1. Literature review
1.1. Introduction
Construction materials are the main resources which are consumed in construction industry. From those widely used construction materials aggregates are the major and are naturally available all over the world. The fine and coarse aggregates generally occupy 60% to 75% of the concrete volume (70% to 85% by mass) and strongly influence properties of fresh and hardened concrete, mix proportions and economy (Abebe 2005).
Fine aggregates usually consists of natural or crushed stone sand which contains particles mostly smaller than 5 mm (0.2 in.). Coarse aggregates consists of gravels or crushed stone particles which contain predominantly larger than 5 mm (0.2 in.) and mostly between 9.5 mm and 37.5 mm (3⁄8 in. and 11⁄2 in.), though, aggregate size more than 37.5 is also familiar. Some natural aggregate deposits, called pit-run gravel, consist of gravel and sand that can be readily used in concrete after minimal standardization and processing (Abebe 2005). Natural gravel and sand are usually dug or dredged from a pit, river, lake, or sea bed. In other side crushed stone is produced by crushing quarry rock, boulders, cobbles, or large-size gravels. Crushed air-cooled blast-furnace slag which is produced from waste of metallurgical industry is also used as fine or coarse aggregate (Steven 2003).
The poor quality of single material could affect the total construction work and may leads to failure, so identifying the quality and property of fine aggregate helps to construct the intended construction projects with good quality.
1.2. Definition of fine aggregate
As most scholar defines that, sand is a naturally occurring granular material composed of finely divided rock and mineral particles. It is characterized by size which is being finer than graver and coarser than silt. Sand can also refer to a textural class of soil type containing more than 85% sand sized particles by mass. The composition of sand varies depending on the local rock sources and conditions, but the most common consistent of sand are silica (silicon dioxide) and calcium carbonate (ACI 2003).
1.3. Use of fine aggregate
Sand has been used widely in the construction industry for many hundreds of years. Even during Roman times, sand was widely used in the production of different tiles and cooking materials (The vital sand museum 2016). Aggregates are used as a stable foundation, as a road/rail base with predictable uniform properties (to help prevent differential settling under the road or building) and as a low-cost extender that binds with more expensive cement or asphalt to form concrete.
As Denamo (2005) clearly states that, aggregates have three basic functions:
• To provide a relatively cheap filler for the cementing material;
• To provide a mass of particles that are suitable for resisting the action of applied loads, abrasion, the percolation of moisture, and the action of weather; and
• To reduce the volume changes resulting from the setting and hardening process and from moisture changes in the cement-water paste.
• For the production of different construction materials. Like; glass, lime
1.4. Source of sands
As many scholars agreed that, sand and gravel are widely distributed and abundant near existing and past rivers and streams, in alluvial basins, and in previously glaciated areas. Even though there is a wide distribution, these aggregates are not universally available for use. Where the locality lacks the aggregate source, the costly alternatives of importing aggregate from outside the area or substituting another material for aggregate is considered. Basically, the sources of natural fine aggregate are of three types (type of sand used in construction 2013; type of sand for construction 2013).
Pit sand (Coarser sand): It is a source for different types of sand which can be obtained from deep pits of abundant supply. It has a property of being grained which is sharp, angular and free from salts. It mostly has a reddish yellow color and mostly employed in concreting.
River sand: These type of sand is mostly obtained from near and beds of rivers. River sand has the property of being fine and consists of fine rounded grains. The color of river sand is almost white and grayish. River sand is usually available in clean condition and issued for concreting and plastering.
Marine sand: This type of sand is taken from sea shores. It has fine rounded grains and it is light brown in color. Sea sand is avoided for the purpose of constructing concrete structure since it contains salt and tends to absorb moisture from the atmosphere and brings dampness.
1.5. Production of natural river sand
River sand is a commonly used natural sand which is available in most river basins which flows through erodible and rocky soils. In Ethiopia the production of sand from rivers are mainly done by the local grouped peoples with no standardized machinery and in a very primitive way. In developed countries there is an organized way of production which is done by using different machineries. The quality of produced sand is depend on the production process, though the nature of sand is the main characteristic of quality. However the production of sand is somewhat different from other mining practices, the general acceptable processes in production of any ore contain the following activities;
Exploration
The success of the project depend mainly on the extent knowledge and value of the mineral ore deposit. A precise information about the location, characteristics and value of the mineral ore deposit is collected during the exploration phase. This includes surveys, field studies, and drilling sample boreholes and other experimental excavations.
