الفهرس 
LITERATURE REVIEWOnly 14 pages are availabe for public view
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Abstract
This research work was carried out to investigate physical, mechanical and bond characteristics of Alkali Activated Slag Concrete (AASC) and compare them with those of similar Conventional Concrete (CC) at ambient temperature and after exposure to elevated temperatures. As mentioned earlier, this work includes three main phases:
The first phase studied the efficiency of utilizing the hybrid cement (Ordinary Portland Cement “OPC” + Ground Granulated Blast Furnace Slag “GGBFS”) to produce Alkali Activated Concrete (AAC) at ambient curing conditions considering the effect of four important parameters (GGBFS to OPC ratio, Na2O ratio, solution modulus “Ms” and water to binder ratio “W/B”) on both workability and compressive strength using Taguchi method. Microstructure analysis using Scanning Electron Microscope (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) was performed to investigate the polymerization products.
The second phase investigated mechanical and physical characteristics of ambient cured AASC after exposure to elevated temperatures. The effect of temperatures (25, 300, 600 and 900 °C) on the ambient cured AASC and on a similar CC (for a comparison purpose) was investigated by observing the physical and mechanical changes. The effect of using polypropylene fibers (PPF) on both AASC and CC was also investigated.
The third phase was designed to investigate the bond behavior between AASC and steel rebars considering some important parameters (rebar diameter and development length to diameter ratio) before and after exposure to elevated temperatures using beam-end bond testing technique. An analytical study was carried out to compare the experimentally obtained results with those obtained from the well-known available equations in the literature and also in the CEB-FIP model code for concrete structures. At the end of this phase, a modified model was proposed to predict the bond behavior of AASC. Also, a comparison between the proposed modified model and the CEB-FIP model with the experimental results was performed.
7.2 Conclusions
Based on the obtained experimental and analytical results and the detailed discussion of these results, the following points can be concluded:
(a) Conclusions of Phase I (Characterization of AAC):
• Using alkali activator with hybrid cement (GGBFS + OPC) is not effective method to produce AAC because of the very low workability obtained. On the other hand, using GGBFS only as a binder material was effective enough to produce AAC with high compressive strength and suitable degree of workability at ambient curing conditions.
• The AAC mix (of 100% GGBFS, 9% Na2O, Ms of 1.0 and W/B ratio of 0.45) achieved the highest 28-day compressive strength the highest degree of workability (f28 was about 43.0 MPa and a slump value was about 230 mm) at ambient curing conditions.
• Among the studied parameters, the most significant parameter that affects the f28 and workability of the developed AAC mixes was the binder ratio (GGBFS:OPC) with participation percentages of 56.19% and 98.83% for f28 and workability, respectively. The highest f28 and slump values were achieved by the developed mixes with a binder of 100% GGBFS.
• The 90-day compressive strength (f90) of all mixes achieved around 2-5% increase compared to the f28 which indicates that the f28 can be considered as the characteristic compressive strength for AAC exactly as in the conventional concrete.
• The inclusion of OPC in mixes increased significantly the cracking level at the microscale; the hydration products of the developed mixes with 100% GGBFS were more homogenous than OPC mixes.
(b) Conclusions of Phase II (Performance of AASC Exposed to Elevated Temperatures):
• Significant spalling occurred in the CC at 900 °C while no spalling occurred in the AASC exposed to the same elevated temperature, minor surface cracks were observed only. This indicates a better resistance to spalling and cracking of AASC compared to CC after exposure to elevated temperatures.
• The unit weight of AASC was slightly higher than that of CC. The inclusion of PPF in both CC and AASC mixes decreased the unit weight of the hardened concrete. The unit weight loss of CC in-creased with elevated temperatures more than that of AASC espe-cially at 900 °C.
• The AASC achieved higher compressive strength (fc) and bond strength (τu) than those of CC for the same binder content. Where-as, the flexural strength (fru) of AASC was lower than that of CC at ambient temperature.
• The fru/fc ratio of AASC is 22.22% lower than that of CC at ambient temperature. For both CC and AASC, the inclusion of PPF decreases this ratio slightly. No significant effect was recorded on this ratio by exposure to the elevated temperatures.
• The AASC achieved ultrasonic pulse velocity (Vu) 11.38% lower than that of CC at ambient temperature.
• The inclusion of PPF in both CC and AASC reduces the values of fc, fru, τu and Vu, and this reduction increased by increasing the PPF content. The decrease of these values for the case of AASC is higher than that of CC for the same PPF content.
• The losses in fc, fru and τu values for CC increased with elevated temperatures more than that of AASC. On the contrary, the loss in Vu value for AASC increased with elevated temperature more than that of CC.
• According to fc, fru and τu values, the AASC mixes have a higher elevated temperature resistance than that of CC mixes. But, adding the PPF in AASC mixes makes this resistance lower than that of CC.
• The cracking patterns of AASC are similar to those of CC at the zone failure of bond strength.
(c) Conclusions of Phase III (Bond Behavior of AASC Exposed to Elevated Temperatures):
• Failure mode for all beam-end specimens was by splitting. There was no significant difference observed in the obtained cracking pattern or failure mode due to exposure to the elevated temperatures.
• The degradation of bond strength when exposure to elevated temperatures above 300 °C was significant. Generally, bond strength decreased and corresponding slippage increased due to the exposure to elevated temperatures.
• The bond strength decreased with the increase of the rebar diameter and increased slightly with the increase of the L/d ratio.
• The CEB-FIP model provides more conservative values of bond strength than the experimental results which increase the safety level when estimating bond strength for AASC in design purposes.
• The proposed modified model achieved a higher correlation with the experimental results than the CEB-FIP model at ambient temperature.
7.3 Recommendations for Further Studies
Although this research focused on understanding of the mechanical and bond characteristics of ambient cured AASC, the behavior of AASC is far away from completion. Additional researches are still needed; therefore, the following recommendations for future work are suggested:
• More studies and investigations related to mix design including mix proportions, mixing procedures and curing regime, with which the AASC could demonstrate the best engineering properties, are required.
• More investigations on the mechanical properties of AASC are needed to include the most of essential properties such as modulus of elasticity, stress-strain relationship and poisson’s ratio.
• Additional experimental, analytical and numerical studies are needed to investigate the bond behavior of AASC to obtain equations and models can predict the bond behavior of AASC at both ambient and elevated temperatures accurately.
• Further studies are needed to address the durability issues of AASC.
• More investigations on AASC structural elements at ambient temperature and after exposure to elevated temperature are needed.
• Additionally, AASC should be investigated for long-term shrinkage and creep characteristics.
• The microstructure of AASC must be well investigated for a well-understanding of the performance of AASC.