A review of salt scaling: II. Mechanisms

doi:10.1016/j.cemconres.2007.03.003

Cement and Concrete Research

Volume 37, Issue 7, July 2007, Pages 1022-1034

https://doi.org/10.1016/j.cemconres.2007.03.003 Get rights and content

Abstract

Salt scaling is a major durability issue for concrete. Despite this, and an extensive research effort, the cause of this damage is unknown. Therefore, no means for preventing salt scaling can be identified. One of the primary reasons for this shortcoming is the lack of a critical review on the state of the research in this field. Such a compilation is presented in this series of articles. In Part I, the characteristics of salt scaling were outlined. In this article, proposed mechanisms are discussed, and their adequacy is judged based on their ability to account for the phenomenology.

Introduction

Concrete is the most widely used material in the world. The raw materials are ubiquitous, so it is relatively inexpensive, but concrete poses numerous durability issues that result in high maintenance costs. The National Research Council estimates that repair of the infrastructure in the United States costs nearly $50 Billion annually [1]. Moreover, the cost of new roadways, bridges, and terminals is estimated at hundreds of billions of dollars annually. Clearly, improving the durability of concrete has huge social and economic implications.

Over the past 60 years, concrete infrastructure has deteriorated by “salt scaling”, which is superficial damage caused by freezing a saline solution on the surface of a cementitious body (Fig. 1). The damage is progressive and consists of the removal of small chips or flakes of material. These characteristics were first revealed in the 1950s through laboratory testing [2], [3], and they were subsequently verified through field tests [4]. In moderate to extreme cases, this damage culminates in the exposure of the coarse aggregate.

In cold climates, salts (NaCl, CaCl₂) are regularly used to de-ice roadways and walkways. Consequently, salt scaling is one of the major durability issues facing cementitious materials in this climate. While salt scaling alone will not render a structure useless, it results in accelerated ingress of aggressive species, such as chlorides, and the propensity for a high degree of saturation. The former renders the body susceptible to corrosion of the reinforcing steel [5], [6], [7], while the latter results in strength loss from internal frost action [8], [9], [10]. Both of these effects diminish the service life of concrete (Fig. 1).

Hundreds of laboratory and field studies have clearly identified the characteristics of salt scaling, but have not explained the cause. In a companion article [11] this work was critically reviewed, and the following list of characteristics was compiled:

1.
Salt scaling consists of the progressive removal of small flakes or chips of binder.
2.
A pessimum exists at a solute concentration of ~ 3%, independent of the solute used.
3.
No scaling occurs when the pool of solution is missing from the concrete surface.
4.
No damage occurs when the minimum temperature is held above − 10 °C; the amount of damage increases as the minimum temperature decreases below − 10 °C and with longer time at the minimum temperature.
5.
Air entrainment improves salt scaling resistance.
6.
The salt concentration of the pool on the surface is more important than the salt concentration in the pore solution.
7.
Susceptibility to salt scaling is not correlated with susceptibility to internal frost action.
8.
The strength of the surface governs the ability of a cementitious body to resist salt scaling.

A number of mechanisms for salt scaling have been proposed. In the next two sections, we consider the ability of these mechanisms to account for the above characteristics. First, in Section 2 we consider mechanisms related to internal crystallization. Then in Section 3 we consider mechanisms related to the role of salt. In Section 4, we consider a new, purely physical mechanism known as glue spalling. Finally, we summarize the implications of our analysis in Section 5.