The exploratory phase may include clearing of wide areas of trees to allow the entry of heavy vehicles mounted with drilling rigs. Many countries require a separate sensitive area or are near previously isolated communities. The mining project which involves in the construction of any access roads should include exhaustive assessment of the environmental and social impacts of these roads. In sand mining project the exploratory phase should contain the assessment on flow of water and effect of mining on the life style of the local community.
Mining
After construction of access roads and preparation of staging areas, mining may proceed. All types of active mining have a practice of the extraction and concentration of a minerals from the earth. In most cases the proposed mining projects differ considerably in the planned method for extracting and concentrating the ore.
Sand and Gravel Processing
After the processing of sand next to excavation, the removal of grit and unwanted material, though the processing became more complex as the uses of sand expanded. In the initial days of quarrying, processing was as basic as a casual throw of the sand through an angled sieve placed over the carriage.
Transportation
The produced sand from the river bed is transported to the construction sites by the consumer or to the storage by the supplier for sale to be ready.
1.6. Quality measurement of fine aggregate
In construction industry, there are different standards which regulate the minimum and maximum requirement with related to construction activities and materials. As other construction materials the quality of sand has an effect on construction industry in the achievement of the project goals, So that, the standard properties have been recommended by different bodies in order to obtain the preferred quality of sand. The specified requirements of sand which is used in construction industry as recommended by different scholars and regulatory standards has discussed below.
Silt Content
In consumption of sand silt and crusher dust may be present either as a surface coating or loose material mixed with sand particles. In both forms silt and clays should not allow to be present in high quantities because of that it decreases the fineness modulus and increase the surface area, further this increases the amount of water necessary to wet all the particle in the mix (Neville 2010). As recommended in Ethiopian standard, the maximum allowable silt content of sand which is used in construction shall be 6%, so that silt content above this value needs to be treated in order to removing the silt or clay.
Grading
The particle size distribution of fine aggregate mostly depends on the type of construction, the richness of the ingredient mixture and the nominal maximum size of coarse aggregate. In leaner mixtures in which small size coarse aggregates are used, a grading that approaches the maximum recommended percentage passing of each sieve is desirable for workability. In a case of that the water to cement ratio is kept constant and the ratio of fine to coarse aggregate is chosen properly, a wide range in distribution can be used without measurable effect on the strength of concrete. However, the best economy, strength and durability will sometimes be achieved by adjusting the concrete mixture to suit the distribution of the locally available aggregates (Steven 2003). In general, the fine-aggregate particle size distribution, which is recommended by ASTM C 33 (2007) and ASTM C 136 (2006) is mostly considered as normal for most concrete structures.
Table 1, fine aggregate grading limit as recommended by ASTM C 33 and AASHTO M 6
Sieve size Recommended percentage passing by mass
9.5 mm (3⁄8 in.) 100
4.75 mm (No. 4) 95 to 100
2.36 mm (No. 8) 80 to 100
1.18 mm (No. 16) 50 to 85
600 µm (No. 30) 25 to 60
300 µm (No. 50) 5 to 30 (AASHTO 10 to 30)
150 µm (No. 100) 0 to 10 (AASHTO 2 to 10)
In addition to the standard percentage pass, ASTM C 33 (2007) recommends the following requirements to be considered in grading of fine aggregate.
• The fine aggregate weigh retained between two consecutive standard sieves should not be more than 45%.
• The fineness modulus must be between 2.3 and 3.1and should be uniform with no more than 0.2 difference from the typical value of the aggregate source. If this value is not attained the fine aggregate should not be allowed for use, except suitable adjustments are done in proportions of fine and coarse aggregate.
• The amounts of sand which pass through the 300 µm (No. 50) and 150 µm (No. 100) sieves affect workability, surface characteristics, air content, and mix water bleeding of concrete. In most specifications allowable passing through 300 µm (No. 50) sieve is 5% to 30% to pass.
The lower limit may be sufficient for relaxed concrete placing conditions and where concrete is mechanically finished. However, for concrete floors to be finished by hand and where a smooth surface texture is wanted, fine aggregate with more than 15% of pass in 300 µm (No. 50) sieve and more than 3% pass in 150 µm (No. 100) sieve should be used (Steven 2003).