EFFECTS OF PLANT ROOTS ON WATERPROOFING · PDF fileEFFECTS OF PLANT ROOTS ON WATERPROOFING...

International Journal on Architectural Science, Volume 8, Number 3, p.76-88, 2011

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EFFECTS OF PLANT ROOTS ON WATERPROOFING MEMBRANE K.K. Tan, M.Y.L. Chew and N.H. Wong Department of Building, School of Design and Environment, National University of Singapore (Received 19 August 2010; Accepted 9 May 2012) ABSTRACT High initial investment cost and the lack of understanding of plant root growth have been the major deterrents of more green roofs installation. This study attempts to address these two issues by exploring the necessity of root barrier. The possibility of accelerating green roof waterproofing test of using pre-grown plants was also studied. Plants were grown in experiment boxes of 1 m by 1 m with pre-form waterproofing membrane laid at base. Three conditions, including controlled moisture, defective waterproofing membrane and increased thickness of fresh planting compound, were compared against the Control sample, which was exposed to natural weather conditions. The moisture of the Control sample and Controlled Moisture sample was between 30-60% and 6-14% respectively. This resulted in a significant difference in the root density and as well as in the dry root masses, measuring an average of 22.3 g and 15.9 g for Control and Controlled Moisture respectively. The dry root masses for the other samples ranged from 19.3 g to 24.0 g. However, there was no outright penetration across all the samples; root growth was within the membrane thickness. Test results for accelerating the test was positive as plants from the increased thickness fresh planting compound and the Control sample exhibited similar root structure and dry root mass. 1. INTRODUCTION 1.1 Background Green roofs are gaining increasing attention due to the numerous advantages they offer. Many cities are exploring ways to increase area of green roofs as ways to reduce the carbon footprint and restore greenery displaced by buildings. However, various building stakeholders raised issues pertaining to green roofs. These issues include [1-9]: - High initial capital investment; - Possible root penetration; - Structural loading capacity; - Ease of maintenance of green roofs; - Guarantees associated with green roofs; and - Aesthetics. It is widely recognized that with these issues tackled, the rate of adaptation of green roofs will be much higher. This study addresses the first 2 issues. Cost of green roofs is high, at USD$150-500/m2, as compared to conventional concrete roofs costing USD$70-160/m2 [10-16]. This significantly higher cost of green roof is deterring many buildings from having greenery on their roofs. Lowering the cost of green roofs will definitely increase green roof coverage significantly, since many building stakeholders place cost as the biggest barrier to their adoption of green roofs. Although many governments, like National Parks Board of Singapore, are giving subsidies for green roofs, this is not a long term solution. The industry has to

resolve this high cost problem and not transfer the cost to another party; this is not sustainable and desirable. Therefore, it is necessary to look at the components of a green roof and the typical cost of each component as well as explore ways to attempt to reduce this cost. In this study, the necessity of root barrier is explored. In general, root barrier layer constitutes 10-15% of green roof cost [1,5,16-19]. Hence if the root barrier can be excluded whenever possible, the cost will reduce immediately and the saving of both investment and resources can be channeled to better use. Many practitioners claim that plant roots do not damage waterproofing and yet they are recommending root barriers or membrane with root retarding compounds be installed in green roofs [18,20-22]. This spans from the lack of understanding of plant roots, leading to unnecessarily high specifications for plants with little or no penetrative attributes. This would ultimately lead to an increase in cost due to precautions that need not be in place [21,22]. On the other hand, though Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau e.V. (FLL, or in English: The German Landscape Research, Development and Construction Society) is looked upon as the standard for green roofs, the testing duration is still rather long, 2 years for use of artificial growing environment, 4 years for natural growing conditions. Thus attempts were being made to reduce the testing period.

International Journal on Architectural Science

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Plant root study is hard due to the lack of visibility of the roots. Plant root studies commonly quantify roots in terms of dry root mass [23-26]. Dry root mass is favored over fresh root mass because the amount of water both in the root and moisture on the surface of the root cannot be controlled. Dry root mass measures the actual amount of mass material of root, hence dry root mass is a more accurate mean of quantifying plant roots. 1.2 Objectives The objectives of this experiment are: 1. To determine if roots would penetrate

waterproofing membrane, including the joints and possible defects.

