Earth as mud bricks, has been used in building construction for thousands of years and approximately 30% of the world’s present population still live in earthen shelters [1] . Mud brick is an inexpensive, environmentally- friendly and abundantly-available building material. It has been used extensively for building construction around the world, particularly in extreme hot, dry desert climates like that of most Arabian countries [2] . In these countries, mud bricks are made by blending mud and water together into a goopy mixture. Traditionally straw is added to improve tensile strength, and may prevent mud bricks from cracking. The mud brick is then shaped in a mold of almost any size or shape, or by hand. The brick mixtures are then laid to dry in the heat of the sun for about 25 days before use [3] - [6] . Figure 1 shows new unlaid mud bricks in the Jordan River, while Figure 2 shows buildings made from mud bricks in an old town in Niger.

Mud bricks have several advantages over other conventional building materials, e.g., concrete masonry. These advantages include: a very minimal manufacturing process; skilled labor is not necessary; mud is available from natural resources; inexpensive construction materials; and mud structures are able to perform satisfactorily under hot environmental conditions [7] . However, there are many disadvantages in using mud bricks as building materials. These include: mud brick may tend to erode under rain impact; absorption of water causes swelling of mud brick, while evaporation of water from the mud brick gives rise to shrinkage and cracking; and mud brick is a relatively fragile material, which cannot resist earthquake hazards.

It is known that mud brick technology has been widely used in desert countries due to the widespread existence of silt and clay deposits in these areas. The importance of using mud brick as a building material, however, has not been seriously investigated through scientific experimentation. Therefore, this study aims to investigate: improvements in mud brick consolidation by increasing its durability; and imparting water penetration to mud brick. In this research, the effect of different mud brick components on strength and thermal conductivity properties are investigated. The basic mixture consists of mud (clay and sand), straw and water. Many additives are provided to the basic mixture in order to improve brick properties.

The overall aim of this study is to determine, through extensive experimental investigation, the effect of different mud brick ingredients on strength and absorption, and utilize these results in a forthcoming study on thermal-conductivity properties. With this in mind, the specific objectives of the study are to improve mud brick consolidation by using materials such as soluble silicate, ethoyl silicate, silanes or siloxanes, isocyanates and various polymers agents to treat mud brick permeability. In addition, the effect of using metallic fibers to improve mud brick durability is investigated.

The raw materials used in this study were mixed together to reach optimum brick properties. These properties include compressive strength and water absorption without sacrificing the superior advantage of the mud brick, which is heat performance.

The testing procedure investigated the strength behavior of the mud brick samples. Experimental measurements were directed to determine the propagation of cracks with an increase of load, the crack pattern before failure, and the ultimate capacity of the brick samples. A California Bearing Ratio (CBR) testing machine was used to determine the compressive strength of the brick samples, Figure 5. Also shown in the figure is the proving ring attached to the vertical jack. This ring, having a capacity of 28 KN was used to monitor the vertically-applied load. All the specimens were loaded until failure.

Tables 1-10 show the failure load for all the specimens. During increases in load, cracks were formed at sides of the tested specimens. Crack width and length increased and eventually lead to final collapse. Figure 6 shows the typical cracking pattern at failure.

Figure 7 shows the effect of increasing the sand/mud ratio on the compressive strength of the produced brick. The results of Group 1 show that average failure load increases with an increase of the sand/mud ratio up to a ratio of 0.15. Increasing this ratio above 0.15 led to a decrease in the compressive strength of the brick samples. At this optimum ratio, the average failure load of the mud brick samples was 15.2 KN with a compressive strength of 1.52 MPa. As such, this sand/mud ratio was kept constant in the subsequent testing stages. Various materials were added to the basic sample consisting of sand, mud, and water with the sand/mud ratio kept constant 0.15.

The ratio of hay was the only variable that changed in group 2 of the tests. This group tested hay percentage ratios of 0.24, 0.48, 0.72, and 1.2. Figure 8 shows the effect of increasing the hay ratio in a basic sample. The figure shows that the optimum hay ratio was 0.48% for the basic sample consisting of sand, mud, and water. This ratio was then kept constant in the following test samples consisting of sand, mud, water, and hay.

In group 3, the hay was exchanged for cement as a bonding material. Figure 9(a) shows that increasing the cement ratio has the effect of increasing the compressive strength.

Comparing Figure 8 and Figure 9(a) shows that the basic sample of sand, mud and water needed cement at 6% to reach the same compressive strength of the basic sample with only 0.48% of hay. This indicates that hay is more effective than cement in increasing compressive strength. Moreover, it is well known that increasing the cement in the mixture has a negative effect on the thermal properties of the produced brick [7] .

