Classification and Properties of Oyster shells

Classification of Oyster shells

Due to the unreliable morphological characters of oyster shells, they are difficult to classify. So, classifications are made according to their shell morphology which varies with environmental conditions and where it seems to be difficult, DNA sequence data has been used to resolve the issue, Reece et al (2008), Wang et al(2008).

Therefore, Oyster shells as classified by Guo et al; (1999) is given below:

True oysters: are mollusks, of the family Ostreidae which mainly belong to the genera Ostrea, Crassostrea (cupped Oyster), Ostreola, and Saccostrea. They are mostly edible and examples include the; Belon Oyster, eastern Oyster, Olympia Oyster, Pacific Oyster, and the Sydney rock oyster.

Pearl oysters: are not closely related to true oysters as they possess feathers, being members of a different family (Pteriidae), they can be obtained naturally or cultured.Others are Thorny oysters (Spondylidae), Pilgrim oysters (a kind of scallop), Saddle oysters (Anomia ephippium)

Oyster shells as coarse aggregates

Several common physical properties of aggregates are relevant to their behavior in concrete and to the properties of the concrete they are made with, therefore, the suitability of oyster shells as light aggregates will be checked by the physical properties as follows;

a) Particle shape: The shape of aggregate can be regarded as regular or smooth aggregate and irregular and angular aggregate as differentiated by Neville and Brooks, (1987).

Flynn (2000) also said the shape of an aggregate influences paste placement characteristics such as workability and pump ability, strength and cost. Shape is related to sphericity, form, angularity, and roundness Quiroga and Fowler (2004); Galloway (1994).

The shape of natural aggregates depends on the strength, abrasion resistance, and on the degree of wear to which they have been subject in their depositional environment. Natural aggregates tend to be more spherical and less angular.

On the other hand the shape of manufactured aggregates depends on the rock type and the crushing equipment. The shape of an aggregate influences the workability of the mixture as well as the void content and packing density. For some amount of paste, a mixture with round or cubical-shaped aggregate will have better workability than a mixture with flaky and elongated aggregates. More over; for the same mass of aggregate, round and cubical aggregate produce mixture with higher packing, which results in a lower void content.

b) Particle surface texture: The surface texture of aggregate is based on the degree to which the particle surface is polished or dull, smooth or rough. These usually affect the bond between aggregate and cement paste, and thus influence the water demand of the mix.

It also influences the workability, quantity of cement and bond between particles and the cement paste. Two independent geometric properties are the roughness or rugosity [degree of surface relief] and the roughness factor [the amount of surface area per unit of dimensional or projected area] [Graves 2006]. Natural aggregate have a smooth surface [Lamond and Pielert 2006]. Natural gravel subject to transport mechanisms tends to be smoother than manufactured aggregates. For instance, gravel would have a surface smoother than a crushed limestone. Nevertheless, there is no reliable method to determine the surface texture of manufactured aggregate [Alin and Fowler 2001]. An improvement in the bond to the matrix is obtained as the surface texture increases [Alin and Fowler 2001].

Rough-texture angular grains bond better with the cement paste to generate higher tensile strengths [O’ Flynn 2000]. Although rough textures lead to better bond between paste and aggregate, they also leads to harsher mixture, as texture roughness increases, the internal friction increases between the aggregates and therefore more paste is needed to achieve a given workability.

c) Soundness of aggregate: This is the ability of the aggregate to resist excessive changes in volume as a result of changes in physical conditions. So aggregate is said to be unsound when its volume changes [Neville, 2000]. Moisture content: The moisture absorbed with the surface water content of aggregate used for making concrete varies considerably.

The water added when mixing must be adjusted to take account of the changes if the surface water content must be kept constant and the required workability and strength of concrete maintained. Coarse aggregate rarely contain more than 1% of surface moisture while fine aggregate can contain excess surface moisture of 10% [Neville, 2000].

d) Grading: The gradation of an aggregate is defined as the frequency of a distribution of the particle size of a particular aggregate [Lamond and Pielert 2006]. Grading limits are specified in ASTM C 33 section 6[ASTM C 33]. Gradation plays an important role in workability, segregation, and pump ability of the concrete. Grading changes are more prevalent than shape and surface texture in the case of coarse aggregate.

For example, uniformly distributed aggregate require less paste which will also decrease bleeding, creep and shrinkage while producing better workability, more durable concrete and higher packing [Quiroga and Fowler 2004]. A graded aggregate, as opposed to a single-size aggregate, will have a greater packing density.

The smaller aggregate will fill in the voids created by the larger aggregates [Lamond and Pielert 2006]. Optimization by blending more than two aggregates at a time is not recommended, especially if each fraction complies with ASTM C 33 grading separately [Quiroga and Fowler 2004].

The void content is affected by the particle size, grading and packing efficiency. When a portion of two aggregates are combined and placed in a single container, the quantity of water needed to fill the voids for the same volume decreases. Thus, combining aggregates of different size fractions reduces the void ratio.

Thermal properties: The three thermal properties of aggregate that can be significant in the performance of concrete are specific heat, coefficient of thermal expansion and conductivity.

The specific heat and conductivity are important in mass concrete while the coefficient of thermal expansion of aggregate influence the value of such coefficient of concrete. The coefficient of concrete depends on the aggregate content that is content on the mix because the higher the coefficient of thermal expansion of aggregate the higher the coefficient of thermal expansion of the concrete [Neville, 2000].

Mechanical properties: Mechanical properties are very important properties in the determination of aggregate and the production of concrete.Below are some of the mechanical properties;

Hardness: This is an important property of concrete used in floor slab subjected to heavy loads.

Toughness: The resistance of aggregate to failure by impact is referred as toughness and it’s determined by aggregate impact value of bulk aggregate.

Oyster shells as substitute for coarse aggregates in normal concrete

Experiences and research studies have shown that under specific conditions, oyster shells has the potential to produce strong, durable structural members suitable for use in the construction industries. The coarse aggregate portion of concrete made with oyster shells has no significant adverse effects on desirable mixture proportions.

From the investigations of Yang et al, (2005); Crushed oyster shells were substituted for fine aggregate in concrete and the investigation revealed that the concrete mixture made with oyster shell did not cause reduction in the compressive strength of concrete at 28 days. Furthermore, it was reported that development of compressive strength was faster as substitution rate of oyster shell increased.It has also been discovered that waste shells such as oyster shells has been in use for civil construction activities like columns, lintels (beams), septic tanks, water well casing ring, bridges, shoreline protection, elevated concrete slabs, speed breaks, road constructions etc as a complete replacements for granite, due to erosion, flooding and lack of granite chippings in the coastal communities thereby, managing the wastes effectively without failure.

Oyster shells as structural member

In the early 1950s, the use of lightweight concrete blocks was accepted in the UK for load bearing inner leaf of cavity walls. Soon thereafter the development and production of new types of artificial LWA (Lightweight aggregate) made it possible to introduce LWC of high strength, suitable for structural work. These achievements further encouraged the structural use of LWA concrete, particularly, where the need to reduce the weight of a structure is essential, for consideration of economy in design.Concrete with full structural efficiency contain aggregates which fall on the other end of the spectrum scale and which are generally made with expanded shale, clay, slates, slag, and fly-ash. Minimum compressive strength is 17.24 N/mm².

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