2. To determine the possibility of accelerating waterproofing tests for Green Roofs.

3. To study root penetrative ability under minimal moisture conditions.

2. METHODOLOGY

2.1 Sampling Adenium Obesum was selected as the experiment plant type because it is one of the commonly used green roof plant locally. 72 plants were divided into 6 groups: 1. Control 2. Controlled Moisture 3. Defective Membrane 4. 30 mm Thick Fresh Planting Compound 5. Potted plants at beginning of experiment 6. Potted plants at end of experiment Of these 6 groups, four were transferred into sample boxes whilst one other group from the group was left on the roof to compare the growth against the other samples. The remaining group had the dry root mass measured upon delivery as a comparison for the rest of the samples to determine growth of the plants. The Controlled Moisture setup achieved moisture control by installation of a transparent canopy over the setup. The sides of the canopy cover to just above the plant level, allowing for ventilation. This prevents excessive heat trapped and hence, allowing for similar sunlight and temperature conditions as the other setups. At the same time, watering is performed to the experiment setup periodically, at 25 liters per week.

2.2 Experiment Boxes Installation Four experiment boxes measuring 1 m × 1 m were laid with sheet bitumen membrane of 2 mm thickness. Specifications of the waterproofing membrane are given below in Table 1. Table 1: Specifications of waterproofing membrane used

Property Specification Weight 1.15 kg/liter Tensile strength 0.3MPa Elongation 824% Water vapor transmission 1.2g/24hrs/m2 Crack bridging up to 2mm Tearing strength 125 N

The setup was with adherence to the FLL guidelines, shown in the following figure. Liquid applied waterproofing was used to seal the joints. 24 hour waterproofing test was carried out, in accordance to Singapore Standard CP82:1999 (ICS 91.120.30). Water was filled in all the sample boxes to a depth of 25mm and left in for 24 hours. Test result showed no signs of leakages.

Fig. 1: Layout of the bitumen membrane according to the FLL guidelines; number in top

diagram indicates order of laying

1

2 3


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Fresh planting compound was then laid at the base of the sample boxes before plants were installed. This is to accelerate the experiment yet provide for root growth just above the membrane and thus, achieving the closest behavioral pattern at the membrane level as compared to having plants grown from seeds. Thickness of this layer is kept at 10 mm for all the samples except the sample to be used to prove the effectiveness of acceleration. The conditions of various experiment boxes are summarized in Table 2. Some damages were deliberately created on the Defective Membrane experiment box to simulate root actions on these damages which may occur on waterproofing membranes. These damages were created on: The membrane itself; Horizontal joints; and Edge of the sample box. Figure 2 shows the defects. 30 mm of fresh planting compound is used instead of 10 mm for the Control sample. This would attempt to identify the difference in membrane damage, if any, as compared to the Control sample and determining if the method of using pre-grown plants is a viable way of accelerating the test. 2.3 Potted Plants To determine if the growth conditions are favorable for the plants, 12 plants were left in the pots and were subjected to the same conditions as the Control sample. The root mass of these plants will then be measured and compared to that of the Control sample. Furthermore, 12 plants chosen at random and the respective root masses were

measured. This will further determine if the conditions on the roof and the sample boxes are suitable for plant growth.

Fig. 2: Defects inflicted on membrane 2.4 Other Factors Other factors considered for exploration in this study include effects of fungi and insects. However, there was very little literature pertaining to these effects, hence there was no direction regarding the data collection. Moreover, it was unclear during the conception stage if fungi would grow and if insects would be attracted to the experiment setups. Therefore, efforts were focused on the study of root action.

Table 2: Condition matrix of experiment setups

Waterproofing membrane condition

Thickness of planting compound

Water provided

Control As per CP82:1999 and FLL 10 mm Subjected to natural rainfall

Defective Membrane

Deliberate defects created 10 mm Subjected to natural rainfall

30 mm Fresh Planting Compound

As per CP82:1999 and FLL 30 mm Subjected to natural rainfall

Controlled Moisture

As per CP82:1999 and FLL 10 mm 25 liters per week


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3. DATA COLLECTION AND RESULTS

