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Non-Destructive Testing of Natural Building Materials

Non-Destructive Testing of Natural Building materials

Table of Contents

1. Introduction

2. Natural Building Materials

2.1 Straw Bale

2.2 Hemplime

2.3 Rammed Earth

2.4 Adobe

3. Non-Destructive Testing

3.1 Moisture Content Meter

3.2 The ‘Schmidt’ Hammer

3.3 Ultrasonic Test

4. Methodology

4.1 Moisture meter performed on Straw-bale

4.2 Compressive tests and Rebound hammer performed on Rammed Earth

4.3 Ultrasonic test performed on Adobe



Non-Destructive Testing of Natural Building Materials

1.    Introduction


This report aims to explore the requirements of performing several types of non-destructive tests on a few natural building materials. Laboratory sessions will be conducted to determine the validity and reliability of three non-destructive tests on three materials. Moreover, the report aims to investigate how these non-destructive tests can be applied to in-built structures.

Natural building materials are often utilized and manufactured by inexperienced workers, making them vulnerable to improper construction, which can affect material properties. In addition, the mechanical properties of materials are subject to degradation as time passes by due to dynamic and dead loads as well as increasing moisture content. A visual inspection is not enough to ensure that materials are not subject to failure and are performing their tasks efficiently.

The word ‘natural’ is ambiguous when talking about building materials. The primary benefit of using natural building materials is to construct an efficient and sustainable building. Therefore, materials that are minimally processed during manufacture, excessively available, or have a renewable nature, are environmentally friendly and can be considered natural. NBM can be used for several purposes in the construction industry, such as a rammed earth wall or Hemplime insulation.

Non-destructive testing is the process of examining and evaluating materials while keeping the product undamaged. There are numerous types of NDTs used in the construction industry on conventional building materials for different purposes. An investigation will be conducted to check whether these tests can be applied to natural in-built structures.

2.    Natural Building Materials


The utilization of natural materials in the construction industry is slowly growing around the global at an increasing rate. The renewable nature of natural building materials makes them inexpensive and widely available. However, the most important advantage of using natural building materials is that they are eco-friendly. The ease of production and construction results in a much lower energy usage compared to other conventional materials. In addition, the carbon emitted to the atmosphere during the production of the natural materials are extremely low, where some materials have a negative embodied carbon.

Because unsustainable materials are most dominantly used as building materials, the construction industry accounts for around 47% of the total emitted CO2 in the UK, while accounting for almost 50% of the total embodied energy in the UK. The energy used in the production, construction, and maintenance of conventional building materials acts as a significant threat to climate change, and thus natural building materials should play a huge role in cutting embodied energy and CO2 emissions.

2.1 Straw Bale

Fueled by the sun, straw is a natural product that is currently used in the construction industry. A single straw has a cylindrical and tough structure, and contains cellulose and silica with high tensile and bending properties.  The availability and easy accessibility makes it a cheap agricultural product. Using straw bale is a sustainable method of recycling as straw is the end product of growing crops and are normally decomposed by burning as not many organisms can decompose straw. Straw bale structures have proven to be energy efficient and durable, as well as being an efficient construction material. Loose straw bales are flammable and have a low fire-resistivity. However, this drawback can be easily overcome by compacting the bales, keeping oxygen to a minimum, thus reducing the risk of combustion. Furthermore, a render layer should avoid the ignition of straw due to externally imposed heat. The render provides an insulating barrier between the straw and a heat source, while preventing the transfer of oxygen into the straw (Ashour et al., 2011).

Straw bale is an excellent material for heat insulation, with a thermal conductivity of 0.067 W/mK.  Research has shown that straw bale houses have kept heat during cold winters and reduced heat during summer. Energy consumption is reduced by about 80% with Straw bale houses. In addition, the thickness of straw bales, which has a minimum of 18cm, provide a soundproof material that insulates sound waves efficiently.

Straw bales are produced by baling straw into a compressed rectangular block, kept together by baling wires or poly-propylene string. Loose ties decrease the compaction of the bales and thus reduces its strength (Ashour and Wu, 2011).

Straw bale is an anisotropic material, where its strength is higher in a vertical orientation than in a horizontal orientation. While the deformation modulus is lower in the horizontal orientation(Ashour et al., 2011).

