This makes the 16 character, letters-only password (91 bits) 8 million times harder to guess than the 12-character (68 bits) one, while the 12-character password with numbers (71 bits) is only eight times harder to crack than the letters-only one.
A four-word passphrase (56 bits) is strong enough for the password that you use to log into 1Password because we hash it well. We estimate that it would cost an attacker about $76 million USD to crack that.
matlab 32 bit crack 21
Paintsan image onto a high-relief plaster surface, producing a fine networkof cracks that follow the contours of the image. Use this filterto create an embossing effect with images that contain a broad rangeof color or grayscale values.
Long-term concrete cracking is unavoidable, and large openings impact concrete durability [4,5]. In hot and dry areas of the world, high air temperature, wind, and low relative humidity are also known to impact durability [6], as they can cause high plastic and drying shrinkage strains in concrete [7,8,9]. ACI 224R-01 [10] attributes early-age concrete cracks to excessive evaporation due to environmental conditions prior to concrete setting. The earlier concrete cracks develop, the shorter the serviceable life of concrete is expected [11,12]. Plastic shrinkage cracks are the earliest to appear, as they occur two-three hours after casting, prior to setting. Subsequent propagation of plastic shrinkage cracks will allow ingress of water and offensive agents such as chlorides and increase the possibility of concrete deterioration and corrosion of steel rebars [13,14]. Plastic shrinkage cracks not only reduce concrete durability but are also aesthetically undesirable [15].
Volume changes in concrete before the hardening of cement-based materials are the main cause of plastic shrinkage strain and cracking [16,17]. Volume loss at the plastic stage is caused by the consolidation of aggregates, bleeding, and evaporation of water. In its plastic state, when undisturbed, the denser solid particles settle and tend to sink down, whilst the lighter-weight materials, such as air and free water, begin to rise to the surface. Air escapes faster, but the escaping water, called bleeding water, escapes slower, and when it reaches the surface, it starts evaporating [18]. When the evaporation rate exceeds the bleeding rate, the concrete surface dries, and at this stage, the possibility of plastic shrinkage cracking increases [19,20,21]. Both environmental conditions and concrete mix composition affect plastic shrinkage, as seen in Figure 1 [22].
Hot weather casting is known to increase plastic shrinkage cracking [23]. It is widely accepted that plastic shrinkage starts when the evaporation rate exceeds the bleeding rate. Several studies reported that environmental conditions such as high air temperature, high wind speed, and low relative humidity have a direct effect on fresh and hardened concrete, as they also accelerate the final set time [24,25].
Eventual drying of the surface leads to a rise in capillary pressure converting it from a mildly compressive to a tensile pressure [27]. When capillary pressure inside the concrete builds up, plastic shrinkage cracking will occur.
To examine the possibility of plastic shrinkage cracking in concrete, ASTM C1579 [28] recommends a set of environmental conditions to be applied: air temperature 36 3 C, wind speed more than 4.7 m/s, and relative humidity around 30 10%. These environmental conditions were selected based on past experimental work [28]. However, Al-Gahtani et al. [30], working in the eastern part of Saudi Arabia, known for high temperature and humidity, found that concrete is more likely to crack with and without the environmental conditions proposed by [28,31].
Nabil et al. [32] examined substrate bases of concrete (50 95 365 mm) for plastic shrinkage cracking in an environmental chamber by covering concrete with plastic sheets. The concrete mixes were exposed to a temperature of 55 C during the first 8 h after casting and 50 C until the end of the test (24 h). The relative humidity (RH) was about 10%, and the wind speed was 10 km/h during the duration of the test. As expected, it was found that covering concrete with plastic sheets was more efficient in minimizing plastic shrinkage cracking and reducing loss of water compared with non-covering. Almutairi et al. [33] did a survey to determine the causes of all early-age cracking in concrete structures in Kuwait city and concluded that the environmental conditions were the main reason for most the concrete cracking, but also high concrete temperature. It was recommended to prevent early-age cracking. The concrete temperature should be controlled by adding ice to the mixing water.
Almusallam et al. [29] and Safiuddin et al. [34] found that plastic shrinkage cracks increase with an increase in the water/cement ratio and content of fine aggregate. Sayahi and Hedlund et al. [26] reported that micro-settlement cracks also occur on the surface of the concrete. Sulakshna et al. [35] examined a Poly Carboxylate Ether (PCE) as shrinkage reducing admixture (SRA) to self-compacting concrete of w/c ratio of about 0.45, with encouraging results.
