04.03.18   R. Charles Ireland, PhD, PE | More by this Author


Whether cast-in-place, pre-cast, reinforced or unreinforced, concrete is the most-widely used construction material in the world.

Detroit’s resurgence has led to repurposing of old warehouses and other legacy buildings. Many of these structures have been neglected without routine maintenance for many years, and subjected to non-climate controlled conditions. Consequently, concrete structural members have experienced significant deterioration, as shown below.

Concrete spall near the base of a column exposing embedded, corroded reinforcing steel.

Original design reference documents containing information about construction materials are often no longer available for these older buildings. For analytical and upgrading design purposes, the compressive strength (f’c) of concrete is important fundamental information to the structural engineer. When compressive strength test information is not available, nondestructive testing (NDT) methods can be used to estimate in-situ concrete strength properties. One such method for establishing concrete compressive strength for in-service concrete is Windsor Probe testing.


The Windsor Probe test was developed in the mid-1960s in a joint effort between the New York Port Authority and the Windsor Machine Company. It is a penetration resistance test based on the surface hardness of a concrete element. Once established, the surface hardness is used to estimate concrete strength.

The test consists of propelling a ¼-inch diameter steel probe into a concrete element using a precision powder 0.32-caliber cartridge, Photo No. 2. The penetration resistance is a measure of the exposed length of the probe.

Windsor Probe device.


Short answer…yes and no. Let’s start with yes.

The American Society for Testing and Materials (ASTM) and the American Concrete Institute (ACI) recognize Windsor Probe testing as an acceptable method to estimate in-situ concrete strength. However, Windsor Probe test results must be correlated with known compressive strength values from concrete cores, or cylinders, obtained from the specific concrete mix being tested. A regression analysis is used to determine a “best-fit” relationship between the penetration resistance and compressive strength.

…and, no.

The Windsor Probe test is considered a ‘surface test’. Therefore, deleterious effects such as carbonation (affecting surface hardness) can result in inaccurately high test values. Other parameters that affect test results include the type, size, and distribution of coarse aggregate, and the location of embedded reinforcing steel. The photos below demonstrate core samples obtained from two different concrete columns in the same structure. Compressive strength test results varied between 440 psi (Photo A) and 1,920 psi (Photo B).

Representative concrete cores depicting variations in aggregate size and distribution.


SME was part of the project team for the redevelopment of a 1920s building with reinforced concrete framing, and a one-way rib-joist floor and roof system. SME obtained eight, four-inch diameter concrete cores from various beam and column members for compressive strength testing. Based on laboratory test findings, concrete compressive strength values ranged from 430 psi to 2,200 psi. These test findings were used to evaluate concrete strength characteristics using the Windsor Probe.

Windsor Probe tests were conducted at five concrete core locations. In-place tests (Windsor Probe) and compressive strength values (concrete cores) were plotted graphically so that a ‘best fit’ relationship equation could be established.

Windsor Probe field test values by themselves varied between 5,300 psi and 6,800 psi. However, after applying the ‘best fit’ equation to field test data, adjusted Windsor Probe test values ranged between 1,000 psi and 1,300 psi. The correlated strength values were more consistent with concrete core compressive strengths.

Compressive strength of concrete cores is the best and most accepted measure of in-place compressive strength. The Windsor Probe test as a supplement to compressive strength testing has its limitations, but can be useful when concrete surface conditions can be suitably considered prior to evaluating field test data. Therefore, it is necessary to obtain and test concrete core samples to verify the mix-specific compressive strength so that a strength correlation with the Windsor Probe test results can be established.

The Windsor Probe test can be reliable! However, it is best-suited as a supplement and complement other test methods.


If you need information about existing structural framing members or strength calculations to establish structural adequacy, look no further. SME’s Structural Materials Services team members can properly assess and identify conditions related to structural members in service.

For more information, contact Shayne G. Giordano, EIT, SMSI


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