Super-High-Strength High Performance Concrete
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- Super-High-Strength High Performance Concrete - CRC Press Book.
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Indian standard recommended methods of mix design denotes the boundary at 35 Mpa between normal strength and high strength concrete. High strength concrete is necessary for the construction of high rise building and long span bridges. It should be remembered that high cement content may liberate large heat of hydration causing rise in temperature which may affect setting and may result in excessive shrinkage.
High Strength Concrete 5. Concrete undergoes plastic shrinkage.
Mixing water creates continuous capillary channels and bleeding, reducing strength of concrete. Revibration removes all these defects and increases the strength of concrete. Concrete cubes made this way have yielded strength up to MPa. The sulphur infiltrated concrete has given strength up to 58MPa. High Strength Concrete 9. When the structure is loaded the micro cracks open up and propagate. The weakness can be removed by inclusion of small, closely spaced and uniformly dispersed fibres in concrete.
High Strength Concrete High performance concrete is an engineered concrete obtained through a careful selection and proportioning of its constituents. The concrete is with the same basic ingredients but has a totally different microstructure than ordinary concrete. In such case concrete failure can start to develop within the coarse aggregate itself.
Several studies have focused on determining whether a specified compressive strength of UHPC can be attained at 28 days under normal moist curing without heat treatment [ 1 , 3 , 7 ]. In some cases, however, the specified strength needs to be obtained within an earlier age of UHPC to accelerate construction speed. Therefore, this study presents experimental results on the characteristics of the early-age strength development of UHPC in various curing conditions conceivable at the site.
Regarding the terminology for the early age, there is not a clear definition of how short the early age of concrete is.
However, 7-day compressive strength, which was determined as the focus of this study by referring to the days required for standard steam curing in plant production, can be regarded as the early-age strength in comparison to day strength that is usually adopted for the design purposes. Factors considered in the experimental program include curing temperature, delay time before the initiation of curing, duration of curing, and moisture condition. The strengths were compared with those of the specimens cured by standard high-temperature steam.
Through the analysis of the test results, several requirements for curing are proposed that are required when the specified strength of UHPC should be attained in an early age even though it is cast in-place. For the mix design of UHPC, the superior mechanical properties need to be considered, such as strength, ductility, and toughness, in addition to the high flowability of fresh concrete and durability. Therefore, various mix proportions of UHPC have been proposed, so far, depending on the target properties. The mixture consists of cement, silica fume, filling powder, fine aggregate, shrinkage reducing agent, expansive agent, superplasticizer, and steel fibers.
Coarse aggregates are not included in the mixture.
The cement in Table 1 is Ordinary Portland cement. The glycol-based shrinkage reducing agent and calcium sulfa aluminate-based expansive agent are added to cope with shrinkage of concrete; in particular, the autogenous shrinkage that is induced by self-desiccation when the water-to-binder ratio is adjusted to be very low to attain high strength. Also, polycarboxylic acid-based superplasticizer is used to ensure high flowability even with a very low water-to-binder ratio.
High Performance Concrete
The steel fibers have the diameter of 0. The length of the fibers can be chosen among 13, 16 and 20 mm depending on the required tensile characteristics. The binder in the water-to-binder ratio in Table 1 indicates cement plus silica fume. The specified compressive strength, cracking strength, and tensile strength of K-UHPC are as high as , 9. In order to ensure these target strengths, initial curing and high-temperature steam curing are recommended in sequence [ 4 ].
High Strength Concrete
It was confirmed that strengths that exceed the specified strengths can be obtained even at an early age immediately after curing if the above curing criteria are fulfilled. These criteria were introduced by taking into consideration the curing of precast members. However, when the K-UHPC is cast in-place, the criteria are barely met in many cases, as a result of the difficulty in controlling the temperature and moisture due to the limited circumstances of the site. This study focuses on the minimum curing conditions of cast-in-place K-UHPC in circumstances where standard steam curing is not available on site that are required to ensure a similar target strength to that of precast K-UHPC at an early age.
Other properties of K-UHPC, such as shrinkage behavior, including the dominant autogenous shrinkage, were investigated in the previous studies [ 9 ]. Since the concept of concrete maturity was established by Carino et al. This is the reason why the high-temperature steam curing is preferred in a precast concrete plant. In UHPC, however, an additional closer relationship exists between curing temperature and strength than that in normal concrete, because most UHPC includes a large amount of silica fume due to the various advantageous characteristics [ 2 , 11 , 12 ], such as a considerable strength increase.
Silica fume is transformed to calcium silicate hydrate by reacting with calcium hydroxide through the pozzolanic reaction. This type of reaction tends to be substantially activated under a high temperature [ 2 , 11 ], which is why it is recommended for most UHPC to be cured under a high temperature to ensure rapid strength development. On the other hand, the moisture condition of UHPC containing silica fume should be given special attention in order to cope with the dominant self-desiccation [ 1 , 11 ]. The Silica Fume Association recommends moist curing of the concrete containing the silica fume for at least 7 days [ 12 ].
