Portal: General

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

It’s 5:30 in the morning in our yard. All of the compressors, trucks, shot-crete pumps, and related equipment are on their way out of the yard for the day™s operations. It™s important that the equipment gets to the various jobsites, which could be anywhere from 10 to 150 miles away, on time and without incident. But what can be done to assure that everything is road-worthy?

I remember doing a job in San Francisco, CA, about nine or ten years ago. The job consisted of building a basement in an existing commercial area. There were buildings on both sides of the new proposed basement. The construction called for the new basement walls to be built on the property line with a 2-in. foam spacer”the perfect shotcrete job!

In 2000, ACI, in cooperation with ASA, estab-lished a formal certification program for certifying shotcrete nozzlemen. Prior to that date, ACI had a guideline document for certifying nozzlemen, but it was not being uniformly applied, it contained outdated features and appli-cations, and it was not being universally accepted nor endorsed.

Where a section of shotcrete is left incom-plete at the end of a shooting shift, some provision must be made to ensure that the joint will not develop a plane of weakness at this point. According to the American Concrete Institute™s œGuide to Shotcrete1 and the œUnified Facilities Guide Specifications,2 construction joints should be tapered to a shallow edge form, about 25 mm thick. (1 in.) Therefore, in North America, contract specifications will typically require that shotcrete construction joints be tapered to a 45-degree edge and be thoroughly cleaned and wetted prior to the application of additional shotcrete. When welded wire mesh re-inforcement is used, some contractors will also overlap two layers of mesh at the expected location of construction joints in the antici-pation that the shotcrete at the joint may be weaker than the unjointed portions of the shotcrete.

The use of calcium aluminate cement as the binder in refractory systems in general, and refractory shotcretes in particular, is widely approach to high-performance refractory shotcrete involves the use of calcium aluminate aggregates along with the calcium aluminate cement. The addition of this unique aggregate imparts enhanced properties to shotcretes, enabling them to endure the toughest of industrial environments.