Reduced retardation

Designated type as given by principal and secondary functions (or the relevant standards), e.g. accelerating, water-reducing or water-reducing/retarding admixture (e.g., ASTM, Type D). 

The rate of slump loss of concrete containing a retarder is similar to that of the equivalent plain concrete mix. Where a water-reducing retarder is used to increase initial workability the time for which the concrete remains workable is extended. 

Retarders enable setting time to be extended thereby preventing the formation of cold joints in large pours. Additionally, water-reducing retarders allow a reduction in cement content with a consequent reduction in maximum temperature rise. 

The delay in setting time and higher initial workability obtainable through use of a water-reducing retarder compensate for the time lost in transit of ready-mixed concrete. 

FIGURE 5.14 The logarithm of the reduced retardation spectrum, Lp, shown as function of the logarithm of the reduced retardation time, A./uj , for the three urethane-end linked polybutadiene elastomers, (o) TB-1, ( ) TB-2, ( )TB-3. The response has been reduced to corresponding state temperatures of 74°C, 0°C, and 17°C, respectively, for the primary softening transition. 

FIGURE 5.15 Double logarithmic plot of the reduce shear creep compUance curves, Jp(t) (i n Pa ), as a function of the logarithm of the reduced retardation time, t/ap, for live elastomers at To = 0°C. Solid line (HTPB-2), long dashed Une (Viton), short dashed line (PB-2), dashed-dotted... 

Protecting the core material from degradation by redncing its reactivity to the outside environment (such as UV Ught, heat, moisture, air oxidation, chemical attack, acids, bases, etc.). Reducing/retarding the evaporation or transfer rate of a volatile active ingredient (the core material) to the outside environment. 

HRB. 1971. Admixtures in Concrete Accelerators, Air Entrainers, Water Reducers, Retarders, Pozzolans. Special Report 119. Washington, DC Highway Research Board. 

FIGURE 13 Logarithmic plot of the retardation spectra as functions of the logarithmic reduced retardation times, A/< t (in seconds). The reference temperature of reduction To is -20.0°C for all... 

Reduced retardation spectra, Lp, calculated from the creep compliance Jpit/ar) curves shown in Fig. 18, are presented logarithmically in Fig. 19 as a function of the logarithm of the reduced retardation time, ijax. The chosen reference temperatures bring the short time spectral contributions together, showing the similarity between the networks of different chain density. The... 

FIGURE 28 Comparison of the [Jp t) - /J curves as a function of the reduced retardation time, t/a-T, of fully cured Epon 828/DDS (O), 1001/DDS (0), 1004/DDS (A), and 1007/DDS (V). The shift factors and reference temperature for Epon 828/DDS are identical to the values used in Fig. 8. For the other three elastomers, horizontal shifts of -0.72, -1.18, and -1.5 have been applied to superimpose the data in the short time regime associated with segmental relaxation. 

FIGURE 12.22. Comparison of retardation spectra for volum-inai Lo and shear deformation U. In this doubie logarithmic piot of the distribution functions of retardation times t the ordinate scaies have been adjusted to superpose the short-time resuits, r/ar is the reduced retardation time.Reproduced from Donaid J. Piazek and Craig A. Bero, Precise Glass Temperature, J. Phys. Condens. Matter. 15 (2003) 5789-5802 with permission from Institute of Physics Publishing. 

Fig. 24. Bilogarithmic plot of the retardation spectra L X) of PS with narrow molecular weight distribution as functions of the reduced retardation time Xr = X/ut om, where is the temperature reduction factor and om is the molecular weight reduction factor [the latter reflects the change in Tg(M)]. The molecular weights and S3rmbols for the various PSs are 4.7 x 10 A25, 0 9.4 x 10, M102 1.9 x 10 , L2, 6.0 x 10 , A19, o and 3.8 x 10 , F380, . The reference temperature Tq = 100°C and the reference molecular weight Mo = 1.9 x 10 .

Fig. 2.27. Recoverable-compliance, Ji(t), data of PPMS 5000 at temperatures -32.2°C ( ), -35.0°C U),-38.6°C (t),-40.0°C ( ), -41.1 °C (x), -42.6 °C n, -44.5 °C ( ), -45.2 °C (V), -46.9 °C (A), and -50 °C (o). The data taken at different temperatures have been shifted horizontally along the log t axis by a temperature-dependent shift factor log ut in order to superpose the curves at the short-time end with the data for -35.0 °C. The inset shows the retardation spectrum, L, as a function of the reduced retardation time X with reference temperature To = -35.0 °C, which was obtained numerically from J lf) data.

Fig. 2.31. The logarithm of the retardation spectrum L of poly(methyl methacrylate) as a function of the logarithm of the reduced retardation time r/ar. The solid curve was calculated from the reduced Jr(t) curve obtained from creep data taken at lower temperatures (14.4-34.7 °C) and longer times (10° s < r < 10 s) and shifted to 13.1° C. The dashed line was calculated from the dynamic compliances obtained by Williams and Ferry at higher temperatures and frequencies To was chosen to be 10.8 °C. From [217] by permission.

The water-reducing-retarding admixtures are used for avoiding cold joints and facilitating large pours. They are used particularly in hot weathering operations. These admixtures should increase the initial set time by at least one hour, with a maximum by 3 A hours. The compressive strength should be at least 110% of the control at 3, 7, and 28 days. 

Generally, water reducers retard both the very early hydration (initial set) and early hydration (final set) of portland cement whereas they increase the degree of hydration at later ages. In some cases, an acceleration of the very early hydration, promoted by carboxylic acids or carbohydrates, causes quick and early stiffening. 

The physico-chemical phenomena associated with the setting of cement is complex. The time of setting is defined in ASTM in terms of the penetration of a needle to a specific depth. Ramachandranl l examined by DSC several cement samples containing water reducers, retarders, and superplasticizers that had just set. In all the mixes, an endothermal peak typical of Ca(OH)2 was detected, suggesting that at the time of set at least some CjS must hydrate. It was also observed that the superplasticized cement paste had hydrated to a lesser extent than the reference cement paste at the time of setting. This indicates that physical forces are also involved in the setting phenomenon. 

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