Portal: General

Much has happened since the last update on the activities of various shotcrete organi-zations and committees in the Summer 2004 issue of Shotcrete. Probably the most important, at least to this author, is that I have retired from full-time employment from SI Concrete Systems and will consult in the industry if anyone will put up with me. I plan to continue my work with the various organizations discussed below. Readers are requested to contact the editor and author if inaccuracies are found or if additional activities should be reported.

Once again the frost is on the pumpkin by guest author and Old Man Winter is at the door. With Todd Bennett less daylight and colder temperatures, we need to adjust our safety programs from avoiding heat exhaustion, dehydration, overexposure, and sun-related illnesses to preparing our field crews to deal with the other extreme.

Often when the subject of safety is brought up, we start thinking of regulations, special protective equipment, programs, inspections, reporting, meetings, insurance, and so on. Common sense tells us that it is in our best interests to have good safety practices and to enforce company safety policies. However, sometimes our attitude toward safety is not very positive, treating safety programs and policies as a necessary evil rather than a critical part of daily operations. Having a safe work place starts with instilling a positive attitude in all employees. Doing even the simplest things safely creates an atmosphere that tells employees that the company is concerned about them beyond what is required by law.
Let™s look at two examples”one obvious, and one less apparent:

After four years of research and development on improving the bonding capabilities of synthetic fibers, we recently patented a high-tensile-strength synthetic fiber that partially fibrillates upon mixing and shooting, increasing its final surface area and bonding capabilities to the concrete.1 The fibers are introduced into the concrete mixer as monofilament units of relatively low surface area (F4-a in Fig. 1), allowing up to 2% vol. (18.5 kg/m3 [31.2 lb/yd3]) fiber dosages. During the mixing process, each fiber transforms into a unit having several fibrils at its ends (F4-b in Fig. 1). The fibrils anchor each fiber so their bonding capabilities are superior to those of

When it comes to preventing water damage and preserving the integrity of concrete structures, waterproofi ng is essential. In of concrete tanks, however, effective, reliable waterproofi ng is especially critical. Concrete tanks play a vital role in many communities. They function as drinking water containers, wastewater treatment plants, and water storage reservoirs. Not only is a leak in a facility like this expensive and time-consuming to repair, it can be inconvenient and even dangerous to the community.

The rising cost of conventional steel reinforcement has dramatically increased the demand for synthetic, as well as steel, fibers as an alternative to wire mesh in shotcrete applications. More importantly, with the shift to the shotcrete industry is discovering that reinforcement yields significant economic advantages, as well as definite engineering benefits for long-term shotcrete durability.
Significant economic benefits result from the elimination of placing wire mesh. In addition, the use of fibrous reinforcement in lieu of wire mesh reduces rebound from the receiving face by up to 20%.
Suitable shotcrete applications include slope stabilization, tunnel liners and water diversion channels, structural repairs, swimming pools, arti-ficial rock, waterscapes, and thin overlays. These applications benefit from the three-dimensional network of reinforcement formed by the fibers, which reduces plastic shrinkage cracking and drying shrinkage cracking. The fibers also provide quanti-fiable toughness and enhanced durability, including increased surface abrasion resistance and impact resistance. Performance is predicated on the proper selection of the fiber type, length, configuration, and addition rate. Elimination of potential voids created by the wire mesh pattern is just one more advantage of using fiber.
General Product Information
There are three fiber types that contribute to the physical properties of shotcrete: steel fibers, micro- synthetic fibers, and macro-synthetic fibers.
Although the price of steel fibers has risen, the fact remains that the in-place cost of steel fiber-reinforced shotcrete is less than the cost of fixing and placing conventional steel. In general, steel fibers must meet the requirements of ASTM A 820 and may be manufactured from either drawn wire or slit sheet steel. Steel fibers, first introduced in the mid 1970s, are generally available in four lengths: 3/4, 1, 1-1/2, and 2 in. (20, 25, 38, and 50 mm). The standard unit of sale is typically 50 lb (22.7 kg) boxes or bags.
Micro-synthetic fibers can be nylon mono-filament or polypropylene monofilament and fibrillated fibers. They have been in use since the early 1980s for secondary temperature-shrinkage

Here is an important safety tip: Reducers and pumping lines should be directed away from the operator, whenever pumping conditions allow. This means reverse the 90-degree reducers and run the pumping system away from the pump operator and the ready-mix truck driver (refer to Fig. 1).
What happens on one-way streets, such as in San Francisco, where you cannot change the way the system can be run? We also have found out that running the shotcrete line system per Fig. 1 can be difficult when pumping shotcrete material that may tend to plug easily. The operator always seems to be going to the other side of the pump to unplug the lines, making it hard to access the ready-mix truck at the back of the pump.
After some serious brainstorming, we came up with a protective solution that would contain the concrete if a hose or reducer broke. We wanted to find something that could be bought almost anywhere and was priced right. Some type of rubber mat seemed to make sense. Our idea was to use a rubber mat from the bed of a pickup truck. To test this idea, we dumped all the tools out of a foreman™s truck and used that mat. We cut it in half and used the special shotcrete attachment device (tie wire) and tied it off. It worked great. We now buy mats for approximately $75, install holes, and attach bungee cords (refer to Fig. 2).
Pump Clean Up
Protecting reducers and couplings is not only a safety concern but an environmental issue as well. We pump a lot of shotcrete

This is an attempt to keep you informed about the activities of various organizations and committees developing standards and recommendations related to the shotcrete industry. Readers are invited to provide supplemental information related to the initiatives of ACI, ASTM International, or any other organization active in shotcreting.

As one who promotes choosing process before product, I believe that the success of any project also depends on matching the appropriate equipment to the process/product chosen. Specialty equipment offers several opportunities to achieve effective results. This article will discuss the predampener, a machine used in dry-mix shotcrete application.
In wet-mix shotcrete application, the water- cement ratio (w/c) is controlled prior to discharge into the mixer-pump apparatus. High-velocity air pressure propels the mixture to the receiving surface. In a dry-mix shotcrete application, the w/c is controlled by the nozzleman. Because it is not desirable for the nozzleman to introduce all the water at the water ring, a predampener must be used to ensure proper hydration of the cement.
The predampener facilitates the process as follows: the bagged product, or œsuper sack material, is dropped into the predampener™s front hopper and moved by an auger system through a fine mist of water that dampens the product prior to it being dropped into the shotcrete gun and propelled by compressed air up to the nozzle, where the appropriate amount of additional water is then added.
When shotcrete is delivered by a ready-mix concrete truck, the mixture is already predampened from the moisture in the sand. Avoid the use of