FIVE TYPES OF PORTLAND CEMENT BASIC AND TUTORIALS

PORTLAND CEMENT TYPES BASIC INFORMATION
What Are The Five Types of Portland Cement?


Portland cement has become the most widely used cement in the world. Portland cement got its name because the cured concrete it produced was the same color as a gray stone quarried in nearby Portland, England.

There are five types of portland cement, each with different characteristics.

■ Type I is a general-purpose cement and is by far the most commonly used, especially in residential work. Type I portland cement is suitable whenever the special characteristics of other types are not required.

■ Type II cement has moderate resistance to sulfates, which are found in some soil and groundwater, and generates less heat during hydration than Type I. This reduced curing temperature can be particularly helpful in large structures such as piers and heavy retaining walls, especially when the concrete is placed in warm weather.

■ Type III is a “high early strength” cement. High early strength does not mean higher strength—only that strength develops at a faster rate. This can be an advantage during winter construction because it reduces the time during which fresh concrete must be protected from the cold. Early strength gain can also permit removal of forms and shoring more quickly.

■ Type IV cement produces less heat during hydration than Type I or Type II and is used only in massive civil engineering structures such as dams, large highway pilings, or heavy bridge abutments. Its strength development and curing rates, though, are much slower than Type I.

■ Type V cement is used in concrete exposed to soil or groundwater that has high sulfate concentrations. This type of cement is usually available only in areas where it is likely to be needed. In the United States, Type V cement is common only in the southwestern states.

Types I, II, and III portland cement can also be made with a foaming agent that produces millions of evenly distributed microscopic air bubbles in the concrete mix. When manufactured in this way, the cements are said to be air entrained, and are designated as Types IA, IIA, and IIIA. Air-entrained cements require mechanical mixing.

Finely ground cement increases the workability of harsh mixes, making them more cohesive and reducing tendencies toward segregation. Coarsely ground cement reduces stickiness. Cement packages that are marked ASTM A150 meet industry standards for both physical and chemical requirements.

Portland cement comes in three colors—grey, white, and buff. The white and buff are more expensive and typically used in commercial rather than residential projects to achieve special color effects.

 Liquid or powder pigments can be added to a concrete mix, and liquid stains can be used to color the surface of cured concrete, but both will add to the cost. For most applications, ordinary gray concrete made with gray cement is suitable. Colored concrete should be reserved for special areas like a front entrance, a patio, or a pool deck

GENERAL APPROACHES TO FABRICATION AND ERECTION OF BRIDGE STEELWORKS BASICS AND TUTORIALS

FABRICATION AND ERECTION OF BRIDGE STEELWORKS GENERAL APPROACHES
What Are The General Approaches To Fabrication and Erection Of Bridge Steelworks

The objective in steel bridge construction is to fabricate and erect the structure so that it will have the geometry and stressing designated on the design plans, under full dead load at normal temperature.

This geometry is known as the geometric outline.

In the case of steel bridges there have been, over the decades, two general procedures for achieving this objective:

1. The “field adjustment” procedure — Carry out a continuing program of steelwork surveys and measurements in the field as erection progresses, in an attempt to discover fabrication and erection deficiencies; and perform continuing steelwork adjustments in an effort to compensate for such deficiencies and for errors in span baselines and pier elevations.

2. The “shop control” procedure — Place total reliance on first-order surveying of span baselines and pier elevations, and on accurate steelwork fabrication and erection augmented by meticulous construction engineering; and proceed with erection without any field adjustments, on the basis that the resulting bridge deadload geometry and stressing will be as good as can possibly be achieved.

Bridge designers have a strong tendency to overestimate the capability of field forces to accomplish accurate measurements and effective adjustments of the partially erected structure, and at the same time they tend to underestimate the positive effects of precise steel bridgework fabrication and erection.

As a result, we continue to find contract drawings for major steel bridges that call for field evolutions such as the following:

1. Continuous trusses and girders

— At the designated stages, measure or “weigh” the reactions on each pier, compare them with calculated theoretical values, and add or remove bearing-shoe shims to bring measured values into agreement with calculated values.

2. Arch bridges

— With the arch ribs erected to midspan and only the short, closing “crown sections” not yet in place, measure thrust and moment at the crown, compare them with calculated theoretical values, and then adjust the shape of the closing sections to correct for errors in span-length measurements and in bearing-surface angles at skewback supports, along with accumulated fabrication and erection errors.

3. Suspension bridges

— Following erection of the first cable wire or strand across the spans from anchorage to anchorage, survey its sag in each span and adjust these sags to agree with calculated theoretical values.

4. Arch bridges and suspension bridges — Carry out a deck-profile survey along each side of the bridge under the steel-load-only condition, compare survey results with the theoretical profile, and shim the suspender sockets so as to render the bridge floor beams level in the completed structure.

5. Cable-stayed bridges

— At each deck-steelwork erection stage, adjust tensions in the newly erected cable stays so as to bring the surveyed deck profile and measured stay tensions into agreement with calculated theoretical data.

There are two prime obstacles to the success of “field adjustment” procedures of whatever type: (1) field determination of the actual geometric and stress conditions of the partially erected structure and its components will not necessarily be definitive, and (2) calculation of the corresponding “proper” or “target” theoretical geometric and stress conditions will most likely prove to be less than authoritative.