LITERATURE REVIEW
2.1 BLOCKS AND BLOCKWORK
After the Second World War, other methods of building houses were abandoned for methods which favour the use of sandcrete blocks. The advantage of using sandcrete blocks is that they have better thermal insulators especially those made from light weight aggregate.
Another reason for preference of sandcrete blocks is availability compared to bricks for instance, the techniques for the production of sandcrete blocks appear simple thus favouring its preference.
Sandcrete blocks are made of dense or light weight aggregate and are formed either hollow or solid. The dense sandcrete has a mixture of cement and aggregates of sand while those of light weight are aggregate of clinker, expanded clay, foamed slag, sintered fly-ash, pumice, expanded vermiculite and aerated concrete as prescribed in BS 6073.
(Hamza, et al, 2009), Sandcrete nine inches blocks must satisfy building specification by laws with respects to the compressive strength. The thickness of the blocks ranges from 50-225mm.
According toNene (2009), British standard BS 2028, 1364 defines blocks as a walling unit with dimensions greatly than brick specified in BS 3921.
According to (Edward 1985),The blocks can cope with thermal and moisture condition and the problem of algae growth on the face of block work during construction is unlikely to affect the strength of the block. The compressive strength of hollow sandcrete nine inches block increases by adding optimum quantity of water which will also have an impact on the mix a and workability.
They are formed in moulds and compacted either by pressing, tamping or vibrating. After moulding, they are allowed to harden naturally. They are used in both external and internal load bearing, curtain, partition, panel walls and many other construction processes.
According to Obande (1990), for the blocks to achieve its constructional purpose, it has to be made in a configuration or shape so as to support its imposed loads both life and dead loads. To achieve this, the blocks are laid in regular patterns such that it is resting at least partly on two blocks and this arrangements must present a pleasant surface appearance and avoid straight (vertical continuous) joints as much as possible when laying and this phenomenon is called bonding.
2.2 DEFINITION OF CEMENT
Cement refers to accurately any adhesive and the material used in connection with block and it is referred to as hydraulic cement. This is because the setting and hardening of cement depends on the presence of water.
The essential properties of cement is the ability to harden by a chemical action when mixed with water which is known as hydration.
According to B.S 12 1978 and the American standard (A.S.T.M.C 150 – 84) cement primarily made from lime (Cao), silica (SIO2) Allumina (AL203), iron oxide (Fe202) etc, these materials that present are mainly a compound being the silicates and aluminates of calcium.
2.3 PORTLAND CEMENT
Portland cement is the name of manufactured material used in making blocks and is mainly responsible for it properties of strength and hardness.
Portland cement is a proper retary name given to the material by its investor Joseph Aspadin. There are several types of Portland cements and several brands of each type. Since each type of cement irrespective of the name confirms to a certain standard, which is described in a British standard specification.
TYPES OF PORTLAND CEMENT
ordinary Portland cement
rapid hardening Portland cement
Portland Blast furnace cement
Low heat Portland cement
2.3.2 CHEMICAL PROPERTIES OF PORTLAND CEMENT
Tricalcium aluminates (3Ca0.AL203(C3A)
The tricalcium silicate (3Ca0.SI02(C3S)
Tetracalcium alumino ferrite (4(a0.AL203.fe203(C4Af)
Dicalcium silicate (2Ca0.SI02(C2S)
Tricalcium aluminate (3Ca0.AL203(C3A)
2.3.3 FUNCTIONS OF THE CHEMICAL PROPERTIES OF PORTLAND CEMENT
Tricalcium aluminates are responsible for initial setting rapid hydration with water.
Tricalcium silicates are responsible for the early gain of strength considerable heat evolved.
Tetracalcium alumino ferrites are responsible for the acceleration of hydration of silicates
Dicalcium silicates are responsible for slow hydration of water and the little evolved.
Tricalcium aluminates are responsible.
Table 1: shows the main compounds of Portland cement
Raw material
Compound
Formular
Lime stone
Lime
Cao
Chalk
Carbondioxide
Co2
Clary
Silica
Sio2
Table 2: Shows the notation of compounds present in Portland cement
Notation of compound
Name of compound
Formular
C3A
Tricalcium alluminate
3 CaoAL203
C3S
Tricalcium silicate
3Ca0Si02
C2S
Dicalcium silicate
2CaOSi02
2.4 AGGREGATES.
This is a collection of items that are gathered together to form a total quantity. In my research I focused on fine aggregates.
