message-icon close-message

    Rodding Effect on Flexural Resistance in a Granite Countertop

    Conventional countertop materials such as granite, stone, marble etc. are inherently brittle and susceptible to fracture under sudden impact type load. The weakness is further aggravated by imperfections such as chips and small cracks typically introduced during fabrication. These in turn become points of crack initiation or stress risers. Sharp impact and repeated (dynamic) loads cause the countertops to break easily during installation and/or transportation.

    A practical and inexpensive approach to counter the weakness is to integrate a stronger and stiffer reinforcing element into the countertop. The function of the reinforcing elements, typically steel or fiberglass rods is to enhance the bending rigidity of the countertop.

    The term Rodding is commonly used to refer to embedding reinforcing rods into the brittle counter top products. Rodding provides a mechanism for transferring force from the weaker granite to the stronger and stiffer rod. In addition, load transfer occurs only if the reinforcing rod is positively and permanently bonded to the countertop. This is achieved with use of structural adhesives specifically formulated to adhere to both the countertop and the metal or polymeric rod. Rod-It is the trade name of a rodding adhesive by for countertop fabrication.

    In practice, rods are embedded in the underside of the countertop, at the front and back of sink and stove top cutouts. Several stone panels with rods imbedded in the underside were submitted to the Durham laboratory for flexural testing. The panels had imbedded stainless steel bars 1/8 inch thick and 1/4 inch wide (with the 1/4 inch inserted).

    Rod-it in work

    Test Parameters:

    Variables Studied:

    1. Rod versus no-rod (reinforced vs. unreinforced stone)
    2. Rod length - long versus short
    3. Surface condition prior to adhesive application (effect of moisture in the stone)

    Results and Conclusions:

    1. The test results show that rodding increases the load resistance significantly
    2. The type of adhesive technology has significant effect on flex strength
    3. Surface condition (slab drying process) prior to bonding showed no significant effect on strength

    Test Summary:

    Marble stone samples measuring (9.5 inch by 4 inch by 1.0 inch) were used for the flexural testing specifically aimed at understanding the effect of rodding on the strength of the slab. The samples were mounted with the rodded bottom surface resting on the supports i.e., bottom surface in tension whereas the finished top surface placed in flexural compression. The test panels are supported at the two end supports with one inch overhang on each end.

    Tests were performed following ASTM D792 method on an Instron 8801 servo hydraulic testing machine with Blue Hill operating system and software. Figure below is a representation of a three point flex test specimen under a flexural loading.

    Rodding Flexural Report

    ASTM D790 - Flexural Testing – 3 Point Flex Strength of adhesive joints

    Results Summary:

    Table 1: Rod versus no rod
    Adhesive Rod-It None
    Substrate Marble Stone Marble Stone
    Rod Rect. Steel -
    Surface Preparation Dry Dry
    Load to Failure (psi) 3084 1562
    Failure Mode Rod bent -
    Effect of Surface Preparation on Flexural Strength

    Table 2: Effect of surface preparation (adhesive cured for 96 hours at room temperature):
    Adhesive Rod-It Rod-It Rod-It
    Substrate Marble Stone Marble Stone Marble Stone
    Steel Rod (length) Short Short Short
    Surface Preparation dry mist water, wipe dry Water submerged, air dry
    Load to Failure 2875 3134 2471
    Failure Mode Bent rod & pulled out Rod bent/embedded Rod bent/embedded
    Effect of Surface Preparation on Flexural Strength

    Failure Modes of Specimens under different surface preparations: