21 Read the text, find additional information and make a report: concrete properties

Tracing some of the fresh and hardened concrete properties back to the influences of the cement can often answer both fundamental questions and more complex problems in concrete construction.

Admixture Compatibility. Admixtures are chemicals added to concrete in small quantities for a specific function (e.g. water-reducing, set-retarding, or set-accelerating). These chemicals affect the hydration and/or are adsorbed by the cement particles. Certain combinations of chemicals and cement properties may adversely affect the setting behavior and be deemed incompatible. Examining the chemistry of the hydration reactions and the components of the cement can give clues to the source of the incompatibility.

Strength Gain. The ultimate compressive strength and rate of strength development of concrete is strongly influenced by the chemical reactivity of the Portland cement. Varying hydration rates of the different cement compounds can help explain how the relative proportions of these compounds affect the rate of strength gain. For instance, the C₂S reacts slowly and contributes to long-term strength gain. C₃S, on the other hand, has a much faster hydration rate, and contributes to higher early-strength gain. Thus, cement with a higher proportion of C₃S – as is the case with most of today’s cements – will tend to have a higher early strength, and allow for early form removal or post-tensioning.

22. Topics for projects and presentations:

1. Cement plants.

2. Environmental impacts of cement.

3. Building with concrete.

4. Use of concrete in infrastructure.

5. The technology and use of hydraulic lime.

“A good building reveals different things about itself when viewed from different distances.”

Matthew Frederick (architect)

Unit 5

a) logging	b) sawn timber	c) wood house

Figure 5.1

1 A) Explain how the following words are connected with “timber”:

boards, lumber, cut wood, foresters, to harvest, loggers, mill, to be processed.

2. Read the information below and compare your answers:

Timber is another name for wood, whether still standing in the form of trees or felled and turned into boards for construction. Some people may also refer to timber as lumber, or differentiate between timber as unprocessed wood and lumber as cut wood packaged for commercial sale. The timber industry around the world is huge, providing wood for a variety of products from paper to particle board.

Before timber is felled, it must be assessed by foresters, who determine how valuable it is and whether or not it is safe to harvest. In many regions of the world, a timber harvest plan must be filed with a government agency, indicating intent to fell trees in a particular region. If the plan is approved, a logging company sends in loggers and support staff to fell the trees and prepare them for shipment to an offsite mill. In some cases, a mill may be established in the forest, if there is a lot of timber which needs to be processed.