Impact of Vibration on Curing and Strength of Concrete

 

Scope

Research Need: During its setting process, fresh concrete transforms from a flowable state, via a plastic state, to a final solid state that includes a large number of crystalline domains formed by ionically and covalently bonded atoms. Early-age concrete thus is vulnerable to vibration damages if the formations of the chemical bonds and crystalline domains are negatively affected, leading to reduced early and ultimate strength. Vibrations could come from a variety of sources, such as passing-by trucks, nearby vibratory soil compactors, and blasting or seismic impulses. As demanded by the fast construction paces today, such vibrations often occur adjacent to newly placed concrete, such as when a soil compactor is used during the placement of concrete for bridge foundations or roadway slabs. Being a pervasive issue that is related to construction speed and structural integrity, weakening of concrete by adjacent vibrations cost stakeholders millions of dollars annually. This issue is becoming more imperative recently, owing to factors such as new design concepts and changes in equipment and construction methods. In the current state of knowledge, however, there has been a surprising scarcity of assembled information on the subject of vibration impact on concrete curing and strength. There exists a large number of different stipulations regarding the nearest allowable locations for vibratory construction and earliest allowable time for vibratory construction that are currently practiced by the different transportation agencies across the country, mainly the State Departments of Transportation. For example, the earliest allowable time for vibratory construction ranges from a few hours to a week or so. The existing stipulations appear to build on different principles, including laboratory experiments, field observations, numerical simulation, and most commonly the borrowing from peer practitioners or close engineering and science fields, which are far from systematic. Work of synthesis on the subject thus is needed to identify, describe, and evaluate the current state of knowledge and practices to benefit the construction of bridge decks, pavement slabs, and overlays.

State of Knowledge: It is generally believed that concrete is most vulnerable to vibrations between the initial and final setting times due to the negative effects of vibration on the bond formations in this critical hydration phase. The setting time of concrete refers to the time required for cement paste to stiffen to a defined consistency, which is closely related to the initial chemical reaction of calcium aluminates of the cement with sulfates within the first few hours after cement-water contact. The initial setting time of concrete measures the time as cement paste starts to lose plasticity, and a minimum value is required to ensure the completion of transportation, placement and compaction of concrete. The final setting time of concrete records the time at which cement paste loses its entire plasticity, hardens sufficiently, and attain the cast shape at mold removal. At normal construction temperature, the initial setting time of concrete could come as early as 60 – 90 minutes and the final setting time could be as late as eight to ten hours. Current practices use two empirical methods, i.e., the Vicat Needle (AASHTO T 131 or ASTM C 191) and the Gillmore Needles (AASHTO T 154 or ASTM C 266) for determining the initial and final setting time.

The strength of concrete can be reduced by vibration beyond its final setting time. It was reported that two-day concrete could lose as much as 9.1% of its 28-day compressive strength under continuous vibration from heavy highway traffic, while the loss of the 28-day compressive strength for 14-day concrete was within 3%. Realizing this post-setting phenomenon, stakeholders have specified conservative time limits before vibrational constructions near freshly cast concrete. As an example, the Wisconsin Department of Transportation is considering to reduce such required curing time from seven days to five days, to enable more rapid construction while still giving sufficient time for concrete to obtain the design strength. If adopted, this modification undoubtedly will mean huge cost savings and convenience to the public.

In addition, vibration seems to have different impacts on different properties of concrete. The same level of vibration can change the compressive strength of concrete by up to 13%, while reduces the tensile strength of concrete by 7%. Based on a study of vibration from highway traffic, the amplitude of vibration seems to be a more important factor than the frequency in causing damage. While a vibration of two Hz and three mm amplitude and a vibration of four Hz and three mm amplitude cause significant reduction in ultimate strength of concrete, the vibration had a negligible strength reduction at a one mm amplitude.

To conclude, a synthesis work is needed to collect and evaluate the current state of knowledge and practices regarding the complex dependence of concrete quality and strength on the nearby vibrations. This work will be useful in the designing of both new and repairing projects, for more accurately determining the time needed before the start of nearby constructions and the allowable intensity and nearness of the vibratory sources.

Information Sources

1. ACI Manual of Concrete Practice (2015). American Concrete Institute. 2015.
2. Research Results Digest 392. National Cooperative Highway Research Program (NCHRP). Jan. 2015.
3. Taylor, P. C., Kosmatka, S. H., & Voigt, G. F. (2006). Integrated Materials and Construction Practices for Concrete Pavement: A State-of-the-Practice Manual (No. FHWA HIF-07-004). Federal Highway Administration. 2006.
4. NCHRP Report 253. Dynamic Effects of Pile Installations on Adjacent Structures (1997). National Cooperative Highway Research Program (NCHRP). 1997.