570.5 Mudjacking (Slab Jacking) by MoDOT Maintenance
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| The WB lanes of I-44 at the Gasconade River open to traffic after being closed for 20 days instead of 60 days because of a new-to-Missouri "bridge jacking" technique. Hydraulic jacks slide a new 670 ft. long bridge deck with underlying girders attached, weighing 2000 tons, into place in one piece during a 10-hour operation. |
The process of raising concrete pavement slabs and filling voids under them is known as "mudjacking", or more recently referred to as "slab jacking". This operation requires the use of a special machine commonly called a mudjack which pumps a grout mixture under the pavement to fill the voids. Another method of filling voids and stabilizing concrete slabs involves the use of a polyurethane material. When either of these methods are used, it can accomplish two jobs very well. It can be used to raise a concrete pavement slab that has settled and to fill holes or cavities which have been formed beneath a concrete slab by pumping action or by settlement of the subgrade, such as to cause bridging by the concrete slab. Generally due to the cost of the material, polyurethane is only used to raise and stabilize the pavement or fill small voids. The polyurethane material should not be used to fill large voids.
When a mudjack is used, the work must be done by an experienced crew under careful supervision. When polyurethane is used the work is usually contracted, since specialized equipment is required to mix and pump the components of the urethane mixture.
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| EPG 948 Incident Response Plan and Emergency Response Management |
Badly cracked or broken slabs cannot be raised effectively, and bituminous patches or decks should be considered for these sections. Pavement leveling or mudjacking to raise settlements is normally done during cool weather in the spring and fall when joints are open, but may be done at any time to fill voids. Mudjacking must be done as soon as settlements or voids are evident. Oil streaks on the pavement or shoulder settlements are usually indicative of voids or settlements. Consideration should be given to mudjacking the most abrupt and deepest settlements and voids. Long gradual settlements are usually not objectionable.
Asphaltic concrete upper deck on concrete sections may be mudjacked, but a longer nozzle will be required and care must be exercised so that the slurry is not pumped between the concrete surface and the asphaltic deck.
| See Also |
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| Maintenance Planning Guideline for Pavement Leveling (Mudjacking or Other Methods) |
| Slab Stabilization |
570.5.1 Mudjacking Using Polyurethane
Mudjacking pavement with polyurethane involves injecting polyurethane material under the pavement at specific locations to fill voids and raise the pavement back to it original elevation.
570.5.1.1 Material
The material for raising and undersealing pavements consists of a water blown, closed cell, high density polyurethane that meets the material specifications in the Missouri Standard Specifications for Highway Construction. The material used must hydrophobic so that the injected product is not significantly compromised by soil moisture or free water under the pavement.
570.5.1.2 Equipment
The equipment used for undersealing generally consists of pumping unit designed and capable of injecting the high density polyurethane material beneath the pavement. The pumping unit is equipped with a dial gauge having pressure increments in 0.1 pound to allow for controlling the rate of flow of material as well as the rate of rise of the pavement. In addition to the pumping unit, pressure and temperature control devices are used to maintain the proper temperature and proportionate mixing of the polyurethane component materials as they are pumped under the pavement.
Pneumatic or electric drills are used to drill 9/16 to 3/4-inch diameter injection holes through the pavement for injecting the material under the pavement. Drilling patterns are generally laid out to avoid damaging the structural integrity of the existing pavement.
Laser levels or dial indicator devices are used to monitor and verify that the pavement is raised to an even plane and to the required elevation.
570.5.2 Mudjacking Using Cementitious Grout
570.5.2.1 Mudjack Crew Requirements
The actual work of mudjacking is performed by a crew of 5 workers plus flagmen under the supervision of a Special Maintenance Supervisor. The crew consists of a skilled nozzle handler, a pump operator, two material feeders and one worker to watch the slab and plug the holes. After start up and when pressure starts to build one material feeder should help the nozzle handler. Mudjacking equipment and hoses should be kept in good condition at all times and cleaned after each use. It should be reconditioned during the winter and hoses replaced on a regular basis to prevent hose "blow outs" on the job.
