A New Kind of Roof Greening System in China

Frontier of Environmental Science June 2015, Volume 4, Issue 2, PP.29-38

A New Kind of Roof Greening System in China Jianhui Yang 1,2, Xiaodan Shen 2, Xinyan Lin 2, Dafang Yang 2, Jinsheng Ye3, Xiao Xue3 1. Opening Project of Key Laboratory of Deep Mine Construction, Henan Polytechnic University, Jiaozuo 454000, China 2. School of Civil Engineering of Henan Polytechnic University, Jiaozuo 454000, China 3. Polytechnic Institute of New York University, male 1013 room, 343 gold street, Brooklyn, NY, USA

Abstract With the wider use of green roofs, new technology and new materials are being applied to the field of building roof greening for buildings. This paper introduces BRGS (built- up roof greening system), a new type of roof greening system that differs from roof greening systems currently used in China in that it integrates a main and an auxiliary water storage capacity into the roof greening system. Compared to other systems currently in use, BRGS offers a simpler, quicker, less labor intensive construction process; lighter floor load; and lower long term maintenance requirements and costs. It also makes full use of rainwater and snowmelt, which provides a significant amount of water to plants. This paper also introduces a planting experiment, the results of which indicate that plants during their early stages of growth tolerate an alkaline environment, and that after a period of time, the pH value level of water stored in BRGS approaches 8.3, so we can conclude that BRGS is suitable for construction engineering.

Keywords: Roof Greening; Built-up Roof Greening System; Planting Experiment; Shale Ceramsite Concrete; Finite Element

1 INTRODUCTION Urbanization is increasing as China’s economy develops. As urban areas expand, causing multiple negative impacts on the environment, such as the reduction of water supply and quality[1], the degradation of air quality[2], and fragmentation and loss of natural habitats[3], ground hardening. Population growth, the shortage of resources, and the deterioration of the natural environment threaten human development. Many studies have been attempted to find and evaluate practical approaches that could mitigate these negative effects[4-6]. Ensuring adequate green space in urban areas and improving access to natural areas surrounding the cities can help to offset these negative effects[3].

However, high-density urban development limits the availability of green spaces, which necessitates a search for new alternatives. In this sense, the exterior surfaces of buildings offer plenty of space to be covered by vegetation, therefore, the planting of green roofs and the greening of facades have become two of the most innovative forms of urban greening[7-9].

Roof greening is well regarded internationally as a form of urban greening[10]. In a recent decade, green roofs consisting of grasses, flowers, and/or other plants have become popular in city planning[11]. In residential and industrial areas it provides technical, economic, climatic, ecological, and social advantages to conventional flat roofs.

The green roof system is one approach that could help to improve urban thermal environment (mitigate urban heat islands)[11,12]. It is well known that the air temperature near a green roof is lower than that near an impervious roof that is composed of concrete, brick, or other impervious materials; The green roof system is a useful tool for reducing urban rainfall runoff[13,14]. During the last two decades, a large amount of research has been published in German on the reduction of rainwater runoff for different types of roof greening[14]. In addition, Roof greening increases urban biodiversity[15-18], provides a visually pleasant environment[19] and enhances architectural interest[20,21] and so on. Roof greening not only improves the quality of urban life, but also enhances our society’s humanity by promoting a harmonious interaction between man and nature in our cities.

Roof greening is the practice of covering roofs with soil and vegetation. Environmental and material scientists recognize that it provides various benefits. It is considered a significant indicator of urban greening construction. It is considered good environmental policy and is popular in developed countries in Europe, the Americas, and Japan.

Roof greening research started late in China, but China has made definite achievements in recent years. Since - 29 http://www.ivypub.org/fes

construction projects in Beijing began implementing roof greening in 2005, Beijing, Shanghai, Shenzhen, and Henan have adopted relevant codes.

Osmundson[22] and Stefan Schrader[23] tell us that roof greening dates back to ancient times. Original examples such as the giant stepped pyramids of Mesopotamia and the Hanging Gardens of Babylon (assuming they actually existed) served primarily aesthetic purposes. In regions with extreme climates, traditional construction benefits from the insulating value of roofs covered with soil and grass. In Norway, for example, sod roofs help keep buildings warm, while in Tanzania they help keep them cool. In the late 19th and early 20th century roofs were gardened in major U.S. cities like New York in response to declining availability and rising cost of land in the inner city.