2.4.1 FINE AGGREGATE (SAND)
This is the product of natural or artificial disintegration of rocks and minerals. Sand is an important constituent of most soil and is extremely abundant as a surface deposit along the course or rivers, on the shores of lakes and the seas and in arid regions. As the term is used by geologist sand particles range in diameter from 0.0625 to 2mm. An individual particle in this size range is termed “sand grain”. The next smaller size class in geology is silt, particles smaller than 0.0625 down to 0.004mm in diameter. Sand is commonly divided into five sub-categories based on size.
Very fine sand (0.0625mm – 0.125mm)
Fine sand (0.125mm – 0.25mm)
Medium sand (0.25mm – 0.5mm)
Coarse sand (0.5mm – 1mm)
Very coarse sand (1mm – 2mm)
(Wikipedia, 2007)
Fine sand is mostly used for plastering work with the sand grains passing through No. 16 ASTM sieve.
Medium sand is used for plastering work with the sand grains passing through the No. 8 ASTM sieve and coarse sand is best for concrete work, its particles must pass through the No. 4ASTM sieve.
2.4.2 GRADING OF FINE AGGRAGATE
There are four zones for the fine aggregate as stated is BS 882 of 1978, which also states that any batch whose grading falls exactly within one zone is suitable. This further stated that the division into zone is based primarily to the percentage passing the 66 sieve shown below:
Table 3: Shows the grading requirement for fine aggregate.
Sieve size
Grading zones (BS 882: 1978)
(BS)
(BS 882: 1983)
Percentage weight passing sieves
Zone 1
Zone 2
Zone 3
Zone 4
9.5mm
100
100
100
100
4.75mm
90-100
90-100
90-100
95-100
2.36mm
60-95
75-100
85-100
95-100
1.18mm
30-70
55-90
75-100
90-100
600mm
15-34
35-59
60-79
80-100
300mm
5-20
8-30
12-40
15-50
2.4.3KINDS/SOURCES OF SAND
Sand is classified based on its source e.g. pit or quarry sand, river sand, sea sand etc. Pit or quarry sand is formed as deposits in soil and has to be excavated out. Grains of it are generally sharp and angular. If it is free from organic matter and clay, it is extremely good for use in mortar and concrete.
Excavated sand: Excavated sand is aggregate excavated from borrow pit in Amagu in Nsude, Udi Local government area of Enugu State. From an engineering point of view, laterite or lateritic soil is a product of tropical weathering with yellow-white, reddish brown and dark brown colour, with or without nodules or concretions and generally found below hardened ferruginous crusts or hard plan (R.C. Okoloekwe).
River sand:It is obtained from the banks and beds of rivers, and may be fine or coarse. There are chances of fine sand having silt and as such should be washed before use. Coarse sand is generally clean and is excellent for all purposes.
Sea sand:It consists of fine rounded grains of brown colour and is collected from sea beach. It usually contains salt which attracts moisture from the atmosphere and causes disintegration of the work in which it is used. It could be used locally after it has been thoroughly washed to remove the salt.
Crushed stone: It is obtained by crushing waste stone of quarries to the particle size of sand. Stone crushed from a good quality stone is an excellent fine aggregate.
2.5 WATER
The strength and workability of sandcrete depends greatly on the amount of water – cement ratio. The purpose of using water is to cause the hydration of cement. Water to be used for the production of concrete or sandcrete must be free of suspended particles, inorganic salts, acids and alkalis, oil contamination and algae. In the production of sandcrete blocks, is necessary for mixing cement and sand, to wash aggregates and in curing of blocks after manufacturing them. Potable water is recommended for use in the production of sandcrete blocks (NIS, 2007).
2.6MIX PROPORTIONS.
Mix used for sandcrete blocks shall not be richer than one (1) part by volume of cement to six (6) parts of fine aggregate (sand) except that the proportion of cement to mixed-aggregate may be reduced to 1:4 or 1:2 (Where the thickness of the web of the block is 25mm or less).
2.6.1 STRENGTH REQUIREMENTS.
Sandcrete blocks shall possess resistance to crushing as stated below and the 28 day compressive strength for a load bearing wall of two or three storey building shall not be less than:-
Average strength of blocks.
