Chapter 1. Introduction

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Curing is the process of deliberate control of moisture and temperature conditions within prescribed limits. The process allows concrete properties to develop and prevents damage as a result of drying and/or thermal gradients during the early history of the structure.

Sometimes concrete may need no additional curing procedures. For example, when environmental conditions and concrete properties are such that no significant drying or thermal stresses develop on the concrete structure, curing is not needed. However, because of their surface-to-volume ratio and exposure, pavements rarely are in this class of concrete.

Significant damage to a concrete pavement can result when conditions exceed these critical limits and when curing practices are improperly applied. Damage typically takes one of several forms: cracking, weak near-surface concrete, and mechanical damage to the concrete surface.

A number of curing-related conditions can lead to cracking. The most common problem seems to be plastic shrinkage cracking (PSC). PSC occurs when concrete is still plastic (i.e. before time of initial setting), and when excessive loss of mixing water causes shrinkage sufficient to crack the plastic concrete. PSC may take the form of relatively large, parallel, well-spaced cracks that begin shallow but may penetrate deeply into the concrete. In other cases, PSC may take the form of a fine pattern of map cracks that penetrate only 15-30 millimeters into the concrete. These are difficult to see on textured or tined pavements. These types of cracks do not seem to cause problems in some situations, but in other cases they provide an entry for deicing salts and may contribute to freezing and thawing damage.

Another cause of cracking is thermal stress. While thermal stress is often thought to be a mass concrete problem, conditions can develop in pavement concrete when steep thermal gradients develop from the surface into the depth of the pavement at early ages.

A third cause of cracking is drying shrinkage of hardened concrete. Hardened concrete is always expected to shrink on drying (unless some kind of shrinkage-compensating component is present in the concrete) regardless of the degree of curing. Ultimately, the amount of drying shrinkage is more dependent on the amount of hydrated cement paste in the concrete rather than on curing practices. This shrinkage is expected and compensated for by planned joint cutting. However, if significant drying of the hardened concrete occurs within the first few days after placement, drying shrinkage can develop that exceeds this expected shrinkage, resulting in cracking at closer intervals than expected and planned for by the contraction joints. Stresses due to drying and temperature loss may be additive, leading to increased potential for uncontrolled cracking. Insufficient curing before opening a pavement to traffic may result in concrete with poor abrasion resistance. Loss of surface mortar can then occur in areas that get heavy traffic. Poorly cured near-surface concrete is also more permeable to liquids and may contribute to more rapid deterioration from scaling following use of deicing salts. Load-bearing capacity is not normally affected by too much drying because the weakened zone is usually confined to the top 50 mm of concrete. Load bearing could be a problem during cold weather, particularly if ice has formed in partially hardened concrete. It could also be a problem if concrete is cured for an insufficient amount of time or if full-depth cracking occurred.

Inadequate curing can result in surface damage to concrete in the form of erosion of paste and spalling of the surface.

Even though poor curing practices may not result in damage, curing is rarely omitted as a construction requirement because critical conditions are difficult to predict with certainty and the damage that can occur is usually irreversible.

Because curing is almost always required in job specifications, this guide does not attempt to give guidance on when or when not to cure. Instead, this guide includes recommendations on the details of curing practice under various conditions.

The major steps in curing portland cement concrete (PCC) pavements are the same as for all concrete, and can be generally summarized as follows:

1. Protect freshly placed concrete from excessive drying and mechanical damage until sufficient physical properties are developed that final curing procedures can be implemented. American Concrete Institute (ACI) publication 308 R calls this the initial curing period, and this step normally takes several hours.(1)
2. Select and apply final curing procedures appropriate to the conditions of the work at the correct time.
3. Maintain final curing procedures (temperature and moisture balance) until sufficient physical properties have developed that deliberate curing is no longer needed. ACI 308 R calls this the final curing period. This step takes 3 C14 days.

Differences between curing concrete pavements and curing other types of concrete are discussed in this general guidance. The information in this document is intended to focus on critical details associated with curing pavements. Many of the critical details can be either determined prior to placing concrete, or at least anticipated as likely events so that corrective action can be taken during the paving and curing operation.

The information in this guide is organized around four major topics:

• Chapter 2. General testing, analysis, and planning before construction.
• Chapter 3. The initial curing period.
• Chapter 4. The final curing period.
• Chapter 5. Termination of curing and verification of curing effectiveness.

Each topic is outlined in an annotated block diagram at the beginning of the chapter.