Questions from the Field

Questions from the Field is a selection of questions NPCA Technical Services engineers received from calls, emails and comments on blogs or magazine articles on precast.org.

If you have a technical question, contact us by calling (800) 366-7731 or visit precast.org/technical-services.

Asabee writes:
With concrete being a mixture of fine aggregate, coarse aggregate, cement and water, is the main cause of concrete volume increase due to the coarse aggregate?

NPCA Technical Services engineers answered:

About 60% to 75% of concrete’s volume is comprised of fine and coarse aggregates. Fine aggregates typically consist of sands or certain types of crushed stone, with most particles being smaller than 5 millimeters. Coarse aggregates are usually made of gravel, crushed stone or a mix of both. The majority of coarse aggregate particles are greater than 5 millimeters. ASTM C33 dictates gradation requirements for both fine and coarse aggregate. The standard requires certain percentages of an aggregate sample (by mass) to pass through various sieve sizes. For example, Table 1 in ASTM C33 states that 100% of a fine aggregate sample must pass the 9.5-millimeter sieve, 95% to 100% of the sample must pass the 4.75-millimeter sieve, 80% to 100% must pass the 2.3-millimeter sieve, 50% to 85% must pass the 1.18-millimeter sieve and so on. The standard also states requirements for coarse aggregate sizes.

When we refer to volume change in concrete, we are talking about changes after the concrete is batched and placed. We are not talking about volumetric changes while materials are being added to the mix. Volumetric changes to concrete after placement and during its life can be caused by many factors. In fact, aggregates play a role in restraining volume change. It is the paste and its constituents that can contribute significantly to volumetric expansion and contraction. This is why water content, for example, is critical in this regard. The more water in the mix, the more chance for shrinkage.

Shamik writes:
Is there a standard test method for fire rating checks of non-shrink grouted joints for precast buildings?

NPCA Technical Services engineers answered:
The specific American Concrete Institute code regarding fire-resistant concrete is ACI 216, “Code Requirements for Determining the Fire Resistance of Concrete and Masonry Construction Assemblies.” However, the code does not include language regarding fire rating testing of grout or grouted joints.

Jerry writes:
We’re pouring a column that is about 123 inches tall and will require about 2,761 pounds of concrete. The reinforcement cages will be made with #3, #5, and #7 rebar and steel plates. We’ll use a 20-foot-long internal vibrator with a 1.25-inch-diameter head. How can we determine the right lifts or layers of concrete to obtain better quality vibration?

NPCA Technical Services engineers answered:
Some important factors to consider are the type of concrete, the flowability of the concrete you’re using, the placement and spacing of the steel, the total volume of the pour and the type of formwork you’re using to frame this column. Concrete in columns is placed in lifts of various depths depending on the aforementioned factors, but generally the lifts range from about 12 to 24 inches. ACI 309, “Guide for Consolidation of Concrete,” recommends that lift depths should not exceed 20 inches.

The key is to vibrate the first lift with the head of the vibrator all the way to the bottom of the form without touching the form. On subsequent lifts, insert the vibrator at least 6 inches into the previous lift to blend the layers together and avoid cold joints.

It also depends on the internal vibrator’s specifications and at what amplitude and frequency the head is vibrating. This will affect the vibrator’s radius of action. We recommend asking the vibrator manufacturer for details on correct use and best practices. Other articles found on precast.org, such as “Production Equipment” and “Top Precast Plant Operational Deficiencies Part 2,” can also be used as references.

JD Anderson writes:
I have a mix strength that exceeded 8,000 psi, twice what it’s rated for. The structure is a pier crane column. Is too strong of a concrete mix a concern?

NPCA Technical Services engineers answered:
Concrete that is breaking at twice the design strength is not necessarily a bad thing, but you should address a few items to ensure there are no major underlying issues.

First, you need to answer the following questions.

• Have you calibrated your strength machine in the past year?
• Are you breaking the cylinders at the rate in accordance with ASTM requirements?
• Are you using the correct end caps and neoprene pads?

Having a machine that is out of calibration, operated incorrectly or using incorrect equipment could lead to inaccurate readings. This is the first place to check to confirm you are in compliance.

The next set of questions to ask are:

• Are you using different materials in your concrete mix?
• Do you have a new cement, admixture or aggregate supplier?

Precasters sometimes use different suppliers for similar products and their strength readings have been greatly improved or reduced. And, the last question – has anything else in the manufacturing process changed? From casting to consolidation to curing, if any of these processes change, strength could be affected.