Zhang and Xiao [36] investigated the effect of recycled sand as fine aggregate for 3D-printed mortar on plastic shrinkage cracks. The replacement ratios tested were at 25%, 50%, 75%, and 100% of natural sand, and we had to use high w/c (0.6) due to the high-water absorption of the recycled sand. The results showed that increased replacement ratios of recycled sand mortar resulted in increased plastic shrinkage cracking. Cohen et al. [37] found that the increase in fine content in concrete (such as fly ash, silica fume, slag, etc.) is not favorable in relation to micro and plastic shrinkage cracking. Lofgren and Esping et al. [38] came to the same conclusion when using silica fume. Zhao et al. [39] examined the influence of clay minerals in manufactured sand and found that as clay lowers the permeability, it also reduces the plastic tensile strength, which leads to an increase in plastic shrinkage cracking.
The capital of Saudi Arabia, Riyadh city, is located in the middle of Saudi Arabia. Temperatures are high in the summer, and the relative humidity is very low in both winter and summer (see Figure 2b). During the months of June to September, the air temperature, relative humidity, and wind speed are at the levels that increase the possibility of plastic shrinkage cracks, as anticipated by ASTM C1579 [28], and concreting at temperatures around 45 C is not uncommon. This (high) temperature level will be examined in this study, as well as the effect of a lower average wind speed of 3 m/s.
Hu et al. [56] showed that RTSF could improve the splitting and flexural strength of concrete and result in comparable (or better) performance to MSF. Baricevic et al. [57] examined the effect of using blends of recycled tire steel and manufactured steel fibers, and the results showed a positive impact in delaying the development of drying shrinkage cracks. Graeff et al. [14] examined the performance of RTSF concrete under cyclic loading and showed that the addition of RTSF can improve the fatigue behavior of concrete and help restrain micro-cracks.
Figure 12, Figure 13 and Figure 14 show the evolution of the cracks for the three variable parameters: temperature, wind speed, and w/c ratio, respectively. The graphs also show the crack widths at 24 h measured both with DIP and a conventional manual optical device. The crack widths obtained for the two methods of measurement are almost identical, confirming that DIP works well. The mean difference between the two measurements was 0.113 mm.
Figure 12 shows that with a temperature increase, cracking starts earlier, and the eventual crack width is wider by almost 100% when comparing Low and High temperatures. The use of 30 kg/m3 of RTSF seems to prevent cracking completely for the Low and Medium temperatures. However, though the addition of RTSF delays and helps control the crack, cracking still develops at High temperatures. For the more extreme temperature tested in this study, the increase in fiber dosage to 40 kg/m3 seems to prevent plastic cracking.
Similarly, to what was observed for increasing temperature values, increasing wind speed causes earlier cracking to develop and eventually leads to larger crack widths. Interestingly, both the Low wind speed and Low-temperature environments, which are below the ASTM C1579 [28] recommendations, led to cracks (see Figure 13).
Again, the use of RTSF at a dosage of 30 kg/m3 prevented cracking in the specimens subjected to the Low and Medium wind speeds only, whilst 40 kg/m3 of RTSF was effective in preventing cracking at High wind speeds.
Effect of (a) temperature; (b) wind speed; and (c) w/c ratio on evaporation rate at crack initiation and time of crack initiation. * higher content of RTSF (40 kg/m3).
As expected, for all mixes, cracking starts earlier, and the evaporation rate is higher with increasing temperature, wind, and w/c ratio, and the final cracks are wider. The plain mid-temperature concrete cracked just before the second hour, as expected by [28].
Cracks develop even at the low temperature of 28 C but crack initiation is delayed by about an hour when compared to specimens exposed to the highest examined temperature. Though surface temperature cannot be changed easily, to reduce plastic shrinkage cracking, the concrete temperature can be cooled down by using cooler materials, including iced water.
At low wind speeds (3 m/s), the evaporation rate reduces but the plain concrete cracks only a bit later and a bit less wide. Hence, at the ranges used, wind speed seems to play a less important role than temperature.
In comparison, Borg et al. [81] examined the use of recycled PET fibers (polyethylene terephthalate) to control plastic shrinkage cracks and found that 1.5% of fiber (by volume) could reduce cracking by up to 57%. Pešić et al. [82] examined the effect of adding recycled high-density polyethylene (HDPE) plastic fibers and found that early-age concrete cracks can be reduced by up to 50% with a fiber volume of 1.25%. Hence, steel fibers appear to be a more robust solution for stopping plastic shrinkage cracks. 2ff7e9595c
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