In group 4, tests for cement and hay were added together to the basic sample. The hay ratio was kept at the optimum ratio of 0.48%, while the cement ratio was varied from 3% to 9%. The results are shown in Figure 9(b). The figure shows that increasing the cement ratio above 6% decreased load capacity. Moreover, at a cement ratio of 6% the load capacity was 15.7 KN with an increase of only 3% in the load capacity when compared to the sample with hay only without cement.

The hay was exchanged by lime as a bonding material in Group 5. The results are shown in Figure 10(a). The results show that increasing the lime ratio increases the load capacity of the brick samples. In Group 5, there was no hay added to the basic mixture.

In Group 6, lime was added to the basic mixture containing sand, mud, and water, in addition to the hay. The hay ratio was kept equal to the optimum ratio of 0.48% in all specimens of this group. Figure 10(b) shows the effect of increasing the lime ratio to the basic components of the brick containing hay. The results show that increasing the lime ratio above 6% had a negative effect on the load capacity of the brick. Comparing Figure 9(b) and Figure 10(b) for specimens with hay in addition to cement or lime, respectively, both figures show a similar trend for compressive strength. This means that in the case of a basic mix with hay, adding an extra bonding material such as cement or lime slightly increases the load capacity up to a limit, after which the strength decreases.

Figure 11 compares the results when adding cement or lime to the basic mixture without hay. The results show that without hay, the cement is more effective than lime in increasing the compressive strength of the produced brick.

The specimens of Group 7 were similar to those of Group 6, except that liquid bitumen was added to the specimens of Group 7. In this case, the brick mix consisted of sand, mud, water, hay, lime, and bitumen. The results

in Table 7 show that the addition of bitumen to the mixture with lime and hay had a negative effect on the compressive strength of the brick.

The ingredients of the samples of Group 8 are shown in Table 8. The samples of this group have no sand. Lime was added with different ratios to mud, water, and hay. The test results of this group are shown in Figure 12. The results show that increasing the lime ratio leads to an increase in the compressive strength. In Figure 13, a comparison is made between the results of Group 6 containing sand and group 8 with no sand. This figure shows that exchanging the sand with lime had a positive effect on the load capacity of the bricks.

The effect of using different bonding materials in the basic sample consisting of sand, mud, and water is shown in Figure 14. Three types of bonding materials, namely, (hay), (steel fibers with no hay), (lime, hay, and steel filings), were used in the comparison shown in Figure 14. The results show that hay acts well when added to mud and sand to produce bricks.

Table 11 shows an overall analysis for the ten groups of samples considered in this study. The average load of each group was compared to control Group 2 where the samples consisted of a sand/mud ratio of 0.15 with a variable hay ratio. The results show that the major change in compressive strength was achieved in Group 3, where hay was replaced by cement as a binding material.

The compressive strength of mud brick was examined in this study. More than ninety tests were conducted to derive the optimum load capacity of the produced brick. The basic contents of the brick samples were mud, sand, and water. Different bonding materials were added to the basic brick mixture to reach the highest compressive capacity without sacrificing the initiative thermal property. The bonding materials considered in this study were: hay, cement, lime, bitumen, steel fibers, and steel filings. The results from the experimental investigation gave the following conclusions:

1) The optimum sand to mud ratio was 0.15 by weight. Increasing this ratio above 0.15 led to a decrease in the compressive strength of the brick samples.

2) Adding hay to the basic sample of mud, sand, and water had a great effect on the compressive strength of the produced brick. The optimum ratio of hay was 0.48% of the total weight of the sand and mud. Adding cement to the basic contents of the brick sample led to an increase in the compressive capacity of the produced brick. However, increasing the cement ratio might not be preferable due to its negative impact on thermal performance.

3) In case of basic specimens with hay, adding an extra bonding material such as lime or cement slightly increased the load capacity up to a certain limit, then compressive strength decreases.

4) Using steel fibers or steel filings as bonding materials did provide a notable increase in the load capacity of the mud brick.

5) The results showed that the major change in compressive strength was achieved in Group 3, where hay was replaced by cement as a binding material.

Farraj Al-Ajmi,Hany Abdalla,Jamal Almatawah,Magdi Abdelghaffar, (2016) Strength Behavior of Mud Brick in Building Construction. Open Journal of Civil Engineering,06,482-494. doi: 10.4236/ojce.2016.63041