Analysis of the effectiveness of thermal insulation of a multi-layer reinforced concrete slab using layer of concrete with low thermal conductivity under the climatic conditions of Vietnam

Multi-layer reinforced concrete slabs with thermal insulation materials are widely used in practice all over the world. In Vietnam, in order to apply modern construction of multi-layer structures with an inner layer of low thermal conductivity concrete into practice, it is necessary to conduct numerical researches on the peculiarities of the country climatic conditions country. In this paper, the authors analyze the heat transfer in multilayer reinforced concrete slabs’ roof under the climatic conditions of Hanoi (Vietnam) using finite element method with computer simulation. The object of the study is the insulating capacity of multilayer reinforced concrete roofs under the influence of changes in the temperature of the outer surface of the structure of the buildings’ roof. The research methods are based on the numerical simulation of heat transfer from the outer to the inner surface of the building roof. From the results obtained, compared to the traditional design solution, multilayer reinforced concrete slab appears to have more advantages, and these advantages are available with the use of heat-insulating material made of polystyrene concrete. These scientific researches enable to choose the rational roof design in Vietnam, which satisfies the demand to reduce influence of temperature.


Introduction
Vietnam is located in the sub-Equatorial monsoon climate. Winter in the North of Vietnam is cold, the air temperature sometimes drops to 1°C. In the Summer in the North of Vietnam, the highest temperature can reach up to 50°C, taking into account the influence of solar radiation. Therefore, the structure of the building roof must provide the required heat transfer resistance for the operating conditions of the buildings with internal temperature between (20-25) 0 C for the climatic conditions of Hanoi (Vietnam) to guarantee the coolness in summer and warmth in winter [1].
In traditional technology, people apply conventional concrete slabs and work progress, in which waterproofing and water disposal -which increases the labor intensity.
Today, there are many new materials and new structures with high thermal insulation and at the same time simplify in the production of works. One of these solutions is a multilayer reinforced concrete slab using heat-insulating material made of polystyrene. Author's publications [2, 3, 4, 5, 6 ...] showed that a reinforced concrete slab with a interior layer of concrete with low thermal conductivity has many advantages including reduction of heat conduction...Under the climatic conditions of Vietnam, it is necessary to assess the effectiveness of application of multilayer reinforced concrete slab about the ability of heat transfer. There are theoretical and empirical formulas for solving heat transfer problems in a multi-layer reinforced concrete slab.  To provide the required insulation, the amount of heat transfer resistance of the building roof layers must be greater than the required heat transfer resistance. However, it is difficult to determine the effectiveness of thermal insulation solutions of roofs for structures with heat-conducting inclusions. In this article, the authors evaluate the effectiveness of some insulation solutions in climatic conditions of Vietnam and evaluate the effectiveness of thermal insulation solution by three-layer reinforced concrete structures with an inner layer of polystyrene using the finite element method (ANSYS) [8]. This is a modern method that enables to analyze and intuitively consider the thermal graph transfering process through the layers of the structure.
The analysis illustrates the potential of using multilayer reinforced concrete structure with an inner layer of low thermal conductivity concrete in Vietnam.

Materials and methods of research
The materials used in the analytical tasks of the study, are shown in table 1 [9,10].  [10] "National technical regulation on Energy efficiency building" R 0yc ≥ 1,00 (m 2 .K/W) is obtained ; the calculated temperature of the interior air t t =20 0 C; Calculated temperature of the outside air t n =5 0 C in winter & t n =30,3 0 C in the summer, when considering solar radiation factors and other conditions, the estimated outdoor temperature is determined by: t tg .t b =t n + φ.I tb /α n =49.64 0 C ≈ 50 0 C; the heat transfer coefficient of the inner surface of the enclosing structure, for smooth ceilings is α t =5,9 W/m 2 .K - (table 3 -TCVN  In this article, the authors use three-layer slabs, which have an external layer of concrete class B12, 5, the thickness of the external layers of 40 mm and the inner layer of concrete with low thermal conductivity, with a compression resistance of the order of 1.5 MPa (D350) [2,3,11,12]. The thickness of the inner layer x (mm) is determined from the required heat transfer resistance in Vietnam.
The size of the sandwich plate 1500x6000x200 mm (figure 5). To assess the possibility of using three-layer panels to improve the required heat transfer for the roof, the authors will solve following problems: -Comparison of the possibility of insulation of three-layer plates (TN-1-in table.2) with thermal insulation of several thermal insulation roof designs, which are commonly used in Vietnam (TN-2, TN-3 and TN-4 in table.2).
-Comparison of the possibility of insulation of three-layer plates (TN-1-in table.2) with thermal insulation of two solutions of thermal insulation roof designs offered in Vietnam standard (TN-5 and TN-6 in table.3).  Table 3. Two solutions of thermal insulation roof structures offered in standard Vietnam (QCVN 09-2013) [10] No. Section of the floor Details of the layers TN-5

Comparison of the possibility of insulation of three-layer plates (TN-1 in table.2) with thermal insulation of several thermal insulation roof designs, which are commonly used in Vietnam (TN-2, TN-3 and TN-4 in table.2)
Using software ANSYS 18.0, the authors analyze heat transfer through roof structures (TN-1, TN-2, TN-3 and TN-4 in table 2) with material parameters in table 1 and climatic conditions in Hanoi, Vietnam. In summer, the calculated air temperature of the outer surface of the enclosing structure t n = 50 0 C and the air temperature of the inner surface of the enclosing structure t t =20 0 C. The results are shown in figures 6, 7, 8, 9 and 10. The results of the analysis illustrate that at the same thickness of 200 mm, a three-layer plate with an interior layer of low thermal conductivity concrete has more possibility of thermal insulation (compared to other types of structures) than other types of structures mentioned. The temperature of the inner surface of three-layer structure of the roof will increase 4,26°C; solid concrete slab will increase 18,83°C; hollow concrete slab (porosity 32%) will increase 9,168° C and reinforced concrete slab (polystyrene blocks inserted, 32% of the area of the plane) will increase 9,891° C.

Comparison of the possibility of insulation of three-layer plates (TN-1-in table.2) with thermal insulation of two solutions of thermal insulation roof designs (TN-5 and TN-6 in table.3) offered in standard of Vietnam [10]
As in sections (a), the authors consider that the temperatures of the internal and external surfaces of the roof structure are t n = 50 0 C and t t = 20 0 C for the analysis of heat transfer processes through the roof structures (in table 3  Similarly, analyze of heat transfer through the roof structure TN-5 and TN-6, when the external temperature of roof structure is t n = -20 0 C, the internal temperature is t t = 20 0 C (corresponding to temperature in Moscow). The results of analysis of heat transfer through the structure with the conditions weather in Hanoi and Moscow are then compared. The results are shown in figures 13a and 13b.
The results of the analysis of heat transfer on some types of structures demonstrate that the three-layer reinforced concrete structure has a smaller thickness than other structural solutions with a thermal insulation layer of polystyrene.
In Vietnam, the biggest problem is eliminating the thermal insulation from outside into the buildings, which is opposite in countries with cold winter period (exp: Russia).