Structural Integrity
While slipform curbing is economically feasible, it must also possess structural integrity. Concrete slipform curbing is, by design and construction, a structurally sound alternative to precast, granite or bituminous curbing.

Design
The profile design of the proposed slipform curb can be varied. Many contractor sites have different designs for their vertical curb and curb and gutter. A slipform machine can be adapted to construct any shape curb specified in the plans for a job.

Slipform curbing is aesthetically pleasing. Transitions between straight portions and multiple radii are seamless. Radii as short as two feet can be poured into one continuous piece.

Strength and Durability
Slipform curbing is stronger and more durable than precast, granite and bituminous curbing. Slipform curb has similar strength qualities as cast-in-place concrete. The slipform curb will be slightly stronger due to its lower water content and low slump placement. Slipform curb can stand up to significant wear and tear from heavy vehicles and plows, unlike bituminous curbing. The continuous nature of the extruded product also gives the curb greater strength against transverse loading. Longer pieces are less likely to be shifted or broken by plows or other large vehicles. These pieces are longer and more stable as a result of the joint construction. Typically, a control joint is sawed every ten feet on center. This cut is designed to provide a location for controlled shrinkage cracking. The saw cut is made 1-3" (25.4-76.2 mm) into the curb and helps direct the shrinkage cracks to the location of the saw cut. Once the cracks form, it becomes a keyed joint. The keyed nature of these controlled joints helps to resist transverse loading. Figure 1 to the right diagrams the keyed contraction joints and how they compare to precast joints. The precast joints are not keyed and therefore do not resist transverse loading like the slipform curbing.

Properly placed and cured slipform curbing will last as long as any well placed and properly cured cast-in-place concrete. Proper curing of slipform curbing can include the use of seal spray to assist the curing process, helping to strengthen material. This seal spray meets ASTM specifications for curing compound. As a results of the high strength qualities of slipform curbing, some contractors have witnessed plow blades breaking on slipform curbing.

Dirigo Slipform uses a standard 4000 psi concrete mix design with fiber reinforcement. This provides more adequate concrete strength for these applications. In some casts we pour over rebar cages or mats.

An advantage of slipform curbing over precast, granite or bituminous is the interface between the subgrade and the curb. Bituminous curbing can easily shear off of the subgrade when snowplows strike it. The bituminous curbing shears easily because it does not bond with the subgrade properly. Bituminous curbing is also a much softer and weaker material, which is why so much of it must be replaced after each winter.

Precast and granite pose similar problems with subgrade bonding. With precast and granite curb, each unit is placed onto a prepared subgrade. The block merely sits on the subgrade. With slipform curb, the extruded mixture fills subgrade voids and bonds the curb to the subgrade. The result is better stability and lessens the possibility of differential settling. Figure 2 to the right demonstrates the difference between precast, granite and slipform interfaces with the subgrade