Early roof gardens used pure soil[23]. Today diverse technologies exist in urban centers around the world. In the late 19th century an architect in Berlin first used vulcanized cement as a non soil-derived growing medium. Today roof gardens are characterized by a stratified substructure of man-made material[22,24]. In principle, basic green roof construction consists of two membranes, one for waterproofing, and one for root-proofing. These are covered by another protective layer that consists of, first, a drainage layer, and then a filter fabric. Finally, the growing medium is spread out onto this technical substructure and is sometimes topped with a mulch layer[23].

The practice of covering a roof with soil or artificial substrate for growing plants is a form of landscape construction.

The design of the green roof differs considerably from that of a conventional roof. In traditional architecture design, the roof must meet functional requirements of providing a waterproof, impact resistant layer of thermal insulation and sunscreen. The roof garden must fulfill not only these requirements, but also provide irrigation and drainage.

Special measures must be implemented to ensure the system remains waterproof, and that plant roots do not damage the roof and cause it to leak[25].

Current practices include changing the soil and maintaining the layers of the roof garden system. Although the concept is simple, the process is laborious and expensive. So not only is the added roof load heavy, the operating and maintenance costs are high. Furthermore, the waterproofing layer in the current systems does not last as long as the structure, which creates a significant risk of leaks and maintenance issues.

The “Built-up Roof-Greening System� (BRGS, Our China patent: 201320158982.8) introduced in this paper conforms to relevant codes, such as "Load code for the design of building structures" GB50009-2012[26], and It offers a lighter floor load and lower construction and maintenance costs than the systems currently in practice. In addition to savings to be realized from the lower maintenance required for the physical structure of the system, BRGS reduces maintenance and labor costs associated with watering by incorporating a primary and an auxiliary water storage capacity into the planting component, and making full use of the significant amount water provided by rain and snowfall.

2 BUILT-UP ROOF GREENING INTEGRATION SYSTEM

FIG.1 MULTIPLE-LAYERED STRUCTURE OF ROOFTOP GARDEN INTEGRATION SYSTEM (RGIS)

In warm climates, BRGS consists of seven components: vegetation, growth medium, empty space (for water storage), permeable concrete cushion layer, waterproof layer, roof structure layer, and drain holes. Fig.1 shows the multiplelayered structure of Roof Greening Integration System. In cold climates, BRGS should add an insulating layer between roof structure layer and waterproof layer.

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2.1 Vegetation Plants suitable for roof gardens include lawn or ground cover, and small to medium sized flowers and shrubs, with as few trees as possible[27]. The soil is thin and the conditions extreme, so the ecological planting type concrete plants grow best meet the following conditions[28-30]: To adapt to the local climate and soil conditions, such as moisture, pH value, soil properties, etc.; Resistance, salt resistance, barren resistance and disease resistance is strong; Rapid growth, and root system developed, the ground part shorter; Perennial and adapt to extensive management; Seeds accessible and reasonable economy, etc..

2.2 Growth medium layer The growth medium layer must provide plants the water, nutrients, and stability they need to grow normally. The growth medium for BRGS is formed from precast shale ceramsite concrete units, or blocks. Fig.2 shows a precast shale ceramsite concrete unit. There are nine blind holes of the same diameter (0.1m) and depth (0.16m), distributed across the top of the unit. Distributed two on each side, and running the full height of the unit (0.2m), are eight semicircular grooves 0.01m in diameter. The blind holes hold enough soil to provide nutrients to the plants, and allow the roots to penetrate into the ceramsite growth medium to absorb moisture held in the porous ceramsite concrete. The grooves allow air to circulate through the growth medium layer, and water to enter the water storage reservoir below the growth medium layer. The soil in the blind holes, the porous growth medium block and the storage capacity below provide the nutrients, water, and rooting room to support plant growth.

FIG.2 SHOWS A PRECAST SHALE CERAMSITE CELLULAR CONCRETE UNIT

2.3 Empty space layer The growth medium is separated from the permeable concrete cushion layer by rows of brick walls 0.18m high, surrounded by a parapet, to create a layer of empty space that serves as a water storage area. Gaps 5.0 cm. wide left at regular intervals in the rows of brick walls allow water to circulate. Incorporating a water reservoir that can capture and hold rainwater and snowmelt into the roof greening system reduces demand for water from outside sources, as well as for labor to water the plants. It mitigates drainage problems and reduces pressure on the city’s storm sewer system.