Visual health of the plants and the moisture content of the Control sample and Controlled Moisture sample are continually monitored throughout the 300 days experimental period. Data regarding roots structure and density, dry root mass and membranes’ conditions are collected at the end of the experiment. 3.1 Visual Health Condition of the Plants The visual health of the plants seems positive 2 weeks after the installation of the plants. It is noted that the plants in the Controlled Moisture sample looks healthier than the other samples, i.e. there are more leaves on the plants. This deviates from the understanding that plants grow well under sufficient moisture condition, vice versa. 3.2 Soil Moisture Content The soil moisture content of the Control and Controlled Moisture samples were recorded using a data logger at 15 minutes interval. Because the defective membrane and 30 mm thick fresh planting compound sample are subjected to similar conditions as the Control sample, hence the data

can be shared. The charts below show the moisture level at various stages of the experiment. It can be seen from the chart that the moisture content of both conditions is very different, with the Controlled Moisture sample reading mainly at 6-14% while that of the Control sample at 30-60%. This is the desired moisture content of both the samples, so as to compare the root growth difference in greatly different moisture content conditions. The moisture content of the Control sample maintained at a high level due to the frequent rain experienced during the experiment period. The longest duration without any rainfall is 18 days. This is close to the most severe dry spell locally of 21 days. Otherwise, it can be seen that there is sufficient rainfall to maintain the moisture content of the samples. On the other hand, the Controlled Moisture sample had much lower moisture content with several sharp increases at the beginning and one at the end of the experiment. These increases were due to watering of the plants excessively, i.e. much more than regular watering. This is aimed at simulating big fluctuations of the moisture levels, given constantly low moisture content.

Moisture Content

0

10

20

30

40

50

60

70

0 50 100 150 200 250 300

Day

Mo

istu

re C

on

ten

t/ %

Control

ControlledMoisture

Fig. 3: Moisture content of samples (%)


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3.3 Demolition of Samples Upon the expiry of experiment duration, the samples were demolished to determine if physical damage was inflicted on the membrane by plant roots. Demolition was carried out by a water jet, at a rate of approximately 0.75 mm/min or 150

cm3/min. The water jet pressure was kept low to prevent unwanted disturbance to the roots, which provides critical information about the integrity of the waterproofing membrane. Key observations at the critical areas for the respective experiment boxes are summarized in Table 3.

Table 3: Summary of observation during removal of soil Sample Control Controlled

Moisture Defective Membrane

30 mm Fresh Planting Compound

Vertical membrane

No signs of damage

No signs of damage No signs of damage No signs of damage

Root density at sides

Some roots observed

Root density is higher than observed in Control sample

Root density is lower Root density is lower

Root density after removal of 15mm of soil

Roots of 2 mm observed

Root of about 1 mm observed; structure and density is less than the Control sample

Root density lesser and size smaller; the observation only matched the Control after removing 50 mm of soil

Root density of this sample exposed is less than the Control sample; more than that of the Defective Membrane sample

Growth through the fresh planting compound

Positive Positive Positive Positive; grown through the 30 mm of fresh planting compound

Root density after further soil removal

Root size was large and structure was dense; root structure was not wide in width

General root structure is similar to Control sample

General root structure is similar to Control sample

General root structure is similar to Control sample; 30 mm of fresh planting compound did not affect the root density

Anchorage of plants onto membrane

Root size of as large as 5 mm grew into the membrane

The region of root growth was also concentrated at the joints; degree of roots anchorage is much lesser, comparatively; majority of roots were no larger than 1 mm

Growth is as intense as the Control sample; substantial growing of the roots into the membrane

Growth rates of the plants’ roots are fast enough to through this depth and subsequently cause some form of damage to the membrane; intensity of damage inflicted by the roots was comparable to that of the Control sample

Damage to membrane

Anchoring force was so strong that the membrane tore when the plant is attempted to be lifted forcefully

Little anchorage of plants onto membrane

Similar to Control sample

Similar to Control sample


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Fig. 3: Roots exposed after removal of 15 mm of soil, Control sample

Fig. 4: Plants left after clearing most soil, Control sample

Fig. 5: Large roots growing into the membrane, Control sample

Fig. 6: Roots growing into membrane of Control sample; figure below shows zoomed in view

point of entry of roots into membrane

Fig. 7: Membrane tore when plant is lifted by shear force, Control sample


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Fig. 8: Roots showed growth across fresh planting compound; method of acceleration is

viable, Controlled Moisture sample

Fig. 9: Membrane was rather cleared of roots, Controlled Moisture sample

Fig. 10: Roots adhering to liquid waterproofing used, Controlled Moisture sample

Fig. 11: Roots growing into waterproofing

membrane, Defective Membrane sample 3.4 Dry Root Mass Roots were cut from each plant then wrapped in aluminum foil and lastly placed in oven to bake to dryness at temperature of 70°C-80°C. The duration of baking depends on the sample and the number of roots in the oven. To better establish if the roots were dry, the masses were measured at constant intervals until there is no further change in mass. The first set of 12 plants (Set A) was selected at random upon delivery by the supplier. This would serve as a base of comparison against the other samples to determine if there is growth of the plants. The second set was the 12 plants (Set B) left


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in pots on the roof and subjected to the same condition as the Control sample. This would serve to determine if the experimental boxes conditions

are favorable for plant’s growth. Table 4 shows the dry root mass measurements.