Lowest strength of Individual block
2.00 N/mm2 (300 psi)
1.75 N/mm2 (250 psi)
National building code 2006
2.6.2 MOULDING AND COMPACTION.
Moulding in sandcrete block production is the casting of the sandcrete into shape with rigid moulds of different sizes (NIS 87:2000).
The initial set of Portland cement takes place from half to an hour after it is mixed with water. This implies that the already mixed sandcrete must be cast into moulds before the initial setting time takes elapses and should be disturbed no further for the production of more efficient blocks.
Once cast into moulds, sandcrete blocks should be thoroughly compacted which is achieved by a vibrator usually powered by a lister diesel engine which is the one used in most block moulding industry. Compaction generally causes entrapped bubbles of air to rise to the surface in order to produce dense, compacted and void free block of uniform strength.
Two methods of compaction are generally acceptable depending on the availability of materials (tools).
(1) By approved (standard) machine compaction usually by vibration.
(2) By metal mould (hand) compaction.
2.6.3PRODUCTION/PROCESSING.
The sandcrete block shall be cast using an appropriate machine with cement/sand ratio of 1:6 measured by volume. Where hand mixing is carried out, the materials shall be mixed until an even colour and 11consistency throughout is attained. The measure shall be further mixed and water added through a fire hose in such sufficient quantity as to secure adhesion. It shall then be well rammed into moulds and smoothed off with a steel face tool.
2.6.4CURING
After removal from the machine, the blocks shall be left on pallets under cover in separate rows, one block high, with a space between each block for at least 24 hours and kept wet by weathering through a fire watering hose. The blocks may then be removed from the pallets and stacked during which time the blocks shall be kept wet. The blocks may be stacked not more than five blocks high under cover at least seven (7) days before use after the previous period.
2.7PROPERTIES OF SANDCRETE BLOCKS.
2.7.1 Strength.
Provided the aggregate is unaltered (i.e. used of specified grade and type of aggregate), a definite relationship exists between strength and density (Neville, 1996). The strength of sandcrete blocks depends on the aggregate type, richness of mix and the degree of compaction (Orchard, 1979).
Cement and sandcrete block association (1970) shows that most sandcrete blocks are about 15% stronger when dry than when wet. BS 6073:(1981) specifies that sandcrete block of thickness 75mm or greater should have an average crushing strength for ten blocks not less than 2.8N/mm2 and that the corresponding crushing strength of any individual block shall not be less than 80% of the minimum permissible average crushing strength i.e. not less than 2.10N/mm2 (BS 6073: 1981).
Obande (1990) also states that it mustn’t be less than 80% at the average value. The National Building Code has specified the minimum strength requirements of 2.00N/mm2 for sandcrete blocks.
2.7.2 THERMAL INSULATION.
The thermal insulation of buildings is necessary to achieve the desired degree of thermal comfort for the occupant at the minimum cost possible. This can be made through the selection of the right materials offering high degree of insulation with little or no extra cost. The insulating property of a building can be controlled from the sandcrete blocks to be used in the construction by instilling the insulating properties from the point of manufacture of the blocks.
The property of sandcrete block that can instill this thermal insulating property is by the production of less dense blocks by employing light weight aggregate. Light weight sandcrete blocks provide good thermal insulating property. The level of insulation is inversely proportional to the density of the material. In general, the lighter and more porous the block, the better will be its insulating value (Oberd, 1990).
2.7.3 FIRE RESISTANCE AND DURABILITY.
Sandcrete blocks are excellent fire resistant material for walling construction. CP111 (1953) and the Building Regulation (1985) calls for an unplastered thickness of 100mm for 2hours fire resistance for sandcrete block walls.
The performance of sandcrete block under fire conditions depends largely on the aggregates used. Studies have shown that a loss of compressive strength begins at temperatures soon after 100oC and blocks may loose as much as 80% of its strength at 4500C (Addleson, 1976).
The durability of sandcrete blocks refers to their ability to withstand alternate wetting and drying and their weather resistant properties.
2.8 WATER ABSORPTION AND EFFECT OF ADMIXTURE OF SAND.
Light weight aggregates used in blocks are porous and have higher water absorption than dense blocks (Andrew and William, 1978). The more the water cement ratio, the more porous the sandcrete block will be.
