防水層下地としてのコンクリートスラブの特性とふくれ形成圧力挙動 : 屋根防水層のふくれ現象のメカニズムに関する研究その2

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1. Introduction The objective of this study is to clarify the blistering mechanism of membrane roofing applied directly on concrete substrate. In the previous paper, part 1, it was found that the main factors on the pressure increase to form a blister are moisture condition and air permeability of concrete. The moisture is the source of vapor pressure increase and the air permeability governs the pressure diffusion. Therefore, it is important to know both properties of concrete deck, and the practical influences of them on the pressure behavior. In this paper, a series of tests and a numerical analysis were conducted using concrete cured under various conditions simulated actual decks. 2. Concrete Specimens and Their Properties The mix proportion and the curing conditions of concrete specimens are shown in Table 1 and 2. The specimens were placed in the cylindrical PVC molds and cured in the molds for 28 days. After being cured, air permeability (Fig. 1(a)), moisture content and pore size distribution of specimens were measured. The air permeability depends entirely on the curing condition (Fig. 2), and it is physically determined by not only the moisture content but also the densification of concrete. The pore size distributions clearly show that the drying from early age obstructs the densification (Fig. 3). Concrete dried from early age has large pores to induce the extremely high air permeability. 3. Pressure Increase Test After the air permeability was measured, the concrete specimen was processed for the pressure increase test being made a void at the center on the top surface (Fig. l(b)). The void was imitated as a small chamber between roofing and concrete substrate to originate a blister. The specimens were put into a heat insulating wall between two environmental rooms (Fig. 4). One of them was controlled to provide a daily temperature change of 20℃-55℃-20℃ for the void, the other to be constant at 20℃, 40 % RH. The change curve of temperature and the maximum of 55℃ (Fig. 5) was supposed as a typical condition of roofing in a hot sunny day. The test was continued for 90 days and the daily pressure behavior in the void was observed by a manometer. When the test ended, the air permeability and the moisture content of the specimens were measured again. The solid lines in Fig. 6 are the results observed on the first five days and the last 90th day. It is clearly seen that the pressure increase is higher as the duration of sealed-curing is longer and the drying environment more humid. The pressure increase on the 90th day of every specimen became a little lower compared with on the first because the moisture content became a little lower and the air permeability a little higher. The high pressure increase, however, was kept in the specimen such as [4 w] or [1w-75]. It thus appears that a blister of membrane roofing grows through such a daily cyclic pressure behavior. 4. Relation Between Pressure Increase and Concrete Properties The relation between blistering and moisture content of substrate has often discussed in the literatures because more moisture contained in concrete can generate higher vapor pressure. From our results, the pressure increase became larger if the average moisture content was more than 6%, or the surface moisture content more than 5 % at the first period of the test (Fig. 7 and 8). At the last of the test, however, the relation between them was obviously different. On the other hand, the air permeability had a close relationship with the pressure increase in spite of the change of moisture content throughout the test period (Fig. 9). It is concluded that the air permeability of concrete has a significant effect on the pressure increase to form a blister, while the moisture content has a slight in practice. It can be said that a sufficient moist curing makes the air permeability low and the pressure increase high. It should be noted, however, that the concrete with a low air permeability never directly promotes the blister growth because such a concrete can prevent the entry of additional air to grow the blister. Further investigations will be required to evaluate the influences of concrete properties on the blister growth. 5. Numerical Calculation of Pressure Behavior The pressure diffusive equation in concrete for the blistering problem proposed by the authors was as follows ; (∂t)/(∂P)=(εμ)/(P_0k)div grad P+(∂t)/(∂P_α)+(∂t)/(∂P_υ) where P is the internal pressure between membrane roofing and concrete or in pores of concrete, P_0 is the initial pressure, k is the air permeability of concrete, e is the effective porosity (gas-permeable, water-unfilled porosity) of concrete, μ is the viscosity of air, P_α is the thermal expansive pressure of dry air and P_υ is the water vapor pressure. Both ∂P_α/∂t and ∂P_υ/∂t are calculated from the temperature change in concrete due to the heat conduction. When the air permeability is low, the pressure diffusion or the air flow from the void into concrete is little then the pressure increase becomes high. The broken lines in Fig. 6 are the results obtained from the numerical calculation based on the equation above and the assumption that the vapor pressure is always saturated. The agreement between experimental and calculated results is fairly good. It is found that the numerical analysis proposed is applicable enough to concrete cured under various conditions and that the daily pressure behavior between a membrane roofing and an actual deck can be simulated. This suggests that the numerical analysis is also available for the investigation on the blister growth, by relating the pressure and/or the air-flow behaviors to the volume change of blister. The dot lines in Fig. 6, [1w-40] and [1w-75], are the same calculated ones but based on the assumption that the vapor pressure is 40 % or 75 % of saturation. They are almost equal to the saturated ones. It is numerically confirmed that the pressure increase depends excellently on the air permeability of concrete. 6. Conclusion 1) The air permeability of concrete deck as a substrate for membrane roofing depends entirely on the curing condition and it is physically determined by not only the moisture content but also the densification of concrete. 2) The pressure increase to form a blister of membrane roofing is more closely effected by the air permeability of concrete than the moisture content because the pressure diffusion, which is governed by the air permeability, is much more distinguished than the vapor pressure increase. 3) The numerical calculation based on the pressure diffusive equation can explain enough the daily pressure behavior to form a blister of membrane roofing.
社団法人日本建築学会の論文
1991-09-30

防水層下地としてのコンクリートスラブの特性とふくれ形成圧力挙動 : 屋根防水層のふくれ現象のメカニズムに関する研究その2

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防水層下地としてのコンクリートスラブの特性とふくれ形成圧力挙動 : 屋根防水層のふくれ現象のメカニズムに関する研究 その2

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防水層下地としてのコンクリートスラブの特性とふくれ形成圧力挙動 : 屋根防水層のふくれ現象のメカニズムに関する研究その2