Concrete Specimen Methods for Strength Testing

Methods of Concrete Specimen Preparation

• Method of preparing cube specimens (BS 1881, Part 108: 1993) is used in Europe and China, while method of preparing cylinder specimens (ASTM C470-81) is used in North America.
• Standard size of cube specimen is 150 × 150 × 150 mm, and it should be prepared by pouring concrete into a cubic mold in three equal layers, with each layer being stroked 35 times by a hemispherical-tipped steel rod.
• Standard size of cylinder specimen is 150 × 300 mm, and it should be prepared by pouring concrete into a cylindrical mold in three equal layers, with each layer being stroked 25 times by a hemispherical-tipped steel rod.

Factors Affecting Compressive Strength

• Factors affecting compressive strength can be categorized into three groups: characteristics and proportions of materials, curing conditions, and testing parameters.
• Compressive strength can be determined by performing a four-point bending test following the procedures.

Relation between Compressive and Tensile Strengths

• Compressive strength of concrete is commonly considered in structural design, but tensile strength is also important for some purposes, such as design of highway and airfield slabs, shear strength, and resistance to cracking.
• As compressive strength increases, tensile strength also increases, but at a decreasing rate.
• Empirical formulas have been suggested to relate tensile and compressive strengths, such as ƒt = 0.3(ƒc)2/3, ƒt = 0.2(ƒc)0.7, and ƒt = 0.12(ƒc)0.7.

Comparison between Cube and Cylinder Strength

• Strength of cube specimen made from identical concrete will be different from the strength of cylinder specimen.
• Normally, strength of cylinder is taken as 0.8 times the strength of the cube, ƒc(cylinder) = (0.8-0.85)ƒcu(cube).

Drying Shrinkage

• Drying shrinkage occurs when concrete begins to dry, and excessive water that has not reacted with the cement begins to migrate from the interior of the concrete mass to the surface.
• Shrinkage strain, ss, is time-dependent, and approximately 90% of the ultimate shrinkage occurs during the first year.
• Shrinkage increase as the ratio of the surface to volume increases.
• Value of shrinkage strain for plain concrete members ranges from 0.0004 to 0.0007 for standard conditions.

Factors Affecting Shrinkage

• Aggregate stiffness (elastic modulus) affects shrinkage, as stiffer aggregates provide a higher restraining effect and lead to smaller shrinkage.
• Aggregate fraction affects shrinkage, as higher aggregate fraction leads to smaller shrinkage and creep due to a restraining effect.
• Theoretical thickness affects shrinkage, as higher theoretical thickness means less contact surface with the atmosphere, making diffusion or migration of water more difficult.
• Humidity affects shrinkage, as higher RH leads to lower shrinkage and creep.

Durability

• Durability of Portland cement concrete is defined as its ability to resist weathering action, chemical attack, abrasion, or any other process of deterioration to maintain its original form, quality, and serviceability.
• Durability is most likely to relate to long-term serviceability of concrete and concrete structures.
• Serviceability refers to the capability of the structure to perform the functions for which it has been designed and constructed after exposure to a specific environment.
• Major durability problems of concrete structures include corrosion of the reinforcing steel, freeze/thaw damage, salt scaling, alkali–aggregate reactions, and sulfate attack.

Factors Influencing Durability

• Durability of concrete depends on its permeability and diffusivity.
• Permeability is defined as the property that governs the rate of flow of a fluid into a porous material under pressure.
• Diffusivity is defined as the rate of migration of ions or moisture in concrete under the action of a higher concentration side to lower concentration side.
• Porosity in concrete can largely affect the permeability.
• Difference between permeability and diffusivity is that permeability is the parameter characterizing water flow when pores inside the concrete are filled up with water, while diffusivity is the parameter describing the migration of ions or water vapor before saturation has been reached in the pores.