Ground penetrating radar image of a slab-on-grade.

{QUESTION} We have a project where the contractor is constantly complaining we are shorting them on concrete. We have checked yield several times during placements and came up with good results. What should we do next?

{ANSWER} Solving this yield issue has two approaches. The first is internal, where the producer reviews internal quality control processes. The second is external, where the volume of concrete actually placed must be reviewed.

Before we dive into these two areas, we must back up and verify the yield calculations. You indicated that yield was checked several times but don't overlook the possibility of incorrect calculations. It is an easy calculation to perform, but it often can be conducted incorrectly. It may be worthwhile to have another set of eyes review the calculations.

For internal review, the producer must be confident the correct amount of material has been placed in each batch. “Profiting from Quality Concrete” on page 35 discusses the cost implications of improper yielding and speculates on the possible reasons that a mix may not yield correctly.

As that article indicates, it is worthwhile to obtain updated properties on the raw materials used for the mix. Changes in specific gravity and absorption of the materials will affect the yield and adjustments to the mix design should be made. Changes in the aggregate moisture can greatly affect yield if not accounted for in batching. A 2 percent change in moisture content can make a difference of 8 cubic yards over a 500-cubic-yard placement. That small change can add up.

As part of an internal quality review, check the scale and pump calibrations. Scale calibrations are reviewed as part of the NRMCA Plant Certification program, but it also requires the producer to have access to standard weights to periodically check the scale calibrations.

For any liquid products pumped into the batch, it is easy to use a graduated container to catch and measure the volume measured in the automated systems. It is obvious that mismeasured amounts reported as correct on the batch ticket will throw off the yield. Yield calculations usually uncover these issues.

Cautiously review the actual volume of concrete placed. These actions may irritate the purchaser, as you are starting to blame him. If all sides of the concrete element are accessible, measuring the dimensions is easy and a volume of the concrete element can be calculated.

Don't forget to consider the volume of any reinforcing steel or embedments. Often, these don't contribute much to the volume, but these are heavily reinforced structures which can have a very large effect.

For structures where only some sides of the element are available for measurement or structures such as large suspended slabs or slabs-on-grade where thickness may vary, a more elaborate approach may be required. Nondestructive testing can produce relatively accurate dimension very rapidly. Two techniques usually recommended for this work are impact echo or ground penetrating radar (GPR).

Impact echo is a nondestructive testing technique that sends a compression wave through the concrete and measures the time required for that wave to return to the source. The relative speed of the wave is known and the thickness of the concrete can then be calculated. The measurement usually only takes a second or two to provide the thickness. This measures only the thickness at that specific location. Lay out a plan to record thicknesses at various locations and calculate an average thickness based on statistical analysis.

GPR will do a similar measurement. Most radar systems are set up to take continuous measurements on a line. This technique may not be as accurate as the impact echo, although it provides a profile and eliminates some of the random measurements.

Often, the yield is okay but there is a dimensional change that requires more concrete than originally anticipated. For example, a slab 200 feet by 200 feet by 6 inches thick would require 740.7 cubic yards of concrete. Adding ¼ inch to the thickness would require 771.6 cubic yards, a difference of more than 30 yards.

This is a dramatic example; it is unlikely a ¼ inch will be added to the entire slab, but if this is a suspended slab and there is some deflection in the forms, it will affect the concrete volume. If this is a slab-on-grade, some small dips and valleys in the subgrade will affect the required concrete volume.

There has to be a reason for the lack of concrete for the placement. You just have to keep digging until you find it.

Contributed by Alfred Gardiner, Principal Engineer with Braun Intertec. more,