WATER TO CEMENT RATIO BASICS AND TUTORIALS

WATER TO CEMENT RATIO BASIC INFORMATION
What Is The Ideal Water To Cement Ratio?

For brittle ceramic materials, including cementitious systems, the strength has been found to be inversely proportional to the porosity. Often, an exponential equation is used to relate strength to porosity; for example,

fc = fcₒe⁻kt

where fc is the strength, fc0 is the intrinsic strength at zero porosity, p is the porosity, and k is a constant that depends on the particular system.

Equations such as this do not consider the pore-size distribution, the pore shape, and whether the pores are empty or filled with water; thus, they are a gross simplification of the true strength vs. porosity relationship.

Nonetheless, for ordinary concretes for the same degree of cement hydration, the strength does indeed depend primarily on the porosity. Because the porosity, in turn, depends mostly on the original w/c ratio, mix proportioning for normal-strength concretes is based, to a large extent, on the w/c ratio law articulated by D.A. Abrams in 1919: “For given materials, the strength depends only on one factor—the ratio of water to cement.” This can be expressed as: fc = K1/ [K2^(w/c)] where K1 and K2 are constants, and w/c is the water/cement ratio by weight.

In fact, of course, given the variability in raw materials from concrete to concrete, the w/c ratio law is really a family of relationships for different mixtures. As stated by Gilkey (1961a):

For a given cement and acceptable aggregates, the strength that may be developed by a workable, properly placed mixture of cement, aggregate, and water (under the same mixing, curing, and, testing conditions) is influenced by the: (a) ratio of cement to mixing water; (b) ratio of cement to aggregate; (c) grading, surface texture, shape, strength, and stiffness of aggregate particles; and (d) maximum size of aggregate.

Thus, in some cases, simple reliance on the w/c ratio law may lead to serious errors. It should be noted that many modern concretes contain one or more mineral admixtures that are, in themselves, cementitious to a greater or lesser degree; therefore, it is becoming more common to use the term water/ cementitious material ratio to reflect this fact rather than the simpler water/cement ratio.

For ordinary concretes, the w/c ratio law works well for a given set of raw materials, because the aggregate strength is generally much greater than the paste strength; however, the w/c ratio law is more problematic for high-strength concretes, in which the strength-limiting factor may be the aggregate strength or the strength of the interfacial zone between the cement and the aggregate.

Although it is, of course, necessary to use very low w/c ratios to achieve very high strengths, the w/c ratio vs. strength relationship is not as straightforward as it is for normal concretes. Figure 1.8 shows a variety of water/ cementitious material vs. strength relationships obtained by a number of different investigators.

A great deal of scatter can be seen in the results. In addition, the range of strengths for a given w/c ratio increases as the w/c ratio decreases, leading to the conclusion that, for these concretes, the w/c ratio is not by itself a very good predictor of strength; a different w/c ratio “law” must be determined for each different set of materials.

ETHICS IN THE CONSTRUCTION INDUSTRY BASICS AND TUTORIALS

CONSTRUCTION INDUSTRY ETHICS BASIC INFORMATION
What Are The Construction Industry Ethics?

In 2004, FMI, the nation’s largest management-consulting firm for the construction industry, teamed up with CMAA to survey project owners, architects, engineers, construction managers, and contractors to gauge their concerns about ethics in the industry.

The results, culled from 270 responses, might be kept in mind as we traverse the design and construction industries in the chapters that follow.

The key concerns expressed by the respondents to the survey were fourfold:

1. There appeared to be a breakdown in trust and integrity.

2. There was a perceived loss of reputation for the industry.

3. There was a need to provide a code of ethics and standards.

4. There was a need to create a more equitable bidding process.

Concerns were voiced by owners, architects, engineers, and contractors; they all seem to point to a need for fairness on the part of each party to the construction process.

Concerns about architects and engineers included the following:

■ Owners stated that architects and engineers do whatever makes the owner happy, often at the expense of the contractor.

■ Architects and engineers need to express fairness when dealing with contractors or making decisions that affect the owner.

■ Design professionals knowingly issue plans and specifications that are deficient. Concerns about contractors included the following:

■ Bid shopping, a practice where contractors use one subcontractor’s price to drive down the price of another to achieve the lowest cost, often an unrealistically low price

■ Change-order games, played by a general contractor who knowingly submits a low bid in the hope of gaining more profit by issuing questionable change orders as construction proceeds

■ Payment games, the receipt of payment from one owner, which should be used to pay for labor, materials, and equipment for that project, commingled with funds to pay for other projects

■ Instituting claims that are vague or specious

■ Engaging subcontractors whose past performance has been unreliable Concerns about owners included the following:

■ Owners who authorize work but argue about paying for it

■ Owners who are very late in their payment of contractor requisitions

■ Owners who pass off responsibility to others when they are the party that should assume responsibility and resolve problems promptly and equitably

■ Owners who lack ethical behavior, such as advertising bogus low bids to drive down the price of bidding contractors

■ Little dialogue between owners and contractors about the expectations of both parties

It appears from this study that there is plenty of blame to go around, indicating the need to maintain and enforce ethical business practices by owner, architect, and contractor alike. So with that in mind, we will now begin the design and construction process.