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12 Twenty- to 24-hour EOT Materials 2.2.1 Boundary Identification and Material Removal Table 5 summarizes the 20- to 24-hour EOT concrete mix- ture characteristics as specified by 11 SHAs. As was true with The first step in the repair process is to identify the extent the 6- to 8-hour EOT concrete mixtures, time to opening with of deterioration and establish the repair boundaries. Guid- the 20- to 24-hour EOT concrete mixtures is frequently stipu- ance is provided in several publications (ACPA 1995, NHI lated in the specifications, most often being linked to strength 2001, FHWA 2003), but the critical factor is to ensure that requirements. The opening criterion presented in Table 5 lists the entire area of deterioration is removed and that minimum time to opening that varied from as early as 12 hours to as repair lengths (1.8 m [6 ft]) are obtained when the repair is late as 48 hours. In two cases, only a time to opening crite- dowelled. The width of the patch should always be a full lane rion is provided, but in all other cases, a strength criterion width for jointed concrete pavements. exists in addition to time to opening, or strength is used as the Saw cutting and subsequent removal of existing materials sole criterion for opening. The range in required compressive depend primarily on the type of pavement being rehabili- strength at opening varied from 17 to 24 MPa (2,500 to 3,500 tated. In the case of jointed concrete pavement (JCP), a full- psi), whereas the range in required minimum flexural strength depth saw cut with a diamond saw blade is recommended. (as measured by third-point loading) was 2.1 to 4.2 MPa (300 The result of this process is a smooth surface with reduced to 600 psi). Although some states (Maryland, Missouri, and potential for spalling during removal. When saw cutting a Ohio) specified the same strength criterion for both the 6- to continuously reinforced concrete pavement (CRCP), two 8-hour EOT concrete and the 20- to 24-hour EOT concrete, cuts are made at each end of the repair. The first is a partial- others (Michigan and Arkansas) reported higher strength depth cut made at the outside edge of the repair area. This cut requirements for the 20- to 24-hour EOT concrete. is followed by a full-depth cut in the interior of the repair at All of the 11 states required use of Type I, II, or III port- a distance dependent on the lap length requirement (610 mm land cement for the 20- to 24-hour EOT concrete. The spec- [24 in.] for tied laps and 200 mm [8 in.] for mechanical or ified minimum cement content ranged from 335 to 502 kg/m3 welded laps) (FHWA 2003). (564 to 846 lb/yd3), with only one state, Texas, specifying Upon completion of the saw cutting, the concrete can be different minimum cement contents for Type I versus Type removed by one of two methods. One method involves break- III (390 versus 335 kg/m3 [658 versus 564 lb/yd3], respec- ing up the concrete into small pieces and removing them tively). The use of accelerators was either not specified or using hand tools or construction equipment. The other method, optional for many of these mixtures. Exceptions were Michi- called the lift-out method, involves removing the existing gan, which specified that calcium chloride be used if ambi- slab section in one or more large pieces, thus causing less ent temperatures fell below 18C (65F), and Georgia and damage to the subbase than the first method does. Illinois, which specified that an accelerator be used. The w/c After removal of the concrete, the subbase must be care- ratio for the 20- to 24-hour EOT concrete mixtures were typ- fully prepared to ensure uniformity of support. If the existing ically higher than those specified for 6- to 8-hour mixtures, ranging from 0.42 to 0.53. None of the states except Indiana subbase was damaged during removal, it may require the approved the use of a supplementary cementitious material addition and compaction of new subbase material. Often it is (fly ash, GGBFS, and silica fume) for use in 20- to 24-hour difficult to adequately compact a disturbed subbase within EOT concrete. Indiana allowed the use of a 10-percent fly ash the confines of a repair area. If the subbase disturbance is or 15-percent GGBFS addition. isolated to the very surface, the disturbed material can be removed and replaced with EOT concrete. 2.2 CONSTRUCTION CONSIDERATIONS In addition to the selection and proportioning of constituent 2.2.2 Load Transfer Restoration materials, specialized construction aspects need to be consid- ered when repairs are constructed with EOT concrete. Con- The restoration of load transfer ensures that spalling of the struction of EOT concrete repairs consists of five basic opera- concrete and damage to the subbase does not occur because tions: repair boundary identification and material removal, of rotation or movement of the patch. This restoration can be load transfer installation, batching, finishing, and curing. accomplished by splicing together existing rebar, installing Although many ways exist to accomplish each task, gener- new rebar (in the case of CRCP), or installing dowel bars in ally accepted guidelines and practices are presented in a num- JCP (FHWA 2003). Current practice is to drill multiple holes ber of publications (ACPA 1994, NHI 2001, FHWA 2003). for dowel bars simultaneously using gang-mounted drill bits. The following is a brief summary of the sequence used to con- Grout is inserted into the hole, the dowel bar is inserted with struct full-depth pavement repairs with a particular emphasis a twist, and a grout retention disk is used to prevent outflow. on EOT concrete installations. Proper dowel bar alignment is critical and must be ensured.