Fig. 12: Roots growing into waterproofing membrane, 30 mm Thick Fresh Planting Compound

Table 4: Dry root mass, arranged in ascending order

Set A Set B Control Controlled Moisture Defects 30 mm soil 1 7.0 16.8 7.3 7.7 13.2 12.4 2 10.4 19.1 15.5 8.9 13.4 14.2 3 12.3 19.5 16.2 9.3 14.6 16.3 4 12.5 20.0 16.4 9.9 15.2 16.5 5 12.9 20.2 18.9 13.8 16.0 20.9 6 14.6 20.5 19.0 14.9 16.6 22.5 7 14.7 20.8 22.6 15.4 16.7 22.6 8 15.1 23.5 22.9 15.5 19.0 25.4 9 16.4 23.5 25.7 20.0 23.3 27.0

10 16.5 27.1 28.8 24.6 27.2 29.8 11 18.4 28.0 31.3 24.7 27.2 34.6 12 18.7 30.5 43.6 25.8 28.9 45.3

Mean 14.1 22.5 22.3 15.9 19.3 24.0 Median 14.6 20.7 20.8 15.2 16.6 22.5

Dry Root Mass

6.0

11.0

16.0

21.0

26.0

31.0

36.0

41.0

46.0

1 3 5 7 9 11

Plant

Ma

ss/

g

Set A

Set B

Control

Controlled moisture

Defects

30mm soil

Fig. 13: Graph of dry root mass


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Table 5: Table of R2 and regression of dry root mass

Set A Set B Control Controlled Moisture

Defects 30 mm fresh planting compound

Regression 0.8948 1.0874 2.421 1.767 1.5174 2.4716 R2 0.9289 0.8995 0.8763 0.9486 0.8961 0.8977

Table 6: Results of unpaired t-test

Set A Set B Control Controlled Moisture Defects 30 mm soil Set A - 5.423 2.873 0.822 2.663 3.410 Set B - -0.037 -2.948 -1.556 0.506 Control - -1.969 -0.974 0.421 Controlled Moisture - 1.346 2.445 Defects - 1.473 30mm soil -

Table 5 shows the regression value of the dry root mass plots. The regression value can be inferred as the larger the value, the wider the spread of the dry root mass, i.e. the spread of Set A and Set B is small, while that of Control and 30 mm Fresh Planting Compound is wide. This cannot be explained with the data collected or the experimental results. It is recommended that future experiments increases the sample size to reduce the effects of such problem. However, the R2 value is consistent across all the samples. This implies that the possibility of extrapolating the data Unpaired t-test was also performed between 2 samples to further describe the statistics. The test is performed to all pairs of data and results shown in Table 6. Comparing 2 sets of 12 data would mean that there is 22 degrees of freedom for the test, with the one-tail test, the critical value is 1.717. It can be seen from the graph and the table that the dry root mass for Set A and the Controlled Moisture sample is the lowest. This is expected for Set A since it has no growth period as compared to the rest of the samples. The result for Controlled Moisture sample is also in line with the observation made during demolition of the sample as it has the lowest root density and has also inflicted the least damage on the waterproofing membrane. In addition, all the t-test values, except the Controlled Moisture sample, were higher than the critical value of 1.717, implying that growth was significant for the all the samples except the Controlled Moisture sample.

Set B’s mass is substantially higher than that of Set A (the t-test value = 5.423). This indicates that the conditions on the roof, with the plants exposed to the natural weather, is suitable for plant growth. The t-test values for Control and 30 mm sample against Set B are -0.037 and 0.506 respectively which are both lower than the critical value of 1.717. This confirms the indication that the growth conditions are similar. Furthermore, though 30 mm sample mass is higher than the Control sample, it is only slightly higher, with t-test value at 0.421. This indicates that the amount of soil makes a small difference in the growth of roots. Although the Defects sample dry root mass is lower than the Control sample and Set B, the t-test value is not high enough establish that the Defects sample growth was lower. The reason for the slightly lower root mass cannot be determined at this stage. A comparison of the linearity of the curves on the graph would illustrate that the gradient of Set A and B are almost the same. However, the gradient of the rest of the samples displayed different rate of increase, especially after plant 7. This showed that some of the plants grew at much faster rate than the rest. This would imply that the conditions are very favorable for some of the plants. Again, the reason for this difference cannot be established at this stage. 3.5 Close Membrane Study The sample membranes were studied at close range after all the demolition works. The general condition of the membrane was examined before detailed study of the areas where root showed growth into the membrane.