Section snippets

Internal crystallization

In spite of evidence that indicates the distinct nature of salt scaling and internal frost action [11], considerable attention has been paid to internal crystallization as a cause of salt scaling. Therefore, in the interest of completeness it is necessary to discuss this topic. Much of the discussion follows analyses by Scherer [9] and Scherer and Valenza [10] that consider the thermodynamics of crystallization in a porous medium. The role of hydraulic pressure is considered in Section 2.1 and

Role of salt

In this section we will consider several proposed mechanisms that concern the role salt plays in salt scaling. In Section 3.1 we will consider the fact that salt reduces the melting point of ice, and how this may result in stress from thermal shock. In Section 3.2 we examine the possibility that damage is caused by the precipitation of salt in the pore system. Section 3.3.1 deals with the effect of salt on the vapor pressure of water and the corresponding degree of saturation of the

Glue spalling

Recently, the glue spall mechanism was proposed as the primary cause of salt scaling [75], [77]. Glue spalling is a technique used to decorate the surface of glass with uniform shallow scallops [78], [79]. The procedure consists of sandblasting a glass surface, spreading a layer of epoxy over the surface at high temperature, and reducing the temperature of the composite (Fig. 9). As the temperature is reduced the epoxy tends to contract much more than the underlying glass, which places this

Conclusions

In Part I of this series a comprehensive review of the experimental studies on salt scaling was presented, and the characteristics of this damage were summarized [11]. In this article, previously proposed mechanisms (including internal frost action) were considered. The feasibility of these mechanisms, as well as the characteristics of salt scaling, was used as a basis for judging adequacy. None of the previously proposed mechanisms is both feasible and capable of accounting for all

Acknowledgement

This work was supported by National Science Foundation Grant CMS-0200440.

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A review of salt scaling: II. Mechanisms

Abstract

Introduction

Section snippets

Internal crystallization

Role of salt

Glue spalling

Conclusions

Acknowledgement

Cem. Concr. Res.

Cem. Concr. Res.

Thermochim. Acta

J. Colloid Interface Sci.

J. Non-Cryst. Solids

Cem. Concr. Res.

J. Mech. Phys. Solids

Cem. Concr. Res.

Cem. Concr. Res.

Int. J. Solid Struct.

Cem. Concr. Res.

Cem. Concr. Res.

Cem. Concr. Res.

Int. J. Solid Struct.

Acta Metall.

Damage on Concrete Pavements by Wintertime Salt Treatment

Studies of “salt” scaling of concrete

Highw. Res. Board. Bull.

Concrete construction or salt — which causes scaling?

Concr. Int.

The corrosion of steel — random thoughts and wishful thinking

Concr. Int.

Influence of carbonation and chlorides on concrete durability

Concrete

The international cooperative test of the critical degree of saturation method of assessing the freeze/thaw resistance of concrete

Mater. Constr.

Mechanisms of frost damage

A working hypothesis for further studies of frost resistance of concrete

J. Am. Concr. Inst.

Void spacing as a basis for producing air-entrained concrete

J. Am. Concr. Inst.

The air-requirement of frost resistant concrete

Theory of volume changes in hardened portland-cement paste during freezing

Proc. Highway Res. Board

Adsorption, Surface Area and Porosity

Theory of propagation of elastic waves in a fluid-saturated porous solid. I. Low frequency range

J. Acoust. Soc. Am.

General theory of three-dimensional consolidation

J. Appl. Phys.

General solutions of the equations of elasticity and consolidation for a porous material

J. Appl. Mech.

The elastic coefficients of the theory of consolidation

J. Appl. Mech.

Elastodynamics of gels

J. Chem. Phys.

Mechanics of Porous Continua

Measuring permeability of rigid materials by a beam-bending method: I. Theory

J. Am. Ceram. Soc.

Microscopic characterization of ice morphology in entrained air voids

ACI Mater. J.

The international cooperative test of the critical degree of saturation method of assessing the freeze/thaw resistance of concrete

Mater. Constr.

The required air content of concrete

Predicting the service life of concrete exposed to frost action through modelling of the water absorption process in the air-pore system

Lund Institute of Technology, Div. of Building Materials, Report TVBM-7085

Moisture uptake and service life of concrete exposed to frost

Theory of crystal growth in undercooled pure liquids

J. Chem. Phys.

Freezing effects in concrete