American Concrete Institute ACI Materials Journal March/April 2017
Bond Strength in Multilayer Casting of Self-Consolidating Concrete
Wael A. Megid and Kamal H. Khayat
Newswise — Placement of self-consolidating concrete (SCC) in mat foundation and large wall elements often necessitates the casting of concrete in multiple lifts that can lead to lift line formation, which could result in aesthetic defects and structural deficiencies. There is a critical need to develop recommendations to quantify bond strength across lift lines in multilayer construction and design SCC mixtures with the necessary rheological characteristics to mitigate this phenomenon. This investigation offers guidelines to evaluate residual bond strength in multilayer casting for SCC of different thixotropic levels. Minimizing the impact of delay in casting successive SCC lifts and designing SCC with adapted rheology should be of interest to material engineers and construction planners. The conventional workability test methods can provide significant contribution to the growing demand of quantifying the structural buildup at rest of concrete, which is a challenge for quality control of SCC on jobsites.
SCC mixtures proportioned with various materials and designed to develop different fresh and hardened characteristics were prepared. The structural buildup at rest was determined by evaluating the variations in the values of workability parameters; slump flow, T50, and J-ring flow; over cretin periods of rest up to 60 minutes. Bond between successive layers was determined using composite specimens cast with two lifts of SCC after rest. Bond strength was determined using the slant shear and direct shear test setups. In both testing methods, a delay time between the casting of the first and the second layers was set to a maximum of 60 minutes. The second layer of SCC consisted of concrete that was properly mixed without any rest, which would represent actual concrete practice. Some samples were cast in one layer to secure monolithic conditions and are considered as control samples. After 7 days of moist curing, all sample were tested under uniaxial compression.
The slanted shear test method is widely employed to evaluate bond strength of resinous repair materials to concrete substrates and involves casting a cylindrical sample made of two identical halves bonded along a slant phase of 30 degree from the vertical direction. In direct shear test method, the interface between the two concrete layers was oriented in the horizontal direction during casting and then in the vertical direction during testing. The use of such direct shear specimen molds can avoid stress concentration at the edge of the bond plane, which is essential to reduce scattering of bond strength results. The fabrication of the direct shear test samples is simple and, unlike the slant shear test, it does not require saw-cutting the sample before testing.
The results show that both residual bond strength test values decrease with the increase in delay time between successive SCC layers. The delay time is found to decrease with the increase in structural buildup at rest of the freshly cast SCC. The maximum allowable delay time required to secure adequate bond strength and determined under slant shear strength is higher than that the direct shear bond strength for SCC of a given thixotropic level. The difference between delay time values determined using the slant and direct shear stress test methods is found to be approximately 55 minutes when SCC mixtures that have low level of structural buildup at rest are used. However, when the SCC is highly thixotropic, the spread in delay time should decrease considerably to secure adequate bond strength. The residual bond strengths, determined under slanted and direct shear stress, dropped drastically to 56% and 15%, respectively, for the SCC mixture that had the highest level of structural buildup at rest. The residual strength determined by the direct shear strength test was more sensitive to defects that can exist at the distinct layer between the two SCC lifts than the slant shear strength.
SCC mixtures that have low level of structural buildup at rest is recommended for casting elements of large dimensions or those where a certain delay prior to casting successive layers is expected. Such SCC can have a minimum initial slump ﬂow of 630 mm (24.8 in.) and maximum rate of loss in slump ﬂow at rest of 1.20 mm/min (0.047 in./min). Such concrete can exhibit low viscosity with an initial T50 of 1.60 sec or less and a rate of increase in T50 at rest of 0.045 sec/min or less as well as a minimum initial J-ring ﬂow of 630 mm (24.8 in.) with a rate of drop in J-ring ﬂow values at rest of 0.85 mm/min (0.033 in.) or less. When the delay time between castings of successive layers of such SCC Category is 15 minutes, the minimum residual bond strength determined under slant and direct shear stress can be 97% and 92%, respectively.
After a certain period of rest, the static yield stress of the SCC can increase and reach a critical value that prevents it from intermixing with the next lift of concrete and thus a decrease in surface roughness between layers. The surface roughness can be improved by increasing the free-fall height (FFH) of the new layer onto the existing material, which would result in reducing its yield stress. Such FFH should be adapted to avoid segregation or bleeding, which can have impaired bond between successive concrete layers. The increase in FFH of SCC belonging to the concrete category mentioned in advance resulted in an increase in residual bond strength from 76% to 86% when the top layer was cast after a delay time of 60 minutes. The increase in bond strength with FFH can be related to an increase in the level of aggregate interlock resulting from greater surface roughness at the interface with the lower concrete lift.
The research can be found in a paper titled “Bond Strength in Multilayer Casting of Self-Consolidating Concrete” published by ACI Materials Journal.
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