Chapter 5. Terminationof Curing and Verificationof Curing Effectiveness

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Figure 21 summarizes major considerations involving length of curing required and verification of effectiveness of curing.

LENGTH OF CURING

Fixed Time Intervals

The traditional prescriptive way of specifying length of curing is with fixed time periods. The requirement is usually accompanied by a minimum temperature during the specified time interval, typically 10 °C.

AASHTO

The AASHTO Guide Specifications for Highway Construction requires 3 days of curing, without comment on temperature.⁽¹⁴⁾

State DOTs

More than half of State guidance reviewed requires 3 days of curing, with no requirements on temperature during the curing period, although some DOTs had coldweather provisions requiring concrete temperatures be 10 °C. One DOT requires the temperature during those 3 days to be at °C. Several States require 4 days, without qualifications on temperature. About 25 percent of States require 7-14 days of curing, but most of these DOTs allow for a shorter period if strength reaches a prescribed level, as determined by field-cured cylinders or maturity methods.

ACI

ACI guidance is quite variable, and some standards provide for several options.

• ACI 318 (Building Code).7 days at T10 °C, or 3 days at T10 °C for high early-strength concrete (not specifically defined).⁽³¹⁾
• ACI 301 (Standard Specification for Structural Concrete).7 days or 3 days for high early strength concrete.⁽³²⁾
  • ACI 308 (Standard Practice for Curing Concrete)⁽⁴⁾
  • 3 days with type III cement.
  • 7 days with type I cement.
  • 14 days with type II cement.
  • Types IV and V, or w/ pozzolan.no recommendation.
  • Pavements.7 days at 5 °C.
• ACI 325.9 R (Guide for Construction of Concrete Pavements and Concrete Bases).7 days at 4 °C.⁽³³⁾
• ACI 330 R (Guide for Design and Construction of Concrete Parking Lots)-3 days for auto traffic, 7 days for all other traffic.⁽³⁴⁾

Time to a Specified Strength

The option to cure until a certain fraction of design strength is attained is common in ACI guidance, summarized as follows.

• ACI 301 (Standard Specification for Concrete).⁽³²⁾
  • Time to 70 percent f'c using field cured specimens.
  • Time to 85 percent f'c using lab specimens, with field temperatures 10 °C.
  • Time to 100 percent f'c using NDT methods (methods unspecified).
• ACI 308 (Standard Practice for Curing Concrete).time to 70 percent f'c— option for pavements.⁽⁴⁾
• ACI 325.9 R (Guide for Construction of Concrete Pavements and Concrete Bases).time to 70 percent f'c.⁽³³⁾
• ACI 330 R (Guide for Design and Construction of Concrete Parking Lots).until compressive strength 21 MPa.⁽³⁴⁾

Maturity

The maturity method is a calculation based on the concept that time-temperature history, rather than simple time, determines the strength development of concrete. By monitoring time-temperature histories of in-place concrete, real-time strength development can be indirectly monitored. The method is calibrated using strength development of laboratoryor field-cured specimens with a known time-temperature history. ASTM C 1074 describes the method.⁽³⁵⁾ Hardware and software are manufactured that automates much of the work, and consulting firms specializing in this procedure exist.

Equations in ASTM C 1074 can be written into a spreadsheet to simplify exploratory calculations.⁽³⁵⁾ Exploratory calculations are useful for approximate planning purposes and investigating likely effects of different temperature histories. For exploratory work, inputs of daily high and low concrete temperatures and of standard laboratory strength determinations can be used to estimate strength development for the first 7 days after placing. Predictions become more prone to error at later ages and should not be used.

In actual field application, the maturity method normally takes temperature input from inplace thermocouples located at critical points in the pavement. Determining critical locations is an important part of the application. Pavement corners, sections of elevated pavement, and most recently placed pavements are particularly sensitive to low temperature events.

VERIFICATION OF CURING

Although strength is the primary variable around which curing specifications are based, verifying adequacy of a curing program on pavements may not be best measured by strength. Several approaches are described below.

Strength of Cores (ASTM C 42)⁽³⁶⁾

The strength of concrete is strongly affected by inadequate curing, and, in theory, could be detected by measuring strength of cores taken from a concrete pavement. However, the effects of poor curing are only strongly apparent in the properties of the top 50 mm of concrete, and sometimes even less. Therefore, only thin pavements are likely to be well represented by strength testing. Compressive strength is not likely to be an effective procedure for typical highway pavements.

