Concrete, noted for its desirable structural characteristics, is unfortunately also noted for nonuniform and aesthetically poor surface appearance. With the growing trend to make man's surroundings more visually attractive, the concrete industry needs to understand the causes of visually unattractive surfaces and then control surface quality. While some want blemish-free surfaces, there are others, particularly in the architectural profession, who feel that a controlled number of voids adds character to the finished surface. To these architects, blemish-free concrete is sterile and uninteresting. The concrete industry does not now have sufficient control to either prevent or produce voids or other blemishes to a predetermined degree. There are six general classifications of factors which can, singly or in combination, cause blemishes. The primary sources of problems leading to blemishes are: general work conditions; the concrete mix; placement techniques; compaction effect; forms; and other influences. The work conditions have a marked influence on the constructor's ability to do what is intended. If a constructor is prevented from properly executing his work as a result of restricted openings in the formwork, he cannot be expected to accomplish optimum results. Workability of the concrete mix affects the occurrence of surface blemishes. Mixes that are properly designed and respond to an appropriate vibrator are necessary to minimize surface problems. Unfortunately, to the best of our knowledge, the industry has no established method of determining when a mix is proportioned for optimum quality. Blemishes can be expected in greater quantities among mixes classified as either sticky or harsh. Mix workability cannot be strictly associated with slump. Objectionable blemishes can originate during the placement of concrete. The concrete must be controlled to flow at the specified consistency with a minimum of disruption by obstruction within the forms. This is a matter of placement technique. Concrete compaction equipment and techniques must be selected that are appropriate for the specific work to be accomplished. To minimize surface blemishes, vibrators must have the proper frequency, amplitude, power source and size. The forms themselves can cause blemishes. Absoptive forms tend to minimize entrapped air and voids in the finished concrete surface.
