Provide a short answer to the following questions. Don’t write a book. a. Why is

Question

Provide a short answer to the following questions. Don’t write a book.

a. Why is air entrainment required in concrete exposed to earth or weather?

b. Name 4 factors that affect the compressive strength of concrete and describe how they

affect it.

c. Name 4 factors that affect the magnitude of drying shrinkage and describe how they

affect it.

d. What percentage range of concrete’s compressive stress is typically taken as its critical

stress and what are the two significant factors pertaining to the critical stress?

e. A graduate student is doing a modulus of rupture test and the beam fractures at a load, P,

of 3.25 kips (a total of 2P acting on the beam). What is the flexrual strength of the

concrete based on this test? How does it compare with the values specified by ACI 318-

05 if the concrete compressive strength is 7300 psi?

f. When doing the modulus of rupture test, what is the significance of using two point loads

over a specified distance rather than a single point load at the mid-span?

g. Split-tension tests are performed on two 4 in. x 8 in. cylinder specimen. The samples fail

at loads of 18500 lb and 19200 lb. Calculate the average split tensile strength of this

concrete to the nearest 1 psi.

Explanation / Answer

1. Air entrainments are one of the admixtures mixed in conrete because to gain extra strength for that mix relavant to the air and weather. Air entrainment is the intentional creation of tiny air bubbles in concrete. A concrete maker introduces the bubbles by adding to the mix an air entraining agent, a surfactant (surface-active substance, a type of chemical that includes detergents).

2. Concrete cube and cylinder casting, curing and testing procedures

The method of casting and capping of cube and cylinder affects the strength ratios of both. The use of rigid and non-rigid moulds affects their strength. Also the method of capping these moulds affects the strength as out of plane surface also influences their strength ratio. Proper curing and testing procedure is necessary to associate a proper relation between concrete cube and cylinder compressive strength ratios, otherwise the ratio obtained will be misleading.

Geometry of the specimen

Geometric factors such as volume of concrete, shape of concrete and h/d ratio (height to lateral dimension) of specimen affects the concrete cube and cylinder strength ratio. The following figure shows the effect of height/diameter ratio to concrete strength ratio.

Level of strength

Nominal strength of concrete has been shown to affect the concrete cube and cylinder strength ratio. Research by Evans indicates that this ratio decreases with increasing concrete strength. Cylinder to cube strength ratio ranges from 0.77 to 0.96 depending on concrete strength level.

  Grading of aggregates.

Grading of aggregates in concrete affects the strength of any structure or specimen. The effect of compression test on concrete specimens are large due to relative size of aggregate particles to specimen dimensions. Most standards sets limits for the ratio of diameter or size of specimen to maximum nominal size of aggregates. Typically this allowable minimum is around 3 to 4.

3.

One of the most important factors that affects shrinkage is the drying condition or in other words, the relative humidity of the atmosphere at which the concrete specimen is kept. If the concrete is placed in 100 per cent relative humidity for any length of time, there will not be any shrinkage; instead there will be a slight swelling. The typical relationship between shrinkage and time for which concrete is stored at different relative humidities is shown in Figure. The graph shows that the magnitude of shrinkage increases with time and also with the reduction of relative humidity.

The rate of shrinkage decreases rapidly with time. It is observed that 14 to 34 per cent of the 20 year shrinkage occurs in 2 weeks, 40 to 80 per cent of the 20 year shrinkage occurs in 3 months and 66 to 85 per cent of the 20 year shrinkage occurs in one year. Another important factor which influences the magnitude of shrinkage is water/cement ratio of the concrete. The richness of the concrete also has a significant influence on shrinkage. Aggregate plays an important role in the shrinkage properties of concrete. The quantum of an aggregate, its size, and its modulus of elasticity influence the magnitude of drying shrinkage.

Harder aggregate with higher modulus of elasticity like quartz shrinks much less than softer aggregates such as sandstone.
Moisture Movement Concrete shrinks when allowed to dry in air at a lower relative humidity and it swells when kept at 100 per cent relative humidity or when placed in water.

Just as drying shrinkage is an ever continuing process, swelling, when continuously placed in water is also an ever continuing process. If a concrete sample subjected to drying condition, at some stage, is subjected to wetting condition, it starts swelling. It is interesting to note that all the initial drying shrinkage is not recovered even after prolonged storage in water which shows that the phenomenon of drying shrinkage is not a fully reversible one.

Just as the drying shrinkage is due to loss of adsorbed water around gel particles, swelling is due to the adsorption of water by the cement gel. The water molecules act against the cohesive force and tend to force the gel particles further apart as a result of which swelling takes place. In addition, the ingress of water decreases the surface tension of the gel.

The property of swelling when placed in wet condition, and shrinking when placed in drying condition is referred as moisture movement in concrete.

4.

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