Practice C31 is to be used for concrete having a measurable slump. It is not intended for "zero slump" concrete, also called dry cast. ASTM C1176 has a comparable procedure for these samples. Also, refer to an article written by Carl Buchman, P.E. on ASTM C1176 by going to this link http://precast.org/tag/astm-c1176/. This practice is used for Self-Consolidating Concrete with the exception that the concrete is placed in one lift and rodding is omitted.
The practice of making and curing concrete cylinders in the field is important for the following reasons:
- (SC) strength acceptance as compared to the specified minimum;
- (SC) mixture proportioning and consistency;
- (SC) quality control;
- (FC) determination of an in-service date;
- (FC) as a method of comparison to other concrete made with the same design;
- (FC) to verify curing methods or methods of protecting the fresh concrete;
- (FC) for form stripping and removal.
(SC) = Standard Curing; (FC) = Field Curing
For the purpose of this post, I will only mention cylinders. Cylinders can vary in size, having two minimum requirements. The length must be two times the diameter, and the minimum diameter is to be three times the nominal maximum size of the coarse aggregate. Cylinders made with a coarse aggregate exceeding 2" must be sieved. The most common cylinder sizes are 6"x12" and 4"x8", with the 4"x8" cylinders becoming the most predominate size.
Concrete used to make compressive test cylinders must be representative of the placed concrete. samples are to be taken from concrete after any water or admixtures are added. Concrete used in other QC testing (i.e.; ASTM C231, air content) is not to be used to make compressive cylinders.
Molds are to be rigid enough to hold their shape. They are to be be capable of holding water. A mallet made of rubber or rawhide weighing 0.75-1.75 pounds is to be used for conventional slump concrete. A rod measuring at least 4" more than the length of the mold having a hemispherical tip on at least one end shall be used. The diameter of the rod shall be 5/8" on molds greater than or equal to 6" in diameter, and a 3/8" diameter rod shall be used for smaller molds.
Concrete compression tests are required to be performed by those individuals holding a certification as an ACI Grade I Field Testing Technician. Conventional concrete cylinders are made by filling the mold in three equal lifts, rodding each layer 25 times. The mallet is to be used to tap the sides of rigid molds 10-15 times after each lift is rodded. An open hand is to be used on light gauge single use molds. Strike a filled mold using the tamping rod. A float or trowel may be used if permitted. Final leveling should be done after moving the filled mold to its final curing location.
Section 10 of ASTM C31 references two methods of curing. Section 10.1 is Standard Curing; Section 10.2 is Field Curing. The method of curing will be specified by the agency with jurisdiction, by the customer, or as stated in the company quality control manual. The NPCA Plant Certification Manual (v10.0) specifies that the "specimens shall be cured in a manner similar to the curing of the concrete products represented by the specimens." This is usually defined as field curing as described in section 10.2.
Standard Curing - The molds will be stored on a surface level within 1/4" per foot. Store cylinders for a period of up to 48 hours in an ambient temperature ranging from 60-80 degrees Fahrenheit. The environment shall also prevent moisture loss. For high strength concretes (6,000 psi or greater), the temperature range shall be 68-78 degrees Fahrenheit. The storage temperature shall be controlled, and a data logger shall document the minimum and maximum temperature throughout this period. This information shall be recorded. There are multiple methods to achieve these requirements. See note 6 of ASTM C31 for a list.
After initial curing of standard cured cylinders, and within 30 minutes of de-molding, place cylinders in water on all surfaces having a temperature of 73.5 +/- 3.5 degrees Fahrenheit. The water tank of storage room must comply with ASTM C511, unless sulfur mortar caps are used.
Field Curing - Store the cylinders of concrete as near the concrete represented as is possible. For precast concrete operations, this can be the plant environment if indoors, or outside if product is cured outside. Protect the cylinders for wind or direct sunlight. The same moisture and temperature of the product should be used for the test specimens. If the product is covered during curing, cover or cap the cylinder molds. If the concrete is not covered, then the concrete in the mold should not be covered. It is important that samples cast for field representation be de-molded when the representative product is stripped. If the concrete is moved outdoors, the concrete samples might also move outdoors at this time. This should be defined in the quality control manual, and may be specified by the customer.
The report shall contain an identification number, a location, the date, time, and technicians name. The slump, air, and fresh concrete temperature shall also be recorded.The curing method (Standard or Field) should be noted of the final record. This record may also be used for recording the strength of the cylinders as specified by ASTM C31. I will review this test method in my next post.
I need to build a set of concrete steps out my kitchen door. I can't imagine it would be durable to build each step individually. Every concrete steps I have seen, make it look like the steps are one solid rock.
ReplyDeletehttp://www.cpcaustralia.com/products
Bryan, it is better to cast all of these steps as one unit. There are several precast concrete plants that cast steps in a factory. Then, these steps can simply be set into place. Perform a google search and look for "precast concrete steps."
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