Rebound Hammer (ASTM C 805)⁽³⁷⁾

The rebound hammer method basically measures the modulus of elasticity of the nearsurface concrete. It is often criticized as being unduly affected by near-surface properties and insensitive to the strength of the entire section of concrete under the test point. This may actually recommend the method for use in evaluating the curing of concrete pavements, where near-surface effects are considered most important. The test method is suitable for in-place measurements and has been found in laboratory tests to be well suited for detecting curing deficiencies in near-surface pavement. There is a considerable amount of scatter in rebound numbers because of the heterogeneous nature of nearsurface properties (principally due to near-surface aggregate particles). The method directs that an average over 10 readings be taken to smooth out this effect. The method requires at least modest maturity of the concrete for the instrument to register readings, typically 1-2 days depending on the concrete mixture and temperature.

A reasonable approach to using this technique for field verification of pavements would be to select one or a few small sections of pavement over which strict curing control could be maintained. Then, using the rebound numbers in these well-cured sections as a reference, the near-surface development of the remainder of the pavement could be evaluated through a random sampling scheme.

Laboratory work has shown that rebound numbers of uncured concrete exposed to modestly severe drying are reduced by about 50 percent at 7 days relative to well-cured concrete.

Surface Water Absorption (ASTM C 1151, withdrawn)⁽³⁸⁾

It has been well established in laboratory work that the amount of water a dry concrete specimen absorbs in the first minute or so after contact with liquid water is related to the quality of the curing of the near-surface zone of the concrete. In theory, then, this method should have direct applicability to verifying curing. A number of field methods have been developed, but most suffer because of lack of control over the moisture content of the in-place concrete. The method is reasonably applied to cores, which can be dried to a constant low moisture content before testing.

The procedure is relatively simple. The top 50 mm of concrete pavement is removed by coring or sawing. The water applied during the short interval of taking the core is not significant if the core is dried in an oven (>60 °C) within no more than a few hours after extraction. The core is so dried for 24 hours, cooled, and weighed, and then the surface of the core representing the surface of the pavement is placed on a towel saturated with water. Sixty seconds is a reasonable exposure time. The core is then reweighed and the mass of water absorbed and the surface area of the concrete are calculated. The result is expressed in units of kg/m². If curing compound is on the surface of the core, it must be removed prior to testing. A powered wire brush is suitable for this. Sometimes a surface cut more than 50 mm from the finished surface is used as a well-cured standard. Although such a surface is probably well cured, it has probably experienced a different type of mechanical action during placing and finishing that make it not strictly comparable with the finished surface.

Well-cured concrete can serve as a control. As with the rebound hammer method, described above, select a small section of concrete over which control of curing can be assured, then take cores and use them as a reference.

Ultrasonic Pulse Velocity

The ultrasonic pulse velocity (UPV) method is an indirect measure of the modulus of elasticity of concrete. The modulus of elasticity of concrete tends to increase with increasing hydration (or quality of curing) of the cement paste fraction of the concrete. UPV testing can be set up in a number of configurations, each of which tends to focus on slightly different features of the concrete. A simple pulse velocity taken through a piece of concrete, which is the traditional way of using UPV to investigate concrete properties, gives information on the average quality of the concrete. This method would be difficult to apply to concrete pavements. UPV testing can be configured to measure the speed of wave propagation in the near-surface zone of the concrete. This configuration should be quite useful for monitoring curing.

Equipment for executing the latter type of analysis is not widely available at the commercial level, but has been mostly used in research applications. The hardware and analysis software could be developed into a practical technology if there were sufficient interest to warrant the commercial development.

Abrasion Resistance

The degree of curing has been shown in numerous research publications to be strongly reflected in the abrasion resistance of the cement-paste fraction of concrete. This truth is easily verified qualitatively using an electrically powered wire brush and observing the ease with which the near-surface mortar can be removed from a small spot of concrete. Poorly cured concrete is easily abraded away, while well-cured concrete is quite difficult to abrade away with such equipment. One major difficulty with this technology is in quantifying the forces involved and the results on the concrete. The test is also sensitive to the moisture condition of the concrete.

These shortcomings could be overcome if cores were taken and standard procedures were developed for laboratory testing, but it is doubtful that the results would be a better indicator than those derived from the other tests described above.