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Table 7: Observations of close membrane study

Sample Visual inspection Underside of the membrane

Slicing along the root growth

Co-relation to dry root mass

Control Root anchorage present at most parts of membrane

No signs of root penetration

Root grew into membrane; up to 10 cm in length

High

Controlled Moisture

Membrane generally clear of root growth

Majority of the roots adhered to the joints where liquid waterproofing was applied

None Extent of damage was much milder as compared to the Control sample, in agreement with the dry root mass results; higher the moisture level, the higher the intensity of root damage to waterproofing membrane

Defective Membrane

- Root density lower - Growth into the

membrane lower - Root anchorage

lower - Damage done

comparable - Significant growth

into the membrane - plants did not take

advantage of the defects created on the membrane

- possible reason for this behavior could be due to the high moisture level present in the sample throughout the experimental period


Root of sizes as large as 3 mm thick can be seen growing into the membrane

High

30 mm fresh planting compound

Root damage was extensive for this sample, comparable to the Control sample; Root anchorage was found on most parts of the membrane; using pre-grown plants as a form of acceleration the experiment is viable



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Fig. 14: Roots left in membrane, Control sample

Fig. 15: Underside of membrane clear of root growth, Control sample

Fig. 16: Slicing of membrane along root growth, Control sample

Fig. 17: Membrane generally cleared of root growth except at regions where liquid waterproofing was applied, Controlled

Moisture sample

Fig. 18: No signs of root at the underside of membrane, Controlled Moisture sample

Fig. 19: Roots did not take advantage of defects inflicted, Defective Membrane sample


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Fig. 20: Damages caused by roots, 30 mm Thick Fresh Planting Compound Sample

4. DISCUSSION AND CONCLUSION/

LIMITATIONS AND CONSTRAINTS From the experiment, it is concluded that there is positive correlation between dry root mass and soil moisture. It was observed that higher amount of soil moisture resulted in higher dry root mass. The measure of dry root mass also coincides with visual observations made during the demolition process; more intensive roots structure associated with higher dry root mass. However, the leaves’ health condition does not reflect the root condition in soil, as reported the difference between Control sample and Controlled Moisture sample. It was also observed that the moisture content had the biggest impact on the anchorage force and hence possibility of membrane damage. This observation is consistent across all the samples. Furthermore, the maximum root size growing into the membrane was smaller for the Controlled Moisture sample, at 3 mm, as compared to 5 mm for the rest of samples. This, again, is stronger evidence as to high moisture content is more likely to cause membrane damage. However, the spread of roots was the highest in the Controlled Moisture sample, as roots density was the highest at the vertical sides of the experiment

box. This could be due to root reaching out further in search for moisture. Interestingly, though all the samples showed roots reaching the base of the sample boxes and some form of anchorage into the membrane, they did not penetrate through the membrane. Growth was within the membrane. Though the waterproofing integrity of all the samples was not challenged, this poses an issue in the long run. At this growth rate, it is almost sure that the roots would grow through the membrane and causing a leakage problem to green roofs. For the Defects sample, plant roots did not take advantage of the defects created on the membrane. Growth into the membrane was seen beside the defects, implying that the defects created did not make growth into the membrane easier. This could be due to the high moisture level in the sample throughout the experimental duration, as the plants have no need to pry into cracks in search for water. Further tests have to be conducted to determine this claim. Lastly, the 30 mm sample root growth was very similar to that of the Control sample. This shows that the method used for accelerating the experiment is viable. Hence, it is recommended that this method be used for future tests. This experiment is conducted over 300 days and has fulfilled the objectives of the study. However, in the process of the experiment, there are several areas that could have room for improvement: further reduction of testing period is desirable; in-situ real time root growth observation; minimize root disturbance during removal of

soil; and increase quantification of magnitude of root

caused damage. The soil removal process has disturbed plant roots significantly, though it was inevitable. This prevented further data collection regarding the direction of root growth because the roots were no longer in the original position of interaction with membrane. Therefore, another method of observation should be considered. REFERENCES 1. Damon Farber Associates, Miller Dunwiddie

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