Const. Materials HW Questions

10. What is a concrete admixture, and why is it used? Discuss the commonly used concrete admixtures

A concrete admixture is defined as a material other than portland cement, coarse and fine aggregates, and water. A commonly used concrete admixture is fly ash. An air-entraining admixture is used to increase concrete's durability against the freeze-thaw effect. It is more commonly used in concrete pavements and bridges in colder climates. The following other admixtures are used as needed: WRA, HRWR, Silica fume, Set accelerator, Set retarder

2. What is the difference between heartwood and sapwood? Explain.

The darker-colored region of the trunk is referred to as the heartwood , and the lighter- colored region is called the sapwood. The word sap is just another term for the food that the tree conducts; hence, the term sapwood merely expresses the function of the light colored outer growth rings in relation to the dark-colored inner rings. The change from sapwood to heartwood is gradual, and as the tree ages, the heartwood portion in the tree increases.

4. What is air-entrained portland cement? Where is it typically specified?

Air entrainment in concrete, mortar, or plaster increases the durability of these materials against freezing and thawing. To reduce freeze-thaw damage, tiny particles of air are introduced in a concrete mix, referred to as air entrainment. Air-entrained portland cement has an air-entraining chemical as its integral part. All five basic types of portland cement are available as air-entrained portland cement. They are identified as Types IA, IIA, IIIA, IVA, and VA. With an air-entraining admixture, normal portland cement (Type I, II, III, IV, or V) is used.

3. List various types of portland cement and where they are typically specified.

Because the requirements for various concretes and other mixes vary, portland cement is manufactured in five different types—Types I, II, III, IV, and V. These types are distinguished from each other by small changes in their chemical composition, giving them different properties. Type I is a general-purpose portland cement. Type III portland cement is high-early strength portland cement. It develops strength at a faster rate than other types in the initial stage, but its final strength is the same as that of the other types. Type III portland cement is generally used in making precast concrete elements, such as hollow-core slabs, double-tees, and concrete pipes. Economic considerations require that the formwork for precast elements be used as frequently as possible. Generally, precast concrete elements are cast one morning and stripped off the form the following morning. Type IV is low-heat-of-hydration portland cement. It is meant for use in massive concrete structures (dam walls or bridge piers), where the temperature rise due to heat generated from hydration must be minimized. Due to its limited use, Type IV cement is produced by manufacturers only on special request. Type V is sulfate-resistant portland cement. Portland cement can be affected adversely by the presence of sulfur. In fact, concrete made out of Type I portland cement will decompose into small fragments (spall) after just a few years in the presence of high-sulfur-containing environments. Some soils and groundwater have a high sulfur content. The use of Type V portland cement is recommended in such environments. Type II is moderate-sulfate-resistant and moderate-heat-of-hydration portland cement. It combines the properties of Types IV and V to a moderate degree. Therefore, most manufacturers make portland cement that meets the requirements of both Types I and II and label the product as Type I/II portland cement. Approximately 90% of all portland cement used is Type I (or Type I/II) followed by Type III (approximately 5%).

6. Discuss the relative advantages and disadvantages of OSB and plywood panels.

Because wood is stronger along the grain than across the grain, cross-graining tends to equalize the strength of a plywood panel in its two principal directions. It also makes a plywood panel dimensionally more stable because shrinkage and swelling of wood are high across the grain and negligible along the grain. For the same reason, a plywood panel is less likely to split than sawn lumber. Therefore, it can be nailed near its edge without splitting. Because OSB generally costs less than plywood, it has become the material of choice for sheathing a wood frame building. OSB provides higher shear strength (racking resistance) than plywood because of the absence of core voids. (The lower shear strength in plywood panels is due to the presence of knot holes and splits in veneers.) Thus, it is not uncommon to see OSB panels used for floor sheathing, roof sheathing, and wall sheathing in a typical wood frame building. OSB panels have a few limitations. Plywood panels, particularly those with higher grades on face veneers, can be stained or painted. This is not true of OSB panels, which are intended only for structural applications. They generally cannot be sanded smooth like plywood panels. OSB panels are also prone to edge swelling if they remain wet for prolonged periods. Additionally, OSB panels cannot be treated with preservatives, whereas preservative-treated plywood is available.

1. Describe the difference between quicklime and slaked lime.

Calcium oxide is quicklime, which is a caustic substance that corrodes metals and damages human skin. However, when mixed with water it forms calcium hydroxide which is called hydrated lime , or slaked lime. It is called hydrated lime because it contains water that is chemically combined with calcium oxide. It is a fairly benign material and is commonly used in building construction.

5. Which type of nail is commonly used for connections between framing members in wood light-frame structures? How do we specify the size of nails?

Common nails are the most commonly used nail type for connecting wood frame members. Thick shank gives greater strength than box nail. The length of common nails in the United States is specified by a penny (abbreviated as d) designation. For example, a 10d nail (called a 10 penny nail) is 3 in. long. The penny designation originated in England centuries ago when 1 poundweight of 10d nails cost 10 pence, 1 poundweight of 12d nails cost 12 pence, and so on. Common nails are available in lengths ranging from 2d to 60d. A 2d nail is 1 in. long, and a 60d nail is 6 in. long. From 2d to 10d, the increase in length is 14 in. per penny. Therefore, a 10d nail is 3 in. long. The next two nail sizes are 12d and 16d, with lengths of 3 1/4 in. and 3 1/2 in., respectively. A 20d nail is 4 in. long. Most commonly used nail sizes in wood frame construction are 6d, 8d, 10d, and 16d.

6. In visually grading of lumber, what do graders typically look for in the piece of lumber? Explain.

Each piece is visually examined on all of its four surfaces for growth characteristics , such as the slope of grain, knots, checks, and shakes. An inspector's trained eyes can determine the grade of a piece of lumber within a few seconds. In visual grading, the inspection agency grades lumber according to the grading rules prescribed by a grading rules-writing agency.

1. Describe the essential differences between softwood and hardwood trees. Give at least three commonly used species of each.

Lumber is divided into two broad categories— hardwoods and softwoods . The hardwoods are generally denser than the softwoods. Because density is a major determinant of the strength of wood, hardwoods are generally stronger than softwoods. The true distinction between softwoods and hardwoods is based on their botanical characteristics. Softwood-producing trees, in general, do not bear flowers and have a single main stem, and most of them are evergreen, with leaves that are needlelike (i.e., conical; hence, softwood trees are also called conifers ). Pines, firs, spruces, cedars, hemlocks, and so on, are softwoods. Hardwood-producing trees are generally flowering trees, have broad leaves, and are typically deciduous, shedding and regrowing leaves annually. Oak, walnut, birch, elm, teak, mahogany, rosewood, and so on, are hardwoods.

3. What does the term SPF mean? What is the difference between SPFs and SPF? Explain.

SPF is the acronym for spruce-pine-fir and is used in woodland region mapping. It means that the wood is from a spruce pine fir woodland community, specifically in the northeastern part of the united states. SPFs designates the spruce pine firs that are from the south.

5. What is lightweight structural concrete? How does it differ from normal-weight concrete? What are the advantages and disadvantages of using lightweight structural concrete?

Structural concrete may either be normal-weight or lightweight concrete. The American Concrete Institute Code defines lightweight structural concrete as one that, in its hardened state, weighs 85 to 115 lb/ft 3. Normal-weight concrete weighs 130 to 155 lb/ft 3. The difference between normal-weight and lightweight structural concretes is only in the type of coarse aggregates used. Lightweight structural concrete is obtained using lightweight aggregate. The fine aggregate in both is the same. Lightweight structural concrete costs more than normal-weight concrete because of the higher cost of producing the aggregate. However, it can yield savings in the overall cost of the structure because it reduces dead loads. Lightweight structural concrete has, however, limitations as to strength. High-strength concrete is difficult to obtain using lightweight aggregates

8. Discuss the importance of the water-cement ratio in concrete

The amount of water required for complete hydration is about 40% of the weight of portland cement. In other words, for complete hydration, the water-cement ratio (referred to as the w-c ratio) should be 0.40. Often, however, a larger quantity of water is needed to provide the requisite workability of concrete. To obtain a normal-weight concrete with a slump of 4 in. or so (used for beams and columns), a w-c ratio of 0.55 to 0.6 is usually needed, which exceeds that needed for complete hydration. In some concretes, the w-c ratio required may be as high as 0.7. The excess water eventually evaporates, leaving air voids in hardened concrete, which reduce concrete's Strength. Experiments have indicated that the strength of concrete is inversely proportional to the w-c ratio, Figure 21.32 . Therefore, a concrete should contain the minimum amount of water that gives it the required workability. Water is an important component of concrete. Portland cement derives its cementing property from its reaction with water. Water used in concrete must be clean. A rule of thumb in the concrete industry is that if the water is fit for drinking, it is fit for use in concrete. Because seawater contains salts, its use leads to corrosion of reinforcing steel. It also modifies the portland cement-water reaction and is, therefore, not appropriate for use.

2. Describe how lime-sand mortar hardens and functions as a cement.

Until the discovery of portland cement, lime-sand mortar was the only masonry mortar available. Egyptians used lime-sand mortar when they built the pyramids, and most historic stone buildings in Europe used lime-sand mortar. In lime-sand mortar, lime is the binder and sand is the filler. Atmospheric carbon dioxide reacts with lime-sand mortar converting the exposed mortar into a hard carbonate crust, resulting in the setting and hardening of mortar. This crust hinders further penetration of carbon dioxide into the interior volume of mortar. Therefore, the setting of lime-sand mortar is slow. The slow setting is also due to the fact that only a small fraction (less than 0.04%) of air is carbon dioxide. Thus, it is difficult to lay more than a few masonry courses per day using lime-sand mortar because the weight of additional courses squeezes the mortar out of masonry.