Free Fall of Concrete in Drilled Shafts Under Dry Conditions
Construction Engineering Services
One recurring question posed to us over the years has been “Is the free fall method for concrete placement in drilled shafts (caissons) ok?” The answer is “Yes!” provided good construction practices are followed and the shaft is essentially dry (an inch or two of water at the bottom is considered acceptable, provided there is no bottom heave or sloughing of the shaft sides and negligible inflow). The interesting thing is, even with research and industry practice both being firmly behind the practice of free fall concrete placement in drilled shafts, we are still regularly asked this question. We hope this Lesson Learned will be of use to you should someone question free fall placement. As always, feel free to contact your local ECS office for project specific assistance.
Studies and practical experience since the 1960’s have shown that the free fall method of concrete placement provided quality drilled shafts for dry placements in excess of 100 feet. Drilled shafts around the world have used free fall concrete placement successfully on shafts as deep as 150 feet. To provide definitive answers to many questions, the Association of Drilled Shaft Contractors (ADSC) along with the Federal Highway Administration Agency (FHWA) sponsored a study in 1994 to put several persistent questions such as the following to rest. The results of the 1994 study were incorporated into the latest FHWA Manual on drilled shafts “Drilled Shafts: Construction Procedures and LFRD Design Methods (FHWA Publication FHWA-NHI-10-016, FHWA GEC 010)”.
Does concrete segregate, or lose strength as a result of extended free fall? The answer is, no, there is not an appreciable loss of concrete strength during free fall placement. The research indicated that by directing the concrete to the center of the shaft only a small, statistically insignificant variation in concrete strength was noted, with all test results meeting or exceeding the design strength of the mix used for the study. The study indicated no negative effects for a rebar cage 2½ feet in diameter and 80 feet deep. Extrapolated, the study supported free fall depths of up to 120 feet, for a 5 foot diameter rebar cage. (Note, full length cores performed by the study’s authors, retrieved from more than 100 drilled shafts constructed to 100 to 150 feet in depth, never indicated weakened or segregated concrete.)
Does significant segregation and loss of strength occur if the concrete hits the rebar cage during placement? Again, the answer is no. For the study, a drilled shaft was actually installed and the concrete was purposefully directed at the rebar cage as well as a permanent corrugated liner near the ground surface. No segregation of aggregate was observed. Ironically, compressive strength was higher in 6 out of 7 direct comparisons to centrally dropped concrete. Although the purposefully sloppy placement procedures did not have a detrimental effect on the concrete strength, the rebar cages experienced distortion. Also, there was evidence of soil intrusion along the side walls resulting from concrete hitting the shaft walls during placement. Therefore, even though sloppy placement did not impact concrete strength, the proper installation method should consist of directing the concrete to the center of the shaft, using a chute, elephant trunk or other applicable device. In other words, good construction practices should, as always, still be followed.
Does rebar spacing (within normal limits) affect whether the concrete flows through and around the rebar? Yet again, the answer is no. The concrete for all shafts was fully formed, though in areas of tightly spaced bars, the exterior of the shaft was described as “rougher”. The study recommends rebar spacing of not less than 3 inches (which is consistent with our recommendation to always follow good construction practices).
Does vibration of a well designed concrete mix affect concrete strength? A small increase (about 7%) in concrete compressive strength was observed between vibrated and non-vibrated concrete. The non-vibrated concrete was still well in excess of its design compressive strength. A properly designed concrete mix should perform well without full length vibration. Vibration of the concrete at the top of the shaft is recommended, as placement at the top of the shaft is essentially conventional concrete placement.
What role does slump, maximum aggregate size, or addition of superplasticizer play in the results? No noticeable difference could be discerned in the compressive strengths between the various concrete mixes placed by varying the properties described above. No segregation was observed in the shafts, even for the sloppy placement procedures.
So, in summary the free fall method of concrete placement can successfully be used in drilled shafts up to 120 feet (or possibly) more in depth without detrimental effects to the compressive strength, integrity of the shaft or segregation of aggregate. The above information should help reduce the concerns of those who have been misinformed or may not have had sufficient experience with drilled shaft construction to be comfortable with free fall placement. Remember, good drilled shaft construction practices should always be followed; such as directing the concrete to the center of the shaft, specifying workable slumps of 5 to 7 inches, providing room for the concrete to flow freely through and around the rebar by maintaining proper spacing (not less than 3 inches).
Free fall placement of concrete is meant for dry shafts. So, don’t throw away those tremie pipes just yet. If the shaft has more than an inch or two of water at the bottom, or a consistent flow of water entering the shaft that can not be controlled with a pump, then the drilled shaft concrete should be placed using proper tremie procedures. And that, our friends, is a Lesson Learned for another day.
We hope this “Lessons Learned” will be helpful to you in planning for your next project.
ECS Corporate Services, LLC