The period from truck to the forms is the most critical time in the life of architectural concrete. The mixer truck must be capable of discharging the specified mix and slump. Some trucks will not discharge concrete with less then a two inch slump; these must be avoided where low slump concrete is specified. The discharge chute should be clean, wet, and steeply inclined to facilitate movement of the concrete and avoid its drying. Architectural concrete mixes with low slumps and high coarse aggregate factors usually require that the mix be transported to the forms in steep-sided buckets. The objective is to place the concrete within the forms without segregation from falling and without splattering concrete on the form as this will tend to harden before the subsequent layer of concrete are cast. The most important single skill in architectural concrete work involves the operation of the vibrator. Although the major function of the vibrator is to consolidate the concrete to its maximum density, it also must blend lifts of concrete within the casting and therefore should be classified as a in-place concrete mixer. If this function is not properly performed, highly objectionable lift lines will be visible on the finished surface. When the vibrator touches the concrete, the frequency causes a particle excitation and gives the impression that the concrete is more fluid than it was before contact. This particle excitation alone makes the concrete appear to flow. However, the actual sequence of events should be noted. Immediately following penetration, the vibrating head strikes particles of the coarse aggregate and, due to its mass, drives these particles away form the vicinity of the head. This accelerated particle drives outward striking another particle which again drives outward striking other particles. The mortar then begins to flow between the particles of the coarse aggregate and the vibrator head must be manipulated up and down by the operator to jiggle the aggregate and facilitate the flow of the mortar between the coarse aggregate particles. A stationary head in the concrete mass will result in a harmonic motion and cause the vibrator head to cavitate and have little influence on the concrete.