Use of Triaxial Compression Test on Mortars to Evaluate Formwork Pressure of Self-Consolidating Concrete

ACI Materials Journal, Sep/Oct 2009 by Assaad, Joseph J, Harb, Jacques, Khayat, Kamal H

A comprehensive research project was undertaken to evaluate the suitability of triaxial testing in determining formwork pressure developed by self-consolidating concrete (SCC) under both drained and undrained conditions. The cohesion (C) and angle of internal friction ([straight phi]) were determined on 10 concrete-equivalent mortar (CEM) mixtures made with various binder types and water-cementitious material ratios (w/cm). Correlations were established with respect to thixotropy and formwork pressure determined using SCC.

Test results show that the cohesiveness of CEM mixtures is greatly affected by the drained and undrained conditions under which the tests are conducted. Mixtures made with ternary cement and/or lower w/cm exhibited higher C and lower [straight phi] values compared to those made with binary or Type I cement, as well as those prepared with increased water content. Lateral pressure of SCC developed right after casting can be predicted using either thixotropy or C values. For pressure drop evaluation over time, however, the CEM triaxial approach was not appropriate given the absence of coarse aggregate, which underestimates the development of internal friction between solid particles.

Keywords: concrete-equivalent mortar; formwork lateral pressure; self-consolidating concrete; thixotropy; triaxial testing.

INTRODUCTION

Since the beginning of using highly flowable concrete, including self-consolidating concrete (SCC), considerable focus has been placed on the magnitude of lateral pressure exerted by the plastic concrete on the formwork. This is due to the significant increase in construction costs that may result if assuming full hydrostatic pressure is exerted on the formwork.

In 2002, Assaad and Khayat initiated a comprehensive research project to quantify the effect of mixture parameters on formwork pressure. This included the evaluation of the type and combination of cementitious materials, chemical admixtures, water content, aggregate particles, and consistency of SCC.1-6 Pressure tests were determined using an experimental polyvinyl chloride (PVC) column measuring 2800 mm (110 in.) in height and 200 mm (7.9 in.) in diameter. The PVC's inner surface was smooth to minimize friction with the cast concrete and watertight to prevent any loss of the fluid phase during testing (undrained conditions). The results have shown that the lateral pressure exerted by plastic concrete is highly affected by the development of shear strength properties which include: 1) the frictional resistance and interlocking among solid particles; and 2) the cohesion bonds among the particles due to cement hydration. The former component of shear strength, termed internal friction, was found to be directly affected by the coarse aggregates and binder types, and it mainly influences the decrease in initial maximum pressure measured right after concrete placement. For longer elapsed periods, the increase in cohesion (whether due to chemical or physical causes) was primarily found to accelerate pressure drop over time by enabling the plastic concrete to develop higher shear strength through the formation of a gel structure capable of carrying an increasing fraction of vertical load.1-6

Assaad and Khayat used a rheological approach to assess the effect of mixture parameters on the development of shear strength properties, and thereby lateral pressure variations.1-6 Such an approach consists of measuring the degree of restructuring (or thixotropy) of fresh concrete using a modified rheometer having an H-shaped impeller and rotating coaxially around the main shaft. The results obtained indicated that thixotropy is indeed an accurate index that can be used to assess the variations in lateral pressure. The greater the degree of thixotropy, the less initial lateral pressure can be measured, and the faster the pressure can decrease with time following casting. This was attributed to the reversible effect of thixotropy, which enables the material to regain its shear strength properties when left at rest without shearing.

Recently, Gregori et al. proposed a new approach to evaluate formwork pressure, which consists of pressurizing a fresh concrete sample placed in a steel cylinder measuring 300 mm (11.8 in.) in height and 150 mm (5.9 in.) in diameter.7 Controlled axial load was applied using a piston of a mechanical testing system to simulate various heads of concrete as well as different casting rates. SCC mixtures having a 680 ± 20 mm (26.7 ± 0.8 in.) slump flow made with different water-cementitious material ratios (w/cm) were investigated. The authors reported that formwork pressures are less than the hydrostatic, with the higher ones obtained for higher w/cm and casting rates. A 50% reduction of formwork pressure compared to the hydrostatic value was recorded for a 0.32 w/cm SCC cast at 7 m/h (23 ft/h). It is important to note that Gregori et al. suggested that, in practice, absence of water drainage and immobility of the fluid phase (cement paste) are reasonable hypotheses for real casting of SCC in watertight formworks.7 This is why the tests were conducted under undrained conditions by taking special measures to avoid loss of fluid during the pressurization process of fresh concrete. The authors reported that the portion of load initially carried by the fluid phase is gradually transferred to the solid skeleton through a phenomenon by which the water tends to no longer be a free component in the system, but is chemically combined by hydration mechanisms.7