2.4 Permeable concrete protective layer The permeable concrete protective layer under the empty space is cast-in-situ LC5 shale ceramsite permeable concrete pad. It bonds well with the existing waterproof layer (waterproof sheet or waterproof paint), and plays an important role in protecting the waterproof layer from penetration by roots. Meanwhile, Being permeable, the concrete also augments the water storage capacity and filters impurities from the water.

2.5 Waterproofing layer The waterproof layer is 2mm thick polyurea elastic, Spray Polyurea Elastic is a new technology that does not require solvents, and so does not create the pollution associated with them. Polyurea is an upgraded product developed from - 31 http://www.ivypub.org/fes

polyurethane. It is applied using advanced spray technology using higher temperature and higher pressure to spray the material directly onto the roof structure. It forms a fast curing elastomer material of excellent chemical properties that is not sensitive to humidity and does not require a catalyst. Neither does it require maintenance after installation, reducing long term maintenance costs. It is critical that the parapet and other ancillary structures be sprayed with polyurea elastic to ensure they are waterproof and moisture proof.

2.6 Insulating layer BRGS uses 60 mm thick shale pottery sand foam concrete to form a lightweight, high performing layer of energy saving thermal insulation.

2.7 Drain hole Several 50mm drain holes are placed around the parapet 20mm above the growth medium to allow excess rainwater and snowmelt to drain to the ground, protecting plants from inundation and prolonged submersion.

2.8 Structural layer Because the structural layer of the roof serves as a reservoir, it does not slope to drain holes like a conventional roof.

It should be level to maximize storage and even distribution of water.

2.9 Comparison of BRGS and a typical roof garden system currently available in China.

BRGS has many advantages over the modern roof garden system typically used in China today. (Tab.1).

TAB.1 COMPARISON OF A TYPICAL ROOF GARDEN SYSTEM CURRENTLY AVAILABLE IN CHINA AND BRGS PROPERTIES

Currently available roof garden system in China

BRGIS

Load (kN/m2) 3.000

drain hole

2.179

50 mm thick PVC foam board layer

0.025

200 mm thick shale ceramsite concrete Growth medium Empty space

20mm-30mm diameter gravel layer

1.800

50mm thick LC5 shale ceramsite permeable concrete protective layer 2 mm thick polyurea elastic waterproof layer 60 mm thick Shale pottery sand foam concrete insulation layer ( cold climates)

0.490

Layer Planting dielectric layer (70% soil and 30% expanded vermiculite or sawdust) Dry laid nonwoven polyester fabric layer

Dry laid nonwoven polyester fabric layer

Layer

Two layers 3 mm thick SBS polymer 0.100 modified bitumen polyester waterproofing membrane 50 mm semi-rigid mineral wool board 0.050 layer 20 mm thick 1:3 cement mortar 0.400 1:8 Cement expanded perlite sloping layer 3.000 Total(warm) 8.375 Total(warm) Total(cold) 8.375 Total(cold) Planting dielectric layer load is the average load of 20mm thick planting dielectric layer.

Load (kN/m2) 0

2.964

0.47

5.633 6.103

(1) BRIGS is recommended for planting flowers and low shrubs, as well as trees, because the plant roots can grow deep into the ceramisite growth medium, which protects the waterproof layer. This structure is conducive to the storage and release of water.

(2) The system currently in use is made up of more components and uses more material, creating a heavier floor load (8.375 kN/m2). BRGS is made up of only four material components, creating a lighter floor load (5.633 kN/m2 or 6.103 kN/m2).

(3) Shale ceramsite concrete is lightweight, with a low modulus of elasticity. In the event of an earthquake it can - 32 http://www.ivypub.org/fes

quickly absorb seismic forces, enhancing the building’s earthquake resistance.

(4) Unlike the current roof garden system, BRGS integrates a primary and secondary water storage system into the green planting model. BRGS not only reduces the number of components and the material requirements of the system; it also makes full use of the water to be derived from rain and snowfall, providing a considerable amount of water to plants.

