Portal: General

Rebound is an essential element in the application of shotcrete. Rebound is defined as follows: œMainly large aggregate with some sand and cement that bounces or ricochets off the receiving surface and falls on to lower surfaces.1 There is a vital function that is achieved in the rebounding of shotcrete. The secret lies in knowing how much rebound is enough. To paint a mental picture for the reader to understand rebound, consider a baseball. If you take a baseball and dip it into some fresh concrete and pull it out, it will be covered with mortar”a paste consisting of the cement and fine aggregate and water”that acts as the glue required to create an artificial rock called œconcrete. If you took this baseball covered with mortar and threw it at a high velocity against a solid block wall at a 90-degree angle to the wall, the ball would strike the surface and bounce off. Because the paste is also in motion at 95 miles per hour and the paste is not securely bonded to the ball, some paste will leave the surface of the baseball, contact the wall, and adhere to the surface. In layman™s terms, it would œsplat onto the wall. The harder the baseball is thrown, the more the paste would leave the surface of the

The purpose of this article is to examine the use of modern wet-mix shotcrete in underground environments. A critical review is provided of aspects such as mixture design, specifications, and placement. More specifically, the intent of this article is to identify the limits of the wet-mix shotcrete process, examine actual mix designs, discuss the various methods of determining (and specifying) the early-age properties of shotcrete (< 1 day), and review shotcrete placement properties, especially pumping the fresh concrete. The presentation treats various subjects taking into account actual field practices as well as the results of on-going research at Laval University, QC, Canada.

The purpose of this article is to examine the use of modern wet-mix shotcrete in under-ground environments. A critical review is provided of aspects such as mixture design, specificcations, and placement. More specifically, the intent of this article is to identify the limits of the wet-mix shotcrete process, examine actual mix designs, discuss the various methods of deter-mining (and specifying) the early-age properties of shotcrete (< 1 day), and review shotcrete placement properties, especially pumping the fresh concrete. The presentation treats various subjects taking into account actual field practices as well as the results of on-going research at Laval University, QC, Canada.

The subject of safety training necessarily starts with the question, œWhat do we need? A training-to answer this question. These answers will begin to define the content, frequency, categories of employees to be trained, and documentation required.
Some training is required by law. Federal, state, and/or local laws detail required training for specific operations. A good first step is to determine what is legally required by various government agencies. On the federal level, the Occupational Health and Safety Administration (OSHA) and the Environmental Protection Agency (EPA) are good examples

There have been a number of articles on shotcrete specifications or guidelines since Shotcrete™s debut issue in 1999.1-5 Because this issue™s theme is Specitcations, I thought it timely to offer an article updating the state of current standards in North America and the ongoing work on shotcrete standards.
As you may know, there are three major groups concerned with shotcrete issues in the U.S.: ACI International, with Committee 506, Shotcreting, and Committee C 660, Shotcrete Nozzleman Certification; ASTM International, with Subcommittee C 09.46, Shotcrete; and, of course, ASA. The first two publish specifications, guidelines, or standards; and, by agreement, ACI International deals with œhow to and design issues, while ASTM International deals with materials specifications, practices, and test methods. Two of the three committees mentioned previously are presently chaired by ASA members”we are working on the third person!”and are required to have a balance of interests of voting members, users, general interest, and suppliers. Documents published by these committees must be approved by oversight committees and sometimes by the entire memberships; thus, they are considered consensus documents in that anyone with an interest may provide input.
ACI International
ACI Committee 506 was organized in 1942 and now consists of approximately 62 members covering diversified interests in shotcrete. Current Chair John H. Pye of the U.S. Nuclear Waste Technical Review Board has divided the committee into subcommittees. This committee has published and is working on the following:

Sufficient water supply is one of the most important variables in the application of good-quality dry-mix shotcrete. It is often overlooked, especially by people who are new to the industry. Although the flow required is relatively minimal (10 to 60 L/min [2.5 to 16 gal./min]), all too often the water pressure requirements are not given the attention they deserve. It is important for the water pressure to be at least 100 kPa (15 psi) greater than the pressure in your conveyance hose. Conveyance hose pressure will vary based on many factors such as length and inside diameter, material gradation, vertical ascents, rpm of the rotor or bowl, and the adjustment of the valves feeding the compressed air to the conveyance hose. Hose pressure will usually vary from as little as 200 kPa (30 psi) to as great as 480 kPa (70 psi). It is best not to rely on an onsite water source. A good-quality water tank and pressure booster pump will easily pay for themselves in the long run.

Shotcrete has been used in construction in North America for almost 80 years. Initially, all shotcrete was applied by the dry-mix shotcrete process, where the majority of the mixed water is added at or near the nozzle just before the shotcrete is pneumatically consolidated by the impact on the receiving surface.

According to the Occupational by Joe Vierra of Safety and Health Admini-Concrete Structures stration, 2.3 million construction workers work on scaffolding on
a frequent basis. Every year, approximately 4500 injuries and 50 deaths occur in scaffold-related accidents with an estimated $90 million in lost workdays in addition to the medical, workers compensation, and other costs resulting from these incidents.

In this section of the fall issue of ASA™s Shotcrete, I introduced some aspects of pool finishing. I receive many calls, questions, and comments from people about the new finishing tools. Since this issue is on architec-tural shotcrete and finishing, I thought it would be interesting to further develop the topic of finishing techniques for shotcrete.

For effective communication and interaction among groups in this industry, the knowledge and use of the Spanish language in the American construction industry has been increasingly recognized as a growing requirement, rather than an exception. According to U.S. Census Bureau reports for the year 2000, of the 281 million inhabitants in the United States, 35 million (12.5%) are Hispanic, which is actually slightly higher than the black population (12.3%). A large increase in the Hispanic population was observed in the last decade. This group has grown by almost 40% since 1990, with much of this growth being attributed to new jobs in construction. To date, it is estimated that Hispanic workers account for up to 15% of the workforce in the construction