Methodology in research

CHAPTER THREE: MATERIALS AND METHODS

3.1. Introduction

In this chapter, the methodology and materials used in the project are represented herein. It puts emphasis on the procedures followed in carrying out the project. It also elaborates the sampling procedures and preparation, data collection, processing and analysis.

Stabilized rammed earth is a form of rammed earth that uses sub soils combined with stabilizing agents to improve the materials physical characteristics. Soils for cement stabilized rammed earth tend to have proportionally higher sand and gravel content and correspondingly lower fines content

In rammed earth construction organic matter content should be avoided, as this may lead to high shrinkage and possible bio deterioration as well as increasing susceptibility to insect attack. Organic material also interferes with action of stabilizers such as cement.  In order to increase the mechanical strength and weathering resistance of soil it is advantageous to minimize the voids ratio in order to increase the contact between soil particles.

3.2. Manufacturing Process of Cement

Main ingredients used in the manufacture of cement are:

1. Limestone
2. Calcium
3. Clay, shale
4. Silica /Alumina
5. Quarrying
6. Local resources necessary:  no market

Limestone (CaCO₃) and Clay are two main raw materials used for manufacturing

3.2.1. Portland cement clinker.

3. Clays have various   amounts   of   SiO₂ and Al2 O
4. In the manufacturing process of  Portland   cement ,   clinker  consist   essentially   of grinding   the raw   materials,  mixing  them   in   appropriate   proportion,  burning  the  raw  material   in a  kiln   at   a  temperature   of  1400 – 1500⁰C  until   material  partially fuses  into  balls  known   as   Clinker   and   grind i ng  cooled   clinker   together   with  a small   amount  of   gypsum
5. The mixture of raw material is burned in a rotary kiln.

3.2.2. Type of cement and its physical properties

The cement used was from Hima cement; 32.5N, whose physical and chemical properties were obtained prior by the manufacturer and the testing certificate delivered.

• Setting Time

Cement  paste  setting  time  is  affected  by  a  number  of  items  including:  cement  fineness, water cement ratio, chemical content (especially gypsum content) and admixtures. Setting tests are used to characterize how a particular cement paste sets. For construction purposes, the initial set must not be too soon and the final set must not be too late. Normally, two setting times are defined but we do have other setting times:

1. Initial set: Occurs when the paste begins to stiffen considerably.
2. Final set: Occurs when the cement has hardened to the point at which it can sustain some load.  Setting  is  mainly  caused  by  C₃A  and  C₃S  and  results  in  temperature  rise  in  the cement paste.
3. False set: No heat is evolved in a false set and the concrete can be re -mixed without adding water. It occurs due to the conversion of un -hydrous/semi -hydrous gypsum to hydrous gypsum(CaSO4.2H2O)
4. Flash Set: is due to absence of Gypsum. Specifically used for under water repair.

The setting time test is conducted by using the same Vicat apparatus, except that a 1mm diameter needle is used for penetration. These particular setting times are just arbitrary points used to characterize cement; they do nothave any fundamental chemical significance.   Both common  setting  time tests,  the Vicat needle and the Gillmore needle, define initial set and final set based on the time at which a needle of particular size and weight either penetrates a cement paste sample to a given  depth  or  fails  to  penetrate  a  cement  paste  sample. The Vicat needletest is more common and tends to give shorter times than the Gillmore needle test as per ASTM C 150.

• Soundness

When referring to Portland cement, “soundness” refers to the ability of a hardened cement paste to retain its volume after setting without delayed expansion. This expansion is caused by excessive amounts of free lime (CaO) or magnesia (MgO).  Most Portland cement specifications limit magnesia content and expansion. The cement paste should not undergo large changes in volume after it has set. However, when excessive amounts of free CaO or MgO are present in the cement, these oxides can slowly hydrate and cause expansion of the hardened cement paste.

Soundness is defined as the volume stability of the cement paste.

ACI prescribe a Soundness Test con ducted by using the Le Chatelier apparatus.

• Fineness

Fineness,  or  particle  size  of  Portland  cement  affects  Hydration  rate  and  thus  the  rate  of strength gain. The smaller the particle size, the greater the surface area-to -volume ratio, and thus, the more area available for water- cement interaction per unit volume. The effects of greater fineness on strength are generally seen during the first seven days. When the cement particles are coarser, hydration starts on the surface of the particles. So the coarser particles may not be completely hydrated.  This causes low strength and low durability.  For a rapid development of strength a high fineness is necessary.  There are various methods for determining the fineness of cement particles. The Blaine air -permeability method is the most commonly used method.

• Strength

Cement paste strength is typically defined in three ways: compressive, tensile and flexural. These  strengths  can  be  affected  by  a  number  of  items  including:  water-cement  ratio, cement -fine aggregate ratio, type and grading of fine aggregate, curing conditions, size and shape of specimen, loading conditions and age.

