Methodology for the estimation of expansive cement borehole pressure

This work is part of a multi-phase project which aims to develop a sound methodology for rock fragmen-tation in underground mines using expansive cement.More specifically,it is the first phase of the project which focuses on laboratory tests to investigate the mechanical performance of expansive cement,also known as soundless chemical demolition agents(SCDA).This paper reports the results of laboratory tests conducted on instrumented thick-walled cylinders filled with expansive cement.Expansive pressure evo-lution and temperature variation with time are first examined for different borehole diameters.The clas-sical analytical method for expansive pressure estimation is validated with direct pressure measurement using high-capacity pressure sensor,and an empirical model is obtained.A new methodology based on iterative procedure is developed using axisymmetric finite element modelling and test results to derive the modulus of elasticity of the expansive cement at peak pressure.The results of this study show that the expansive pressure increases with borehole diameter when the rigidity of the steel cylinder is constant reaching 83 MPa for a 38.1 mm borehole.It is also shown that the expansive pressure decreases signif-icantly with increased cylinder rigidity for the same borehole diameter.The newly developed methodol-ogy revealed that the modulus of elasticity of expansive cement at peak pressure is estimated at 8.2 GPa.A discussion on the extension of the findings of this work to hard rock mining applications is presented.

The Hydration Characteristics and Expansion Machanism of Expansive Cement at Low W/B Ratio

The hydration characteristics and expansion impetus of three kinds of cement paste under free- and confined-curing conditions were investigated, which were respectively mixed with three different kinds of expansive agent at low W/B ratio.The results show that the hydration products of pure cement paste and paste mixed with expansive agent are same,but the amount of hydration products,un-hydrated C 3S and C 2S are obviously different at the same hydration age.At 3d age,the amount of CH in pure cement paste is less than that of paste mixed with expansive agent,but it is reverse when at 28d age.The amount of AFt at 3d and 28d age in pure cement paste is less than those of paste mixed with expansive agent.Regardless of under free- or confined-curing condition,the amount of ettringite produced varies little since 3d age.The joint effect of the tumefaction of gel-ettringite due to water absorption and the expansive pressure on the pore caused by the crystalloid ettringite is the cause of the volume expansion of cement paste,and the former effect is much greater than the latter.

Experimental Study of Concrete Column Shape Modification Using Fiber Reinforced Polymer Composite Shells and Expansive Cement Concrete

Fiber Reinforced Polymer （FRP） composites are an effective material for strengthening circular concrete columns. The effectiveness of FRP confinement for square and rectangular columns is greatly reduced due to stress concentrations at the sharp comers and loss of the membrane effect at the fiat sides of the cross-section. Shape modification can eliminate the effects of column comers and flat sides, and thereby restore the membrane effect and improve the compressive behavior of FRP-confined square and rectangular concrete columns. Shape modification using chemical post-tensioning, achieved by using expansive cement concrete, is described and several mix designs for obtaining the optimal level of expansion are presented. In addition, parametric studies regarding the optimal geometry of the shape-modified cross-section are presented utilizing the analytical model.

Mix design and expansion behavior of sulpho-aluminate expansive cementitious composite

Sulpho-aluminate expansive cementitious composite is proved to be one of the most effective ways to prevent concrete cracking too soon. Mix design of sulpho-aluminate expansive cementitious composite completely depends on experience and experiments at present. This method ignores the influence of expansion behavior which hinders the application of sulpho-aluminate expansive cementitious composite. The workability,free expansion property,flexural strength and compressive strength of sulpho-aluminate expansive cementitious composite have been investigated by tests. The relationship between expansion ratio and mix design parameters such as curing age,water-cement ratio and cement abundance coefficient is deduced according to the experimental statistics. A new simplified approach to mix design of sulpho-aluminate expansive cementitious composite is proposed as a reference for construction application,which avoids experiments and experience.

A novel V-cut method for explosive-free breakage of biaxially loaded rock using soundless chemical demolition agents

Interest in soundless chemical demolition agents(SCDAs),also known as expansive cements,as potentially viable alternatives to explosives for rock fragmentation,has been growing in recent years.Consequently,there is an increasing amount of literature on the use of SCDA for the breakage of rock blocks and boulders.Limited research has been conducted so far on the breakage of excavation fronts,such as tunnel or drift faces,using SCDA.This is due to the perception that the planar compressive in-situ stresses in the face would inhibit the creation and propagation of fracturing due to expansive pressure.This study proposes a novel V-cut method for demolishing rock panels under biaxial stress using SCDA.This method was examined through large-scale tests and numerical modelling.The rock panels were subjected to high biaxial confinements of 26 MPa and 40 MPa.Such a level of confinement corresponds to an in-situ stress state 1000 m below the surface in the Canadian shield.The V-cut drillhole pattern employs two sets of three SCDA holes angled at 45
from the face of a Stanstead granite panel.The drillhole arrangement aims to create a V-shaped wedge in the plane of major principal stress.When angled drillholes are subjected to expansive pressure,they tend to cast out of the panel face,causing fragmentation.Two panels of 1 m1 m0.25 m were successfully demolished using the proposed method.The three-dimensional fast Lagrangian analysis code FLAC3D modelling was used to reconstruct the panel failure mechanism owing to the V-cut.This study demonstrates the feasibility of fragmenting an excavation front,such as a rock excavation face,with SCDA using a V-cut drill hole pattern while subjected to high biaxial confinement.