Zain 1,2 and Roszilah Hamid 1.
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- Autogenous Shrinkage of Concrete at Early Ages.
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The optimum particle grading increases the flowability of UHPC and eliminates entrapped air. This study presents a simplified particle grading design approach that positively influences the strength, autogenous shrinkage, and microstructure characteristics of UHPC. Carbon nanofibers CNFs of superior mechanical properties were added to enhance the strength of UHPC and to reduce its autogenous shrinkage.
Test results showed that the presence of homogeneously dispersed CNF increased the compressive strength and compensated the autogenous shrinkage of UHPC. The findings indicated that an ideal particle distribution, which is close to the modified Andreasen and Andersen grading model, contributed to achieving high compressive strength and CNFs were capable of providing nano-bridges to compensate the shrinkage caused by GGBS.
Introduction The recent developments in concrete engineering and technology have facilitated researchers around the world to synthesize ultra-high performance concrete UHPC with advanced engineering properties. UHPC is characterized by ultra-high strength and durability with or without fiber reinforcement and exhibits a day compressive strength of more than MPa [ 1 , 2 , 3 ]. However, concrete becomes less ductile when its strength is increased. Therefore, steel fibers or synthetic fibers are commonly used in UHPC to create ultra-high performance fiber-reinforced concrete UHPFC with ductile behavior and enhanced mechanical properties.
The consistency and quality control of the materials of UHPC are major concerns for its applications. The design of UHPC can be formed with eight to ten different ingredients. Due to the complexity of the concrete mix and the involvement of different material parameters, the properties of UHPC vary and it needs to be subjected to special mix design and curing regime.
FA and GGBS are among the most common supplementary cementitious materials which allow a high percentage of cement replacement. Several particle grading models have been discussed in literature [ 9 , 10 ], and the shape and angularity of every material differ. Lower material variability contributes to achieving a much precise result in terms of optimum grading based on the particle size distribution.
Yu et al. Fennis et al. However, UHPC shows an extremely high autogenous shrinkage, thereby leading to cracking at early ages. The early-age cracking due to restrained autogenous shrinkage affects and limits the progress of concrete construction [ 12 ]. In particular, the autogenous shrinkage strain observed at later ages, that is, 60—90 days, depends on the mix design [ 13 ].
FA decreases autogenous shrinkage in concrete. In contrast, the inclusion of GGBS increases autogenous shrinkage [ 8 ]. The rate and magnitude of autogenous shrinkage for all concretes increase with the increase in curing temperature.
The use of SF further increases the autogenous shrinkage as a result of its high surface area; the effect is critical in low water content concrete, which thereby undergoes a significantly decreasing internal relative humidity RH in cement paste during hardening. In addition, self-desiccation occurs in the absence of an external source of water [ 12 , 13 ].
Nanomaterials such as nano-silica, nano-calcium carbonate, graphite nanoplatelets, carbon nanotubes CNTs , and carbon nanofibers CNFs are used to enhance concrete properties due to their high surface area and fineness [ 1 , 15 , 16 ]. CNTs were first introduced by Iijima in and subsequently adopted in various industries to complement and improve the characteristics of different materials [ 17 ]. They are potential candidates as nano-reinforcement in concrete due to their nano-dimensional nature and good coverage ability in the cement matrix [ 16 ]. CNTs and CNFs are also challenged by their dispersion issue, which is usually addressed by using a surfactant [ 18 , 21 ].
Shimoda et al. The dispersion of CNFs greatly influences the concrete properties; the micrographs obtained from Scanning Electron Microscope SEM have revealed that the presence of individual nanofibers with a good dispersion in the matrix leads to better mechanical properties [ 23 ]. Well-dispersed CNFs prolong the post-tension cracking under bending [ 15 , 24 ]. Kim et al. The present study explores the simplified version of UHPC by optimizing the packing density in the matrix of concrete. GGBS was used as a pozzolanic supplementary cementitious material to increase the strength of concrete.
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After achieving the two best particle packing options, a new type of CNFs was incorporated to investigate the strength and shrinkage properties of the designated UHPCs under normal curing temperature. The results are summarized in Table 1 , along with their specific gravity and specific surface area. The grading particle size distribution curves of cement, GGBS, silica flour, and silica sand are shown in Figure 1. The selected new type of CNFs with large diameter and length was produced using catalytic chemical vapor deposition technique.
Due to a controlled chemical vapor deposition process, the resulting CNFs consisting of the nano-structural filaments in a herringbone structure have stronger and more stable characteristics [ 27 ]. Before using in concrete, CNFs were dispersed in distilled water through ultrasonication without surface modification and surfactant.
This table shows that silica sand and silica flour were used to produce a number of concrete mixes. The morphology of silica sand is shown in Figure 3 a; it was much coarser and flaky in shape. In contrast, silica flour was much finer and spherical in shape; its morphology is shown in Figure 3 b.
