Portal: General

It seems like every time we take delivery of a new pump, everybody wants to climb all over it. They love to check out all controls and features, comparing them to the equipment they have been using.

Shotcrete construction joints are required in situations such as the following. When the volume of shotcrete is greater than that which can be conveniently applied and finished in a single shift. In the construction of top-down shotcrete installations, such as soil-nailed walls, where the element is constructed in a series of horizontal lifts and in segmental shotcrete construction, such as used in the underpinning of buildings.

The shotcrete pump. Every shotcrete crew needs one. Unfor-tunately, the pump that pushes all the material we need to do our job gets more abuse than any of the other equipment. Pumps come in various sizes and shapes, anywhere from a trailer single axle to a boom truck or robot. They all have one thing in common: they pump concrete at very high

Wet-mix shotcrete is a mixture of cement, aggregates, and water that may contain admixtures, which is hydraulicallypumped to the nozzle where compressed air is added to provide high velocity for placement and consolidation of the material on the receiving surface. The purpose of this article is to illustrate the uses of wet-mix shotcrete for small (repair), medium, and large output applications. There are several factors that dictate good wet-mix shotcrete practices: the right concrete pump for the job; a suitable mix design; the right nozzle (for the best material velocity); the distance the material has to be pumped; crew experience; good nozzling technique; and implementation of proper safety precautions for the pump and the hoses, including the clamps; and the overall safety of the workers.

Basically, three different types of shotcrete guns exist for the dry-mix process, all of which work on the suspension-conveying principle. Used occasionally today, was invented by Carl E. Akeley in 1907.1 It employs two connected chambers arranged one above the other, with the discharge outlet at the bottom of the lower one. The feed opening of the upper chamber and the connection between the two chambers can be closed off hermetically with bell-shaped valves independently of one another. The bottom chamber is subjected to the same air pressure as the discharge line. The procedure starts with dry mix being filled into the upper chamber with the bell valve between the two chambers closed. Next, the feed opening is closed hermetically and the upper chamber is pressurized just like the lower one. Now the valve between the two chambers can be opened, allowing the mixture to slide from the upper into the lower chamber. After this has happened, the valve between the two chambers can be reclosed and the pressure released in the upper chamber to permit reopening of the inlet valve. In the meantime, the dry mix is discharged from the lower chamber by a pneumatically driven feed wheel and is picked up by the air stream in the discharge line. The upper chamber is refilled at the same time, and the cycle is repeated.

Construction sites can be very noisy places”kind of like a Rolling Stones concert, but much longer in duration. A lot of the equipment we use on a regular basis in the construction industry can produce sufficient levels of noise to cause hearing loss when a worker is exposed to that noise for extended periods of time. Complicating this loss of hearing is the fact that the loss comes over an extended period of time. A worker who cannot hear may be in more danger on the job site. He also may develop tinnitus (a ringing sound in the ears), increased stress levels, and increased blood pressure. The ability to hear what is happening around us is an important part of working safely.
Noise levels are measured in decibels (dBA). The scale used for measuring decibels is a bit like the scale used to measure earthquakes. The noise at 73 decibels is about 2 times louder than the noise at 70 dBA. The Occupational Safety and Health Administration (OSHA) has developed rules regarding how long a worker can be exposed to noise levels before hearing protection becomes mandatory.

As a member of the American Shotcrete Association (ASA) since its inception, and as a member of the shotcrete industry for even longer, I have been an active participant in the debate over the importance of shotcrete nozzleman certification. For years, very few throughout the industry disagreed with the need for a nozzleman certification program. The vehicle by which a certification program could be delivered, however, was argued and debated by industry members everywhere!
American Concrete Institute
In 1997, the American Concrete Institute (ACI) established ACI Committee C 660, Shotcrete Nozzleman Certification, with the mandate to develop, maintain, and update programs for use in the certification of persons performing as shotcrete nozzleman. As Chair of this committee, it is my pleasure to announce that this certification program is now fully operational, and since its launch at the World of Concrete in Las Vegas in 2001, almost 300 nozzlemen throughout North America have been granted certification by ACI.
For years, the shotcrete industry has recognized the need for stringent standards with respect to the quality of the shotcrete process. In addition to good quality, well-proportioned shotcrete mixtures and suitable equipment, shotcrete nozzlemen are certainly a key element in this process. ACI Committee C 660 has delivered a credible and thorough program with strict policies, guidelines, and procedures that respond to the demands of our industry. A select group of experienced and acknowledged shotcrete experts have been approved by ACI to act as examiners for the certification program. Today, certification of a shotcrete nozzleman through this ACI program provides nozzlemen with not only improved knowledge and skills but also with international recognition as craftsmen.

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.