Chapter Five: Design Standards and Specifications

1 Chapter Five: Design Standards and Specifications

1.1 DESIGN STANDARDS

A Standard is the limit of a measure of quality of a product prepared for judgement and compliance by an authoritative agency, professional or a recognized body. According to (Business Dictionary, 2020) Standards can be classified as: Government or statutory agency standards and specifications enforced by law,

Proprietary standards developed by a firm or organization and placed in public domain to encourage widespread use, and

Voluntary standards established through consultation and consensus and available for use by any person, organization, or industry. Once established, standards (like bureaucracies) are very difficult to change or dislodge. Standards that apply for Tanzania water projects will be from the Tanzania Bureau of Standards, and for construction works, British standards shall be used.

A list of institutions whose standards are recommended for use in water supply and sanitation projects design include the following:
(a) Tanzania Bureau of Standards,
(b) British Standards (BS),
(c) American Society for Testing and Materials (ASTM)
(d) Deutsches Institut für Normung (DIN); German institute for standardisation,
(e) American Association of State Highway and Transportation Officials (AASHTO),
(f) European Standards (ES).

Table 5.1 describes the standards codes of practise and relevant area for application in construction works.

1.2 SPECIFICATIONS

Specification is a detailed description of how work is to be performed or requirements to be achieved, dimensions to be met, materials to be used, standards to be followed and tests to be carried for the product to meet acceptance criteria. Specifications are normally drafted by the client to suit the need for a particular work. For the purpose of construction of water and sanitation projects standard specifications have been prepared for various woks as follows:
(a) Standard Specifications for Civil Works,
(b) Standard Specifications for Electrical works,
(c) Standard Specifications for Mechanical works and
(d) General Specifications.
These documents can be downloaded from the Ministry’s Website and customized to fit the needs of particular works.

1.3 MATERIALS

1.3.1 Building Materials

Building material is any material used for construction purposes such as materials for structures. Wood, cement, aggregates, metals,sand, bricks, concrete, clay. These are the most common types of building material used in construction. The choice of these materials is based on their quality and cost effectiveness for building projects.

1.3.2 Materials Testing

Before a material is to be used for construction work, it is imperative to conduct appropriate tests as per applicable standards. The following are the minimum tests proposed to be conducted on various construction materials.

Aggregates
Test of aggregates explained below includes both fine and coarse aggregates.

Flakiness indextest
Flaky particles are those whose least dimension is 0.6 times lesser than the mean size. Thickness of these particles are comparatively smaller than the other two dimensions.

Maximum allowable limit of the flaky particles in the mix is 30%. If it exceeds this value then the mix is considered unsuitable for construction purpose.

Flakiness index is the percentage by weight of flaky particles in a sample. The flakiness index is calculated by expressing the weight of flaky particles as a percentage of the total weight of the sample , test procedure is as outlined in BS – 812 ,1995.

Elongation index test
Elongated particles are particles having length considerably larger than the other two dimensions, also one dimension is 1.8 times greater than the other two dimensions.

Maximum allowable limit of the flaky particles in the mix is 30%. If it exceeds this value then the mix is considered unsuitable for construction purpose.

Elongation index is the percentage by weight of elongated particles in a sample. The elongated Index is calculated by expressing the weight of elongated particles as a percentage of the total weight of the sample, test method is expalined in BS– 812,1995

Flaky and elongated particles lower the workability of concrete mixes due to high ratio of surface area to volume. The presence of flaky and elongated particles also may cause inherent weakness in concrete with possibilities of breaking down under heavy loads.

Abrasion (Los Angeles Abrasion Test)
Abrasion test is the measure of aggregate toughness and abrasion resistance on crushing, degradation and disintegration. Tests for abrasion is conducted based on BS 812: Part 113: 1990.

Organic impurities test
Sand should be checked for the presence of organic impurities such as decayed vegetation, humus, and coal dust as these affect the quality of concrete. Test for organic impurities should be conducted as per. BS 812: Part 4: 1976.

Crushing value (ACV) test Aggregate crushing value test on coarse aggregates is a relative measure of the resistance of an aggregate crushing under gradually applied compressive load. The method for the determination of Aggregate Crushing Value (ACV) is the Code: BS 812 Part 110.