Oat and wheat straw bales have a density ranging from 81-106.3 kg/m3, where barley straw bales densities range from 54.6-78.381-106.3 kg/m3. While other experiments have shown that the optimal density for wheat and barley is about 112kg/m3 (Ashour and Wu, 2011).

The fiber content and type of straw affect the erosion, strength, and durability properties of earth plaster. As fiber content increases, the risk of shrinkage crack in the plaster decreases. In addition, thermal conductivity of earth plaster decreases as the fiber content increases (Ashour and Wu, 2011).

Careful design of moisture is required as a moisture content greater than 70% can lead to decomposition. Moisture trapped within a wall envelope has a negative effect on the insulation efficiency. Barley straw has higher insulation properties due to its higher resistivity to moisture. The safe moisture content is about 15% (Lawrence et al., 2009).

2.2 Hemplime


Hemplime, or Hempcrete, is a mixture of hemp and lime-based binder used as an efficient insulation material in the construction industry. It is an outstandingly environmentally friendly as it can be considered carbon negative. Although Carbon Dioxide is emitted during the production of lime, this is overcome as CO2 is absorbed during the growth of hemp. Hemplime concretes have an exceptional thermal performance with a low thermal conductivity resulting in an excellent insulation proficiency. It has an average U-value of 0.17W/m2K, however, the value is true for all Hemplime walls as it varies with different thicknesses and binder type. This significantly decreases the amount of fuel needed to keep a building heated or cooled over its lifetime. UK manufacturers of Hemplime materials have announced that about 165kg of CO2 are absorbed per m3 for hand-placed concrete and 110kg for spray-applied Hemplime. Because hemp rapidly grows and can go up to 4.5 meters in only four months, its absorbs a significant amount of CO2 to create a hard stem to support is self-weight. Formerly a waste by-product, the wooded cellulose of the stem is now used for building purposes. Compared to timber, the rapid growth of the hemp plant makes it much more renewable. While it is also more sustainable than timber as it refills itself in only 4 months. Hemp is a local crop around the world and can be grown in most climates and in a wide range of soils, minimizing the energy used in the transport of the plant. There is no need for chemical fertilizers as hemp is weed-suppressant and pest-resistant, reducing pollution (Walker et al., 2014).

Hemplime binders are made from at least 70% pure lime, produced by burning limestone. It is burnt at 900°C, about 300°C-600°C lower than the required temperature to burn Portland cement, reducing the energy needed for production. In addition, the energy needed for the transport of the product is less than that of cement because it has a much lower density. Conventional insulating materials are normally made from unsustainable materials, such as fossil fuels. The energy used in the production of Hemplime is much less than the intensive energy used in the production of other conventional materials (Walker et al., 2014).

2.3 Rammed Earth


Rammed earth is a mixture of soil types compacted between temporary formwork. It has been used for centuries in several regions. The utilization of rammed earth construction is only recently being used again in construction developments. Rammed earth is formed of soils with a wide range of sizes. Per the British Standard grading limits, gravel sizes range between 2-60mm, sand 0.06-2, silt 0.002-0.06, and clay less than 0.002 Although rammed earth soils are available in many colors, red is the preferred option. Using multiple color soils can lead to an undesired non-uniform finish. The voids ratio in the material affects the strength and durability properties. Therefore, it is highly useful to decrease the voids ratio as much as possible. Rammed earth uses a small amount of silt as a void filler and enough clay to act as a binder and increase the strength of the material. Sand and gravel dominate the soil content, where soils coarser than 5-10mm are usually sieved out before the utilization of the soil. Experiments have shown that compressive strength weakens when gravel size increase. Coarse materials result in porous gaps that are susceptible to freeze and thaw, which can result in cracking. Contact with water results in swelling of the clay particles, and shrinkage when the water dries. The loads that can be carried by a structure is dependent on the dry density of the material, which varies from 1700 kg/m3 to 2200 kg/m3 for rammed earth. It depends on the compaction efficiency, moisture content during compaction, and soil type. Maximum density is achieved when the moisture content is at an optimum. The Proctor tests are usually used to determine the maximum dry density and the optimum moisture content. Like concrete, rammed earth is much stronger in compression than in tension. Other factors affecting the compressive strength are cohesive strength and aggregate strength. Rammed earth is designed to have no shear or bending strength, as both are extremely low (Maniatidis and Walker, 2003).