(5) In the current system, the life of the waterproof layer is about 15 years. Long term saturation, extreme temperatures, and root penetration contribute to the deterioration of the waterproof layer. The consequences of root penetration and leakage would be serious and costly In BRGS, the polyurea resin protected by the layer of permeable concrete is maintenance-free for the lifetime of the building.

3 ROOF GARDEN LOAD VALUES AND PRECAST SHALE CERAMSITE CONCRETE UNITS. The permanent and variable load for a roof garden is considerably greater than that of a conventional roof, which affects the underlying structure, the stability of the foundation, and construction cost of the project. Therefore, determining the permanent load and temporary load is a very important problem in roof garden structure design.

3.1 Live load According to the "Load code for the design of building structures" GB50009-2012[26], we can determine the nominal value of uniform live loads for roof garden roofs to be 3kN/m2. Live load does not include the weight of the flower beds (vegetation layer), nor that of snow and rainfall.

3.2 Permanent load According to the “Code for Roof Greening” DB11T 281-2005[31], the roof greening permanent load includes the structural layer of the roof; the structure of the roof greening system; and the soil, water and vegetation contained in the system. Because the roof garden live load does not include the weight of the vegetation and other materials, determining the permanent load of the roof garden is comparatively complicated. This paper examines the greening load (i.e. the vegetation and growth medium loads) associated with BRGS (see Tab.1).

3.2.1 Vegetation load The load value of lawn cover is 0.05 kN/m2. For short bushes and small clumps of woody plants it is 0.01 kN/m2.

The load value for 1.5m tall trees and shrubs is 0.2 kN/m2, and for 3.0m tall shrubs and trees it is 0.3kN/ m2.

3.2.2 Growth medium load The wet density of shale ceramsite concrete is 1300 kg/m3. The wet density of the nutrient soil is 850kg/m3.The average load is given by

 abc  9 R h  4 r c   2

Ggm 

2

c

 9 R 2 h s

(3-1) g ab where a, b and c represent the length, width, and height, respectively, of the shale ceramsite cellular precast concrete unit, R and h represent the radius and depth, respectively, of a blind hole; r is the radius of the ventilation and circulation channel; g is the acceleration due to gravity.

So we can calculate the average growth medium load for BRGS to be 2.179 kN/m2. This value is less than 3.0 kN/m2, the average load when growth medium is soil[9], so the growth medium load of BRGS is lighter than that of the typical roof garden system currently available in China (see Tab.1).

4 CHECKING THE BEARING CAPACITY OF THE CELLULAR PRECAST SHALE ERAMSITE CONCRETE UNIT As part of this research we tested two shale ceramsite cellular precast concrete units under concentrated load (Fig.3).

Fig.4 shows the failure pattern at a maximum pressure of 30.25 kN. As the component is still usable, this failure - 33 http://www.ivypub.org/fes

pattern does not constitute real damage. Fig.5 shows the failure pattern at a maximum pressure of 35.67 Kn. This is real damage. Results: the maximum pressure that the precast shale ceramsite concrete unit can bear is 35.67 kN. This value is adequate for it to perform its function in the roof garden.

FIG.3 BENDING TEST

FIG.4 THE FIRST COMPONENT AFTER A CRUSH

FIG.5 THE SECOND COMPONENT AFTER A CRUSH

In order to further determine whether the component meets bearing capacity requirements, ANSYS software was employed to conduct three-dimensional simulation analysis of the precast shale ceramsite concrete unit. The responses of the structure under self-weight stress and under 3.05 kN live load were analyzed. Fig.6 shows the threedimensional finite element model.

FIG.6 THE THREE-DIMENSIONAL ďŹ NITE ELEMENT MODEL

FIG.7 DISPLACEMENT VECTOR SUM UNDER SELF-WEIGHT

FIG.8 DISPLACEMENT VECTOR SUM UNDER LIVE LOADS

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FIG.9 FIRST PRINCIPAL STRESS CONTOUR UNDER SELF-WEIGHT

FIG.10 FIRST PRINCIPAL STRESS CONTOUR UNDER LIVE LOADS

The results show that: the max displacement of the structure under self-weight stress is 0.000190 mm, and under live load stress is 0.000224 mm. These data suggest the total deformation is very small. This component fulfilled the requirements of normally utilized limit status; the largest principal tensile stress of the structure under self-weight stress is 0.0121 MPa, and under live load stress is 0.0146 MPa. These data suggest the principal tensile stress is very small. So this component can fulfill the requirements of normally utilized limit status.

