Pre-blended dry or damp materials are placed into the delivery equipment. Compressed air conveys material through a hose at high velocity to the nozzle, where water is added. Material is consolidated on the receiving surface by the high-impact velocity.
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Shotcrete is just a placement method for concrete. Thus, the in-place material will have the same characteristics as concrete. You should consult the coating supplier to establish how long they want the concrete surface cured. They may also specify a certain limit for surface moisture conditions. There are several methods for evaluating the moisture content of the in-place concrete. Here are the ASTM standards that deal with surface moisture:
In addition, The International Concrete Repair Institute (ICRI) has a certification program, “ICRI Concrete Slab Moisture Testing Technician—Grade 1.”
There are many variables that would need to be considered to answer your question. Water flow, depth of the application, and overall geometry can influence the required thickness. This is a question that should be addressed by an experienced engineer who can ascertain the required structural properties to resist the hydraulic loads for the depth and geometry of your application. You may find consultants who are ASA Corporate Members in our Buyers Guide.
Shotcrete is a placement method for concrete. Thus, generating the maturity curves are based on the concrete mixtures. There are several online resources about the maturity method. The Minnesota DOT provides a PDF resource that discusses production of the curves.
Shotcrete is simply a placement method for concrete. The specified concrete cover over reinforcing bar is usually included in contract documents for construction and values vary depending on exposure conditions. ACI 318 provides cover requirements for structural concrete in buildings, and ACI 350 provides cover requirements for concrete liquid-containing structures. Local building codes and fire codes may also require specific cover in concrete construction. If your project doesn’t specify the cover requirements, we recommend you consult with a professional engineer experienced in the type of project you are working on to learn what the code requirements may be.
By “gunite batch truck” we assume you are referring to a volumetric mixer supplying material for a dry-mix shotcrete operation. Gunite is the original tradename for what we now call dry-mix shotcrete. Here’s the description from an ACI Materials Journal (January-February 1991 issue) article about the calibration of volumetric mixers:
“To insure production of quality concrete, each volumetric-measuring unit must be calibrated for each respective concrete ingredient, following the manufacturer’s recommendations and ASTM C 685. These ingredients must be the same as those to be used in actual concrete production. The measuring devices for aggregates, cement, and dry admixtures are calibrated by weighing the discharged ingredient. Devices for water, latex modifier (if required), and liquid admixtures such as air-entraining and water-reducing admixtures generally are calibrated by weighing or measuring the volume of the discharged ingredient. The objective of calibration is to coordinate the discharge of all concrete ingredients to produce the proper mixture.”
ASTM C685/C685M states, “The proportioning and indicating devices shall be individually checked by following the equipment manufacturer’s recommendations as related to each individual concrete batching and mixing unit. Adequate standard volume measures, scales, and weights shall be made available for the checking accuracy of the proportioning mechanism.” Thus, you need to check with your equipment supplier for their recommended procedures to verify batching. Because concrete mixtures always are based on weight of ingredients there you will need to weigh a given volume to confirm the batching is accurate.
The air content test is a measure of total air so includes both entrapped and entrained air. Ten percent is definitely high. The 10% air is likely not representative of the in-place shotcrete. It may have been an issue with the lab mixer introducing more entrapped air for some reason. Estimating the air content from the unit weight test requires a good value for the theoretical unit weight. I’m not sure if you have that with the volumetric batching. You should run the air meter test (ASTM C231/C231M) to measure the air content to get a more accurate assessment.
Regarding the verification of mixture proportions, this is from ASTM C685/C685M:
“7.5 Proportioning Check—Whenever the sources or characteristics of the ingredients are changed, or the characteristics of the mixture are noted to have changed, the purchaser is permitted to require a check of the fine aggregate content and the coarse aggregate content by use of the washout test. Essentially, in the washout test, 1 ft3 [0.03 m3] of concrete is washed through a No. 4 [4.75-mm] sieve and through a No. 100 [150-µm] sieve; that retained on the No. 4 sieve is normally considered coarse aggregate whereas that passing the No. 4 and retained on the No. 100 sieve is considered fine aggregate. Corrections to the quantity of aggregates (per cubic foot or cubic meter of concrete) shall be made if the original sieve analysis of each aggregate is available.”
Because you are only interested in the sand and cement, you can simply weigh the sample of concrete, then wash out all the cement, and then weigh the remaining sand. You would need to bring the sand to roughly the same moisture content as the sand in the truck, so you aren’t including in the weight of excess water in the sand. You should note that with shotcrete impact during placement we will generally lose 50% of the air content, so your final in-place air should be around 5%. That is a reasonable value for good freezing-and-thawing durability.