Despite having a low sound transmission due to the thicknesses of the walls, rammed earth walls have a low thermal resistance, with U-value of about 1.9W/m2K.

To improve wet strength and erosion resistance, stabilizers such as cement are added to the mixture. Stabilization can be avoided by careful design, as natural rammed earth construction is preferred environmentally due to the low embodied carbon footprint involved. The most commonly specified cement proportions are between 6% and 10%. The amount of cement required depends on the grading of the soil, where excessive clay content hinders the effectiveness of cement stabilization. Other types of stabilizers are non-hydraulic lime, which work with clay, fibers, and sodium silicate (Maniatidis and Walker, 2003).

Another property that affects the strength of the material significantly is suction, which is the difference between the pore air and pore water pressures. It is dependent on temperature T, gas constant R, gravity g, molecular mass of water vapor wv, and relative humidity RH (El Hajjar et al., 2018). The suction can be calculated using Kelvin’s equation:

= − = −(RT/g wv) ln( )

2.4 Adobe

Adobe is a building construction method that uses cheap, renewable, and accessible materials to build walls. It can be produced from a wide range of soils and has a simple method of construction. It is a mixture of various types of soils, water, and reinforcement, most commonly straw. The ease of transportation and simple manufacturing means that the embodied energy and carbon in constructing adobe bricks are extremely low, where the energy used to build a concrete block is 300 times higher than that used for the manufacturing of an adobe brick. Adobe bricks also have high thermal properties and are soundproof, decreasing the cost and energy used for heating and cooling a building. However, if the bricks are not reinforced with other materials, they tend to have a low compressive and tensile strength, resulting in low seismic resistance. It can be stabilized mechanically, or chemically. Enhanced compaction of the material results in increased strength as well as decreasing porosity. Additives and stabilizers are usually integrated into the material during manufacturing to increase cohesion, strengthening its mechanical properties and durability. There are several types of additives that can be used such as sand, cement, straw, and animal fat (Rouhi and Hejazirad).

Adobe is produced by laying a mould on a surface and compacting the wet adobe mixture firmly. The surface is the levelled and scratched by hand. The mould is then removed and the brick dries in a shaded place for about 2 days. Exposing the bricks to the sun increases the risk of cracking and rate of evaporation (Rouhi and Hejazirad).

Adobe consists of three soil types: sand, silt, and clay. The ideal clay content is 20%, where a much lower percentage result in high permeability and low compressive strength. Meanwhile, if the clay content is excessive, the wall is susceptible to cracking due to shrinkage. Straw adds tensile strength and absorbs moisture, reducing shrinkage cracks. However, straw decreases the compressive strength, and if it is decomposed, voids are created (Rouhi and Hejazirad).

Moisture content in adobe increases due to capillary absorption of water in the ground, condensation, or rainfall, affecting swelling clay. When water enters, strong hydrogen bonds of the material are broken causing the cohesion of clay particles to decrease. When excess moisture is removed, the clay particles reform the hydrogen bonds. However, these cycles result in shrinkage of the soil and cracks are propagated.  Water trapped in pores are also at risk of freezing or thawing, increasing pore pressures. High pore pressures lead to decreased cohesion and detaching of a structure (Illampas et al., 2013).


3.    Non-Destructive Testing


Buildings need to be monitored regularly to prevent structural deformation and distortion of a structure. A reduction in the strengths of a material can be detrimental in the failure of a material. Degradations of structural and insulation properties can occur due to an increase in moisture content or due to the presence of a discontinuity in the material.

Non-destructive tests are usually much less expensive than destructive tests, and are preferred in the construction industry due to their non-invasive methods, while the tester is unharmed. They also play an important role in preventing malfunctions, as a few tests can alarm the responsible engineer when variations in a material performance occur, helping in maintaining quality controls. While conducting these tests, operations keep running as there is no need to shut down assets while performing the tests.


