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Tutorial 6 Answers 1. What is a saturated surface dry (SSD) condition of an aggregate and its significance? A: (Equal to potential absorption) Saturated surface dry (SSD) is defined as the condition of an aggregate in which the surfaces of the particles are "dry", but the inter-particle voids are saturated with water. When mixing concrete, adding SSD aggregates does not affect free water content. If dry aggregate is used to mix concrete, more water needs to be added to make the aggregate reach SSD, otherwise the aggregate absorbs moisture and affects concrete strength.

2. How is the moisture content of coarse aggregate determined? A: Aggregate particles have internal pores that absorb water. Aggregate stored in a stockpile or bin is typically in a wet condition, meaning it contains absorbed water and moisture coating the surface, called free or surface moisture. The total moisture content of an aggregate sample is the sum of the absorbed moisture and the free surface moisture.

Moisture Content (MC) = (M-Md)/Md x 100% M: Mass of aggregate Md: Oven dry mass of aggregate

3. What is meant by the absorption capacity of coarse aggregates and how is it measured?

A: Absorption capacity represents the maximum amount of water the aggregate can absorb. It is calculated from the difference in weight between the SSD and OD states, expressed as a percentage of the OD weight:

Absorption Capacity (AC) = (Mssd-Md)/Md x 100% Mssd: Saturated surface dry (SSD) mass of aggregate Md: Oven dry mass of aggregate

4. The moisture content of an air-dried coarse aggregate sample of 500 g had a moisture content of 0.5%. The absorption capacity is 1.5%. Find the SSD mass of this sample.

A:

500 x (

1+1.5% 0.5% 100%

1+

) = 504g.

5. A sand stock pile is exposed to wet weather. When 800 g of this sample is tested it gave a moisture content of 2.5%. Its absorption capacity is 2.0%. a. (i) What is the SSD mass of this sample? A:

800 x (

1+2.0% 1+

2.5% 100%

) = 796.1g

b. (ii)What is the amount of free moisture (in grams) available?

800-796.1 = 3.9g

6. What are the fresh concrete properties that are affected by the quality of mixing water and how are they affected?

A: Setting time and workability; Decrease setting time and workability therefore increases delay of total project.

(Extra Info) Quality of water is important because bad quality and impurities in water lead to the following: interfere with the setting of the cement; affect the strength of the concrete; cause staining of its surface; leads to corrosion of the reinforcement.

7. What are the hardened concrete properties that are affected by the quality of mixing water and how are they affected? A: Strength Development and surface appearance. Reduce later strength and increases concrete staining. 8. What are the dangers of using sea water as mixing water? A: total salinity is around 3.5% (dissolved solids are typically NaCl, MgCl2 and MgSO4) in sea water. Sea water contains organic impurities and also high chlorine amount and sulphate amount which will cause corrosion in the bars. Sea water increases the risk of corrosion of the reinforcement. Seawater leads to a slightly higher early strength but a lower long- term strength.

As chemical attack like chloride attack, sulphate attack will occur and lead to corrosion of steel thus failing the integrity of the structure Sea water as mixing water causes problems for concrete: - dampness - efflorescence - low strength - risk of steel reinforcement corrosion - damage due to volume expansion. 9. What are the problems associated with the use of calcium chloride as an accelerator? A: Excess calcium chloride can cause flash set (...