5 PLANTING EXPERIMENT

FIG.11 ON THE DAY OF PLANTING

FIG.13 A MONTH AFTER PLANTING

FIG.12 A WEEK AFTER PLANTING

FIG.14 A YEAR AFTER PLANTING

A planting experiment begun in May 2012 placed a shale ceramsite cellular precast concrete unit in a water tank and filled the blind holes with nutrient rich soil, where bluegrass seeds were planted. Fig 11 shows the newly completed planting. After three days, the seeds started to germinate, and after one week germination was above 80%. Fig.12 shows a week after planting. During the first week there was no rain, and the grass was not watered. After a month, the plants were 10cm tall. Fig.13 shows a month after planting. During this month it rained twice. The plants were not watered in addition to the rainfall. In the year since the planting the weather has been mostly sunny and dry, with occasional rain. If it doesn’t rain for a long time, the plants only need to be watered once a month, perhaps even less frequently. BRGS can significantly water consumption and labor requirements associated with long term maintenance.

During the planting experiment we measured the changes of pH values in the water tank. The relationship of the - 35 http://www.ivypub.org/fes

water’s pH values with the days of the experiment is given in Fig 15, where we can see that the pH values increased rapidly during the first several months after which it gradually decreased until it reached an appropriate value. Now, after a year the pH value is 8.3.

FIG.15 THE RELATIONSHIP BETWEEN THE PH VALUES IN WATER TANK AND TIME

The planting experiment shows rapid plant growth and a high survival rate. During the early stages of growth the plants tolerate the alkaline environment created by the concrete growth medium, and take root easily. It is necessary to choose plants with a deep root system so that the roots can grow through the soil and penetrate well into the permeable ceramsite concrete growth medium. The cellular concrete growth medium units protect the soil from washing and improve water retention, improving the ability of the plants to withstand drought and flood.

6 CONCLUSIONS (1) BRGS integrates a main and an auxiliary water storage capacity into the roof greening system. Not only does BRGS reduce the construction process and reduce the floor load, it also makes full use of rainwater and snow, which provides quite a lot of water to plants, thereby reducing maintenance costs.

(2) Plants with fibrous, strongly developed root systems that can adapt to the conditions of the shallow soil layer should be chosen for roof gardens. Because it is porous, the precast shale ceramsite unit has advantages over soil.

The soil in the blind holes and the growth medium block itself can provide nutrients, water, and rooting room to support plant growth. Water in the empty space layer further augments the water supply.

(3) This new approach is suitable for flowers, low shrubs, or trees. Plant roots can penetrate deep into the cermasite concrete growth medium, which protects the waterproof layer. BRGS is conducive to storage and release of water.

During long periods with little or no rain, plants only need to be watered once a month, or even less frequently.

BRGS can significantly reduce water consumption from outside sources, and long term labor and maintenance costs.

In the event of inundation, it resists soil shifting.

(4) BRGS is lighter than the roof garden system currently used in China, demanding less of the structure upon which it is installed.

(5) A planting experiment shows that plants in early stages of growth tolerate an alkaline environment, and that the water’s pH value in BRGS approaches 8.3 after about a year.

(6) This analysis shows that BRGS is suitable for practical applications.

ACKNOWLEDGEMENT This study was supported by the National Science Foundation of China (41172317). This work was also Supported by the Opening Project of Key Laboratory of Deep Mine Construction, Henan Polytechnic University (2011KF-01), and Henan Provincial Department of Education Scientific and Technological Project (2010A560010). edited by Ross - 36 http://www.ivypub.org/fes

Gent (U.S.A), Henan Polytechnic University.

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Authors Jianhui Yang (1969-), male, Han nationality, Postdoctor, professor, mainly engaged in research on engineering materials and structures and dynamic response. In June, 2003, he graduated from Structural Engineering major in Dalian University of Technology, receiving a doctor degree in engineering. In June, 2007, he finished postdoc training from the postdoctoral research station of water conservancy projects at Wuhan University.

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