Cryogenic Suction Research Articles

A Hypothesis for Salt Frost Scaling in Cementitious Materials

In this study freezing characteristics was investigated experimentally on adequately air-entrained cementitious materials (Powers’ spacing factor is between 60-160 µm) with variables including water-binder (w/b) ratio, slag cement addition and a wide range of air content. Results confirm cryogenic suction as the primary transport mode of the external moisture into the surface region to amplify continuous ice growth in capillary pores therein. This leads to pore pressure build-up and eventually surface scaling. Three key factors are identified to govern this cryogenic suction mechanism: (I) the initial ice nucleation in capillary pores and (II) the presence of unfrozen moisture transported through (III) the capillary network. Air entrainment, although needed for protecting internal frost damage, yields no added gain in improving scaling resistance in concrete and is consistent with the capillary suction dominated scaling mechanism. Addition of salt in the pore solution suppresses the ice nucleation and subsequent growth which may explain the pessimum effect.

Laboratory X-ray Tomography: A Valuable Experimental Tool for Revealing Processes in Soils

AbstractThe use of high-resolution tomographic techniques has allowed for unprecedented observations and a renewed understanding of geomaterials and processes. A laboratory x-ray scanner is used to explore the potential of the technology in the context of complex geotechnical systems. Tests benefit from the fast and non-destructive nature of x-ray measurements and the micrometer-scale resolution that is attainable. Several first-time observations are reported here. In this paper we demonstrate the following: subsurface volume loss in sandy soils can cause the formation of sharply defined low-density pipes; cryogenic suction consolidates sediments next to ice lenses during ground freezing; root growth involves transverse expansion, and the stress relaxation at the tip facilitates further longitudinal invasion; blade insertion causes successive shear localizations; and the incipient formation of desiccation cracks is not necessarily along a planar front—in fact, the fracture plane may split as it encounters heterogeneities at the tip. Finally, it is shown that x-rays can be used to monitor chemical processes that cause coupled mechanical effects, such as osmotic consolidation induced by ionic diffusion and mineral dissolution. Although brief events may not be tomographically imaged, single x-ray radiographs can be analyzed and compared to gain extensive process information.

Sorptivity as a Measure of Salt Frost Scaling Resistance of Air-Entrained Concrete

Salt frost scaling is the progressive deterioration of the concrete surface associated with freezing and thawing in the presence of a salt solution. Its major mechanism has been confirmed to be cryogenic suction that promotes continuous ice growth in the surface region. In this paper, salt frost scaling of different air-entrained concretes is investigated, alone with the room-temperature sorptivity measurement. A clear correlation is found between the two properties, which is supported by a theoretical analysis on the sorptivity. These findings explain why HPC of low w/b ratio and associated low capillary porosity has much improved scaling resistance. Similar benefits are obtained for regular w/b ratio concrete containing high cementitious replacement level of slag cement.

収縮低減剤を混和したセメント硬化体の低温吸水効果による凍害劣化機構

For the formation of ice and its melting in the microstructure, Gibbs free energy for ice are lower than that of pore liquid. This results in the movement of water from fine pore to large pore and air voids. This phenomenon is so-called cryogenic suction pump. For coexistence of shrinkage reducing agent (SRA) in pore water, the movement of water into ice in air void is promoted by osmotic pressure effect as well as cryogenic suction pump effect. The authors used two different types of SRAs：one in the traditional one（SRA-1）, the other in the newly developed one（SRA-2）. The later in adsorbed strongly on cement was compared with traditional SRA-1. The authors prepared three different specimens of hardened cement paste（HCP：without SRA（OPC）, with SRA-1, with SRA-2. Length changes of HCP specimens increased with an increase of freezing/thawing cycles, and depended on the types of paste：SRA-1＞SRA-2＞OPC. However, the megascopic observation of frost damage did not correspond to the order of length changes：SRA-2＞SRA-1＝OPC. For the most serious damaged specimen added SRA-2, the gel pore less than 4nm increased during freezing/thawing cycles, where capillary pore decreased. The osmotic pressure due to present of SRA in pore liquid emptied the large gel pore and shrinkaged it providing increase of small gel pore. On the other hand, capillary pore was destroyed during freezing/thawing cycles, and then the capillary pore decreased. From these results, strong adsorbed SRA improves the resistance against frost damage, which is almost the same on specimens without SRA.本論文は、収縮低減剤が混和されたセメント硬化体の凍結融解劣化機構を解明するため、ギブス自由エネルギーの差によって働く低温吸水ポンプ作用と未凍結水移動の駆動力の増大によって気泡に隣接する毛細管空隙が断裂するという仮説を立て、凍結融解時の細孔構造変化及び長さ変化測定によって検証を行ったものである。その結果、収縮低減剤を混和したセメント硬化体において未凍結水の移動による毛細管空隙の収縮と比較的早いサイクルでひび割れが内部まで伝播したことを確認し、前述した仮説の妥当性を裏付けるとともに、収縮低減剤の種類によっては凍結融解抵抗性を損なわないことが確認された。

