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General Guidance

  This is an informational resource on hot mix asphalt (HMA). It contains a general overview of all HMA aspects. It is intended to assist those who work with HMA in any way including architects, engineers, contractors, students, and homeowners. Pavement Checklist Pavement Distress Top-Down Cracking References Suggested Reading Asphalt Institute. (2001[1]). HMA Construction. Manual … Read more »

Figure 3. Limerock base course undergoing final grading.

HMA Pavement

HMA pavements are classified as “flexible” pavements because the total pavement structure deflects, or flexes, under loading. A flexible pavement structure is typically composed of several layers of material. Each layer receives the loads from the above layer, spreads them out, then passes on these loads to the next layer below. Thus, the further down … Read more »

Porous concrete. Note the rough texture.

Permeable Pavements

Description Permeable pavements allow water to infiltrate through surfaces that would normally be impermeable, such as asphalt or portland cement concrete parking lots. (NCDENR, 2007[1]) Rain and snow may then recharge the groundwater table, and air may pass through to nourish the roots of grass and trees. Pervious surface treatments reduce the risk of stormwater … Read more »

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Asphalt Additives

Additives Asphalt additives are any of a number of substances that can be added to the asphalt binder to alter the characteristics of the final blend of HMA. Sasol Sasol production meets about 30% of South Africa’s transportation fuel needs. There are two major plants in South Africa, one at Sasolburg, which is a natural … Read more »

Figure 1. Anti-stripping modifier example.

Asphalt Modifiers

Some asphalt cements require modification in order to meet specifications. Asphalt cement modification has been practiced for over 50 years but has received added attention in the past decade or so. The added attention can be attributed to the following factors (Roberts et al., 1996[1]): Increased demand on HMA pavements. Traffic volume, loads and tire … Read more »

Figure 2. Dowel bars in slots.

Dowel Bar Retrofit

Dowel bar retrofitting is a method used to restore or provide better load transfer across transverse joints or cracks using dowel bars. Usually, dowel bar retrofits are necessitated by excessive faulting due to a loss of aggregate interlock over time. It is interesting to note that much of the rigid pavement built in the U.S. … Read more »

Figure 2. Side tie bar inserter.

Tie Bar Placement

Tie bars are typically placed after PCC placement either by hand or using a tie bar inserter attachment (slipform paving only). When one lane at a time is paved, tie bars are inserted at mid-slab depth and bent back until the adjacent lane is ready to be paved (see Figure 1). On slipform pavers, tie … Read more »

Figure 1: Dense-Graded HMA Up Close

Mix Types

HMA Mix Types The most common type of flexible pavement surfacing in the U.S. is hot mix asphalt (HMA). Hot mix asphalt is known by many different names such as hot mix, asphalt concrete (AC or ACP), asphalt, blacktop or bitumen. For clarity, this Guide makes a conscious effort to consistently refer to this material … Read more »

Bituminous Surface Treatments_FI

Bituminous Surface Treatments

A bituminous surface treatment (BST), also known as a seal coat or chip seal, is a thin protective wearing surface that is applied to a pavement or base course. BSTs can provide all of the following: A waterproof layer to protect the underlying pavement. Increased skid resistance. A filler for existing cracks or raveled surfaces. … Read more »

Figure 1: 6-inch ACB layer under 1.5 inches of State Mix IV (Coin Shown is a Quarter)

Asphalt Concrete Base (ACB)

Asphalt concrete base (ACB), also called asphalt treated base (ATB), is a dense-graded HMA with a larger nominal maximum aggregate size (1 inch) intended for use as a base course or binder course (see Figure 1). In addition to site paving benefits, ACB can be advantageous because it can provide: A waterproof barrier to prevent … Read more »

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