Surface Deterioration due to Aggregates and D-Line Cracking

In addition to pop-outs (described in Chapter 3) which are due to the presence of soft or laminated aggregates such as shale close to the surface of the concrete, another form of deterioration due to frost action is directly related to the presence of aggregates near the finished surface of concrete. This common phenomenon (referred to as type II pop-outs in Chapter 3) can occur when a saturated aggregate particle located just a few millimetres below the surface is subjected to freezing. In this case, the water expelled from the aggregate towards the surface creates disruptive pressures inside the thin layer of mortar above the aggregate particle. If these disruptive pressures are large enough, the mortar layer will break away leaving a shallow depression over the particle as illustrated in Figure 3.4. Even if it does not affect the structural integrity of the concrete, this type of deterioration damages the appearance of the concrete. This phenomenon is related to the characteristics of the inner pore structure of the aggregates, and also to the characteristics of the transition zone between the paste and the aggregates. Aggregates having a high porosity are the most likely to cause type II pop-outs because they can expel a relatively high amount of water. The size of the aggregate particles is also important. The use of smaller particles reduces the risks involved because the amount of water expelled per unit surface area is correspondingly reduced.

The occurrence of this type of deterioration (type II pop-outs) is not only related to the characteristics of the aggregate itself but also to the characteristics of the mortar cover. The use of a low water/cement ratio significantly reduces the permeability of cement paste and correspondingly reduces the probability that the aggregate particle will become critically saturated under natural exposure conditions. An increase of the cover thickness will have the same effect. Table 5.1 shows the results obtained by Verbeck and Landgren (1960) for the same dolomite aggregate (with a 6.68% absorption) used in the making of two concrete mixtures (with water/cement ratios of 0.70 and 0.45) and three different mortar cover thicknesses (3, 6 and 9 mm). The table gives the number of days of wetting required to produce failure of the mortar cover when subjected to freezing. This number ranges from 111 days (for a 0.70 water/cement ratio and a 3 mm mortar cover) to 980 days (for a 0.45 water/cement ratio and a 9 mm mortar cover). The differences are significant and are of practical importance because deterioration will occur only if, under field conditions, the concrete is kept at a high humidity level for a period sufficiently long for the aggregates to become critically saturated. Of course, it is not really possible to control the mortar cover in practice, but the data in Table 5.1 indicate that the water/cement ratio exerts a great influence, especially for the smaller cover thicknesses (for a 3 mm mortar cover, the period of wetting before failure moves from 111 days for a 0.70 water/cement ratio to 477 days for a 0.45 water/cement ratio). However, the use of a low water/cement ratio cannot prevent a really poor aggregate from creating frost damage.

Table 5.1 Number of days required for a dolomite aggregate to reach the degree of saturation which will cause failure of the mortar cover during freezing, as a function of the water/ cement ratio and the thickness of the mortar cover (after Verbeck and Landgren, 1960).

Water/cement ratio

(by mass) (× 10Permeability −9 _cm/s) Days of wetting sustained before _{failure (pop-out) at various}

thicknesses of mortar cover

3 mm 6 mm 9 mm

0.70 3000 111 879 792

0.45 1 477 885 980

Another type of deterioration often related to coarse aggregates is D-line cracking. D-line cracking (briefly described in Chapter 3) is defined by the ACI Manual of Concrete Practice as ‘a series of cracks in concrete near and roughly parallel to joints, edges and structural cracks’ (Figure 3.1). D-line cracking, which is not merely a surface problem but, most of the time, goes through the entire thickness of the concrete slab, very often results from the volume change of coarse aggregates (or the expulsion of water at the aggregate–paste interface) during freezing. The increased volume of the aggregate during freezing (due to ice formation in the inner pore system), or the expulsion of water, creates disruptive stresses in the surrounding cement paste which produce microcracks. The repetition of freezing and thawing cycles causes the propagation of these microcracks which progressively coalesce to form macrocracks and then become apparent at the surface of the concrete slab. The phenomenon is generally very slow, and sometimes more than 15 years will be required for macrocracking to be observed at the surface of the concrete. D-line cracking is generally

observed near gutters, joints, and edges or structural cracks, simply because these locations usually provide a higher level of relative humidity. Concrete structures which are frequently exposed to wetting, such as roads and airfields, are thus particularly affected by this type of deterioration.

The use of low water/cement ratios can be helpful in preventing D-line cracking, because the saturation of aggregates is more difficult when the paste is less permeable. Calcareous and siliceous sedimentary rocks are generally considered as the types of aggregates most likely to cause D-line cracking. With certain aggregates, D-line cracking may even occur without any freezing and thawing cycles, since the volume changes of the particles due to wetting and drying cycles are sufficient to initiate microcracks. If they are present in sufficient number, aggregate particles that cause pop-outs can also be expected to cause D-line cracking, but particles that cause D-line cracking do not necessarily cause pop-outs. D-line cracking is less likely to occur with smaller particles, because the total amount of water expelled from the particle per unit area (and thus the disruptive stresses generated into the mortar fraction) is lower.