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13 TABLE 6 Blanket use recommendations (FHWA 1994) Minimum Ambient Opening Time, Hr Temperature in Period 8 16 24 36 48 <10oC (50oF) Yes Yes Yes Yes No 1018oC (5065oF) Yes Yes Yes No No 1827oC (6580oF) Yes No No No No >27oC (80oF) No No No No No 2.2.3 Batching early strength gain of EOT concrete materials. Protection against moisture loss becomes critical for EOT concrete Regardless of whether the EOT concrete materials are repairs if high temperature, low humidity, high winds, or a batched at the job site or at a batching facility, it is important combination of these environmental conditions exists. that the concrete produced be uniform in consistency and that Many SHAs use AASHTO M 148 Class A liquid curing the constituent materials be intimately blended through adher- compounds for accelerated concrete paving under normal ence to a proper mixing sequence and time. Also, admixtures placement conditions. Among these curing compounds, white- must be added to fresh concrete in appropriate dosages and pigmented compound (Type II Class A) is the most com- order to avoid potential harmful effects. Concrete-containing, monly used. This material has the potential to create a seal air-entraining admixtures must be sufficiently mixed to ensure that minimizes evaporation of mixing water when it is applied the development of an adequate air-void system. Delays in to the surface and exposed edges of concrete. The white color placing the concrete, especially after the accelerator has been also assists in reflecting solar radiation during bright days to added, must be avoided because early setting may negatively prevent excessive heat development on the concrete surface. impact consolidation of the repair. This might not be a desirable outcome for EOT concrete repairs where heat generated by solar radiation accelerates hydration and thus early strength gain. Concrete repairs 2.2.4 Finishing located in mountainous and arid climates may require heav- Finishing operations should be performed in a timely fash- ier dosage rates of resin-based curing compound meeting ion. It is imperative with EOT concrete, as with other types AASHTO M 148, Type 2, Class B requirements. This is of concrete, that the surface not be overworked. However, largely because concrete in harsher climatic conditions is because the highearly-strength materials set rapidly, timing more susceptible to plastic-shrinkage cracking. An applica- of this step is even more critical. Surfaces that are over- tion rate of 5.0 m2/L (22 yd2/gal) is recommended for these worked often become brittle, more susceptible to abrasion materials (FHWA 2003). and/or freeze-thaw damage, and may exhibit a lowered resis- In addition to curing compounds, insulating blankets are tance to chemical attack. Trapping of bleed water must also often used in conjunction with EOT concrete materials to be avoided. assist in holding in heat produced by the rapidly hydrating cement paste, thereby aiding in early strength development of the concrete. These blankets are often essential when cool 2.2.5 Curing ambient temperatures are present. Insulating blankets do not reduce the need for a curing compound, as blankets typically Internal concrete temperature and moisture directly influ- do not decrease the likelihood that plastic-shrinkage crack- ence early and ultimate concrete properties, and thus curing ing will occur. Table 6 indicates when insulation is recom- takes on special importance in EOT concrete installations. mended based on ambient temperatures and desired opening Proper curing provisions are necessary to maintain a satisfac- time (FHWA 1994). It is not recommended to place blankets tory moisture and temperature condition for a sufficient time too soon after applying a curing compound. In warm condi- to ensure proper hydration (FHWA 1994). Compared with tions, waiting several hours and placing the blankets as the normal paving concrete, curing is even more essential to work progresses is acceptable. Concrete exposed to temper- retain moisture and heat necessary for hydration during the ature below 4C (40F) may need additional blankets.