570.5.2.2 Size and Location of Holes
Holes for mudjacking are generally 2 5/8 in. in diameter and spacing should be done by experienced personnel. Each settlement to be raised may vary and joints and cracks must be considered. A normal spacing for two lane (24 ft. pavement) where both lanes have settled would be to place holes approximately 6 ft. apart and 36 in. from the centerline and edges of the pavement in both lanes. Holes along both sides of the centerline should be staggered. Past experience and field conditions provide the best guide.
If a slab is down on only one side or at only one end and only a few holes in the settled part would be needed to raise that part, it is desirable to locate the holes as if it were necessary to raise a much larger portion of the slab. When this plan is adopted, the holes that are drilled first will fit into the regular arrangement of holes in case further settlement should make it necessary to drill and pump a larger area at a later date.
When grout is pumped through the hole, it will tend to flow about the same distance in all directions form the hole and will; thus fill a circular area with the hole in the center. It is, therefore, desirable to space all the holes so that they will be about the same distance apart or will lie in circular patterns about one another. Holes in adjacent slabs can be located so as to conform to this arrangement in a general way, although it may be necessary to shift the holes at the end of the slab from the regular pattern. If the slab is badly broken, or the portion to be raised is amll, it is advisable to locate the holes at about the spots at which jacks would be placed if it were possible to insert jacks under the slab.
570.5.2.3 Materials for Mudjacking
Mudjacking is usually done with a mixture of selected top soil or loam, free from stones, roots, and other foreign material which would clog the equipment, and Portland cement and water. The soil should have a low shrinkage factor and 9 sacks of cement per cubic yard of soil is to be added to reduce the percentage of shrinking, making it less permeable to free water, and to serve as a hardening agent. Where a suitable loam or top soil is not available, "fine blow" sand, commercial agricultural lime, mineral filler or other fine sands, having well rounded particles may be used. However, these materials will accelerate wear of the mudjack cylinders. If finely ground limestone is used, it should contain a minimum of 90% calcium carbonate with 100% passing the #50 sieve, and not less than 60% passing the #200 sieve. The recommended proportion of cement to these other materials is 6 sacks of cement to one cubic yard of material. The amount of water needed in the mixture will vary with the consistency desired.
When a mudjack is used to correct pumping of a concrete slab, it is best to fill the holes under slab with a thin grout. This grout should be a little more fluid than that used for raising a slab so that it will seek out and fill the cavities more readily.
When the mudjack is employed for raising a slab which is settled, it is necessary to use a somewhat stiffer mix than that which is used for the correction of pumping. Great care must be taken, however, to make sure that the slurry is not too stiff. The use of a mix that is too stiff can cause "stools" (build-up) to form under the slab around each drill hole. This leaves an unsupported slab area between drill holes which will probably result in damage to the slab. In other words, a grout that is too stiff will not flow far enough from the point of entry at the drill hole.
570.5.2.4 Order of Pumping
After all holes have been drilled, the next step is to raise the settled slab. In this operation, care must be taken to make sure that the slab is raised only a very small amount and not more than 1/4 in. at each hole at a time, and that the grout is applied under the settled portion in such a way that no great strain is developed at any one place. It is usually necessary to proceed back and forth along slab. As a portion of the slab under which grout is being pumped is brought back to its original shape, the pumping is extended in each direction until the entire dip is at the desired elevation.
It is not advisable to bring the portion of the slab near the middle of the dip up to the level indicated by the straight dotted line in the side view of the Order of Pumping figure below. A sharp bend would then be produced at Section B and the slab would crack there. Naturally, the middle portion must be raised faster than a portion nearer an end of the dip, but lifting must be carried on along the dip in such a way as to give the shape shown on Line C at an intermediate stage.
Because of the many changeable factors, no definite order of pumping can be followed strictly in lifting a slab. However, good results will be obtained if a certain system is followed in a general way. For example, the condition shown in the figure could be corrected by use of a mudjack. The holes would be located as shown in the top view. As stated above, raising must be started at the middle of the dip. The numbers near the holes indicate the order in which the holes are used for pumping grout under the slab.
