Reinforced concrete masonry wall is constructed by assembling of masonry units for example concrete block or bricks, mortars, reinforcing, and sometimes grout that is a kind of soupy concrete. In this article properties of materials employed in the construction of reinforced concrete masonry walls are discussed.
Properties of Materials for Reinforced Concrete Masonry Walls Construction
Following are the types of materials used in reinforced concrete masonry walls construction:
- Concrete masonry units
- Steel reinforcement
Properties of Concrete Masonry Units for Reinforced Concrete Masonry Walls
ASTM C90 (Standard Specification for Loadbearing Concrete Masonry Units) regulated the properties of concrete masonry units. Solid and hollow masonry units which are made from Portland cement, water, and mineral aggregates, are covered by the ASTM C90.
Minimum thickness limits of face shell and web (shown in Figure 1) of hollow concrete masonry units are depend on the masonry wall thickness.
Figure-1: Hollow Concrete Masonry Unit
Moreover, compressive strength of concrete masonry unit specified as 13.1 MPa which is an average for three units and minimum of 11.72 MPa for single unit. For example, if compressive strength of three units were 12 MPa, 13 MPa, and 13.85 MPa respectively, then the test would not be acceptable and fail because the average compressive strength is 12.95 MPa which is below 13.1 MPa.
Furthermore, it is uncommon to designate compressive strength larger than minimum value because availability and prices of units with higher strength will become an issue when high compressive strength is specified.
ASTM C90 specification states that, dry shrinkage of unit should not surpass 0.065 percent otherwise walls which are constructed from those concrete masonry units will probably develop shrinkage cracks.
Another limitation that the ASTM C90 determined is water absorption of concrete masonry units and it is provided in Table-1.
As it can be seen from the table, more absorption is allowed for lightweight unit compare to medium and normal units because aggregates of the lightweight unit absorb more water. Figure-2 show assembled concrete masonry units.
Table-1: Water Absorption Requirement of Reinforced Masonry Units
Figure-2: Placed Masonry Units
Properties of Mortar for Reinforced Concrete Masonry Walls
Mortar keeps concrete masonry units apart and keeps them together at the same time. Generally, the mortar thickness between units is about 9.5 mm but it could be modified to make rooms for concrete masonry dimension variations; in this way mortar keep units apart.
Moreover, mortar holds concrete masonry units together by developing a bond between them through which flexural and direct tensile strength is produced. The tensile strength might not be significant in reinforced concrete masonry as in unreinforced masonry; nonetheless it is still crucial for the wall.
A horizontal joint, which is called bed joint, is laid in face shell and mortar is placed on face shell only not the webs. Furthermore, vertical joints are called head joints, and mortar is poured into the head joints to a depth equal to the thickness of the face shell on both sides of the wall. Both head joint and bed joint can be seen from Figure-3.
Figure-3: Head and Bed Joints in Reinforced Concrete Masonry Wall
Standard Specification for Mortar for Unit Masonry (ASTM C 270), governs mortar that is used in the construction of reinforced masonry wall and the ASTM C 270 specifies three cementitious systems namely masonry cement, cement and lime, and mortar cement.
Masonry cements are a mixture of Portland or other cements, plasticizing, fillers agents, and other materials that are intended to improve the mortar achievement. Cement-lime mortar is composed by blending cement such as Portland cement or blended cement, hydrated lime, sand, water, and sometimes admixtures.
Mortar cement is the most desirable one because of its extra flexural bond strength which is lacking in other types of mortars. That is why it can be applied in high seismic regions for instance seismic design category D and higher. This application is not allowed for other types of mortar.
ASTM C 270 classified mortars into M, S, N, O types and the strongest type is M then others respectively. The type of mortar used in the construction of majority of reinforced masonry wall is either Type S or Type N. additionally, Type S or Type M is applied in the seismic category D or higher.
Finally, mortars are classified by either proportion or by property, not both. Categorizing mortars by proportion is a default in the case the designer does not determine the mortar classification.
Properties of Grout for Reinforced Concrete Masonry Walls
ASTM C 476, Standard Specification for Grout for Masonry, govern gouts employed in reinforced concrete masonry walls. The components of masonry grout are cement, water which might contain lime, sand, coarse aggregate, or admixtures.
The most prime reason of grout application is for bonding concrete masonry units to the steel reinforcement. The steel reinforcement is installed in the masonry unit cell, which is about 100 mm2, after that remaining void of the hollow is filled with grout.
Due to the absorptive property of masonry units, the slump of grout must be greater than that of concrete. The poured grouts in masonry cell is subjected to suction and the mixture will lose water.
The space in masonry cells is not filled properly unless the grout is soupy at the beginning of pouring. Water absorption can be observed on the side of masonry unit which its color changes to grey, so lacking this color after grouting might be evidence that the grout has not reached to the bottom of the masonry units.
The grout is appropriate for placing if its slump is between 20 cm to 28 cm. The slump of grout might seem a lot but site engineer should be aware that it is required to pour the grout properly. The mixture will lose water short time after placing and it will be much stronger.
Finally, proper vibration equipment is applied to compact the grout. Figure-4 shows cells in concrete masonry units in which steel reinforcement is installed then filled with grout.
Figure-4: Cells in Concrete Masonry Units
Properties of Steel Reinforcement for Reinforced Concrete Walls
Steel reinforcement used for reinforced concrete masonry wall construction is same that is used in other reinforced concrete constructions.
Joint steel bars as per ASTM A 951, is exclusive to masonry and it is galvanized to be protected against corrosion. The main task of joint reinforcement is to control shrinkage cracks.
Steel bars are installed vertically in the concrete masonry cell and it is advised to employed spacer to place bars in the exact position. Reinforcement is placed in one layer so it installed at the center of the unit.
Steel reinforcement can also be used on one side of the masonry units. For example, when lateral load like soil acts on the reinforced masonry wall or in the case of wind load if the wall is not subjected to wind suction.
In either case, the designer should be aware to put steel bars in right position. For wall thickness of 30 cm or more, two layer of reinforcement can be used in seismic areas.
Finally, reinforcement can be placed horizontally such as in walls that span horizontally between masonry columns embedded in the walls.