The effect of the preference of the admixture of quartz-river sand over light weight sandcrete mixes is a common practice to increase the compressive strength and the bond of the resulting sandcrete blocks. This improves the durability, workability and reduces the cement requirements and the shrinkage of the block. The shrinkage of light weight sandcrete blocks is reduced by the addition of sand which acts as a stability element in the structure of the matrix (Andrew and William, 1978).
2.9.1 FACTORS AFFECTING LIQUID ABSORBILITY
Absorbing the liquid depends on the hiberstitial arrangement of the particles of the constituent materials at macro level. If is therefore necessary instigates the effects of cement with other constituents in the molding of nine inches block. Some of the properties are:
Porosity
Presence of admixtures may increase, decrease or maintain the porosity of the main material depending on the aggregate sizes. When imposed to persistent flooding, a highly porous block could absorb much water consequently become weak ended and eventual fall. The volume of liquid absorbed by a porous medium is an indication of its pore volume and it is a good approximate measure of its porosity.
Permeability
The term permeability is often loosely used to cover a number of different properties. In this paper, it is defined as the property of porous medium which characterizes the ease with which a fluid will flow in a permeable medium empresses permeability in terms of measurable quantities and states that the steady state rate of flow is directly proportional to the hydraulic gradient.
Sorptivity
The use of blocks for external walls in tropical humid climate, water – resistance ability of the blocks must be considered in order to minimize penetration of moisture or rain water into the building. Many times, block work is used in the construction of channels for drainage. Blocks to be used in such purposes must have low sorptivity value. Sorptivity is a measure of the capacity of a porous medium to absorb liquid by capillarity.
(Hall, 1989), the absorption of water under capillary action is directly proportional to the square root of time. Various test method are used to determine hygrothermal properties of a material. However, in most cases, the test method chosen for a particular property is always the one appropriate to the predominant transport mechanism acting on the block.
After evolution of various methods capillary rise method is employed in this investigation due to its simpliarity and accuracy. Basically, a sample of sandcrete block is place with one edge filling in contact with water surface. Through that area, water is absorbed. The height of capillary rise is then measured at increasing time intervals.
The funeness of the capillary pores in sandcrete blocks promotes absorption of water by capillary attraction; hence a measure of the rate of absorption provides a useful indication of the pore structure. If water is absorbed rapidly it shows that the pore are either large or straight; it the absorption rate is slow, then the pore are small or not easily accessible.
THERMAL PROPERTIES
Thermal properties of most cementitious materials are found to change with the presence of admixtures (Cisse and Laguabe, 2000; Okpala, 1993). The change is found to depend on the admixtures grain structure or interstitial arrangement within the main material and other micro structural parameters including the volumetric fraction of each constituent the shape of the particles, and the size distribution of the particles. In predicting the thermal performance of buildings, it is necessary to consider the dynamic effects of this variation. The thermal properties investigated in this study are:
Thermal conductivity (k):
Thermal conductivity is a measure of the quantity of heat that flows through a material per unit time. Thermal conductivity of most materials is found to change with the presence of impurities or admixtures. From the fourier’s steady- state heat conduction equation, thermal conductivity is determined as:
Specific heat capacity (c)
It is a measure of the thermal storage capacity of the material. The specific heat capacity of a sandcrete block indicates the relative amount of heat energy of wall but with it is capable of storing per unit mass. Walls with high specific heat capacity can store more energy, have a larger thermal lag and thus, generally be more effective for thermal storage and peak load shifting. This time lag effect contributes to shifting demand to off- peak periods and improves overall thermal efficiency specific heat capacity of the sandcrete block is determined from the classical heat capacity equation.
Thermal diffusivity
Thermal diffusivity is a measure of the materials ability to undergo a temperature charge it describes the heat transfer capability of a material relative to its heat storage ability materials with a low thermal diffusivity have a slow rate of heat transfer relative to heat storage. Thermal diffusivity is obtained through the relation.
Thermal effusivity:
Thermal effusivity represents the capapcity of a material to absorb and release heat. The value of the thermal effusivity is useful in calculating the heat – accumulation capacity of materials with high thermal effusivity cannot hold heat long enough because heat will quickly disspate from its surface as soon as surrounding temperature drops. On the other hand, material with low thermal effusivity (But with high thermal inertia) will hold heat much longer.