3.1 Moisture Content Meter


A moisture meter is a meter that determines the moisture content in various types of wood. It has a small screen that displays the moisture content once the two pins on the remote make slight contact with the material surface. The remote has information of properties of different types of wood. It is a portable device used as a non-destructive method in surveying. The remote works by measuring the resistivity or conductance of the currents travelling from one pin to the other, affected by the material in between. The resistivity/conductance are then related to the moisture content and the value is displayed on the screen. The device is suitable for rough and irregular surfaces of structures (Fujita, 1983).

A surface moisture meter is preferred to a pinned moisture meter. Although the destruction caused by a pinned meter is extremely low, the surface moisture meter is non-destructive. In addition, the surface moisture meter can be used to detect moisture in the subsurface as well. However, surface moisture meter can only be performed on smooth, regular surfaces (Fujita, 1983).

Moisture readings should not be taken using these devices near the edge of the material, as it would result in inaccurate readings.

3.2 The ‘Schmidt’ Hammer


The Schmidt hammer, invented by Ernst Schmidt, or the rebound hammer, is a rapid test used to indicate the compressive strength of concrete. In addition, the rebound hammer is used to assess the quality and uniformity of the concrete.

Moreover, water can enter concrete through porous regions in the material, resulting in an increase in the water to cement ratio and reducing its compressive strength. Porous regions are present if the surface was not finished properly or if the concrete was not compacted adequately. Inadequate compaction of the concrete also means that there is trapped air in the material, resulting in a reduction of its designed compressive strength after it has dried.  A reduction in the compressive strength of a material means that the material can no longer resist high stresses that it previously could, making it susceptible to deformation and cracking (Breysse and Martínez-Fernández, 2014).

The rebound hammer is a cylindrical device consisting of a spring controlled mass sliding on a plunger. With constant energy, the hammer strikes the concrete surface and rebounds back. For the rebound hammer to work properly, the hammer must strike the surface at right angles only. The rebound of the hammer is measured, where the value mainly depends on the surface hardness of the concrete, and can be correlated to the compressive strength of the concrete. The extent of rebound is measured on a graduated scale, where a low rebound value indicates that a high amount of energy was absorbed by the concrete, meaning that the strength of the concrete is low (Breysse and Martínez-Fernández, 2014).

The rebound hammer is not suitable to determine the strength of concrete in the earlier stages to determine whether the concrete has reached the desired strength. The concrete strength increases after it has been cast in place as time passes by and with increasing temperatures, where it should reach 90% after 14 days and 99% after 28 days (AbdElaty, 2014).


3.3 Ultrasonic Test


An ultrasonic test is a non-destructive method of evaluating detrimental characteristics of a material. The test uses sound waves of a high frequency that is propagated through a substance. The test is not only limited to solids, but can also be applied to liquids and gas. In addition, the test is used to detect flaws in a material, as well as determining dimensions and properties of a material. A pulser produces electrical pulses with a high voltage to the transducer, which generates the sound waves that travel through the material. If a sound wave encounters a void or a crack as it travels through the material, it will be reflected to the transducer. The reflected sound wave is converted to an electrical signal which is shown on a monitor, where the strength of the wave and the time at which it was reflected are displayed. The travel time is the used to determine the depth at which the defect is located. A defect will be detected whether it is at the surface or subsurface. It provides instantaneous results with high degree of accuracy, and is not difficult to set-up (Garnier et al., 2011).

However, there are several limitations to performing an ultrasonic test. The surface of the material must be smooth as rough material are difficult to transmit sound waves through. In addition, materials with low sound transmission are not suitable for this test. The test cannot be performed on thin or irregular in shape materials either (Garnier et al., 2011).


4.    Methodology

Three non-destructive tests will be performed, on a different natural material each. As mentioned before, the moisture content of straw-bale plays a key role in the compressive strength of the material. Therefore, an investigation will be conducted to determine whether the moisture meter is suitable to determine the moisture content of an in-built straw-bale wall. Moreover, rammed earth shares a few similar characteristics with concrete during the construction of walls. The rebound hammer test will be performed on rammed earth samples, and the appropriateness of the test on the material will be determined. Meanwhile, an ultrasonic test will be performed on an adobe sample to check whether defects will be detected.