3.2.3. Duration of Testing

Typically, Durations of testing are:

1. 1 day (for high early strength cement)
2. 3 days, 7 days, 28 days and 90 days (for monitoring strength progress)
3. 28 days strength is recognised as a basis for control in most codes.

When considering cement paste strength tests, there are two items to consider:

1. Cement mortar strength is not directly related to concrete strength. Strength tests are done on cement mortars (cement + water + sand) and not on cement pastes.
2. Soil used to form rammed earth structures was free from organic material and other non – soil substances, such as rubbish, deleterious material, etc.

Soils for rammed earth had about 50% to 70 % (can be achieved by blending) fine gravel and sand, 15% to 30 % silt and 5% to 15% clay. This was obtained through the particle size distribution. This was done in accordance with BS Soil mixtures were tested by the ‘roll’ method, and the break off should be between 80 mm and 120 mm. Deleterious material in this context means soil containing salts such as sulphates which interfere with the setting of the binder.

In this case we used soil excavated from the site and it already was identified with components and whether it needs other additives to enhance its characteristics. A series of field tests were conducted using a sample from the site taken from a depth of 1m, to ensure that the surface organic materials are not included.

3.3. The jar test “Particle size test”

In order to know the proportions of different particle sizes of the soil, the jar test was used. That is to get a preliminary assessment of the ratio of coarse to fine particles in the soil. Two thirds of a bottle is filled by soil taken from the site, and water was added to fill the bottle. The bottle was shaken till all the soil particles were suspended then it was left to settle for a few hours. As the water cleared, you can see the formation of different soil layers separated by clearly visible lines. The sand layer settled at the bottom as its particles are heavier, then layers of silt and clay stays on top.

The material attained was excavated to depth of 7m below the ground surface within the structural vicinity, it was then piled up into heaps so as not to lose the peripheral soil when it gets mixed up with top soil and impurities. We ensured that the heap is compact so as it is free from external influences. Also, ensured that it does not get too wet or too dry, otherwise it would require that it comes back to the OMC (optimum moisture content).  Thereafter the material was sieved in order to aerate the soil and as well remove lumps, stones and pebbles.

The material i.e. soil, sand and stabiliser components were then measured, filled in the container with accuracy per specification. The material was then mixed by hand tools, i.e.; hoes and spades.

The mixing process was done in the following order;

1. Pour in order i.e. soil, stabiliser(cement)
2. The material is 1^st mixed dry 2 to 3 times
3. Add water and mix it wet, 2 to 3 times

When mixing the material, we ensured that it is at its Optimum Moisture Content (OMC). This helps attain Maximum Dry Density (MDD), therefore attaining maximum strength.

Tests were as well carried out in the lab to obtain compressive tests for rammed earth structure which are included in the appendices. The mixed material was then transported from the mixing point to the building area using wheel barrows. Then a mechanism was developed of pouring the material into the formwork.

Formwork was set out and checked with the plumb line to ensure the panels were vertical. The material was then poured into the form work, the soil present must be loosened and slightly humid and pour layers of only 120mm thickness.

Checking the layer thickness, before ramming, we used a layer gauge (6mm diameter MS void) to check the thickness of the loose soil, level the layer evenly (add or remove some soil)Ramming; we first rammed on the sides of the panels and then in the center and rammed the loose soil until it produces a clear sharp sound.

Removing the panels; after finishing the ramming, we gave it about 2-3 hours to set then we dismantled the formwork and proceeded further in the same way. Never keep a formwork in place over night.

The rammed earth structure was constructed with a wall thickness of 450mm on the ground floor and 350mm on the upper floor. The walls were constructed with this thickness to cater for thermal mass especially in the most tropical parts of the country.

During this construction there were false walls constructed as well which were used to accommodate high end home entertainment systems to project screens and conceal speakers.

Ratio used was 1:7 (1 bag of cement and 7 wheel barrows of excavated soil). Column sizes of 400 * 400mm and beam sizes of 400 * 150mm.

In the Traditional burned clay bricks construction consists of building structures by laying individual masonry units with bricks. These are laid with cement mortar, which binds them together to create a structure; these type of construction provides beautiful walls and floors usually at economical prices.

Using a2MPa compressive strength criterion as the measure of stabilization success, soil property value ranges were related to the proportion of samples exceeding the criterion. Linear shrinkage (LS) and plasticity index (PI) are found to be the best discriminators of soil predisposition, with textural variables being useful secondary discriminators. “Favourable” soils, with stabilization success rates of ≥80%, include those with: (1) LS<6.0% and PI<15%; and (2) LS 6.0–11.0%, PI 15–30%, and sand content <64%. These soils were stabilized with treatments averaging 4.2% cement and 1.8% lime, with individual treatments ranging from 4–8% total cement and/or lime. “Unfavourable” soils, with stabilization success rates of <60%, include those with LS 6.0–11.0, PI 15–30, and sand content ≥64%, or with LS>11.0, PI>30. These findings should assist rammed earth engineers to more easily select a suitable soil and to minimize resources spent on preconstruction stabilization trials.