Understand what causes concrete surface defects and how to prevent this from occurring.
In this age of technology, it seems unacceptable that newly poured concrete often has to be patched. A large amount of masonry cement is consumed for patching, and often contractors just accept the patching expense in the bidding process.
Several years ago, Wyco began a research project into the surface defect causes and first investigated micro air separation in concrete pavement, which is caused by vibrator speeds- they initially thought this produced the undesirable concrete surfaces. Since concrete mix only contains between 1 to 3 percent micro air and less than 3 percent macro air, it does not generate enough air volume in the mix to cause the size of the surface voids that were observed. Wyco realized early on in their research project that they needed committed industry partners to aid in investigating related mix design properties, applied physics and objective field data.
Wyco partnered with quality contractors who were looking for solutions. These contractors would patch walls almost immediately after stripping the forms to hide surface defects, and they wanted a better to handle this problem. Charting pours were monitored to determine when the forms were stripped. Wyco checked the mix that was designed, read the pressure pot test and interviewed the vibrator operator, but they still could not find any abnormal data that would help solve the surface defect puzzle.
A sampling of industry vibrators, constructed glass forms, applied collected field data were monitored, and eventually a discovery was made. Researchers found that the water in the mix was separated from the rest of the materials in the concrete. The faster they ran the vibrator or the longer they kept it in the concrete, the more water they would pump to the form faces. It became apparent that using a widely variable tool on widely variable material produced a consistently disappointing result.
To identify tendencies in these flowable mix designs, Portland Cement Association (PCA) and Construction Testing Laboratories (CTL) investigated the mix properties of concrete commonly placed by a pumping operation. PCA's and CTL's knowledge and experience with water reducing agents that delay set times gave Wyco a greater understanding of the water, cement and fly ash in these types of mixes. The process of retarding the hydration leaves water sitting in the concrete and available to be driven out by vibration. The difference in water's density compared to other materials shaped its behavior to vibration velocities. We took a controlled speed vibrator back to the contractor to evaluate speed and time spent per cubic foot in the concrete.
Wyco found in their research that surface defects were reduced by matching controlled vibration speeds with flowable concrete mixes. To combat the surface defect problem, employees must be educated.
Many business owners realize that committing to employee education is crucial to their ultimate success. Concrete contracting firms often spend resources on educating employees about jobsite safety, first aid, placement practices, equipment use, employee benefit use, wellness programs and even concrete mix design.
Vibration education has been in catch-up mode for a while. The key player in driving a company's commitment to education has been the project engineer. The success of placing well-consolidated concrete with minimal surface defects comes from a free exchange of information between the vendor and the project engineer, job superintendent, labor foreman and vibrator operator. Adjusting a product's use in concrete will depend on support from education partnerships within the concrete industry. The concrete vibrator's important functions in relation to surface defects have to resonate throughout a company from architects and consulting engineering to a well-informed vibrator operator. The vibration dynamics and expectations need to be well communicated to everyone involved so that the results are understood and improvements in concrete surfaces can be made.
There is a definite cause and effect in vibrating flowable mix designs. The ultimate goal of the vibration industry and tool manufacturers should be to develop products that are quantifiable, data-collectable and evolving.