4.1 Moisture meter performed on Straw-bale


Five straw-bale samples with different densities will be used to determine the reliability of performing the rebound hammer on the material. Each moist sample will be weighed prior to drying the samples in the oven. After approximately 24 hours, the weight of the dry sample will be measured, and the moisture content will be calculated using the following formula:

Moisture content (%)= (Wet mass-dry mass)/dry mass,

Afterwards, the two pins of the moisture meter remote will make minimal contact with the straw-bale at 4 different spots of the vertical plane for each sample. The moisture content will be measured and recorded. Lime render will then be applied to the vertical surface of each sample and the moisture content will be re-measured using the same device. The same procedure will be applied but with the non-destructive pin less moisture meter. The calculated moisture content and the four measured ones will be compared to each other and analyzed to determine the reliability of performing the moisture meter on a straw-bale wall and if it can work in-situ.


4.2 Compressive tests and Rebound hammer performed on Rammed Earth

The second investigation will be determining whether the rebound hammer is a suitable non-destructive method of determining the compressive strength of a rammed earth wall. The dry densities of five rammed earth samples of distinct dimensions will be measured.  will be compacted in formwork and will be left to dry. After the material is dry, the formwork will be removed, and the rebound hammer will strike the surface of the rammed earth sample at five different locations. The rebound index will be used to determine the compressive strength. To determine whether the results are reliable, a destructive compression test will be applied to each sample, and the values will be compared and evaluated.

4.3 Ultrasonic test performed on Adobe

Finally, five samples of adobe brick with different thicknesses will be produced in the same way that the rammed earth samples were. Artificial voids will then be created in at least one spot for each sample. The ultrasonic test will then be performed on each sample and a monitor will be used to determine and record the time the sound waves were reflected to the receiver.  The bricks will then be rendered and the ultrasonic test will be re-conducted to check that it is a suitable method of detecting voids or cracks on in-built adobe walls.


AbdElaty, M. (2014) ‘Compressive strength prediction of Portland cement concrete with age using a new model’, HBRC journal, 10(2), pp. 145-55.

Ashour, T., Georg, H. and Wu, W. (2011) ‘Performance of straw bale wall: A case of study’, Energy and Buildings, 43(8), pp. 1960-1967.

Ashour, T. and Wu, W. (2011) ‘Using Barley Straw as Building Material’, Nova Science Publisher, Inc., 28pp.

Breysse, D. and Martínez-Fernández, J. L. (2014) ‘Assessing concrete strength with rebound hammer: review of key issues and ideas for more reliable conclusions’, Materials and structures, 47(9), pp. 1589-1604.

El Hajjar, A., Chauhan, P., Prime, N. and Plé, O. ‘Effect of suction on the mechanical characteristics of uniformly compacted rammed earth’. IOP Conference Series: Earth and Environmental Science: IOP Publishing, 012045.

Fujita, T. 1983. Electric resistance type wide range moisture meter. Google Patents.

Garnier, C., Pastor, M.-L., Eyma, F. and Lorrain, B. (2011) ‘The detection of aeronautical defects in situ on composite structures using Non Destructive Testing’, Composite structures, 93(5), pp. 1328-1336.

Illampas, R., Ioannou, I. and Charmpis, D. C. (2013) ‘Overview of the pathology, repair and strengthening of adobe structures’, International Journal of Architectural Heritage, 7(2), pp. 165-188.

Lawrence, M., Heath, A. and Walker, P. (2009) ‘Determining moisture levels in straw bale construction’, Construction and Building Materials, 23(8), pp. 2763-2768.

Maniatidis, V. and Walker, P. (2003) ‘A review of rammed earth construction’, Innovation Project “Developing Rammed Earth for UK Housing”, Natural Building Technology Group, Department of Architecture & Civil Engineering, University of Bath.

Rouhi, J. and Hejazirad, F. ‘The Experience of Adobe-Mud Bricks Strengthening in Bam Citadel after the 2003 Bam Earthquake’.

Walker, R., Pavia, S. and Mitchell, R. (2014) ‘Mechanical properties and durability of hemp-lime concretes’, Construction and Building Materials, 61, pp. 340-348.

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