Based on these previous studies, the control of curing temperature and moisture condition would have a crucial effect on the strength development of cast-in-place UHPC. However, it is usually difficult to apply an ideal curing scheme in terms of temperature and moisture when the UHPC is cast on site, because the construction site has an inferior condition to a laboratory or a precast concrete plant; a realistic curing scheme should, thus, be devised on site. Some researchers have focused on determining whether the UHPC can attain the specified compressive strength at 28 days when subjected to ambient or room temperature and sufficient moisture for a certain period [ 1 , 3 ].
However, sometimes the specified strength needs to be ensured within a shorter period in order to advance the completion date of the structure, even under inferior site conditions, as investigated in this study. Ishii et al. Koh et al. Ahlborn et al. Additionally, they studied the effect of delay time before the steam curing and concluded that a delay time of even as long as 10 or 24 days did not significantly affect the strength after the steam curing.
Schachinger et al. They showed that the gradual increase of the degree of hydration of silica fume in the specimen cured at a relatively low temperature delayed the specified strength development by as much as several years. Nakayama et al. Matsubara et al. Honma et al. The difference of strengths at each curing temperature was significant at 7 and 28 days, but became less significant at 91 days. The chemical mechanism and microstructure during curing and hydration of concrete are presented in some references [ 18 , 19 ].
As discussed previously, the quality of UHPC is largely affected by curing conditions, such as curing temperature and moisture condition, etc. However, preparing the steam curing system on a construction site which is necessary for ensuring rapid strength development would be uneconomical and involve some difficulties due to its temporary use during curing and the required movability along the casting place of concrete.
Therefore, it would be very important to determine an efficient curing method for cast-in-place UHPC by taking into account the site condition, construction period, economy, and required strength of UHPC. In this study, the test variables are determined by relaxing the conditions of the prototype curing method of K-UHPC [ 4 , 13 ], which has been deonstrated to be sufficient for ensuring the specified compressive strength of MPa immediately after curing.
The moisture conditions during curing are categorized into four types. The enclosed or sealed condition is realized by tightly wrapping the specimen with polyethylene sheet to ensure that the internal moisture does not evaporate. A dry condition is provided by a dry heating chamber, while a constant temperature and humidity chamber as shown in Figure 3 is used to apply the water or steam condition. The curing time is evaluated based only on the period of constant temperature. Although moisture is continuously supplied during the initial curing period with the standard curing method of K-UHPC, considering any adverse site situation, it is assumed in this study that the specimen is subjected to a dry condition during the initial curing, regardless of the form removal conducted at 12 h after casting.
Therefore, the initial curing period is also called the delay time in this study because the curing is not actually performed during this period. In the enclosed condition, however, the specimen is sealed immediately after the form is removed, as can be expected on site. Furthermore, while other moisture conditions last only as long as the curing time, the enclosed condition is maintained until the strength is measured at 7 days since this situation can be easily applied on site.
Because the purpose of this study is to examine how closely the strength attains the specified compressive strength within an early age, 7-day compressive strengths are measured according to the standard test method [ 4 , 20 ]. The average compressive strength is calculated by averaging the strengths of the three specimens for each test variable. The shape of the specimen is a cylinder with mm diameter and mm height according to the relevant specifications [ 4 , 21 ]. As will be shown in the comparison provided in the later part, these are the most widely-used dimensions, as far as the cylindrical shape is concerned.
Although a specimen with different size was used in some previous studies, the size did not exceed mm in diameter and mm in height at most. It can be sufficiently assumed that the internal temperature of these small-sized specimens used in practice, whether it is a cylinder or a cube, is uniformly distributed according to the ambient curing temperature; and, thus, the effect of the shape and size of a specimen on the temperature distribution and related strength development is negligible. The test specimens were prepared by following the related specifications [ 4 , 21 ] in terms of placing, consolidation, finishing, and ensuring plane ends.
The test variables of this curing test basically include four cases of curing temperature, three cases of delay time before the initiation of main curing, three cases of main curing time and four cases of moisture condition. These are summarized in Table 2 and Figure 4 with explanations for several abbreviations. For example, T-M indicates the cases that are cured for 48 h starting from 24 h after casting, with all the curing times and moisture conditions included. The test results, as affected by various curing temperatures, with other conditions remaining the same as those of the standard steam curing, are presented in Table 3.
Figure 5 shows the characteristics of strength development according to curing temperature and moisture condition with other conditions remaining the same. The compressive strength was proportional to the curing temperature, regardless of the moisture condition. Overall, the enclosed condition resulted in fairly good strength development when compared with other moisture conditions, especially at lower temperatures, although only passive measures were taken to prevent the evaporation of water in concrete.
As mentioned previously, the enclosed condition was maintained until the strength was measured at 7 days, so that the remaining water in the concrete could be used for hydration and strength development. However, other moisture conditions were maintained only during the curing time, which means the specimens were exposed to a dry environment during the remaining time before strength measurement.