Effect of air-void system on frost expansion of highway concrete exposed to deicer salt

The net frost expansion (dilation) was determined by monitoring the change in length of air-entrained concrete specimens undergoing a freeze–thaw cycle between 20°C and − 20°C. Two dilation processes were found during freezing: an initial dilation at the freezing point and a subsequent dilation which continues during the entire freezing period. The second dilation dominates when the surface is exposed to a deicer solution. For both exposure conditions, a good correlation was found between the dilation and the spacing factor, which fits well with the hydraulic pressure theory. The results show that a lower spacing factor is required for frost protection against salt frost attack, consistent with the current practice of recommending higher total air content as compared to freezing without salt exposure. Concretes that had excellent field durability had frost dilation values below the paste fracture strain of about 250 micro strains. The dilation results suggest that cryogenic suction of external liquid exacerbates internal hydraulic pressure development which in turn may explain the surface scaling phenomenon.

Thermal conductivity of hydrate‐bearing sediments

A thorough understanding of the thermal conductivity of hydrate‐bearing sediments is necessary for evaluating phase transformation processes that would accompany energy production from gas hydrate deposits and for estimating regional heat flow based on the observed depth to the base of the gas hydrate stability zone. The coexistence of multiple phases (gas hydrate, liquid and gas pore fill, and solid sediment grains) and their complex spatial arrangement hinder the a priori prediction of the thermal conductivity of hydrate‐bearing sediments. Previous studies have been unable to capture the full parameter space covered by variations in grain size, specific surface, degree of saturation, nature of pore filling material, and effective stress for hydrate‐bearing samples. Here we report on systematic measurements of the thermal conductivity of air dry, water‐ and tetrohydrofuran (THF)‐saturated, and THF hydrate–saturated sand and clay samples at vertical effective stress of 0.05 to 1 MPa (corresponding to depths as great as 100 m below seafloor). Results reveal that the bulk thermal conductivity of the samples in every case reflects a complex interplay among particle size, effective stress, porosity, and fluid‐versus‐hydrate filled pore spaces. The thermal conductivity of THF hydrate–bearing soils increases upon hydrate formation although the thermal conductivities of THF solution and THF hydrate are almost the same. Several mechanisms can contribute to this effect including cryogenic suction during hydrate crystal growth and the ensuing porosity reduction in the surrounding sediment, increased mean effective stress due to hydrate formation under zero lateral strain conditions, and decreased interface thermal impedance as grain‐liquid interfaces are transformed into grain‐hydrate interfaces.

Experimental design for a pilot study on bedrock weathering near the permafrost table

An experimental design is described to test the hypothesis that ice segregation near the top of permafrost and in the lower part of the active layer can brecciate frost-susceptible bedrock. Seasonal temperature cycles in the active layer and the top of permafrost were simulated to a first approximation within a block of chalk measuring 0·30 m × 0·31 m wide and 0·33 m high and insulated around the sides to minimize lateral heat transfer. The block, moistened first by capillary rise, was initially frozen from the surface downwards, simulating permafrost aggradation. Thereafter, the upper half of the block was cycled above and below 0°C (simulating seasonal freezing and thawing of the active layer) while the lower half remained below 0°C (simulating permafrost). During thaw cycles water was supplied to the surface and base of the simulated active layer. Temperature, unfrozen water content, surface frost heave and porewater pressure within the chalk were monitored during the experiment. Preliminary data from three freeze–thaw cycles indicate that: (1) both downward and upward freezing occurred during freeze-back of the simulated active layer; (2) 78 per cent of the surface frost heave (total = 4·8 mm) took place during the thaw periods, when the active layer was approaching or at its maximum depth; and (3) negative porewater pressure was recorded in some instances during freeze-back, suggesting the occurrence of cryogenic suction. The thaw-related frost heave suggests that ice segregation within the chalk occurred beneath the descending thawing front, in the still frozen part of the simulated active layer and the uppermost permafrost, and marked the commencement of rock brecciation. Copyright © 2000 John Wiley & Sons, Ltd.

Heat and mass flow associated with a freezing front

A one-dimensional mathematical model for heat and moisture flow in saturated porous materials is given. The model is formulated using capillary theory and depends upon the inter-relationship between pore radius, freezing point depression and the cryogenic suction. The conditions at the frost line include a criterion which determines whether heave and/or penetration of the frost line occurs.Numerical results are given which were computed using an expanding—contracting finite-difference approximation. These results compare well with temperature and heave measurements made in the laboratory using a precise freezing cell.