Both silica sand and silica flour were used in producing UHPC mixes because they together provide a better filler effect and improve the microstructure of concrete. The flow was measured for all concrete mixes and the test conducted for Mix 1 is shown in Figure 4. The cured cube specimens were tested at the ages of 1 day, 7 days, and 28 days to determine compressive strength in accordance with ASTM C [ 29 ].
The results reported herein are the average of three specimens. Over the years, many researchers studied the best packing of cement or binder cement plus GGBS and aggregates through optimum particle grading.
The combined grading curves of cement or binder and graded sand particles associated with optimum packing for different concrete mixes have been presented in Figure 5. Different types of concrete can be designed using different values of the distribution modulus, q. The optimum design of a high content of fine materials is required in UHPC [ 10 , 30 ]; the value of q was fixed at 0. In the UHPC mix design, the modified Andreasen and Andersen model Equation 1 plays the major role to optimize the composition of granular materials.
External drying was prevented by sealing the specimens immediately after casting using two layers of polyethylene sheets. The free end of the specimen had a linear variable displacement transducer attached to measure the autogenous deformation continuously. The data were recorded every 10 min. The average strain values were reported. This test was performed in accordance with the recommendations given by Wei et al. The entrapped air content of different concrete mixes was determined based on image analysis. For each concrete, the prism was cut to a thickness of 5 mm at the center section and oven-dried for 30 min.
The cross-sections of all specimens were analyzed using image enhancing and analysis software, ImageJ. All images were set as 8-bit and analyzed using binary grayscale analysis, which manually adjusts the threshold level to determine the optimum threshold range to differentiate the surface and depth.
In this analysis, the white background represents the concrete surface, whereas the dark background represents the air voids on the surface [ 31 ]. The sum of the dark portion represents the entrapped air content, which is calculated as a percentage of the total area. Carbon dioxide present in the atmosphere reacts in the presence of water with hydrated cement. Calcium hydroxide [Ca OH 2 ] gets converted to calcium carbonate and also some other cement compounds are decomposed.
Such a complete decomposition of calcium compound in hydrated cement is chemically possible even at the low pressure of carbon dioxide in normal atmosphere. Carbonation penetrates beyond the exposed surface of concrete very slowly. The rate of penetration of carbon dioxide depends also on the moisture content of the concrete and the relative humidity of the ambient medium.
Autogenous Shrinkage of Concrete at Early Ages
Carbonation is accompanied by an increase in weight of the concrete and by shrinkage. Carbonation shrinkage is probably caused by the dissolution of crystals of calcium hydroxide and deposition of calcium carbonate in its place. As the new product is less in volume than the product replaced, shrinkage takes place. Carbonation of concrete also results in increased strength and reduced permeability, possibly because water released by carbonation promotes the process of hydration and also calcium carbonate reduces the voids within the cement paste.
As the magnitude of carbonation shrinkage is very small when compared to long term drying shrinkage, this aspect is not of much significance. Other factors that have an effect on the magnitude of shrinkage include mix proportions, material properties, curing methods, environmental conditions, and geometry of the specimen. Water content has been found to affect the magnitude and rate of drying shrinkage in concrete. Other factors include elastic properties of the aggregate used in the concrete mix; in general, aggregate with a high elastic modulus will produce low shrinkage concrete.
Aggregates which contain clay minerals will affect shrinkage behavior as well. One of the most important factors that affects concrete shrinkage is the drying condition or in other words, the relative humidity of the atmosphere at which the concrete specimen is kept. If the concrete is placed in per cent relative humidity for any length of time, there will not be any shrinkage; instead there will be a slight swelling.
The typical relationship between shrinkage and time for which concrete is stored at different relative humidities is shown in Figure. The graph shows that the magnitude of shrinkage increases with time and also with the reduction of relative humidity. The rate of shrinkage decreases rapidly with time. It is observed that 14 to 34 per cent of the 20 year shrinkage occurs in 2 weeks, 40 to 80 per cent of the 20 year shrinkage occurs in 3 months and 66 to 85 per cent of the 20 year shrinkage occurs in one year.
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The richness of the concrete also has a significant influence on shrinkage. Aggregate plays an important role in the shrinkage properties of concrete.
Dimensional Stability - Shrinkage
The quantum of an aggregate, its size, and its modulus of elasticity influence the magnitude of drying shrinkage. Harder aggregate with higher modulus of elasticity like quartz shrinks much less than softer aggregates such as sandstone. Concrete shrinks when allowed to dry in air at a lower relative humidity and it swells when kept at per cent relative humidity or when placed in water. Just as drying shrinkage is an ever continuing process, swelling, when continuously placed in water is also an ever continuing process.
If a concrete sample subjected to drying condition, at some stage, is subjected to wetting condition, it starts swelling. It is interesting to note that all the initial drying shrinkage is not recovered even after prolonged storage in water which shows that the phenomenon of drying shrinkage is not a fully reversible one.
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