Answers
Tensile and compressive residual-strength tests were performed over the range of impact energies, two energy levels being selected for evaluation of the fatigue performance. Fatigue load amplitudes were chosen to be fractions of the static strengths. The behaviour of impact-damaged specimens was investigated under tension-tention (R = 0) and tension-compression (R = −1) fatigue loads. The sequence of loading (impact/cycle or cycle/impact) was changed to study its effect on the strength or the life of the composite materials.
Ans.2Concrete cracking strength can be defined as the tensile strength of concrete subjected to pure tension stress. However, as it is difficult to apply direct tension load to concrete specimens, concrete cracking is usually quantified by the modulus of rupture for flexural members.
Indirect tensile strength tests.
A cylindrical specimen is loaded diametrically across the circular cross section. The loading causes a tensile deformation perpendicular to the loading direction, which yields a tensile failure.
We are prefered 4 point bending test because in it we got batter result compare to 3 point bending.
Ans.4 plastic shrinkage
When a concrete's tensile strength is exceeded by an applied stress, a crack forms in the concrete. Concrete has a relatively low tensile strength compared to its compressive strength and experiences a variety of volumetric changes depending on environmental conditions, curing conditions, and applied stresses. In general, the "gel" structure of the cementitious paste in concrete undergoes swelling when it is wetted and shrinkage when it is dried. The ability of the concrete to resist cracking depends primarily on 1) the magnitude of shrinkage strains due to carbonation shrinkage and drying shrinkage, 2) the stress induced in the concrete, 3) the stress relief associated with creep and relaxation, and 4) the tensile strength.
Most of the problems associated with the shrinkage of concrete are related to the shrinkage of the hardened concrete after it has set. However, shrinkage may occur during the first few hours after placing while the concrete is still plastic and the concrete has not reached any significant strength.
Description
The primary cause of "plastic shrinkage" cracks is the rapid evaporation of water from the surface of the concrete. Immediately after the concrete has been placed, the particles within the concrete begin to settle. When the particles settle, the water within the concrete displaces and rises to the top. This process is better known as "bleeding." Not all of the water within the concrete displaces.
Under most weather conditions, some of the water on the surface of the concrete evaporates. The rate of evaporation depends on factors such as the temperature of the concrete, temperature of the air, relative humidity, and wind velocity surrounding the concrete. Table 1 shows how different weather conditions and properties effect the rate of evaporation. The highest evaporation rates are obtained when the concrete and air temperatures are high, when the relative humidity of the air is low, when the concrete temperature is high compared to the air temperature, and when a strong wind is blowing over the surface of the concrete. The rapid evaporation of water at the surface is most associated with placing concrete in hot weather conditions.
However, plastic shrinkage cracks can also form in cold weather conditions when the temperature of the concrete is high compared with the surrounding air temperature. When the temperature at the surface creates an evaporation rate that exceeds the rate of water produced by the bleeding process, the water film disappears and the top surface of the concrete begins to shrink. When the evaporation rate exceeds1.0 kilogram per square meter per hour, it is almost certain that plastic shrinkage cracks will develop (Figure 1). When the evaporation rate is greater than 0.5 kilogram per square meter per hour cracking is possible.
Plastic shrinkage cracks typically occur on horizontal surfaces exposed to the atmosphere. These cracks are different from other early cracks because they are deeper and wider.
Plastic shrinkage cracks are typically two to four inches deep and approximately one-eighth inch wide. They may also extend several feet in length adopting a crow’s-foot pattern. These cracks form before any bond has developed between the aggregate particles and mortar. Therefore, the cracks tend to follow the edges of large aggregate particles or reinforcing bars and never break through the aggregate particles. Although plastic shrinkage cracks usually do not impair the structural performance of the slab, cracks in some building floors have been blamed for leakage.
Prevention
There are several corrective procedures listed below to reduce the risk of experiencing plastic shrinkage cracks.
Moisten subgrades and forms to prevent absorption.
Dampen dry aggregates that are absorptive.
Reduce the temperature of the concrete by
Precooling aggregate with water.
Cooling the cement.
Using chipped ice to cool mixing water.
Shading aggregates, water tanks, and lines.
Avoid overmixing.
Place concrete early in the morning or late afternoon.
Construct temporary walls to reduce wind velocity.
Provide sunshades for concrete.
Reduce time between placing and start of curing by working efficiently during construction.
Use evaporation retardant (usually polymers).
Use fog sprays to keep the humidity high and the air temperature low.
Drying shrinkage
Drying shrinkage is defined as the contracting of a hardened concrete mixture due to the loss of capillary water.
This shrinkage causes an increase in tensile stress, which may lead to cracking, internal warping, and external deflection, before the concrete is subjected to any kind of loading. All portland cement concrete undergoes drying shrinkage or hydral volume change as the concrete ages. The hydral volume change in concrete is very important to the engineer in the design of a structure. Drying shrinkage can occur in slabs, beams, columns, bearing walls, prestressed members, tanks, and foundations.
Drying shrinkage is dependent upon several factors. These factors include the properties of the components, proportions of the components, mixing manner, amount of moisture while curing, dry environment, and member size.
Concrete cured under normal conditions will undergo some volumetric change. Drying shrinkage happens mostly because of the reduction of capillary water by evaporation and the water in the cement paste. The higher amount of water in the fresh concrete, the greater the drying shrinkage affects. The shrinkage potential of a particular concrete is influenced by the amount of mixing, the elapsed time after the addition of water, temperature fluctuation, slumping, placement, and curing. The makeup of concrete is also very important.
Each aggregate and cement type has distinctive characteristics, each contributing to concrete shrinkage. The amounts of water and admixtures used during mixing also have direct and indirect effects on drying shrinkage of concrete. Concrete shrinkage occurs mostly due to the evaporation of the mixing capillary water. The severity of this shrinkage depends on the physical properties of the concrete including size of the structure, location of the structure, and the surrounding temperature.
Properties and Proportions of Components
The compositional makeup of concrete contributes directly to the drying shrinkage of concrete. Loss of moisture in the hydrated cement paste results in shrinkage.
Different compositions and fineness of cements have variable effects on the shrinkage of cement paste. Difference in shrinkage is reduced significantly due to the adjustment of the amount of gypsum added to the different cement compositions. The size of aggregate is not as important, but has an indirect influence on the water content of concrete. Shrinkage decreases with the volumetric increase of aggregate concentration causing a linear relationship between free shrinkage and crack width. High density aggregates and high modulus of elasticity of aggregates will decrease the compressibility and increase the shrinkage of concrete.
The use of admixtures may alter the hydration reaction, which results directly in a high increase of drying shrinkage.
Moisture
The concrete properties influence on drying shrinkage depends on the ratio of water to cementitious materials content, aggregate content, and total water content. The total water content is the most important of these. The relationship between the amount of water content of fresh concrete and the drying shrinkage is linear. Increase of the water content by one percent will approximately increase the drying shrinkage by three percent. Constant water to cementitous materials ratio coincides with changes in the amount of aggregate used.
Dry Environment
The amount of drying shrinkage depends on the environmental conditions; relative humidity, temperature, and air circulation.
Concrete subjected to a dry atmosphere will, in most cases, have a greater drying shrinkage than if subjected to an alternative wetting and drying. Lower temperatures generally produce a decrease in drying shrinkage because of higher humidity and slower evaporation.