Timber and timber products

Unit 5

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 (for example 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)

TIMBER

a) logging	b) sawn timber	c) wood house

LEAD-IN  

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.

b) 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 particleboard.

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.

2 Speculate on the areas of timber application.

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"Good timber does not grow with ease. The stronger the wind, the stronger the trees." (J. William Marriott)

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“Out of the crooked timber of humanity no straight thing was ever made.” (Immanuel Kant)

3 Discuss the following quatations:

• Write a paraphrase of each proverb. • Say whether you agree or not, and why.

READING  

4 a) Transcribe the following words:

Versatility, diversity, fuel, cellulose, lignin, polymers, glucose, crystalline, deciduous, knots, rustic, lacquer.

b) In what context do you think the following words and phrases will appear in the text?

•building material •versatility •diversity •paper production •photosynthesis •dry weight •commercial timbers •flooring •knots •sealed with oil or lacquer •external timber cladding

c) Read the text and check your answers:

Timber, arguably the original building material, retains its prime importance within the construction industry because of its versatility, diversity and aesthetic properties. About 20% of the earth’s land mass is covered by forests, divided roughly two thirds as hardwoods in temperate and tropical climates and one third

as softwoods within temperate and colder regions. Approximately a third of the annual worldwide timber harvest is used in construction, and the rest is consumed for paper production, as, or wasted during the logging process.

The main constituents of timber are cellulose, hemicellulose and lignin, which are natural polymers. Cellulose, the main constituent of the cell walls, is a polymer made from glucose, a direct product of photosynthesis within the leaves of the tree. Glucose molecules join together to form cellulose chains containing typically 10,000 sugar units. Alternate cellulose chains, running in opposite directions to each other, form a predominantly well-ordered crystalline material. It is this crystalline chain structure which gives cellulose its fibrous properties, and accounts for approximately 45% of the dry weight of the wood.

Commercial timbers are defined as hardwoods or softwoods according to their botanical classification rather than their physical strength.

Hardwoods are from broad-leafed trees, which in temperate climates are deciduous, losing their leaves in autumn, although in tropical climates, when there is little seasonal variation, old leaves are constantly being replaced by new. Hardwood flooring has a proven track record for durability and aesthetic impact. Both solid timber and plywood laminates with a 4 mm hardwood wearing layer are commercially available. The standard timbers are the European oak, beech, birch, ash, chestnut, walnut and maple, but additionally some imported hardwoods with darker grain colours are available and interesting effects are produced with bamboo. The timbers are frequently offered with minimal knots and uniform graining or as rustic with knots and a larger variation of colour. Laminates are usually prefinished but solid timber may be sealed with oil or lacquer after installation on site.

Softwoods are from conifers, characteristically with needle-shaped leaves, and growing predominantly in the northern temperate zone. Mostly they are evergreen, with the notable exception of the European larch and they include the Californian redwood, the world’s largest tree with a height of over 100 metres. Western red cedar has long been the preferred timber for external timber claddingbecause of its durability and warm colour. However, recently the popularity of larch and Douglas fir as softwood cladding has increased as greater emphasis is placed on the use of renewable resources from sustainable forests due to its moderate durability to decay and more resistance to impact damage.

5 Read the text again and answer the questions that follow (1-7):

1. Why is timber considered to retain its prime importance within the construction industry?

2. What climate areas do hardwoods and softwoods cover?

3. Which are natural polymers? Characterize each of them.

4. What is commercially available among hardwoods?

5. What kinds of trees are mentioned in the text?

6. How can you characterize softwoods?

7. What softwoods are preferable for external timber cladding? Why?

Follow-up

6 a) Find the synonyms for the words given in italics.

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