Because of the shape of the dip, the holes through which grout is pumped first must receive more than the holes on either side. Pumping may be started at either edge of the pavement. In the sample shown, it will be assumed that the edge shown towards the top of the page is raised first. After some grout has been pumped through the hole where the no. 1 is placed, the next pumping is done through the hole at 2. This pumping at 2 will relieve the strain that was set up in the slab by the pumping at 1. As indicated by the position of the no. 3, the next pumping is again at the first hole. Then the positions of the nos. 4, 5, 6, 7 and 8 show that the raising proceeds back and forth on each side of the starting hole. Thus, there will be four pumpings at the starting hole while there are only two at each hole next to it. The nos. 4 and 2 in circles represent the numbers of pumpings at the holes. As previously stated, only a small amount of grout is pumped through a hole at one time, not more than enough to raise the slab 1/4 in.
The portion of the slab in the middle of the pavement width is now raised by pumping the grout through three or four holes in that area. The holes must be used in proper order. If both edges of the slab were about the same elevation, the next pumping must be done at the other edge shown near the bottom of the figure above. As indicated by the sequence of numbers from 1 to 13, the raising procedure is similar to that near the other edge, but it is carried a little farther. The pumping can be continued by going back to the middle of the pavement and then to the edge first raised. The operations are repeated in a similar manner until the entire slab has been brought to the desired elevation.
If raising were started by using one hole near each edge of slab and one hole near the center of the width, the slab would probably crack. The slab must be carefully watched at all times during the raising operations because the amount of lift cannot always be determined by the amount of grout pumped and serious cracking will certainly develop if the slab is raised too far in one place at any time. The procedure for pumping just described is intended to serve only as a somewhat general guide. On a particular job, the elevation must be checked with a stringline or a straight edge or by sighting.
The last hole at each end of a dip is never used until the slab is at the desired grade. Then a very thin grout is pumped through each end hole to assure completely filling of the thin wedge shaped opening, which will have then formed at this part of the dip. Slabs that are not badly cracked before or during mudjacking operations can be raised several inches.
570.5.2.5 Mudjacking Bridge Approaches
Refer to Mud Jacking Bridge Approach in the Bridge Preventative Maintenance Guidelines.
570.5.2.6 General Rules To Use When Mudjacking
Use 2 5/8 in. bits to drill new holes. Holes must be located at the right place.
Use of a thick grout causes "stools" under the pavement. These are very troublesome if the spaces between them are not filled with grout. The grout must be thin. If it cannot be seen in a mixing box, its consistency can be judged by listening for the correct "slouching" sound.
The best time for pumping is in cool (not cold) weather, while the slabs are not at maximum expansion. Before pumping is started, the surface of the pavement should be wetted. This will make the clean-up work much easier.
During the raising operation care must be taken to select the order of using the hole so that cracking of the slab will be prevented. A hole may be used as many times as necessary.
Pumping must proceed from the area of greater lift to the area of lower lift by rotation. If a portion of a slab being raised ends at a joint, it is usually advisable to cross over the joint and drill two holes in each lane about 2 in. from the joint. These holes must be filled last.
A string line or surveyor's level must always be used when a long slab is being raised to grade. A straight edge must be used at each joint.
The maximum permissible rate of pumping is 1 ft3 per minute for a thick mixture and about 3 ft3 per minute for a grout of average consistency. Be certain that the slab is not raised more than 1/4 in. at any one hole during one pumping. A surveyor's level should be used if necessary to ensure this.
After a drill hole has been used, it must be filed with a wooden plug. The plug can be taken out easily when the hole must be used again.
Care must be taken to make sure that each hole is used as often as necessary to provide proper support for the slab.
After all pumping has been completed all drill holes must be cleaned out and filled with a mortar composed of one part of cement to three parts of concrete sand by volume.
When a mudjack hose shows signs of excessive wear, it must be replaced promptly. Failure of a hose under pressure may cause personal injuries and damage to passing traffic. The man who handles the hose during the pumping operation must wear goggles.
570.5.3 Maintenance Planning Guideline
See Maintenance Planning Guideline for Pavement Leveling (Mudjacking or Other Methods).
Index of all Maintenance Planning Guidelines.