighted. This Chapter gives the reader an overview of this research document, the methodology used and the objectives of this research. In Chapter 2 an introduction to the effect of fire in structures is provided. This Chapter also contains a literature review of alkali-activated materials exposed to fire. Chapter 3, 4 and 5 describe the methodology and results associated with the production of an optimal paste, mortar and concrete. The mix designs produced corresponded to the optimal formulations based on alkali-activated material and with the goal to enhance mechanical properties and fire behavior. Chapter 6 describes the testing procedure and results of an experimental campaign where 18 historical masonry walls were reinforced with textile reinforced alkali-activated mortar, exposed to fire and after tested in diagonal compression to determine residual shear strength. Chapter 7 tests the residual post fired behavior of two concrete slabs corresponding to high strength concrete and an alkali-activated concrete of equivalent strength. Two control specimen results are also provided and a comparison of results of the four specimens is reported. Chapter eight summarizes the conclusions of the project and recommendations for future research.
This project was successful in fine tuning mix designs to produce alkali-activated materials with fire resistant capabilities and the verification of these mixes in medium and large-scale structural members tests. Key conclusions obtained from this project are provided below:
Mixture design of experiment was successfully used to determine the optimal balance of ferronickel slag, silica fume, potassium hydroxide and potassium silicate. The paste produced was characterized by compressive and flexural strengths of 80 MPa and 6.8 MPa, respectively. The post heat exposure (to 900 °C for 2h) compressive strength dropped by 80% and the flexural strength was reduced almost by 100%. It was noted that low post heat exposure properties were related to cracking damage, which was expected to be much lower after the inclusion of fine aggregate in the mortar phase. Carbon emissions savings when compared to ordinary Portland cement (OPC) paste were found to be about 90% when the ferronickel slag was considered a waste material and 55 % when it was taken as a by-product.
The optimal combination of water sand and the previously developed optimal binder was studied using the same methodology. The results indicated a much higher retention of mechanical properties after adding fine aggregate. Compressive strength reduction (after exposure to 900 °C for 2h) was 75%, while a 25% increase of flexural strength was observed. It is important to mention that his mortar was optimized for flexural strength to achieve a better bond behavior as it was intended as a basis for textile reinforced mortar (TRM) applications. CO2 savings corresponded to 77% compared to OPC-based similar products.
Optimization of aggregate sizes for the concrete recipe was carried out and the optimal mix resulted in 38% of post heat exposure (600 °C for 2h) compressive strength. The CO2 savings corresponded to 77% compared to a similar product based on OPC.
The optimal mortar recipe was tested for application in strengthening applications of masonry walls and the post fire behavior was studied. An average increase of about 255% in peak in-plane shear strength in the case of one layer, and 330% in the case of carbon textile alkali activated mortar reinforcement was observed. Peak shear strength remained high even after exposure to 550 °C. The residual strength was found to increase at least 200% and 290% for the one- and two-layer systems at both temperature levels.
Four-point bending tests executed on four concrete slabs, two of alkali-activated concrete and two of high strength concrete (one for control and one for post heat exposure residual strength), indicated a similar load bearing capacity of all configurations. While the ultimate load remained similar, the deformation behavior had a higher variation in the Portland cement slabs. A pseudo-ductility coefficient was calculated and the values indicated a 0.1% drop of ductility for the alkali-activated concrete slab, while a 34% drop in deformation capacity was observed in the high strength concrete slab.
Overall, the produced alkali-activated material construction products showed a high reduction in CO2 emissions when compared to the Portland cement counterparts and also demonstrated a comparable or even better performance than typical existing products after exposure to high temperatures.
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Keywords
Alkali-activated material (AAM), Fire behavior, Design of experiment, Life cycle analysis