Reduction of Cracking and Shrinkage in Compressed Clay Beams during Drying

Uncontrolled evaporation of moisture from compressed clay beams can cause surface cracks, resulting in reduction of strength. This paper presents various treatments applied to clay beams during the process of casting, compacting and drying in order to curtail the possibility of cracking and to decrease percentage of drying shrinkage. Following treatments were applied to the beams during casting and drying: (i) a steel plate and double layer of plastic sheet was provided between the beam and the plank, (ii) the beam was enveloped with a propylene fabric sheet during casting and (iii) beams were covered with plastic sheet during drying. Using these treatments, the clay beams were cast and compacted at various intensities of compaction. The results show that the drying shrinkage was reduced to minimum and the cracks were curtailed. The rate of drying shrinkage was decreased depending upon the level of compaction. Thus at the higher degree of compaction, more density of clay beams was achieved, which resulted in higher degree of compressive strength in baked and unbaked state.

of Moses [3]. In order to increase the performance of moulded earth blocks in unbaked state, it was tamped and compressed since18 th century [4] and is called CEB (Compressed Earth Blocks).
In order to use indigenous clay as low cost material of construction, beams were cast and compacted from a mixture of clay and pit sand in a ratio of 70:30, in addition to 22% of potable water. During the process of drying of these clay beams, shrinkage cracks occurred due to evaporation of moisture. Due to this shrinkage, tensile stresses may develop. If these tensile stresses are constrained, they get intensify and reach its tensile INTRODUCTION S helter is one of the basic requirements of human being besides food. The modern tendency is towards construction of structures with reinforced cement concrete and steel which is not affordable for the poor masses [1]. Thus low cost and indigenous material of construction is to be sought and used in original or modified state for the construction of affordable houses. Clay is low cost and indigenous material of construction and is used, in different forms, for construction of low cost houses since ages [2].
Clay has been used in the form of moulded earth block called bricks in baked and unbaked state since the time strength, as a result cracks may occur [5][6][7][8][9]. Consequently the mechanical strength is significantly reduced [10]. Due to complex behaviour of soil, cracking is also affected by factors such as mineral composition, content of clay, humidity, temperature and relative thickness of layers etc [11][12][13][14][15]. The object of this study is to decipher the treatments by application of which the problems of drying shrinkage and cracking in the clay beams could be curtailed.

Compacting System
A stiff and strong mechanical system was designed and fabricated to cast and compact clay beams. This mechanized system is shown in Fig 1. The details of which are presented elsewhere [16]. The moist clay beams were

Beam Specimen
The indigenous clay was quarried at the depth of more than 1200 mm lest the particles of organic matter like leaves and roots of the plants, debris and aggregates may not be mixed with the quarry.
Indigenous clay was mixed with pit sand in the ratio of 70:30, in addition to 22% of potable water, in a pan mixer for a period of 15 minutes. A detailed study regarding the percentage of pit sand and its effect on the compressive strength of the baked clay was conducted by Ansari [17].
This study [17], suggested that 30% of pit sand mixed with 70% of indigenous clay gave: (i) maximum cube crushing strength of baked clay and (ii) the percentage of shrinkage was decreased with increase of ratio of pit sand.
Hence, the same ratio was used in this study. Generally, workability of clay depends on percentage of moisture.
At the moisture content of 22%, it was very easy to mix properly this clay pit sand mixture in a pan mixer and cast the beams easily.
This moist clay was allowed to mature for 24 hours. The moist clay-sand mixture was filled in the mould in five equal layers and each layer was properly tamped. The mould was filled to its total depth of 460 mm and railed in to the compacting chamber of the system for the compaction of beam.
The section of beam of 150x300mm is considered to be standard laboratory size for testing concrete beams.
Beams of 4000 mm length are generally used in various engineering construction works. It is difficult to cast,

FIG. 1. MECHANIZED SYSTEM FOR CASTING AND COMPACTING CLAY BEAMS
compact, bake, transport and test clay beams of this much length. Therefore, in this study, beams of size 150x300x2000mm were cast, compacted and tested.
In order to decrease the percentage of cracking and to curtail possibility of cracking in the beams, following treatments were applied during casting and drying process: (i) A finished steel plate underneath a double layer of plastic sheet was provided between the beam and the wooden plank on which the beam was cast (Fig. 2).The double layer of plastic sheet was provided between the steel plate and the beam in order to provide a friction free medium.
This steel plate was provided to reduce the possibility of adhesion of clay with wooden plank during the process of drying shrinkage.
(ii) The beam was properly enveloped with a propylene fabric sheet during the process of compaction (Fig. 3). The compaction of beam using this treatment is called filter press condition.
(iii) During drying, the compacted moist clay beams were covered with plastic sheet (Fig 4).

RESULTS AND DISCUSSION
After applying the compacting load of 1.94 MPa, the beam was demoulded and allowed to dry in shed. On drying it showed vertical cracks which further opened with time as shown in Fig. 5. The cracking was due to un-controlled rate of evaporation of moisture from the surface of clay beam. To curtail the possibility of cracking of clay beam during drying, it was necessary to reduce the rate of evaporation. Thus the other beams were covered properly with a plastic sheet (Fig. 4) to control and reduce rate of evaporation. With this treatment, the cracking in the beam was eliminated except at the base which was in contact with the wooden plank (Fig. 6). A smooth steel plate with a double layer of plastic sheet was provided between the beam and wooden plank (Fig. 2) to eliminate the possibility of cracking at the base. With the application of these two treatments, none of the beams cracked during the process of drying (Fig. 7). With this treatment of covering the clay beam with plastic sheet during drying, the time of drying was increased.

FIG. 4. THE MOIST CLAY BEAMS COVERED WITH PLASTIC SHEET DURING DRYING
Due to oozing out of moist clay from the crevices and openings, when no treatment was applied to the beam during casting and compacting, the degree of compaction could not be increased above 1.94 MPa. Thus, in this condition, the density of clay beam could not be increased.
In order to achieve more density of the beam, it is to be

CONCLUSIONS
The main conclusions drawn from this study are: (i) The clay beams were cast, compacted and dried in shed showed a number of vertical cracks.
(ii) When the beams were cast and compacted using the devised treatments, possibility of cracking was completely curtailed and shrinkage was also decreased depending upon the level of compaction.
(iii) When the beam was compacted in filter press condition; water, added in the clay during mixing, was made ooze out. The amount of this water expelled due to compaction depended upon the degree of compaction.
(iv) The beams shrunk averagely up to 6.4% when they were compacted at 1.94 MPa. When the degree of compaction was increased to 7.2 MPa, the percentage of shrinkage was decreased to 0.64%.