10% finer test
The 10 per cent Fines Aggregate Crushing Value (10% FACT) is determined by measuring the load required to crush a prepared aggregate sample to give 10% material passing a specified sieve after crushing. The test procedure is outline in line with code BS 812: 1990 Part 111.

Impact resistance value (AIC) test
The aggregate impact resistance value is a measure of resistance to sudden impact or shock. This value may differ from resistance to gradually applied compressive load. The procedure of Aggregate impact resistance value is provided in code BS 812 : Part 112 : 1990.

Grading–sieve analysis test
This is the classification of a coarse-grained soil based on the different particle sizes it contains. This aspect is important as it indicates the compressibility properties, shear strength and hydraulic conductivity. The standard gradation and sieve analysis test is provided under: BS 812: Section 103.1: Sieve Analysis of Fine and Coarse Aggregates.

Absorption test
Water absorption is the measure of the porosity of an aggregate. It gives an indication of the strength of aggregates. When more water is absorbed, the aggregates is more porous in nature and generally considered unsuitable unless found to be acceptable based on strength, impact and hardness tests. The standard method for Testing aggregates to water absorption test is according toBS 812-120:1989.

Specific gravity test
The specific gravity of an aggregate is the ratio of its mass to that of an equal volume of distilled water at a specified temperature. The standard method for Testing aggregates to determine the density is BS 812 : Part 2: 1995.

Chemical content (pH, chloride and sulphate) in aggregates test
This test aims at establishing the permissible levels of chlorides and sulfates in aggregate. Theigh levels of chemicals may result in deterioration of concrete by corrosion of steel reinforcement.Corrosion of steel affects serviceability and strength of concrete structures. The test to determine the content of chemicals in aggregates is conducted as perBS 812-Part 117 & 118:1988.

1.3.3 Water

Impurities test
Water for washing aggregates and for mixing concrete shall be in accordance with DIN 4030 and DIN 1045 and shall be clean and free from objectionable organic matter, alkali, salts and other impurities.

Chemical content (chloride, PH values, sulphate) in water
Samples of water being used or which is proposed for use for mixing concrete shall undergo testing for quality to determine the concentration of sulphates and chlorides, which shall be such that the concrete mix as a whole complies with the specified limit for salt content. Chemical content in water may be determined through procedure explained in the code APHA 21st:2005/ICP OES.

1.3.4 Cement

Setting time test
The settling time is the time required for cement to convert from a plastic paste to a non-plastic and rigid mass. The cement settling time is determined through procedure explained in the AASHTO T 131 and ASTM C 191: Time of Setting of Hydraulic Cement.

Compressive strength test
The compressive strength of cement is the measure of the strength it provides to the mix after it has hardened. The test enables the identification of the quantity of cement required and how much strength it will provide. The compressive strength of cement is the basic data needed for concrete mix design. Cement is basically identified by its compressive strength as grade 53 grade, 43 grade, 33 grade of cement. The test procedure is as per code of practice BS EN 196- 1:2005.

1.3.5 Concrete Works

Tests conducted for concrete includes:

Slump test
Concrete slump test or slump cone test is done to determine the workability or consistency of concrete mix prepared at the laboratory or the construction site during progress of the work. Concrete slump tests should be carried out batch to batch to check the uniform quality of concrete during construction.The slump is carried out as per the procedures mentioned in ASTM C143 in the United States, and EN 12350-2 in Europe.

Compressive strenght test
Compressive strength of concrete is the ability of material or structure to carry the loads on its surface without any crack or deflection. The standard test method for Compressive Strength of Cylindrical Concrete Specimens is carried out by procedure as stated in American Society for Testing Materials ASTM C39/C39M.

Concrete voids test
This test method is related to the susceptibility of the cement paste portion of the concrete to damage by freezing and thawing. The test estimates the likelihood of damage of concrete due to cyclic freezing and thawing. The parameters of the air-void system of hardened concrete determined by the procedures described in the code AASHTO T 269.

1.3.6 Steel

Tensile strength The tensile strength of steel is the measure of maximum amount of stress that can be taken before failure. Tensile strength should be conducted as per standards methods as provided in code of practise DIN 15018.

1.3.7 Other Materials

Testing for materials used in construction such as sands, bricks/blocks, etc. should be done according to the recommended standards specified in volume I of DCOM or as may be recommended for a specific project.

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