Scanning Electron Microscopy and Other Techniques to Investigate Low-Strength Concrete

Fracture surface scanning electron microscopy (SEM), X-ray microcomputed tomography (X-ray CT), microhardness testing, contact angle goniometry, and foam drainage testing were used to investigate the cause of low compressive strength concrete in South Dakota. SEM indicated that air voids were collecting in the vicinity of aggregate surfaces. These voids reduced the area over which stress could be transferred from the paste to the aggregate. Also, SEM showed that paste between the air voids was of poor quality. X-ray CT corroborated the SEM findings concerning the air void distribution, whereas microhardness testing indicated that the paste in higher-strength samples was three times as hard as the paste in low-strength samples. Air bubbles may attach to the aggregate surfaces due to the aggregate becoming hydrophobic when airentraining admixtures were used in the concrete. Contact angle goniometry confirmed this phenomenon as model aggregate surfaces had contact angles between 10 and 40 degrees for all admixtures tested, and model aggregate treated with vinsol resin–based air-entraining agents exhibited significantly lower (by 5 to 20 degrees) contact angles. Finally, examination of three-phase foams produced by mixing cement, air-entraining admixture, and water showed that vinsol resin–based foams had larger bubble sizes and kept more water within the foam than did nonvinsol resin-based admixtures.