Superplasticizers are high-range water-reducing admixtures that meet the requirements of ASTM C494 Type F or G. They are often used to achieve highstrength concrete by use of a low water-cement ratio with good workability and low segregation. They also may be used to produce concrete of specified strengths with less cement at constant water-cement ratio. And they may be used to produce self-compacting, self-leveling flowing concretes, for such applications as longdistance pumping of concrete from mixer to formwork or placing concrete in forms congested with reinforcing steel. For these concretes, the cement content or watercement ratio is not reduced, but the slump is increased substantially without causing segregation. For example, an initial slump of 3 to 4 in for an ordinary concrete mix may be increased to 7 to 8 in without addition of water and decrease in strength. Superplasticizers may be classified as sulfonated melamine-formaldehyde condensates, sulfonated naphthaline-formaldehyde condensates, modified lignosulfonates, or synthetic polymers. 4.13.2 Air-Entraining Admixtures These create numerous microscopic air spaces within concrete to protect it from degradation due to repeated freezing and thawing or exposure to aggressive chemicals. For concrete exposed to repeated cycles of freezing and thawing, the air gaps provide room for expansion of external and internal water, which otherwise would damage the concrete. Since air-entrained concrete bleeds to a lesser extent than non-air-entrained, there are fewer capillaries extending from the concrete matrix to the surface. Therefore, there are fewer avenues available for ingress of aggressive chemicals into the concrete. The Standard Specification for Air-Entraining Admixtures for Concrete, ASTM C260, covers materials for use of air-entraining admixtures to be added to concrete in the field. Air entrainment may also be achieved by use of Types IIA and IIIA portland cements (Art. 4.2.2). 4.13.3 Set-Accelerating Admixtures These are used to decrease the time from the start of addition of water to cement to initial set and to increase the rate of strength gain of concrete. The most commonly used set-accelerating admixture is calcium chloride. Its use, however, is controversial in cases where reinforcing or prestressing steel is present. The reason is that there is a possibility that the accelerator will introduce free chloride ions into the concrete, thus contributing to corrosion of the steel. An alternative is use of one of many admixtures not containing chloride that are available. 4.13.4 Retarding Admixtures To some extent, all normal water-reducing admixtures retard the initial set of concrete. A Type B or D admixture will allow transport of concrete for a longer time before initial set occurs. Final set also is delayed. Hence, precautions should be taken if retarded concrete is to be used in walls. Depending on the dosage and type of base chemicals in the admixture, initial set can be retarded for several hours to several days. A beneficial side effect of retardation of initial and final sets is an increase in the compressive strength of the concrete. A commonly used Type D admixture provides higher 7- and 28-day strengths than a Type A when used in the same mix design. 4.13.5 Mineral Admixtures Fly ashes, pozzolans, and microsilicates are included in the mineral admixture classification (Arts. 4.9 and 4.10). Natural cement (Art. 4.4) is sometimes used as an

The Brick Trowel. This is the 'main stay' of any bricklaying operation is the large brick trowel. These trowels come in a range of sizes from around 9 to 13 inches and the length is measured along the underside of the blade. So which one? The choice is determined by your ability to control it. Obviously the larger the trowel the more mortar you will be able to lift and place. For the person new to bricklaying the best bet is to go for the smaller version; in time you can always progress upwards. Look for a trowel which has a welded joint where the handle meets the blade, it is best not to go for a version which has been riveted at that point. In general a riveted joint will become loose in a short period of time due in part to tapping bricks into place. Now trust me on this, there are left and right handed trowel.... yes, yes I here you say, just like a left handed screwdriver. Well its true and its all to do with the shape of both sides of the trowel. One side is straight and is used to cut off excess mortar after laying the brick, the edge has a rounded edge which can be used for trimming bricks down to side. Left and right handed trowels have these edges reversed.

The maximum glass area recommended for glazing with standard H-type lockstrip gaskets may be obtained from Fig. 11.37. Loads for the chart are based on minimum glass thicknesses allowed in Federal Specification DD-G-451c. Glass producers should be consulted to obtain the latest thickness recommendations. A wide range of window types are available to satisfy the ever-increasing scope of architectural requirements. There are many materials, sizes, arrangements, details, and specific features from which to choose. Types of windows (see symbols Fig. 11.38) include: Pivoted windows, an economical, industrial window used where a good tight closure is not of major importance (Fig. 11.39). Principal members of metal window units are usually 13/8 in deep, with frame and vent corners and muntin joints riveted. However, when frame and vent corners are welded, principal members may be 11/4 in deep. Vents are pivoted about 2 in above the center. These windows are adaptable to multiunits, both vertical and horizontal, with mechanical operation. Bottom of vent swings out and top swings in. No provision is made for screening. Putty glazing is placed inside. Commercial projected windows, similar to pivoted windows (Fig. 11.40). These are an industrial-type window but also are used for commercial buildings where economy is essential. Vents are balanced on arms that afford a positive and easy control of ventilation and can be operated in groups by mechanical operators. Maximum opening is about 35
. Factory provision is made for screens. Security windows, an industrial window for use where protection against burglary is important, such as for factories, garages, warehouses, and rear and side FIGURE 11.36 Structural gasket glazing: (a) and (b) gasket fitted into a groove; (c) and (d) H shape locked to continuous metal fin; (e) and ( f ) weep holes for factory sealed double-glazed